Queiroz, M
For earlier publications, go to McQuay, MQ
1994
Evaluation of a Dimensionless Group Number to Determine Second-Einstein Temperatures in a Heat Capacity Model for All Coal Ranks
Coimbra, C.F.M. and Queiroz, M.
Combustion and Flame, (in press). Funded by ACERC.
A dimensionless group number is proposed to characterize the differences in chemical structures and physical properties between coal-like materials varying from lignites to anthracites, including graphite as a limiting case. This dimensionless number provides a simple and efficient correlation to determine second-Einstein temperatures (Ø2) in a specific heat capacity © model for all coal ranks, using information derived directly from the chemical composition (proximate and ultimate analysis) and the calorific value (H) of each substance. The nondimensional correlation has the form RØ2/H = f(FC), where R is the gas constant for the heterogeneous material and FC is the amount of fixed carbon in the parent coal. Properties of fifty coal-like materials were used to obtain this functional dependence. It was found that a linear function provides a good fit of the experimental data. This dimensionless correlation allows calculations of the behavior of the specific heat capacities of the materials studied here with an average value of 3/55% for the mean deviation in relation to experimental curves in the important temperature range of 300-600 K. The applicability of Einstein theory of heat capacity is analyzed for the special case of coal-like materials, and a generalization of Merrick's model for all coals of industrial interest is presented.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reacting Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
ASME Journal of Heat Transfer, 116:761-764, 1994. Funded by ACERC.
The understanding of turbulent reacting flows, which are characterized by the Navier-Stokes equations along with conservation equations of mass and energy, is one of the most challenging fields of engineering science. Various theoretical models based on simplifying assumptions have been developed to predict the behavior of such flows. In some of the models proposed, the problem of modeling the mean reaction rate is exchanged for the problem of describing a scalar dissipation function. Thus the scalar dissipation, which describes the destruction of the fluctuations of a passive scalar at the finest scales existing in a given flow, is an important parameter in modeling turbulent reacting flows.
The objective of this work is to present the statistics of dissipation of a conserved scalar from the DNS of a turbulent reacting shear layer. Since experimental data on the statistics of temperature dissipation in a reacting shear layer of jet flame is readily available, a comparison of this data with the DNS results will also be made.
Time-Resolved Temperature Measurements in an Elliptic Cross-Section, Turbulent, Diffusion Jet Flame
Cannon, S.M. and Queiroz, M.
1994 ASME Winter Annual Meeting, 296:37, 1994. Funded by ACERC.
An experimental study to determine the thermal structure of an elliptic cross-section, turbulent, diffusion jet flame using time-resolved temperature measurements was performed. The 2:1 aspect ratio fuel inlet allowed a fully developed turbulent flow (Reynolds number = 6000) of propane to exit into near ambient air. Measurements of mean and rms temperature, as well as power spectral densities (psd) and probability density functions (pdf) of temperature were obtained along the centerline and radially along the major and minor axis at axial stations ranging from 5<=z/Dh<=30. Similar to observed behavior in axi-symmetric diffusion flames, the non-axi-symmetric flame showed evidence of small-scale, momentum-driven vortices inside the flame zone and large-scale, buoyancy-driven vortices outside the flame zone. Differences in these turbulent shear layers along the major and minor axis were observed, as the minor axis had 25% higher temperature fluctuations along the fuel side mixing layer and the major axis had 20% higher temperature fluctuations along the air side mixing layer. A weaker fuel side shear layer along the major axis allowed more radial movement and a more radially stretched reaction zone in the near-burner region along this same major axis. This greater radial movement was sufficiently strong to cause a faster destruction of the inner vortex structures, such that less mixing would be observed along the fuel side of the major axis.
1993
Turbulent Reacting Flows
McMurtry, P.A. and Queiroz, M.
Chapter 7, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter summarizes current technologies and developments for treating reacting turbulent flows. Accurately predicting the effects of turbulence on combustion processes ranks among the most challenging problems in the engineering sciences. Since most combustion occurs in a turbulent environment, effects of turbulence must be treated in a realistic manner in any predictive method. In addition to the effects of turbulence on the chemical species transport, density changes resulting from exothermic chemical reactions can alter the structure and development of the flow field. As such, the fluid mechanics, thermodynamics, and chemistry are strongly coupled, making the description of the turbulent combustion process extremely difficult. Moreover, other complications often arise. Among these are the effects of multiple phases, an inherent aspect of entrained solids and liquids. This chapter illustrates some of the characteristics of turbulent flows and outlines some of the methods used to treat turbulence in reacting flows, while pointing out the need for improved capabilities in this field of study. First, general background on turbulent flows is provided. The most popular approaches used to model turbulence in reactive systems for engineering applications are discussed, along with a few newly introduced techniques. Some of the complications that arise in multi-phase turbulent flow are also discussed. More computationally intensive numerical approaches, used primarily in fundamental research applications, are presented. These methods include Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). Direct simulation results for simplified flow configurations are discussed in light of the information they have provided concerning the physics of turbulent reacting flows.
Thrust Area 4 Research Program: Turbulent, Reacting Fluid Mechanics and Heat Transfer
McMurtry, P.A. and Queiroz, M.
Energy & Fuels, 7 (6):814-816, 1993. Funded by ACERC.
Two of the main reasons for poor turbulence model performance are the lack of understanding of the basic physical mechanisms acting in turbulent reaction flows and a lack of reliable data needed to "tune" model parameters. In addition, the physical complexity of the turbulent mixing process does not lend itself easily to packaged models. There is a need for true innovations and new approaches to treat the turbulence in reacting flows. The work in this thrust area consists of a combination of experiments and numerical studies aimed at (1) directly improving the performance and extending the generality of comprehensive combustion computer codes; (2) developing new approaches to modeling turbulent flows; and (3) conducting fundamental research to improve our understanding of the physical processes acting in turbulent reacting flows and heat transfer. The key products anticipated from the research in this thrust area include (1) new turbulent mixing models that can be incorporated in comprehensive predictive codes for turbulent combustion processes; (2) documented experimental data on planar sprays and particle dynamics in turbulent flows; (3) documented experimental data on radiation properties in multiphase combustion applications; (4) model improvements in the ACERC computer codes to handle spray flames and pulverized particles; (5) three-dimensional radiation submodels for incorporation into the comprehensive code; and (6) fundamental data on the mechanisms of turbulent mixing and reaction.
Parametric Evaluation of a Particle Dispersion Submodel Used in a Two-Dimensional Pulverized-Coal Combustion Code
Shirolkar, J.S. and Queiroz, M.
Energy & Fuels, 7 (6):919-927, 1993. Funded by ACERC.
Particle dispersion data collected with a laser-based diagnostic technique are used to evaluate the performance of a particle dispersion submodel incorporated in a two-dimensional, comprehensive, pulverized-coal combustion code. In this code, the gas-phase mechanics is formulated in an Eulerian framework, whereas the particle phase is based on a Lagrangian scheme. The turbulence is described by the two-equation kappa-epsilon model. The experimental data available include radial profiles of small (0.4-3.5 µm) and large (3.5-98 µm) particle velocities and size-resolved particle number density of pulverized-coal particles for both reacting and isothermal conditions at different axial stations in a laboratory-scale, axisymmetric, controlled-profile reactor. Various parameters were varied during the reacting cases: the secondary air flow rate, secondary swirl number, and the initial coal-size distribution. For the isothermal conditions, only the secondary air temperature is varied for both swirling and nonswirling cases. It is observed that a reduced swirl number of 0.8 instead of the experimental swirl number of 1.4 gave relatively better predictions for the gas-phase aerodynamics, particle number density, and particle trajectory calculations for all the swirling cases. Some limitations of the particle dispersion submodel as well as the experimental data are acknowledged.
Experimentally Determined Particle Number Density Statistics in an Industrial-Scal, Pulverized-Coal-Fired Boiler
Queiroz, M.; Bonin, M.P.; Shirolkar, J.S. and Dawson, R.W.
Energy & Fuels 7:(6) 842-851, 1993. Funded by ACERC.
A study on the variations of particle data rate statistics and the probability density function (PDF) of cumulative particle number density has been completed in a full-scale, tangentially fired, 85 MWe pulverized-coal-fired boiler. Variables in the tests included boiler load and coal type. It was observed that particle data rate fluctuations were greater in magnitude for small particles (<3.5 µm) and that the PDFs of particle data rate were well approximated by normal distributions. Furthermore, there were no preferential frequencies in the large (<3.5 µm) or small particle data rate fluctuations anywhere in the boiler. The PDFs of cumulative particle number density for the small particles were negatively skewed and, as compared to the large particle PDFs, were less sensitive to boiler location. The large particle PDFs were more negatively skewed near the walls and more Gaussian as distance from the wall increased. Broader distributions of cumulative particle number density with peaks at higher values were observed for the small particles for the coal with lower volatiles and higher ash content. Moreover, for the large particles, a noticeable shift of the PDFs, longer "tails" toward higher cumulative number densities, and a substantial flattening of the PDF curves were observed for the same coal. The shape of the PDF profiles did not change substantially as the boiler load changed. The effect of a lighter load on the small particle PDFs was to slightly broaden the distribution, mostly in the direction of large cumulative particle number densities. For the large particles, a shift toward higher cumulative particle number densities, a slightly broadening effect, and a reduction in the maximum PDF values were observed at lighter load.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reacting Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
ASME Journal of Heat Transfer, 1993 (in press). Funded by ACERC.
