ACERC
Abstracts - 1993 |
ACERC
Abstracts - 1993 |
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Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
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.
Mengüç, M.P. and Webb, B.W.
Chapter 5, Fundamentals of Coal Combustion: For Clean and Efficient Use,
(L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded
by ACERC.
This chapter presents methods and data for reacting radiative heat transfer in coal combusting systems. The key factors to be considered in pulverized coal combustion have been outlined as 1) turbulent fluid mechanics, 2) gaseous turbulent combustion, 3) particle dispersion, 4) heterogeneous char reactions, 5) radiation heat transfer, 6) coal devolatilization, 7) ash/slag formation, and 8) pollutant formation. In most global coal combustion prediction methodologies, each of these facets is modeled separately, and then coupled with the others in a global prediction scheme. An intimate coupling exists between these different phenomena in a heterogeneous combustion system. The radiation transport lies at the very heart of this coupling, particularly since it is the dominant mode of heat transfer even in moderately scaled pulverized coal combustion systems. In this chapter, we concentrate on the fundamentals of radiation transfer and its application to pulverized-coal combustion systems.
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.
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.
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.
McMurtry, P.A.; Menon, S.
and Kerstein, A.R.
Energy & Fuels, 7, (6):817-826, 1993. Funded by National Science Foundation
and ACERC.
The use of the linear eddy mixing model in application to reacting flows is discussed for several different combustion geometries. The unique feature of this model is the explicit distinction made among the various physical processes (convection, diffusion, and reaction) at all scales of the flow. This is achieved by resolving all relevant scales of motion through a reduced, one-dimensional statistical description of the scalar field in a linear domain. The advantages of this modeling approach over other conventional modeling approaches are pointed out. Applications to both "stand-alone" formulations and subgrid formulations for use in large eddy simulation (LES) are discussed.
Kerstein, A.R. and McMurtry,
P.A.
Physical Review, E, 1993 (in press). Funded by US Department of Energy,
National Science Foundation and ACERC.
Two mean-field theories of random advection are formulated for the purpose of predicting the probability density function (PDF) of a randomly advected passive scalar subject to an imposed mean scalar gradient. One theory is a generalization of the mean-field analysis used by Holzer and Pumir to derive the phenomenological model of Pumir, Shraiman, and Siggia governing PDF shape in the imposed-gradient configuration. The other theory involves a Langevin equation representing concentration time history within a fluid element. Predicted PDF shapes are compared to results of advection simulations by Holzer and Pumir. Both theories reproduce gross trends, but the Langevin theory provides the better representation of detailed features of the PDF's. An analogy is noted between the two theories and two widely used engineering models of turbulent mixing.
Frankel, S.H.; Madnia, C.K.;
McMurtry, P.A. and Givi, P.
Energy & Fuels, 7 (6):827-834, 1993. Funded by National Science Foundation,
Office of Naval Research, and ACERC.
The Linear Eddy Model (LEM) of Kerstine is used to simulate the mechanism of scalar mixing from an initial binary state in incompressible, homogeneous turbulence. The simulated results are used to measure the limiting rate of mean reactant conversion in a chemical reaction of the type F + rO --> (1 + r) Products under isothermal and nonpremixed conditions. The objective of the simulations is to access the performance of the closed form analytical expressions obtained by Madnia et al., based on the Amplitude Mapping Closure for the evaluation of the mean reactant conversion rate. This assessment is made for various flow conditions with different asymptotic statistical behavior.
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.
Denison, M.K. and Webb,
B.W.
J. Quant. Spectrosc. Radiat. Transfer, 50 (5) 499-510, 1993. Funded by
ACERC.
An absorption-line blackbody distribution function for H2O that provides an efficient means for total radiative transfer calculations is presented. The function eliminates the need to specify a path-length required by current narrow and wide bank models since the basic radiative property is the locally defined absorption coefficient. This allows the model to be used with arbitrary solution methods of the radiative transfer equation that requires the absorption coefficient as input. A simple mathematical correlation is presented for use in computer algorithms. A few sample calculations of total emissivity as well as numerical solutions to the radiative transfer equation with the use of the distribution function are performed. The model shows good agreement with Hottel's total emissivity data. There is also very good agreement between the model and computationally intensive line-by-line calculations in isothermal media of uniform composition. The function may also be used for approximate calculations in non-uniform media.
Butler, B.W. and Webb, B.W.
