1994
Thrust Area 4: Turbulent, Reacting Fluid Mechanics & Heat Transfer
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.
Effects of Turbulence Length-Scale Distribution on Scalar Mixing in Homogeneous Turbulent Flow
Cremer, M.A.; McMurtry, P.A. and Kerstein, A.R.
Physics of Fluids, 6:2143, 1994. Funded by ACERC and US Department of Energy.
The linear eddy-mixing model is used to study effects of the turbulence length-scale distribution on the transient evolution of a passive scalar in a statistically steady homogeneous turbulent flow. Model simulations are carried out using both wide-band length-scale distributions reflecting high-Reynolds-number scaling, and narrow-band (in effect, low-Reynolds-number) distributions. The two cases are found to exhibit qualitative differences in mixing behavior. These differences are interpreted mechanistically. The narrow-band case yields the best agreement with published direct numerical simulation results, suggesting that those results are, in effect, low-Reynolds, number results not readily extrapolated to high-Reynolds-number mixing.
Low-Wave-Number Statistics of Randomly Advected Passive Scalars
Kerstein, A.R. and McMurtry, P.A.
Physical Review E, 50:2057, 1993. Funded by ACERC, National Science Foundation and US Department of Energy.
A heuristic analysis of the decay of a passive scalar field, subject to statistically steady random advection, predicts two low-wave-number spectral scaling regimes analogous to the similarity states previously identified by Chasnov. Consequences of their predicted coexistence in a single flow are examined. The analysis is limited to the idealized case of narrow band advection. To complement the analysis, and to extend the predictions to physically more realistic advection processes, advection diffusion is simulated using a one-dimensional stochastic model. An experimental test of the predictions is proposed.
K-Distributions and Weighted-Sum-of-Gray-Gases - A Hybrid Model
Denison, M.K. and Webb, B.W.
Heat Transfer-1994, 2:19-24, 1994. (Also presented at the 10th International Heat Transfer Conference, Brighton, England, August, 1994.) Funded by ACERC.
The weighted-sum-of-gray-gases (WSGG) model is shown to be related to k-distributions resulting in a hybrid approach that draw on the strengths of each model. The WSGG weights are related to the cumulative k-distributions through an integration of Planck's function over wave number weighted by the cumulative d-distributions. This allows a single quadrature over the absorption cross-section (k-space) of each arbitrarily large band to account for non-gray boundaries or particulates. Predictions from the hybrid model compare well with line-by-line benchmarks.
The Spectral Line-Based Weighted-Sum-of-Gray-Gases Model in Non-Homogeneous Media
Denison, M.K. and Webb, B.W.
ASME Journal of Heat Transfer, 1994 (in press). Funded by ACERC.
An approach is developed to extend the previously developed spectral-line weighted-sum of gray gases (SLW) model to non-isothermal, non-homogeneous media. The distinguishing feature of the SLW gas property model is that it has been developed for use in arbitrary solution methods of the radiaive transfer equation (RTE). A spatial dependence results in significant improvement over the use of spatially uniform gray gas absorption cross-sections in comparisons with line-by-line benchmarks.
Development and Application of an Absorption-Line Blackbody Distribution for CO2
Denison, M.K. and Webb, B.W.
International Journal of Heat Mass Transfer, 1994 (in press). Funded by ACERC.
An absorption-line blackbody distribution function for CO2 is presented which permits efficient and accurate calculation of total heat transfer rates. The model allows the local absorption coefficient to be the basic radiative property permitting its use in arbitrary solution methods of the radiative transfer equation (RE). A mathematical correlation is presented to approximate the function for use in computer codes. Total emissivities calculated with the correlation agree well with Hottel data. Excellent agreement is also demonstrated with line-by-line solutions of the RTE.
The Spectral-Line Weighted-Sum-of-Gray-Gases Model for H2O/CO2 Mixtures
Denison, M.K. and Webb, B.W.
ASME Journal of Heat Transfer, 1994 (in press). Funded by ACERC.