The understanding of turbulent reacting flows, which are characterized by the Navier-Stokes equations along with conservation equations of mass and energy, is one of the most challenging fields of engineering science. Various theoretical models based on simplifying assumptions have been developed to predict the behavior of such flows. In some of the models proposed, the problem of modeling the mean reaction rate is exchanged for the problem of describing a scalar dissipation function. Thus the scalar dissipation, which describes the destruction of the fluctuations of a passive scalar at the finest scales existing in a given flow, is an important parameter in modeling turbulent reacting flows.
The objective of this work is to present the statistics of dissipation of a conserved scalar from the DNS of a turbulent reacting shear layer. Since experimental data on the statistics of temperature dissipation in a reacting shear layer of jet flame is readily available, a comparison of this data with the DNS results will also be made.
The Effect of Monsized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
Int. Journal of Exp. Heat Transfer, 6:80-89, 1993. (Previously presented at The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990). Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
1992
Distributed Combustion Effects on Acoustic Growth Rates in a Modified Rijke Burner
Barron, J.T. and Queiroz, M.
Heat Transfer in Fire and Combustion Systems, 199:183-188, 1992. Funded by Air Force Office of Scientific Research and ACERC.
The effects of distributed combustion on acoustic growth rates in a modified Rijke burner has been investigated. Three different particle types were used (25 and 43-µm aluminum, 7-µm zirconium carbide and 10- and 43-µm aluminum oxide) at mass loadings between 0% and 10%. The results indicate that the degree of acoustic driving or damping is a function of frequency of oscillation and the type, size and concentration of particles. Acoustic driving was produced by 25- and 43-µm aluminum, and 7-µm zirconium carbide particles. The greatest amount of driving was produced by the 25-µm zirconium carbide particles. The effects of distributed combustion and particle damping were also found to be strongly dependent on frequency. Aluminum oxide particles at low concentrations (<=2%) caused acoustic driving most probably because of flame catalytic effects. At high concentrations, the acoustic oscillations were damped because of viscous damping effects.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reaction Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
Heat Transfer in Fire and Combustion Systems, 223:97-103, 1992. (Also presented at the ASME National Heat Transfer Conference, San Diego, CA, August 1992). Funded by ACERC.
Data from a three-dimensional numerical simulations for a binary single step chemical reaction in a temporally developing turbulent shear layer were used to study the dissipation statistics of a conserved scalar as well as of temperature. Two specific chemical reaction cases were considered: isothermal reaction and chemical reaction with moderate heat release. The average and rms profiles of the mixture fraction dissipation are presented. The study indicated that in both cases the mixture fraction dissipation is related to the reaction rate. The temperature dissipation was found to be lognormally distributed in a region where there was significant chemical reaction. Also features of the temperature dissipation compare qualitatively well with experimental results reported in the literature.
The Effect of Fuel Droplets on Temperature Dissipation Statistics in a Gaseous Jet Flame
Dawson, R.W. and Queiroz, M.
IV Encontro Nacional de Ciencias Termicas (ENCIT 92), Rio de Janeiro, Brazil, December 1992. Funded by ACERC.
Presented in this paper is an experimental investigation of the effects of fuel droplets on temperature dissipation. For this purpose, temperature dissipation measurements have been completed in a lifted premixed propane flame issuing from a converging nozzle. Hexane droplets with initial diameter of 50µm were then introduced into the core of the flame while dissipation measurements were repeated. Profiles of dissipation show that the evaporating droplets cause a reduction in the temperature gradient at the core of the flame and at the outer shear layer. Deviations from the lognormal distributions in the outer shear layer are shown to be less significant and occur closer to the centerline when droplets are present.
The Effect of Monosized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
Int. Journal of Exp. Heat Transfer, 1992 (in press). [Previously presented at The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990.] Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
Results of Goudey Station Boiler Testing 1989, Interim Report
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Advanced Combustion Engineering Research Center, Empire State Electrical Energy Research Corp. Report, 1992. Funded by Empire State Electric Energy Research Corp. and ACERC.
To validate the three-dimensional computer code under development by Brigham Young University, three-dimensional experimental data is needed. Experimental data from large-scale industrial furnaces is virtually nonexistent in the open literature. This study was directed to obtaining some of the needed experimental validation data that began with testing at NYSEG's Goudey Station in June 1989. Four operating parameters were varied during the 1989 Goudey tests: 1) load, 2) excess air, 3) coal fineness, and 4) burner tilt angle. The experimental data was obtained during two test sets. Twenty-six, 2-3 hour time blocks where operating parameters changed from test to test and a single, 24-hour period where operating parameters were held constant. Experimental data obtained during this study was taken by three teams. Results indicate that the data is internally consistent and accurate enough to validate the trends expected from the three-dimensional combustion code. Validation with the data will begin as soon as coal-qualified PCGC-3 comprehensive code is available from ACERC.
1991
Temperature Dissipation Measurements in a Lifted Turbulent Diffusion Flame
Boyer, L. and Queiroz, M.
Combustion Science and Technology, 79:1-34, 1991. Funded by ACERC.
Temperature dissipation measurements have been completed in a lifted turbulent non-premixed propane flame issuing from a converging nozzle at several axial stations and along the centerline. The radial and axial components of the temperature dissipation were measured directly. The assumption of log normality is shown to be a good approximation of the temperature dissipation character in the core of the flame out to ro, the radius where the mean temperature is at a maximum. Significant deviations from the lognormal distribution are observed on the outer side of the shear layer. The radial and axial components of the dissipation are reasonably isotropic in the region close to the jet's centerline. However, in direct contrast to the results found in nonreacting flows, detailed comparisons of the radial and axial temperature dissipation profiles in the off-axis region of 0.1 < r/ro < 1.3 indicate that there are definite anisotropic and non self-similar characteristics in the temperature dissipation. Furthermore, joint probability density functions between temperature and its dissipation components have shown that they are uncorrelated on axis, yet both negative and positive correlation existed further outward from ro at all the axial stations measured.
The Effect of Heat Release on Various Statistical Properties of a Reacting Shear Layer
Son, S.F.; McMurtry, P.A; and Queiroz, M.
Combustion and Flame, 85:51-67, 1991. Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
Local Particle Velocity, Size and Concentration Measurements in an Industrial Scale Pulverized Coal Fired Boiler
Bonin, M.P. and Queiroz, M.
Combustion and Flame, 85:121-133, 1991. Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
A Dual Thermocouple Probe for Measuring Temperature Dissipation Statistics in Reacting Flows
Dawson, R.W.; Boyer, L. and Queiroz, M.
Heat Transfer in Combustion Systems, 166:59-67, (R.J. Santoro and J.D. Felske, eds.), The American Society of Mechanical Engineers, 1991. Funded by ACERC.
This paper discusses the design of a dual thermocouple probe to measure temperature dissipation statistics in turbulent reacting flows. The probe is described in detail and an analysis is made of its advantages and accuracy, including a parametric study on the effect of wire spacing, sampling frequency, time constant, and cutoff frequency on the calculations of temperature dissipation statistics. Also, measurements performed with the probe in a partially premixed, lifted propane flame are briefly discussed to demonstrate the probe's capability. The measurements reported include mean and rms profiles and normalized probability density function of the lognormal radial component of temperature dissipation, as well as joint statistics between the temperature and its dissipation, represented by joint probability density functions.
Testing of a Tangential Coal-Fired Power Plant with Water Cooled Probes
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Fossil Fuel Combustion, 33:49-56, (R. Ruiz, ed.), The American Society of Mechanical Engineers, 1991. Funded by Empire State Electric Energy Research Corp. and ACERC.
Brigham Young University (BYU)/ACERC testing of an 80 MWe corner fired coal power plant in June of 1989 sought data to validate 3-D comprehensive combustion computer codes. The major controlled variables during the two weeks of testing were coal grind fineness, O2 content in the flue-gas, plant load, and burner tilt. A full complement of board data was taken along with coal fineness, coal split between coal feed lines and on-line trial instruments on carbon-in-ash and air flows. Selective spatially resolved data was collected throughout the radiant section of the boiler from the near field in the burners to the platen area with water cooled probes for gas temperature, gas composition, particle size distribution and composition, velocity (magnitude, direction and turbulence) measurements, and radiant flux. The water cooled probe results yielded spatially resolved data in the near and far combustion field that will be compared to computer modeling runs in the future at BYU. The effects of O2, coal fineness, load variation and burner tilt are clearly discernable on the spacially resolved gas temperature, gas composition, velocity, radiant flux and particle size and composition measurements and are reviewed in this paper. Centrifugal effects on particle size are also discernable as are particle size effects on composition.
