ACERC
Abstracts - 1986-1988 |
ACERC
Abstracts - 1986-1988 |
|
|
Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
Sloan, D.G.; Smith, P.J.
and Smoot, L.D.
Progress Energy Combustion Science, 12, (3), 63?250, 1986. 188 pgs. Funded
by US Department of Energy.
The standard k-e equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the k-e equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.
McMurtry, P.A.; Metcalfe,
R.W.; Jou, W.H. and Riley, J.J.
AIAA Journal, 24, (6), 962, 1986. Funded by NASA Lewis Research Center.
To study the coupling between chemical heat release and fluid dynamics, we have performed direct numerical simulations of a two-dimensional mixing layer undergoing a simple single-step chemical reaction with heat release. The reaction is function only of the species concentrations and does not depend on temperature. We have treated the fully compressible equations, as well as an approximate set of equations that is asymptotically valid for low Mach number flows. These latter equations have the computational advantage that high-frequency acoustic waves have been filtered out, allowing much larger time steps to be taken in the numerical solution procedure. A derivation of these equations along with an outline of the numerical solution technique is given. Simulation results indicate that the rate of chemical product formed, the thickness of the mixing layer, and the dynamics are studied to analyze and interpret some effects of heat release on the fluid motion.
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.
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.
McMurtry, P.A.; Riley, J.J.
and Metcalfe, R.W.
To appear in Journal of Fluid Mechanics, 1988. Funded by NASA Lewis Research
Center and Office of Naval Research.
The effects of chemical heat release on the large-scale structure in a chemically reacting, turbulent mixing layer are investigated using direct numerical simulations. Three-dimensional, time-dependent simulations are performed for a binary, single-step chemical reaction occurring across a temporally developing turbulent mixing layer. It is found that moderate heat release slows the development of the large-scale structures and shifts their wavelengths to larger scales. The resulting entrainment of reactants is reduced, decreasing the overall chemical product formation rate. The simulation results are interpreted in terms of turbulence energetics, vorticity dynamics, and stability theory. The baroclinic torque and thermal expansion in the mixing layer produce changes in the flame vortex structure that result in more diffuse vortices than in the large-scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers are shown to result from vorticity generation by baroclinic torques.
McMurtry, P.A. and Givi,
P.
To appear in Combustion and Flame, 1988. Funded by Air Force Office of
Scientific Research.
Direct numerical simulations have been performed to study the mechanisms of mixing and chemical reaction in a three-dimensional, homogeneous turbulent flow under the influence of the reaction of the type A + B = Products. The results are used to examine the applicability of Toors hypotheses [1] and also to determine the range of validity of various coalescence/dispersion (C/D) turbulence models that have been used previously to model the effects of turbulent mixing in such flows [2]. The results of numerical simulations indicate that the probability density function (PDF) of a conserved Shvab-Zeldovich scalar quantity, characterizing the mixing process, evolves from an initial double-delta distribution to an asymptotic shape that can be approximated by a Gaussian distribution. During this evolution, the PDF cannot be characterized by its first two moments; therefore, the application of Toor's hypothesis is not appropriate for the prediction of such flows. The results further indicate that the initial stages of mixing are well represented by the Dopazo-O'Brien C/D model, whereas, at intermediate times, the results obtained by DNS fall between those obtained by the two closures of Dopazo and O'Brien [3] and Janicka et al. [4], and deviate the most from those of Curl [5]. Therefore, a C/D model between the two closures of Dopazo-O'Brien and Janicka et al. is expected to result in favorable comparison with our data. The final stages of mixing are not well predicted by any of the C/D closures in that none of the models is capable of predicting a Gaussian asymptotic PDF for the Shavab-Zeldovich scalar variable. The results of our numerical simulations may be used to generate a C/D model (or models) that can predict all the stages of mixing accurately.
Riley, J.J. and McMurtry,
P.A.
To appear in Analysis of Reacting Flows, Springer-Verlag, 1988. Funded
by NASA Lewis Research Center, Office of Naval Research, University of Washington,
and US Office of Basic Energy Sciences.
At the present time the role of direct numerical simulation as applied to turbulent, chemically reacting flows is twofold; to understand the physical processes involved, and to develop and test theories. In this paper we present and example of the former. We employ full turbulence simulations to study the effects of chemical heat release on the large-scale structures in turbulent mixing layers. This work not only aids in understanding this phenomenon, but also gives insight into strengths and limitations of the methodology.
We find, in agreement with previous laboratory experiments, the heat release is observed to lower the rate at which the mixing layer grows and to reduce the rate at which chemical products are formed. The baroclinic torque and thermal expansion in the mixing layer are shown to produce changes in the flame vortex structure the act to produce more diffuse vortices than in the constant density case, resulting in lower rotation rates of the large scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers are shown to result from vorticity generation by baroclinic torques. The density reductions also lower the generation rates of turbulent kinetic energy and the turbulent shear stresses, resulting in less turbulent mixing of fluid elements.
Calculations of the energy in the various wave numbers shows that the heat release has a stabilizing effect on the growth rates of individual modes. A linear stability analysis of a simplified model problem confirms this, showing that low-density fluid in the mixing region will result in a shift in the frequency of the unstable modes to lower wave numbers (longer wavelengths). The growth rates of the unstable modes decrease, contributing to the slower growth of the mixing layer.
Finally, we find that this methodology can be confidently applied for Reynolds numbers less than several hundred and for Damkohler numbers less than about ten. With some modification it is possible to treat the infinite Damkohler number case using a conserved scalar.