Energy & Fuels, 7:835-841, 1993. Funded by Empire State Electric Energy
Research Corp and ACERC.
This paper reports experimental measurements of local particle and gas temperatures in a full-scale utility boiler. The boiler is a nominally 80 MWe tangentially fired unit operated by New York State Electric and Gas. Gas temperatures were measured using a 4-m-long triple-shielded suction pyrometer. Particulate temperature data were collected using a two-color pyrometer. Temperature data were acquired at all levels in the boiler for two different coals. Results show reacting coal particle temperatures above those of the local gases only at selected positions in the burner region. At the top of the radiant section and just above the boiler nose, the particle temperatures were below the local gas temperatures by about 150 K. The results illustrate the strong three-dimensionality of the temperature field primarily in the near-burner region. Above the flame zone both particle and gas temperatures were nearly invariant across the boiler cross-section. Differences in particle and gas temperatures resulting from different fuels and firing conditions were revealed. Radiant heat flux profiles from the two tests illustrate significant dependence of the boiler radiant energy field on the coal type and mass mean particle size. Wall incident fluxes of over 500 kW/m² were measured in the near-burner region.
Butler, B.W., Denison, M.K.
and Webb, B.W.
Experimental Thermal and Fluid Science, 1993 (in press). (Also presented
at the Experimental Heat Transfer, Fluid Mechanics and Thermodynamics Conference,
Honolulu, HI, 1993.) Funded by US Department of Energy and ACERC.
This paper reports local gas and particle temperature and radiant and total heat flux measurements made in a 0.8 m diameter cylindrical down-fired laboratory-scale reactor fired at approximately 0.1 MWt with a high-volatile bituminous coal pulverized to a mass mean diameter of 55µm. Spatially resolved gas temperatures were measured using a triple-shielded suction pyrometer and particle cloud temperatures with a specially designed two-color pyrometer. Hemispherical wall radiant heat fluxes were measured using an ellipsoidal radiometer and total (convective plus radiative) heat fluxes with a plug-type heat flux meter. The particle and gas temperature profiles exhibit a strong spatial dependence on reactor fluid dynamics. Additionally, the difference between the gas and particle temperatures varies significantly with location relative to the burner inlet streams and recirculation zones. Maximum radiant fluxes of 110 kW/m² were observed, with differences between radiative and total heat flux being less than 10% at all axial locations. Maximum heat fluxes occur downstream from the location of the maximum gas and particle temperatures and exhibit a generally decreasing trend as distance from the flame increases. Predictions of the radiation heat transfer in the reactor were carried out using the discrete ordinates method. Both spectral and gray radiative transfer calculations were performed. Predicted radiant fluxes agree well with the experimental data. The sensitivity of the model predictions to the uncertainties in the input data is explored.
McMurtry, P.A.; Gansuage,
T.C.; Kerstein, A.R. and Krueger, S.K.
Physics of Fluids A., 5 (4):1023 - 1034, 1993. (Also presented at the
Thirteenth Symposium on Turbulence, Rolla, MI, September 1992.) Funded
by US Department of Energy and ACERC.
The linear eddy-mixing model is used to predict the evolution of a decaying scalar field in statistically steady homogeneous turbulent flow over a wide range of Reynolds and Schmidt numbers. Model results at low Reynolds number and order unity Schmidt number are shown to be in good overall agreement with direct numerical simulations. Results at higher Schmidt and Reynolds numbers reproduce conventional scaling properties of the scalar statistics.
Predictions of Schmidt number and Reynolds number sensitivity of the evolution of the scalar concentration probability density function are presented and interpreted.
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.
Ma, J.; Dean, M.; Rossman,
J.; Sastrawinata, T.; Webb, B.W. and Fletcher, T.H.
Meeting of the Western States Section of the Combustion Institute, October
1993, Menlo Park, CA. Funded by ACERC.
Soot properties and formation mechanisms have been extensively studied in gas flames such as acetylene and propane. However, relatively little information is known concerning soot properties in coal combustion. Coal tar is the precursor to soot in coal combustion, so that the aromatic ring structures are already present. Experiments are presented to show the size of soot particles generated from coal tar at high temperature. A flat flame burner is used to provide the high temperature environment. Coal particles are entrained along the centerline of the reactor, and release pyrolysis products into the hot surrounding gas. The tar/soot cloud diffuses radially away from the centerline as it is convected axially in the flow reactor. The soot sampling system inserts a carbon-coated microscope grid radially into the soot cloud at different residence times, and the soot particle deposit thermophoretically. Soot particles are then analyzed using transmission electron microscopy (TEM) at magnifications as high as 150,000. Distinct soot particles with approximate diameters of 25 nm were observed along with particle agglomerates consisting of multiple primary particles. The observed agglomerate size increases with residence time in the reactor. Liquid-like unstable deposits (believed to be condensed tar) were also observed. These qualitative observations are important for descriptions of soot radiation from coal flames.