Results of Goudey Station Boiler Testing 1989, Final Report
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Advanced Combustion Engineering Research Center, Empire State Electrical Energy Research Corp. Report, 1991. Funded by Empire State Electric Energy Research Corp. and ACERC.
The Goudey Station testing has direct purpose for validation as well as an indirect purpose in future boiler design and operation. All of the spatially resolved data are gathered by research type instruments under controlled and common conditions. Data are not taken at every internal node (i.e. every 6 inch in spacing), but at sample locations where boiler internal access is available.
For the second series of tests in 1991, spatially resolved data were gathered from seven, four-inch ports and thirty, two inch ports, during twenty-five different tests. The data gathered consisted of gas temperature, gas and particle composition, particle velocity, concentration, size distribution, and radiation flux, all of which are combustion code predicted variables. The major test variables were load, excess air and coal grinded fineness. All three operating variables had significant influence on the test variables. In addition, burner tilt had a strong influence on the spatially resolved variables. Soot blowing was constrained during these tests but could not eliminate the effects of ash depositions during the tests. Predictions are expected to show the same trends.
The radiant flux showed significant differences from port to port and boiler level differences. Direct probing of the burner plume was of sufficient accuracy to identify coordinated O2 concentration and gas temperature variation through the burner plume. This compared favorably with the two-dimensional PCGC-2 predictions for the first 1.5 meters of the burner plume before three-dimensional effects altered the two-dimensional flow characteristics. The particle size distribution, concentration and velocity data were accurate enough to see the centrifugal effects of a corner-fired furnace on cyclone type particle separation. In addition, a pulsating particle concentration could be identified that was not related to any causal source but required temporal averaging. Gas temperature and radiation flux data averaged by level showed a mid-furnace flattening that was unexpected. Three-dimensional, gas only, predictions did not provide any explanation of the flattening nor were the predictions expected necessarily to resolve the observations. Carbon burnout was correlated with furnace level enough to indicate nearly complete combustion by the seventh level in the furnace.
1990
An Analysis of Single Stream Droplet Combustion Through Size and Velocity Measurements
Bonin, M.P. and Queiroz, M.
The American Society of Mechanical Engineers, New York, HTD-142, 57-66, Farouk, B. et al., Eds., Heat Transfer in Combustion Systems, 1990. Funded by ACERC.
A commercially available, laser-based particle analyzer capable of measuring the distribution of size and velocity of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid droplets burning in a turbulent, co-flowing air stream. Previous applications of this instrument have focused on light absorbing particles such as pulverized coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to light droplets larger than 100 µm, including the development of an instrument response function specific to non-absorbing particles in this size class.
A parametric study which investigated the influence of gas-phase turbulence, fuel type, and initial droplet size on the droplet vaporization rate was conducted. Gas phase temperature and velocity measurements were made using thermocouples and hot wire anemometry. Comparisons between measured and predicted droplet size using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Limitations in single stream measurements have been encountered in the lower portion of the flame, relative to the uniform particle flux requirement in the sample volume. Under certain experimental condition, droplet coalescence was also observed downstream of the ignition point.
Experimental Exploration of the Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M. and Yao, S.C.
Accepted for publication in Combustion and Flame, 1990. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of turbulence intensity and fuel vapor pressure on the thermal structure in a linear array of burning droplet streams has been investigated, using two levels of turbulence intensity and fuel vapor pressure. Particular attention has been focused on the relationship between the dynamic motion of the flame front and the thermal structure of the flame. Bare microthermocouples, digitally compensated for thermal inertia effects, were used to measure fluctuating gas-phase temperatures in this dilute spray flame with 407 mm nominal diameter hexane droplets. The flame was formed by nine vertical streams oriented in a plane and horizontally separated by a distance of 4 mm. Increasing the vapor pressure of the fuel caused higher flame temperatures at the average location of the premixed-gas flame. However, the essential features of the average and fluctuating temperature profiles as well as the pdf surfaces were unchanged over the range of vapor pressures investigated. The higher turbulence intensity level promoted higher temperature fluctuations because the fluctuating combustion zone of the flame was widened. The dynamics of the drifting motion of the flame as well as the instantaneous temperature profiles across the flame were also influenced, causing the disappearance of the bimodal shape in the pdf surfaces.
The Effect of Lateral Spacing on the Combustion Dynamics and Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M.
Accepted for publication in Comb. Sci. & Tech., 1990. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 mm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially-premixed flame which acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Local Particle Velocity, Size and Concentration Measurements in an Industrial Scale Pulverized Coal Fired Boiler
Bonin, M.P. and Queiroz, M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
The Effect of Lateral Spacing on the Combustion Dynamics and Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M.
Comb. Sci. & Tech., 72, 1-16, 1990. Funded by ACERC.
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 µm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially premixed flame that acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Temperature and Concentration Measurements in a Turbulent Spray Flame
Montgomery, C.J.; Son, S.F. and Queiroz, M.
Heat Transfer in Combustion Systems, 142, 1990. Funded by ACERC.
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
The Effect of Heat Release on Various Statistical Properties of a Reacting Shear Layer
Son, S.F.; McMurtry, P.A. and Queiroz, M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
The Effect of Monosized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990. Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
Full-Scale Testing of an 80 MWe Tangential Coal Fired Power Plant
Cannon, J.N.; Queiroz, M. and Webb, B.W.
Symposium on Effects of Coal Quality on Power Plants, Sponsored by Electric Power Research Institute, St. Louis, MO, 1990. Funded by ACERC and Empire State Electrical Energy Research Corp.
The major thrust of this paper is to present the data from research type instrumentation used to probe a full-scale utility boiler. The results will be used to validate Comprehensive Combustion Codes (CCC). This paper reviews how CCC can be integrated into the Coal Quality Impact Model (CQIM) to assist in accuracy. The paper presents selected portions of the spatially resolved gas velocity, gas temperatures, and composition profiles, radiation plus particle size, velocity, and concentration profiles for common sample ports to allow data comparison between data sets. The data are analyzed to show the effects of coal particle grind, load, burner tile, and excess air.
The data show particle burnout time, centrifugal force influence on the particles, near burner field effects as distinguished from far field effects, incident radiation imbalance due to coal pipe splits, and load changes on heat transfer surfaces and flow field. In addition, a full set of operator board data was taken to connect the research results to standard utility operating instrumentation.
Testing of a Tangential Coal-Fired Power Plant with Water Cooled Probes
Cannon, J.N.; Webb, B.W. and Queiroz, M.
14th Annual Energy-Sources and Technology Conference and Exhibition, Houston, Texas, 1990. Funded by ACERC and Empire State Electrical Energy Research Corp.
Brigham Young University (BYU)/ACERC testing of an 80 MWe corner fired coal power plant in June of 1989 sought data to validate 3-D comprehensive combustion computer codes. The major controlled variables during the two weeks of testing were coal grind fineness, O2 content in the flue-gas, plant load, and burner tilt.
A full complement of board data was taken along with coal fineness, coal split between coal feed lines and on-line trial instruments on carbon-in-ash and air flows. Selective spatially resolved data was collected throughout the radiant section of the boiler from the near field in the burners to the platen area with water cooled probes for gas temperature, gas composition, particle size distribution and composition, velocity (magnitude, direction and turbulence) measurements, and radiant flux.
The water cooled probe results yielded spatially resolved data in the near and far combustion field that will be compared to computer modeling runs in the future at BYU. The effects of O2, coal fineness, load variation and burner tilt are clearly discernable on the spatially resolved gas temperature, gas composition, velocity, radiant flux and particle size and composition measurements and are reviewed in this paper. Centrifugal effects on particle size are also discernable as are particle size effects on composition.
1989
Scalar Measurements in a Simplified Spray Flame
Montgomery, C.J.; Son, S.F. and Queiroz, M.
Western States Section, The Combustion Institute, Pullman, Washington, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
Non-Intrusive Analysis of Single Stream Droplet Combustion Through Size and Velocity Measurement
Bonin, M.P. and Queiroz, M.
Western States Section, The Combustion Institute, Pullman, Washington, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A newly-developed, laser based instrument which is capable of non-intrusively measuring the size and velocity distribution of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid fuel droplets burning in a turbulent, co-flowing air stream. This instrument determines the size distribution of particles having diameters ranging from 0.5 to 200 mm with corresponding velocities as high as 400 m/s. Therefore, the instrument is a valuable diagnostic tool for the investigation of both simplified and more complex spray flames. Measurement uncertainty is typically ten percent of the indicated droplet size.
Previous applications of the instrument were primarily concerned with size measurement in light absorbing environments consisting of solid particles such as coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to liquid fuel droplets. Before actual measurements were made, an appropriate instrument response function specific to non-absorbing (liquid) particles was created. With the corrected response function, a parametric examination of a simplified spray flame was undertaken to demonstrate the sizing capability of the instrument under non-absorbing conditions. The parametric study was designed to track the influence of variable turbulence intensity, fuel type and initial droplet size on the droplet vaporization rate. Temperature measurements made with digitally compensated thermocouples further quantified the mechanisms affecting the droplet size history. Hot-wire measurements were also performed to characterize the co-flowing air stream. Comparisons between measured and predicted droplet sizes using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Under certain experimental conditions, droplet agglomeration was observed downstream of the ignition point. The cause of the agglomeration is not clear, however it is thought to result from droplet collision and subsequent coalescence in the turbulent flow.