Givi, P. and McMurtry, P.A.
Combust. Sci. and Tech., 57, 141-147, 1988. 7 pgs. Funded by Air Force
Office of Scientific Research.
The probability density functions of a passive scalar quantity are calculated in a perturbed mixing layer by means of direct numerical simulations. The results indicate that the two-dimensional rollup of the unsteady shear layer, and the pairing process in particular, contributes greatly to the generation of the predominant peak of the PDFs within the mixing region.
Smith, P.J. and Baxter,
L.L.
Western States Section, 1986, The Combustion Institute, Banff, Canada.
20 pgs. Funding source US Department of Energy, Brigham Young University and
Electric Power Research Institute.
The location of the coal particles in pulverized coal combustion and gasification processes is a dominant factor in determining flame stability, combustion/gasification efficiency and pollutant concentrations. Experimental and predicted data show that particles do not mix at the same rate as the gas phase and that this difference in turbulent mixing is important to the coal reaction process. The environment around a given particle during heat up, devolatilization and heterogeneous reaction controls the overall combustion process. In flows of interest to pulverized coal combustion, the particle dispersion is dominated by the random fluctuations of the gas phase turbulence as opposed to the mean drag on the particle. A mixed Eulerian-Lagrangian mathematical model for predicting mean trajectories of expected values for ensembles of representative particles is presented, including the turbulent dispersion effects. The mean turbulent velocity is modeled as a Fickian diffusion process from the man number density as calculated from the Eulerian part of the formation. This model is discussed and compared to alternate procedures. Applications of the model are shown for particle dispersion in non-reacting flow and pulverized coal combustion. An evaluation by comparison with experimental data is presented. Conclusions regarding the significance of turbulent particle dispersion are discussed.
Brewster, B.S. and Smoot,
L.D.
Western States Section, 1988, The Combustion Institute, Salt Lake City,
UT. 22 pgs. Funded by Morgantown Energy Technology Center through subcontract
from Advanced Fuel Research Co.
The predicted effects of turbulence on the mean gas properties in coal combustors have been shown previously to be significant (Smith and Fletcher, 1986; Brewster, et al., 1988). These effects have been incorporated in comprehensive models by the statistical, coal-gas mixture fraction method (Smoot and Smith, 1985), which is based on the scalar approach for gas diffusion flames. Previously, only a single progress variable has been used for tracking coal off gas, including all material originating in the coal and transferring to the gas during the reaction process. Simple weight-loss models for devolatilization have been adequate for this approach that assumes constant off gas composition. With recent technological advances, the incorporation of detailed devolatilization models into comprehensive codes has become a practical consideration. However, detailed models that predict varying off gas composition require improved and/or extended methods of accounting for the effects of turbulence/chemistry interactions for successful incorporation.
In a recent paper, Brewster et al. (1988) presented a generalized formulation of the coal-gas mixture fraction method which allows for an arbitrary number of progress variables, thus allowing each element, or group of elements that evolve at similar rates, to be tracked independently. Independence of the progress variables was assumed, although it would not be difficult to include correlation coefficients if they were known. Results were presented for a slightly fuel-lean, swirling combustion case to illustrate the method. Two progress variables were used to separately track coal volatiles and char off gas, and significant decreases in the size of the fuel-rich region and in coal burnout were noted. These differences were attributed to the enriched oxygen content and (apparently) decreased heating value of the early off gas in the case of two progress variables. In this paper, additional results with multiple progress variables tracking coal off gas are reported and discussed. Calculations using an alternative method of calculating char off gas enthalpy agree qualitatively with previous observations of the effects of tracking volatiles and char off gas separately in combustion. Similar calculations in an oxygen-brown, fuel-rich gasification case show relatively little effect. Even less effect is observed when hydrogen from the coal is tracked separately and all of the other elements are lumped together. The effect of tracking volatiles separately decreased when ultimate volatile yield was increased from 40 to 80 percent, and noticeable differences existed with 50 percent heat loss (uniform) from the reactor compared to no differences when the reactor was assumed to be adiabatic. See also 1-88-J09 which relates to this Thrust Area.
Baxter, L.L. and Smith,
P.J.
Western States Section, 1988, The Combustion Institute, Salt Lake City,
UT. 21 pgs. Funded by ACERC consortium: Babcock & Wilcox, Combustion Engineering,
Consol, Electric Power Research Institute, Empire State Electrical Energy Research
Corp., Foster Wheeler, Pittsburgh Energy Technology Center, Tennessee Valley
Authority, and Utah Power & Light.
An approach to describing the transport of particles in turbulent flows is presented which is derivable from first principles. The approach is independent of the particular turbulence model used. It is valid in applications ranging from entrained flow dispersion to fluidized bed applications. It involves no adjustable parameters and requires a description of the turbulence in terms of the mean square velocities and Lagrangian autocorrelation functions.
The model has been rigorously evaluated by comparison to exact solutions, accurate alternative approaches, and experimental data. The model is able to reproduce the exact solutions both analytically and numerically, predict results which are both more accurate and precise than those of the leading alternative models, and predict experimental data within its inherent error under a range of different conditions.
The incorporation of the model into a comprehensive combustor simulator is also demonstrated. The model is found to be more efficient and robust, both in terms of the particle dispersion portion of the comprehensive code and in terms of the overall code performance. Predictions using this description of turbulent particle dispersion yield results that agree in detail with previous predictions using a calibrated particle dispersion model.
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.
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.
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.
|