Menon, S.; McMurtry, P.A.
and Kerstein, A.R.
AIAA 31st Annual Aerospace Sciences Meeting, 93-0107, Reno, NV, January
1993. Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
Accurate treatment of turbulent mixing processes is among the most important unsolved problems in developing numerical predictive schemes for reacting flows and propulsion systems. In this paper, the development and implementation of a new subgrid modeling approach to treat turbulent mixing and chemical reactions is described. The model is applicable to both nonreacting and reacting flows, with and without heat release. In addition, the same subgrid model with some minor changes is applicable to both nonpremixed and premixed reacting flows. The application of the subgrid model to simulate thin premixed flame propagation in a turbulent media is demonstrated in this paper.
Frankel, S.H.; McMurtry,
P.A. and Givi, P.
Presented at the APS Division of Fluid Dynamics Conference, Albuquerque,
NM, November 1993. Funded by the National Science Foundation and ACERC.
The Linear Eddy Model (LEM) is utilized for statistical predictions of stationary, homogeneous turbulent flows under the influence of isothermal chemical reactions. Nonpremixed reacting systems are considered with two reaction mechanisms: A binary, irreversible single-step reaction of the type A + B --> P, and the series-parallel reaction A + B --> R, A + R --> P. For both systems, the influence of various flow parameters on the rate of reactant conversion is elucidated. For the second reaction scheme, the effects of the flow parameters on the "selectivity" are also investigated. The trends portrayed by LEM are shown to be in accord with those produced by Direct Numerical Simulation (DNS) at moderate values of the Reynolds number, the Schmidt number and the Damköhler number. The advantage of LEM is its capability to extend the parameter range well beyond that currently attainable by DNS. The LEM generated results for a wide range of Schmidt and Damköhler numbers are presented and their effects on the chemical selectivity are discussed. These results are also used to examine the performance of some of the existing closures for the modeling of selectivity. It is shown that none of the closures considered are capable of reproducing LEM results accurately. In view of the agreement of LEM predictions with DNS results and the previous success of the model in reproducing known statistical features of scalar mixing, the use of the model is recommended for statistical modeling and analyses of chemically reacting turbulent flows.
Denison, M.K. and Webb,
B.W.
ASME Heat Transfer, 115, 1004-1012, 1993. (Also presented at the William
Maxwell Reed Seminar, Department of Mechanical Engineering Conference, Lexington,
KY, April 1993. Funded by ACERC.
This paper presents an approach for generating weighted-sum-of-gray gases (WSGG) models directly from the line-by-line spectra of H2O. Emphasis is placed on obtaining detailed spectral division among the gray gases. Thus, for a given model spectrum, the gray gas weights are determined as blackbody fractional functions for specific subline spectral regions at all temperatures. The model allows the absorption coefficient to be the basic radiative property rather than a transmissivity or band absorptance, etc., and can be used with any arbitrary solution method for the Radiative Transfer Equation (RTE). A single absorption cross section spectrum is assumed over the entire spatial domain in order to fix the subline spectral regions associated with a single spectral calculation. The error associated with this assumption is evaluated by comparison with line-by-line benchmarks for problems of nonisothermal and nonhomogeneous media.
Denison, M.K. and Webb,
B.W.
Proceedings from the National Science Foundation Joint US-Russian Workshop
on Radiative Heat Transfer in Highly Coupled Physical Systems, Austin, TX,
October, 1993. Funded by ACERC.
This paper presents a hybrid spectral-line-based weighted-sum-of-gray-gases (SLWSGG) model obtained directly from the line-by-line spectra of H2O and CO2. The model allows the absorption coefficient to be the basic radiative property rather than a transmissivitiy or band absorptance etc., and can therefore be used with any arbitrary solution method for the Radiative Transfer Equation (RTE). The model is based on a novel absorption-line blackbody distribution function. Predictions from the hybrid model compare well with line-by-line benchmarks.
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