1988-1987
Effects of Droplet Spacing on the Flame Front Motion and Thermal Structure of a Planar Spray Flame
Queiroz, M.
ASME Winter Annual Meeting, 1988, Chicago, Illinois. Funded in part by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamic nature of flame propagation and on the thermal structure of a planar spray flame at ambient conditions has been studied. The flame was made up of 300-micron average-diameter hexane droplets injected through ten droplet streams in a plan, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and bare micro-thermocouples, digitally compensated for thermal inertia effects, were used to perform measurements of gas-phase temperatures in this dilute flame.
The reactive flow in the present study was characterized by an inlet pre-ignition zone, followed by a bluish premixed flame that acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front intermittent combustion zone increased due to an increase in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Wider intermittent combustion zones caused an increase in temperature fluctuations and a decrease in the average temperature gradient across the flame front.
Digital Compensation of Thermocouples for Thermal Inertia Effects
Son, S.F.; Queiroz, M. and Wood, C.G.
Western State Section of the Combustion Institute, 1988 Fall Meeting, Dana Point, CA, Paper No 88-103. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A digital technique, using a Fast Fourier Transform (FFT) algorithm, has been implemented to accurately and quickly compensate thermocouples for thermal inertia effects. The digital compensation technique seems to be more accurate, less sensitive to signal-to-noise ratio problems, and more flexible than the traditional method of electrical compensation. The proposed method is described in detail and an analysis is made of its advantages and accuracy. Errors that may occur using this method are quantified by comparing results with the analytical compensation of a generated temperature signal. Realistic noise is also added to the generated sign to evaluate the method in a more practical environment. Digital filtering is implemented to minimize the effects of noise on the compensated signal. The technique is shown to be easily implemented and accurate. Finally, temperature measurements made in a turbulent spray flame, are compensated. The effect of varying the average time constant on the compensated temperature is demonstrated.
Experimental Investigation on the Influence of Fuel-Vapor Concentration and Gas-Phase Turbulence Intensity on the Thermal Structure of a Planar Spray Flame
Queiroz, M. and Yao, S.C.
Western State Section of the Combustion Institute, 1988 Spring Meeting, Salt Lake City, Utah. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of gas-phase turbulence intensity and fuel vapor concentration on the thermal structure of a planar spray flame has been investigated. Particular attention has been focused on the relationship between the dynamic motion of the flame front characterized by an intermittent combustion zone (15) and the thermal structure of the flame. Microthermocouples were used to perform dynamic measurements of gas-phase temperatures in this dilute flame. The flame was made up of 400-micron nominal diameter hexane droplets injected through nine droplet streams in a plane, horizontally separated by a distance of 4 cm.
Good agreement was found between previous observations on the combustion dynamics of the flame and its thermal structure. The overall average temperature gradient, root-mean-square of temperature fluctuations, and shape of the probability density function surfaces were related to the intermittent combustion zone where the flame front presented a different drifting motion for different spray conditions. The ignition mechanism was also found to affect the thermal structure of the flame. Higher temperature fluctuations and shallower overall average temperature gradients across the flame were observed in flames having a relay ignition mechanism compared to the ones having a premixed flame ignition mechanism.
Two transitional points in the average temperature profile were observed for flames with a strong premixed flame ignition mechanism. The first point associated with a change in the average temperature gradient, is related to the thermocouple being located in the region between the premixed flame (burning of the fuel vaporized in the pre-heat zone ignited by the pilot flame) and the diffusion flame (burning of the vaporizing droplets ignited by the premixed flame). In this region, the local gas phase cools down because of the vaporizing droplets and possibly because of entrained air, causing the change in the local average temperature gradient. The second point, associated with the highest average temperature in the profile, is related with the thermocouple crossing the average location of the diffusion flame in the spray plane.
At low turbulence intensity, higher fuel-vapor concentrations in the gas phase caused higher average temperature at the average location of the premixed-gas flame. Temperature fluctuations decreased, but the overall shape of the pdf surfaces was unaffected. From consideration of the bimodal PDFs at low turbulence intensity levels, higher fuel vapor concentrations seems to have caused the instantaneous gradient across the flame to smooth out.
A higher turbulence intensity level caused smoothing of the average temperature profile at the location of the premixed-gas flame because of the reduction of the premixed-gas flame ignition mechanism strength. It also promoted higher temperature fluctuations by widening the intermittent combustion zone, and changed the dynamics of the drifting motion of the flame as well as the instantaneous temperature profile across the flame, causing the disappearance of PDF surfaces with a bimodal characteristic.
Experimental Study on the Influence of Lateral Stream Spacing on the Flame Front Dynamics and Thermal Structure of a Planar Spray Flame
Queiroz, M. and Yao, S.C.
ASME Winter Annual Meeting, 1988, Chicago. 10 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamic nature of flame propagation and on the thermal structure of a planar spray flame at ambient conditions has been studied. The flame was made up of 300-micron average-diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and bare micro-thermocouples, digitally compensated for thermal inertia effects, were used to perform measurements of gas-phase temperatures in this dilute flame.
The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish premixed flame that acted as the ignition source. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an increase in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Wider intermittent combustion zones caused an increase in temperature fluctuations and a decrease in the average temperature gradient across the flame front.
A Parametric Exploration of the Dynamic Behavior of Flame Propagation in Planar Sprays
Queiroz, M. and Yao, S.C.
Accepted for publication in Combustion and Flame, 1987. Funded by ACERC (National Science Foundation and Associates and Affiliates).
An exploration of the dynamic nature of flame propagation in planar sprays at ambient conditions has been performed. The parametric influence of three different levels of fuel-vapor concentrations (corresponding to three liquid fuel initial temperatures of 43ºC, 25ºC, and 15ºC), two droplet sizes (305, and 407 mm), three fuel compositions (hexane, iso-octane, and a 50/50 by weight mixture of these fuels), and two levels of gas-phase turbulence intensity 17%, and 2.5%) on the average speed of flame propagation and its fluctuations has been investigated suing sequential photographic information. It was found that higher fuel-vapor concentrations, smaller droplet sizes, or more volatile fuels used the average flame speed to increase while reducing its fluctuations. The effect of higher gas-phase turbulence intensity also caused the average flame speed to increase, but increased its fluctuations. Some of the observed phenomena can be qualitatively explained through the ignition process in a spray. When the presence of fuel vapor in the gas phase becomes significant due to the evaporation of droplets before the flame front, a premixed-gas type of ignition process prevails. At this condition the variations of average flame speed and its fluctuations may be explained through the effect of gas-phase equivalence ratio on the premixed-gas average flame speed. When the presence of fuel vapor in the flame front is reduced at high turbulence, a relay type of ignition process occurs. In this latter case, the increase of average flame speed and of flame speed fluctuations may be interpreted from the enhanced turbulent diffusion of thermal energy and from the turbulent fluctuations caused by the eddy transport phenomenon in the flow field, respectively.
Queiroz, M
For earlier publications, go to McQuay, MQ
1994
Evaluation of a Dimensionless Group Number to Determine Second-Einstein Temperatures in a Heat Capacity Model for All Coal Ranks
Coimbra, C.F.M. and Queiroz, M.
Combustion and Flame, (in press). Funded by ACERC.
A dimensionless group number is proposed to characterize the differences in chemical structures and physical properties between coal-like materials varying from lignites to anthracites, including graphite as a limiting case. This dimensionless number provides a simple and efficient correlation to determine second-Einstein temperatures (Ø2) in a specific heat capacity © model for all coal ranks, using information derived directly from the chemical composition (proximate and ultimate analysis) and the calorific value (H) of each substance. The nondimensional correlation has the form RØ2/H = f(FC), where R is the gas constant for the heterogeneous material and FC is the amount of fixed carbon in the parent coal. Properties of fifty coal-like materials were used to obtain this functional dependence. It was found that a linear function provides a good fit of the experimental data. This dimensionless correlation allows calculations of the behavior of the specific heat capacities of the materials studied here with an average value of 3/55% for the mean deviation in relation to experimental curves in the important temperature range of 300-600 K. The applicability of Einstein theory of heat capacity is analyzed for the special case of coal-like materials, and a generalization of Merrick's model for all coals of industrial interest is presented.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reacting Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
ASME Journal of Heat Transfer, 116:761-764, 1994. Funded by ACERC.
The understanding of turbulent reacting flows, which are characterized by the Navier-Stokes equations along with conservation equations of mass and energy, is one of the most challenging fields of engineering science. Various theoretical models based on simplifying assumptions have been developed to predict the behavior of such flows. In some of the models proposed, the problem of modeling the mean reaction rate is exchanged for the problem of describing a scalar dissipation function. Thus the scalar dissipation, which describes the destruction of the fluctuations of a passive scalar at the finest scales existing in a given flow, is an important parameter in modeling turbulent reacting flows.
The objective of this work is to present the statistics of dissipation of a conserved scalar from the DNS of a turbulent reacting shear layer. Since experimental data on the statistics of temperature dissipation in a reacting shear layer of jet flame is readily available, a comparison of this data with the DNS results will also be made.
Time-Resolved Temperature Measurements in an Elliptic Cross-Section, Turbulent, Diffusion Jet Flame
Cannon, S.M. and Queiroz, M.
1994 ASME Winter Annual Meeting, 296:37, 1994. Funded by ACERC.
An experimental study to determine the thermal structure of an elliptic cross-section, turbulent, diffusion jet flame using time-resolved temperature measurements was performed. The 2:1 aspect ratio fuel inlet allowed a fully developed turbulent flow (Reynolds number = 6000) of propane to exit into near ambient air. Measurements of mean and rms temperature, as well as power spectral densities (psd) and probability density functions (pdf) of temperature were obtained along the centerline and radially along the major and minor axis at axial stations ranging from 5<=z/Dh<=30. Similar to observed behavior in axi-symmetric diffusion flames, the non-axi-symmetric flame showed evidence of small-scale, momentum-driven vortices inside the flame zone and large-scale, buoyancy-driven vortices outside the flame zone. Differences in these turbulent shear layers along the major and minor axis were observed, as the minor axis had 25% higher temperature fluctuations along the fuel side mixing layer and the major axis had 20% higher temperature fluctuations along the air side mixing layer. A weaker fuel side shear layer along the major axis allowed more radial movement and a more radially stretched reaction zone in the near-burner region along this same major axis. This greater radial movement was sufficiently strong to cause a faster destruction of the inner vortex structures, such that less mixing would be observed along the fuel side of the major axis.
1993
Turbulent Reacting Flows
McMurtry, P.A. and Queiroz, M.
Chapter 7, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter summarizes current technologies and developments for treating reacting turbulent flows. Accurately predicting the effects of turbulence on combustion processes ranks among the most challenging problems in the engineering sciences. Since most combustion occurs in a turbulent environment, effects of turbulence must be treated in a realistic manner in any predictive method. In addition to the effects of turbulence on the chemical species transport, density changes resulting from exothermic chemical reactions can alter the structure and development of the flow field. As such, the fluid mechanics, thermodynamics, and chemistry are strongly coupled, making the description of the turbulent combustion process extremely difficult. Moreover, other complications often arise. Among these are the effects of multiple phases, an inherent aspect of entrained solids and liquids. This chapter illustrates some of the characteristics of turbulent flows and outlines some of the methods used to treat turbulence in reacting flows, while pointing out the need for improved capabilities in this field of study. First, general background on turbulent flows is provided. The most popular approaches used to model turbulence in reactive systems for engineering applications are discussed, along with a few newly introduced techniques. Some of the complications that arise in multi-phase turbulent flow are also discussed. More computationally intensive numerical approaches, used primarily in fundamental research applications, are presented. These methods include Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). Direct simulation results for simplified flow configurations are discussed in light of the information they have provided concerning the physics of turbulent reacting flows.
Thrust Area 4 Research Program: Turbulent, Reacting Fluid Mechanics and Heat Transfer
McMurtry, P.A. and Queiroz, M.
Energy & Fuels, 7 (6):814-816, 1993. Funded by ACERC.
Two of the main reasons for poor turbulence model performance are the lack of understanding of the basic physical mechanisms acting in turbulent reaction flows and a lack of reliable data needed to "tune" model parameters. In addition, the physical complexity of the turbulent mixing process does not lend itself easily to packaged models. There is a need for true innovations and new approaches to treat the turbulence in reacting flows. The work in this thrust area consists of a combination of experiments and numerical studies aimed at (1) directly improving the performance and extending the generality of comprehensive combustion computer codes; (2) developing new approaches to modeling turbulent flows; and (3) conducting fundamental research to improve our understanding of the physical processes acting in turbulent reacting flows and heat transfer. The key products anticipated from the research in this thrust area include (1) new turbulent mixing models that can be incorporated in comprehensive predictive codes for turbulent combustion processes; (2) documented experimental data on planar sprays and particle dynamics in turbulent flows; (3) documented experimental data on radiation properties in multiphase combustion applications; (4) model improvements in the ACERC computer codes to handle spray flames and pulverized particles; (5) three-dimensional radiation submodels for incorporation into the comprehensive code; and (6) fundamental data on the mechanisms of turbulent mixing and reaction.
Parametric Evaluation of a Particle Dispersion Submodel Used in a Two-Dimensional Pulverized-Coal Combustion Code
Shirolkar, J.S. and Queiroz, M.
Energy & Fuels, 7 (6):919-927, 1993. Funded by ACERC.
Particle dispersion data collected with a laser-based diagnostic technique are used to evaluate the performance of a particle dispersion submodel incorporated in a two-dimensional, comprehensive, pulverized-coal combustion code. In this code, the gas-phase mechanics is formulated in an Eulerian framework, whereas the particle phase is based on a Lagrangian scheme. The turbulence is described by the two-equation kappa-epsilon model. The experimental data available include radial profiles of small (0.4-3.5 µm) and large (3.5-98 µm) particle velocities and size-resolved particle number density of pulverized-coal particles for both reacting and isothermal conditions at different axial stations in a laboratory-scale, axisymmetric, controlled-profile reactor. Various parameters were varied during the reacting cases: the secondary air flow rate, secondary swirl number, and the initial coal-size distribution. For the isothermal conditions, only the secondary air temperature is varied for both swirling and nonswirling cases. It is observed that a reduced swirl number of 0.8 instead of the experimental swirl number of 1.4 gave relatively better predictions for the gas-phase aerodynamics, particle number density, and particle trajectory calculations for all the swirling cases. Some limitations of the particle dispersion submodel as well as the experimental data are acknowledged.
Experimentally Determined Particle Number Density Statistics in an Industrial-Scal, Pulverized-Coal-Fired Boiler
Queiroz, M.; Bonin, M.P.; Shirolkar, J.S. and Dawson, R.W.
Energy & Fuels 7:(6) 842-851, 1993. Funded by ACERC.
A study on the variations of particle data rate statistics and the probability density function (PDF) of cumulative particle number density has been completed in a full-scale, tangentially fired, 85 MWe pulverized-coal-fired boiler. Variables in the tests included boiler load and coal type. It was observed that particle data rate fluctuations were greater in magnitude for small particles (<3.5 µm) and that the PDFs of particle data rate were well approximated by normal distributions. Furthermore, there were no preferential frequencies in the large (<3.5 µm) or small particle data rate fluctuations anywhere in the boiler. The PDFs of cumulative particle number density for the small particles were negatively skewed and, as compared to the large particle PDFs, were less sensitive to boiler location. The large particle PDFs were more negatively skewed near the walls and more Gaussian as distance from the wall increased. Broader distributions of cumulative particle number density with peaks at higher values were observed for the small particles for the coal with lower volatiles and higher ash content. Moreover, for the large particles, a noticeable shift of the PDFs, longer "tails" toward higher cumulative number densities, and a substantial flattening of the PDF curves were observed for the same coal. The shape of the PDF profiles did not change substantially as the boiler load changed. The effect of a lighter load on the small particle PDFs was to slightly broaden the distribution, mostly in the direction of large cumulative particle number densities. For the large particles, a shift toward higher cumulative particle number densities, a slightly broadening effect, and a reduction in the maximum PDF values were observed at lighter load.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reacting Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
ASME Journal of Heat Transfer, 1993 (in press). Funded by ACERC.
The understanding of turbulent reacting flows, which are characterized by the Navier-Stokes equations along with conservation equations of mass and energy, is one of the most challenging fields of engineering science. Various theoretical models based on simplifying assumptions have been developed to predict the behavior of such flows. In some of the models proposed, the problem of modeling the mean reaction rate is exchanged for the problem of describing a scalar dissipation function. Thus the scalar dissipation, which describes the destruction of the fluctuations of a passive scalar at the finest scales existing in a given flow, is an important parameter in modeling turbulent reacting flows.
The objective of this work is to present the statistics of dissipation of a conserved scalar from the DNS of a turbulent reacting shear layer. Since experimental data on the statistics of temperature dissipation in a reacting shear layer of jet flame is readily available, a comparison of this data with the DNS results will also be made.
The Effect of Monsized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
Int. Journal of Exp. Heat Transfer, 6:80-89, 1993. (Previously presented at The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990). Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
1992
Distributed Combustion Effects on Acoustic Growth Rates in a Modified Rijke Burner
Barron, J.T. and Queiroz, M.
Heat Transfer in Fire and Combustion Systems, 199:183-188, 1992. Funded by Air Force Office of Scientific Research and ACERC.
The effects of distributed combustion on acoustic growth rates in a modified Rijke burner has been investigated. Three different particle types were used (25 and 43-µm aluminum, 7-µm zirconium carbide and 10- and 43-µm aluminum oxide) at mass loadings between 0% and 10%. The results indicate that the degree of acoustic driving or damping is a function of frequency of oscillation and the type, size and concentration of particles. Acoustic driving was produced by 25- and 43-µm aluminum, and 7-µm zirconium carbide particles. The greatest amount of driving was produced by the 25-µm zirconium carbide particles. The effects of distributed combustion and particle damping were also found to be strongly dependent on frequency. Aluminum oxide particles at low concentrations (<=2%) caused acoustic driving most probably because of flame catalytic effects. At high concentrations, the acoustic oscillations were damped because of viscous damping effects.
Statistical Properties of Scalar and Temperature Dissipation in a Turbulent Reaction Shear Layer
Shirolkar, J.S.; Queiroz, M. and McMurtry, P.A.
Heat Transfer in Fire and Combustion Systems, 223:97-103, 1992. (Also presented at the ASME National Heat Transfer Conference, San Diego, CA, August 1992). Funded by ACERC.
Data from a three-dimensional numerical simulations for a binary single step chemical reaction in a temporally developing turbulent shear layer were used to study the dissipation statistics of a conserved scalar as well as of temperature. Two specific chemical reaction cases were considered: isothermal reaction and chemical reaction with moderate heat release. The average and rms profiles of the mixture fraction dissipation are presented. The study indicated that in both cases the mixture fraction dissipation is related to the reaction rate. The temperature dissipation was found to be lognormally distributed in a region where there was significant chemical reaction. Also features of the temperature dissipation compare qualitatively well with experimental results reported in the literature.
The Effect of Fuel Droplets on Temperature Dissipation Statistics in a Gaseous Jet Flame
Dawson, R.W. and Queiroz, M.
IV Encontro Nacional de Ciencias Termicas (ENCIT 92), Rio de Janeiro, Brazil, December 1992. Funded by ACERC.
Presented in this paper is an experimental investigation of the effects of fuel droplets on temperature dissipation. For this purpose, temperature dissipation measurements have been completed in a lifted premixed propane flame issuing from a converging nozzle. Hexane droplets with initial diameter of 50µm were then introduced into the core of the flame while dissipation measurements were repeated. Profiles of dissipation show that the evaporating droplets cause a reduction in the temperature gradient at the core of the flame and at the outer shear layer. Deviations from the lognormal distributions in the outer shear layer are shown to be less significant and occur closer to the centerline when droplets are present.
The Effect of Monosized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
Int. Journal of Exp. Heat Transfer, 1992 (in press). [Previously presented at The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990.] Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
Results of Goudey Station Boiler Testing 1989, Interim Report
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Advanced Combustion Engineering Research Center, Empire State Electrical Energy Research Corp. Report, 1992. Funded by Empire State Electric Energy Research Corp. and ACERC.
To validate the three-dimensional computer code under development by Brigham Young University, three-dimensional experimental data is needed. Experimental data from large-scale industrial furnaces is virtually nonexistent in the open literature. This study was directed to obtaining some of the needed experimental validation data that began with testing at NYSEG's Goudey Station in June 1989. Four operating parameters were varied during the 1989 Goudey tests: 1) load, 2) excess air, 3) coal fineness, and 4) burner tilt angle. The experimental data was obtained during two test sets. Twenty-six, 2-3 hour time blocks where operating parameters changed from test to test and a single, 24-hour period where operating parameters were held constant. Experimental data obtained during this study was taken by three teams. Results indicate that the data is internally consistent and accurate enough to validate the trends expected from the three-dimensional combustion code. Validation with the data will begin as soon as coal-qualified PCGC-3 comprehensive code is available from ACERC.
1991
Temperature Dissipation Measurements in a Lifted Turbulent Diffusion Flame
Boyer, L. and Queiroz, M.
Combustion Science and Technology, 79:1-34, 1991. Funded by ACERC.
Temperature dissipation measurements have been completed in a lifted turbulent non-premixed propane flame issuing from a converging nozzle at several axial stations and along the centerline. The radial and axial components of the temperature dissipation were measured directly. The assumption of log normality is shown to be a good approximation of the temperature dissipation character in the core of the flame out to ro, the radius where the mean temperature is at a maximum. Significant deviations from the lognormal distribution are observed on the outer side of the shear layer. The radial and axial components of the dissipation are reasonably isotropic in the region close to the jet's centerline. However, in direct contrast to the results found in nonreacting flows, detailed comparisons of the radial and axial temperature dissipation profiles in the off-axis region of 0.1 < r/ro < 1.3 indicate that there are definite anisotropic and non self-similar characteristics in the temperature dissipation. Furthermore, joint probability density functions between temperature and its dissipation components have shown that they are uncorrelated on axis, yet both negative and positive correlation existed further outward from ro at all the axial stations measured.
The Effect of Heat Release on Various Statistical Properties of a Reacting Shear Layer
Son, S.F.; McMurtry, P.A; and Queiroz, M.
Combustion and Flame, 85:51-67, 1991. Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
Local Particle Velocity, Size and Concentration Measurements in an Industrial Scale Pulverized Coal Fired Boiler
Bonin, M.P. and Queiroz, M.
Combustion and Flame, 85:121-133, 1991. Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
A Dual Thermocouple Probe for Measuring Temperature Dissipation Statistics in Reacting Flows
Dawson, R.W.; Boyer, L. and Queiroz, M.
Heat Transfer in Combustion Systems, 166:59-67, (R.J. Santoro and J.D. Felske, eds.), The American Society of Mechanical Engineers, 1991. Funded by ACERC.
This paper discusses the design of a dual thermocouple probe to measure temperature dissipation statistics in turbulent reacting flows. The probe is described in detail and an analysis is made of its advantages and accuracy, including a parametric study on the effect of wire spacing, sampling frequency, time constant, and cutoff frequency on the calculations of temperature dissipation statistics. Also, measurements performed with the probe in a partially premixed, lifted propane flame are briefly discussed to demonstrate the probe's capability. The measurements reported include mean and rms profiles and normalized probability density function of the lognormal radial component of temperature dissipation, as well as joint statistics between the temperature and its dissipation, represented by joint probability density functions.
Testing of a Tangential Coal-Fired Power Plant with Water Cooled Probes
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Fossil Fuel Combustion, 33:49-56, (R. Ruiz, ed.), The American Society of Mechanical Engineers, 1991. Funded by Empire State Electric Energy Research Corp. and ACERC.
Brigham Young University (BYU)/ACERC testing of an 80 MWe corner fired coal power plant in June of 1989 sought data to validate 3-D comprehensive combustion computer codes. The major controlled variables during the two weeks of testing were coal grind fineness, O2 content in the flue-gas, plant load, and burner tilt. A full complement of board data was taken along with coal fineness, coal split between coal feed lines and on-line trial instruments on carbon-in-ash and air flows. Selective spatially resolved data was collected throughout the radiant section of the boiler from the near field in the burners to the platen area with water cooled probes for gas temperature, gas composition, particle size distribution and composition, velocity (magnitude, direction and turbulence) measurements, and radiant flux. The water cooled probe results yielded spatially resolved data in the near and far combustion field that will be compared to computer modeling runs in the future at BYU. The effects of O2, coal fineness, load variation and burner tilt are clearly discernable on the spacially resolved gas temperature, gas composition, velocity, radiant flux and particle size and composition measurements and are reviewed in this paper. Centrifugal effects on particle size are also discernable as are particle size effects on composition.
Results of Goudey Station Boiler Testing 1989, Final Report
Cannon, J.N.; Webb, B.W. and Queiroz, M.
Advanced Combustion Engineering Research Center, Empire State Electrical Energy Research Corp. Report, 1991. Funded by Empire State Electric Energy Research Corp. and ACERC.
The Goudey Station testing has direct purpose for validation as well as an indirect purpose in future boiler design and operation. All of the spatially resolved data are gathered by research type instruments under controlled and common conditions. Data are not taken at every internal node (i.e. every 6 inch in spacing), but at sample locations where boiler internal access is available.
For the second series of tests in 1991, spatially resolved data were gathered from seven, four-inch ports and thirty, two inch ports, during twenty-five different tests. The data gathered consisted of gas temperature, gas and particle composition, particle velocity, concentration, size distribution, and radiation flux, all of which are combustion code predicted variables. The major test variables were load, excess air and coal grinded fineness. All three operating variables had significant influence on the test variables. In addition, burner tilt had a strong influence on the spatially resolved variables. Soot blowing was constrained during these tests but could not eliminate the effects of ash depositions during the tests. Predictions are expected to show the same trends.
The radiant flux showed significant differences from port to port and boiler level differences. Direct probing of the burner plume was of sufficient accuracy to identify coordinated O2 concentration and gas temperature variation through the burner plume. This compared favorably with the two-dimensional PCGC-2 predictions for the first 1.5 meters of the burner plume before three-dimensional effects altered the two-dimensional flow characteristics. The particle size distribution, concentration and velocity data were accurate enough to see the centrifugal effects of a corner-fired furnace on cyclone type particle separation. In addition, a pulsating particle concentration could be identified that was not related to any causal source but required temporal averaging. Gas temperature and radiation flux data averaged by level showed a mid-furnace flattening that was unexpected. Three-dimensional, gas only, predictions did not provide any explanation of the flattening nor were the predictions expected necessarily to resolve the observations. Carbon burnout was correlated with furnace level enough to indicate nearly complete combustion by the seventh level in the furnace.
1990
An Analysis of Single Stream Droplet Combustion Through Size and Velocity Measurements
Bonin, M.P. and Queiroz, M.
The American Society of Mechanical Engineers, New York, HTD-142, 57-66, Farouk, B. et al., Eds., Heat Transfer in Combustion Systems, 1990. Funded by ACERC.
A commercially available, laser-based particle analyzer capable of measuring the distribution of size and velocity of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid droplets burning in a turbulent, co-flowing air stream. Previous applications of this instrument have focused on light absorbing particles such as pulverized coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to light droplets larger than 100 µm, including the development of an instrument response function specific to non-absorbing particles in this size class.
A parametric study which investigated the influence of gas-phase turbulence, fuel type, and initial droplet size on the droplet vaporization rate was conducted. Gas phase temperature and velocity measurements were made using thermocouples and hot wire anemometry. Comparisons between measured and predicted droplet size using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Limitations in single stream measurements have been encountered in the lower portion of the flame, relative to the uniform particle flux requirement in the sample volume. Under certain experimental condition, droplet coalescence was also observed downstream of the ignition point.
Experimental Exploration of the Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M. and Yao, S.C.
Accepted for publication in Combustion and Flame, 1990. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of turbulence intensity and fuel vapor pressure on the thermal structure in a linear array of burning droplet streams has been investigated, using two levels of turbulence intensity and fuel vapor pressure. Particular attention has been focused on the relationship between the dynamic motion of the flame front and the thermal structure of the flame. Bare microthermocouples, digitally compensated for thermal inertia effects, were used to measure fluctuating gas-phase temperatures in this dilute spray flame with 407 mm nominal diameter hexane droplets. The flame was formed by nine vertical streams oriented in a plane and horizontally separated by a distance of 4 mm. Increasing the vapor pressure of the fuel caused higher flame temperatures at the average location of the premixed-gas flame. However, the essential features of the average and fluctuating temperature profiles as well as the pdf surfaces were unchanged over the range of vapor pressures investigated. The higher turbulence intensity level promoted higher temperature fluctuations because the fluctuating combustion zone of the flame was widened. The dynamics of the drifting motion of the flame as well as the instantaneous temperature profiles across the flame were also influenced, causing the disappearance of the bimodal shape in the pdf surfaces.
The Effect of Lateral Spacing on the Combustion Dynamics and Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M.
Accepted for publication in Comb. Sci. & Tech., 1990. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 mm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially-premixed flame which acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Local Particle Velocity, Size and Concentration Measurements in an Industrial Scale Pulverized Coal Fired Boiler
Bonin, M.P. and Queiroz, M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Parametric, in-situ, particle velocity, size and number density measurements have been made in a full scale, coal burning power plant using an optical diagnostic technique. Available ports in the boiler allowed measurement at three locations above the burner level. Variable test parameters included furnace load, excess air, and burner tilt, using a medium volatile bituminous coal. Higher particle velocities were observed when the boiler was operated at a maximum capacity due to increased air and coal flows. Port-to-port velocity variations were attributed to the rotational nature of the mean flow, changes in gas density with changing gas temperature, and the interaction of the flow with the boiler nose. Measured particle number density profiles were characterized by high values in the small particle size class (< 2 µm), decreasing exponentially with increasing particle size. The measured number density profiles indicated that the combustion process is largely complete at locations 7 m above the burners and that the particles measured consisted primarily of ash, a conclusion that is also supported by the percent carbon-in-ash data. The mass-mean and number-mean particle sizes for all tests varied between 10 and 45, and 0.5 and 0.85 µm, respectively. The characteristic similarity between the particle size distribution of the ash and that of the parent char, previously documented in laboratory scale investigations, was also observed in the present study. Cumulative mass distribution profiles indicated that a significant centrifugal effect is exerted on the condensed phase by the rotating flow. An increase in small particle number density (~ 0.5 µm) was also apparent at lower boiler loads due to changes in the combustion process occurring at these operating conditions, which affect the various modes of ash particle formation.
The Effect of Lateral Spacing on the Combustion Dynamics and Thermal Structure of an Array of Burning Droplet Streams
Queiroz, M.
Comb. Sci. & Tech., 72, 1-16, 1990. Funded by ACERC.
The effect of lateral stream separation distance on the dynamics of flame propagation and thermal structure of a simplified spray flame has been studied. The flame was made up of 300 µm average diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and micro-thermocouples were used to perform measurements of gas-phase temperatures. The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish partially premixed flame that acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an augmentation in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Increased lateral spacing resulted in higher temperature fluctuations and lower average temperature gradients across the flame front.
Temperature and Concentration Measurements in a Turbulent Spray Flame
Montgomery, C.J.; Son, S.F. and Queiroz, M.
Heat Transfer in Combustion Systems, 142, 1990. Funded by ACERC.
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
The Effect of Heat Release on Various Statistical Properties of a Reacting Shear Layer
Son, S.F.; McMurtry, P.A. and Queiroz, M.
Combustion and Flame, 1990 (In press). Funded by ACERC.
Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.
The Effect of Monosized Hexane Droplets on the Thermal Structure of a Lifted Gaseous Flame
Rasmussen, K.G. and Queiroz, M.
The III Encontro Nacional de Ciencias Termicas (ENCIT 90), Santa Caratina, Brazil, 1990. Funded by ACERC.
Gas temperature measurements have been completed in a lifted propane flame with and without droplets issuing from a contoured nozzle at several radial stations in the developing region of the jet. A general decrease in the average gas temperature was observed in the flame when the droplets were introduced, due to local evaporative cooling effects. Rms temperature profiles with two local extremes at either side of the average reaction zone were observed for 5 < x/D < 20 in the gaseous flame. Further downstream along the centerline, this region disappeared because the jet's fuel-rich, central core ceased to exist. The droplets prolonged the axial region where these double-maxima rms temperature profiles existed, due to an extension of the flame core. A comparison of power spectral densities measured with and without the droplets suggests that the droplets substantially changed to flow field in the core region, to the point of inhibiting a complex vortical structure existent in the gaseous flame.
Full-Scale Testing of an 80 MWe Tangential Coal Fired Power Plant
Cannon, J.N.; Queiroz, M. and Webb, B.W.
Symposium on Effects of Coal Quality on Power Plants, Sponsored by Electric Power Research Institute, St. Louis, MO, 1990. Funded by ACERC and Empire State Electrical Energy Research Corp.
The major thrust of this paper is to present the data from research type instrumentation used to probe a full-scale utility boiler. The results will be used to validate Comprehensive Combustion Codes (CCC). This paper reviews how CCC can be integrated into the Coal Quality Impact Model (CQIM) to assist in accuracy. The paper presents selected portions of the spatially resolved gas velocity, gas temperatures, and composition profiles, radiation plus particle size, velocity, and concentration profiles for common sample ports to allow data comparison between data sets. The data are analyzed to show the effects of coal particle grind, load, burner tile, and excess air.
The data show particle burnout time, centrifugal force influence on the particles, near burner field effects as distinguished from far field effects, incident radiation imbalance due to coal pipe splits, and load changes on heat transfer surfaces and flow field. In addition, a full set of operator board data was taken to connect the research results to standard utility operating instrumentation.
Testing of a Tangential Coal-Fired Power Plant with Water Cooled Probes
Cannon, J.N.; Webb, B.W. and Queiroz, M.
14th Annual Energy-Sources and Technology Conference and Exhibition, Houston, Texas, 1990. Funded by ACERC and Empire State Electrical Energy Research Corp.
Brigham Young University (BYU)/ACERC testing of an 80 MWe corner fired coal power plant in June of 1989 sought data to validate 3-D comprehensive combustion computer codes. The major controlled variables during the two weeks of testing were coal grind fineness, O2 content in the flue-gas, plant load, and burner tilt.
A full complement of board data was taken along with coal fineness, coal split between coal feed lines and on-line trial instruments on carbon-in-ash and air flows. Selective spatially resolved data was collected throughout the radiant section of the boiler from the near field in the burners to the platen area with water cooled probes for gas temperature, gas composition, particle size distribution and composition, velocity (magnitude, direction and turbulence) measurements, and radiant flux.
The water cooled probe results yielded spatially resolved data in the near and far combustion field that will be compared to computer modeling runs in the future at BYU. The effects of O2, coal fineness, load variation and burner tilt are clearly discernable on the spatially resolved gas temperature, gas composition, velocity, radiant flux and particle size and composition measurements and are reviewed in this paper. Centrifugal effects on particle size are also discernable as are particle size effects on composition.
1989
Scalar Measurements in a Simplified Spray Flame
Montgomery, C.J.; Son, S.F. and Queiroz, M.
Western States Section, The Combustion Institute, Pullman, Washington, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
Non-Intrusive Analysis of Single Stream Droplet Combustion Through Size and Velocity Measurement
Bonin, M.P. and Queiroz, M.
Western States Section, The Combustion Institute, Pullman, Washington, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A newly-developed, laser based instrument which is capable of non-intrusively measuring the size and velocity distribution of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid fuel droplets burning in a turbulent, co-flowing air stream. This instrument determines the size distribution of particles having diameters ranging from 0.5 to 200 mm with corresponding velocities as high as 400 m/s. Therefore, the instrument is a valuable diagnostic tool for the investigation of both simplified and more complex spray flames. Measurement uncertainty is typically ten percent of the indicated droplet size.
Previous applications of the instrument were primarily concerned with size measurement in light absorbing environments consisting of solid particles such as coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to liquid fuel droplets. Before actual measurements were made, an appropriate instrument response function specific to non-absorbing (liquid) particles was created. With the corrected response function, a parametric examination of a simplified spray flame was undertaken to demonstrate the sizing capability of the instrument under non-absorbing conditions. The parametric study was designed to track the influence of variable turbulence intensity, fuel type and initial droplet size on the droplet vaporization rate. Temperature measurements made with digitally compensated thermocouples further quantified the mechanisms affecting the droplet size history. Hot-wire measurements were also performed to characterize the co-flowing air stream. Comparisons between measured and predicted droplet sizes using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Under certain experimental conditions, droplet agglomeration was observed downstream of the ignition point. The cause of the agglomeration is not clear, however it is thought to result from droplet collision and subsequent coalescence in the turbulent flow.
1988-1987
Effects of Droplet Spacing on the Flame Front Motion and Thermal Structure of a Planar Spray Flame
Queiroz, M.
ASME Winter Annual Meeting, 1988, Chicago, Illinois. Funded in part by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamic nature of flame propagation and on the thermal structure of a planar spray flame at ambient conditions has been studied. The flame was made up of 300-micron average-diameter hexane droplets injected through ten droplet streams in a plan, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and bare micro-thermocouples, digitally compensated for thermal inertia effects, were used to perform measurements of gas-phase temperatures in this dilute flame.
The reactive flow in the present study was characterized by an inlet pre-ignition zone, followed by a bluish premixed flame that acted as the ignition source of the fuel streams. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front intermittent combustion zone increased due to an increase in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Wider intermittent combustion zones caused an increase in temperature fluctuations and a decrease in the average temperature gradient across the flame front.
Digital Compensation of Thermocouples for Thermal Inertia Effects
Son, S.F.; Queiroz, M. and Wood, C.G.
Western State Section of the Combustion Institute, 1988 Fall Meeting, Dana Point, CA, Paper No 88-103. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A digital technique, using a Fast Fourier Transform (FFT) algorithm, has been implemented to accurately and quickly compensate thermocouples for thermal inertia effects. The digital compensation technique seems to be more accurate, less sensitive to signal-to-noise ratio problems, and more flexible than the traditional method of electrical compensation. The proposed method is described in detail and an analysis is made of its advantages and accuracy. Errors that may occur using this method are quantified by comparing results with the analytical compensation of a generated temperature signal. Realistic noise is also added to the generated sign to evaluate the method in a more practical environment. Digital filtering is implemented to minimize the effects of noise on the compensated signal. The technique is shown to be easily implemented and accurate. Finally, temperature measurements made in a turbulent spray flame, are compensated. The effect of varying the average time constant on the compensated temperature is demonstrated.
Experimental Investigation on the Influence of Fuel-Vapor Concentration and Gas-Phase Turbulence Intensity on the Thermal Structure of a Planar Spray Flame
Queiroz, M. and Yao, S.C.
Western State Section of the Combustion Institute, 1988 Spring Meeting, Salt Lake City, Utah. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of gas-phase turbulence intensity and fuel vapor concentration on the thermal structure of a planar spray flame has been investigated. Particular attention has been focused on the relationship between the dynamic motion of the flame front characterized by an intermittent combustion zone (15) and the thermal structure of the flame. Microthermocouples were used to perform dynamic measurements of gas-phase temperatures in this dilute flame. The flame was made up of 400-micron nominal diameter hexane droplets injected through nine droplet streams in a plane, horizontally separated by a distance of 4 cm.
Good agreement was found between previous observations on the combustion dynamics of the flame and its thermal structure. The overall average temperature gradient, root-mean-square of temperature fluctuations, and shape of the probability density function surfaces were related to the intermittent combustion zone where the flame front presented a different drifting motion for different spray conditions. The ignition mechanism was also found to affect the thermal structure of the flame. Higher temperature fluctuations and shallower overall average temperature gradients across the flame were observed in flames having a relay ignition mechanism compared to the ones having a premixed flame ignition mechanism.
Two transitional points in the average temperature profile were observed for flames with a strong premixed flame ignition mechanism. The first point associated with a change in the average temperature gradient, is related to the thermocouple being located in the region between the premixed flame (burning of the fuel vaporized in the pre-heat zone ignited by the pilot flame) and the diffusion flame (burning of the vaporizing droplets ignited by the premixed flame). In this region, the local gas phase cools down because of the vaporizing droplets and possibly because of entrained air, causing the change in the local average temperature gradient. The second point, associated with the highest average temperature in the profile, is related with the thermocouple crossing the average location of the diffusion flame in the spray plane.
At low turbulence intensity, higher fuel-vapor concentrations in the gas phase caused higher average temperature at the average location of the premixed-gas flame. Temperature fluctuations decreased, but the overall shape of the pdf surfaces was unaffected. From consideration of the bimodal PDFs at low turbulence intensity levels, higher fuel vapor concentrations seems to have caused the instantaneous gradient across the flame to smooth out.
A higher turbulence intensity level caused smoothing of the average temperature profile at the location of the premixed-gas flame because of the reduction of the premixed-gas flame ignition mechanism strength. It also promoted higher temperature fluctuations by widening the intermittent combustion zone, and changed the dynamics of the drifting motion of the flame as well as the instantaneous temperature profile across the flame, causing the disappearance of PDF surfaces with a bimodal characteristic.
Experimental Study on the Influence of Lateral Stream Spacing on the Flame Front Dynamics and Thermal Structure of a Planar Spray Flame
Queiroz, M. and Yao, S.C.
ASME Winter Annual Meeting, 1988, Chicago. 10 pgs. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The effect of lateral stream separation distance on the dynamic nature of flame propagation and on the thermal structure of a planar spray flame at ambient conditions has been studied. The flame was made up of 300-micron average-diameter hexane droplets injected through ten droplet streams in a plane, horizontally separated by a distance varying from 1 to 6 mm. Sequential photography was used to document the flame front motion and bare micro-thermocouples, digitally compensated for thermal inertia effects, were used to perform measurements of gas-phase temperatures in this dilute flame.
The reactive flow was characterized by an inlet pre-ignition zone, followed by a bluish premixed flame that acted as the ignition source. Further downstream, a pattern of yellowish diffusion flames surrounding individual streams or groups of them was established, depending on the lateral separation of the streams. As the lateral spacing of the streams was increased, the vertical region swept by the flame front increased due to an increase in the flame propagation unsteadiness associated with larger variations in the local fuel-vapor concentration. Wider intermittent combustion zones caused an increase in temperature fluctuations and a decrease in the average temperature gradient across the flame front.
A Parametric Exploration of the Dynamic Behavior of Flame Propagation in Planar Sprays
Queiroz, M. and Yao, S.C.
Accepted for publication in Combustion and Flame, 1987. Funded by ACERC (National Science Foundation and Associates and Affiliates).
An exploration of the dynamic nature of flame propagation in planar sprays at ambient conditions has been performed. The parametric influence of three different levels of fuel-vapor concentrations (corresponding to three liquid fuel initial temperatures of 43ºC, 25ºC, and 15ºC), two droplet sizes (305, and 407 mm), three fuel compositions (hexane, iso-octane, and a 50/50 by weight mixture of these fuels), and two levels of gas-phase turbulence intensity 17%, and 2.5%) on the average speed of flame propagation and its fluctuations has been investigated suing sequential photographic information. It was found that higher fuel-vapor concentrations, smaller droplet sizes, or more volatile fuels used the average flame speed to increase while reducing its fluctuations. The effect of higher gas-phase turbulence intensity also caused the average flame speed to increase, but increased its fluctuations. Some of the observed phenomena can be qualitatively explained through the ignition process in a spray. When the presence of fuel vapor in the gas phase becomes significant due to the evaporation of droplets before the flame front, a premixed-gas type of ignition process prevails. At this condition the variations of average flame speed and its fluctuations may be explained through the effect of gas-phase equivalence ratio on the premixed-gas average flame speed. When the presence of fuel vapor in the flame front is reduced at high turbulence, a relay type of ignition process occurs. In this latter case, the increase of average flame speed and of flame speed fluctuations may be interpreted from the enhanced turbulent diffusion of thermal energy and from the turbulent fluctuations caused by the eddy transport phenomenon in the flow field, respectively.