The weighted-sum-of-gray-gases model, first introduced by Hottel and Sarofim (1967) for expressing total gas emissivities and in e context of the zone method, has recently been extended to the general form of the radiative transfer equation (RTE). The fundamental radiative property of the model is the locally defined absorption coefficient that permits the use of arbitrary solution methods of the RTE. Denison and Webb developed a spectral line-based weighted-sum-of-gray-gases (SLW) model by constructing a histogram representation of the high-resolution spectra of H2O. Subsequently, a novel absorption-line blackbody distribution function was developed which easily allows the blackbody weights of aj of any desired number of gray gases to be determined by simple differencing rather than accessing detailed spectral line information. The distribution function also provides the means of incorporating a spatial dependence of the gray gas absorption cross-sections on temperature, pressure and species mole in non-isothermal, non-homogeneous problems.
The development of the SLW model in the above references has only considered individual species (H2O and CO2) independent of one another. In most practical gas flames a mixture of gases must be considered. Previous treatment of gas mixtures with the weighted-sum-of-gray-gases models has involved either determining a single set of absorption coefficients and blackbody weights from emissivities of the mixture or determining the weights of the mixture as a product of the weights of the individual species under the assumption of random positions of absorption-lines. In this paper the SLW model is formulated for H2O/CO2 mixtures.
Prediction of NOx Production in a Turbulent Hydrogen-Air Jet Flame
Menon, S.; McMurtry, P.A.; Kerstein, A.R. and Chen, J.
AIAA Journal of Propulsion and Power, 10:161-168, 1994. Funded by ACERC, NASA and US Department of Energy.
A major concern in the numerical study of turbulent nonpremixed flames is the accurate prediction of trace species. The production of pollutants such as NOx during unsteady combustion needs to be understood and predicted accurately so that the design of the next generation's combustion systems can meet the forthcoming stricter environmental restrictions. Numerical studies using steady-state methods cannot account for the unsteady phenomena in the mixing region, and therefore, fail to accurately predict the NOx production that could occur. A novel unsteady mixing model is demonstrated here that accounts for all the length scales associated with mixing and molecular diffusion processes. Finite-rate kinetics in the form of a reduced mechanism have been used to study hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparisons with experimental data and PDF calculations show good agreement, thereby, providing validation of the mixing model.
Mean-Field Theories of Random Advection
Kerstein, A.R. and McMurtry, P.A.
Physical Review, E, 49:474-482, 1994. Funded by ACERC, National Science Foundation and US Department of Energy.
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.
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.
Radiation Heat Transfer in a Laboratory-Scale, Pulverized Coal-Fired Reactor
Butler, B.W.; Denison, M.K. and Webb, B.W.
Experimental Thermal and Fluid Science, 9:69-79, 1994. Funded by ACERC and US Department of Energy.
This article 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.
The Structure of Premixed Flames in Isotropic and Shear Driven Turbulent Flows
Smith, T.M.; Menon, S. and McMurtry, P.A.
Proceedings of the 32nd Aerospace Sciences Meeting, Reno, NV, January 1994. Funded by ACERC and NASA.
The characteristic properties of constant density premixed flames in turbulent flows are analyzed using the results of direct simulations. A thin flame model is used to simulate the propagation of the flame sheet in both decaying isotropic turbulent flow and temporal mixing layers with three-dimensional instability. The study so far has focused on characterizing the geometry of the flame surface that evolves in different types of turbulent fields. The curvature of the flame surface and the strain field in the plane of the flame has been determined and the results show that the most probable shape of the scalar structure in three-dimensional flow fields is cylindrical (i.e. two-dimensional), in both isotropic turbulence and turbulent mixing layers. This agrees with the results of Ashurst, who showed that in constant energy systems, the flame sheet tends to align itself in the direction of the most compressive strain and attain a cylindrical shape. The preferential two-dimensional nature of the flame structure seen in different types of turbulent flows suggests that realistic premixed combustion studies could be carried out using two-dimensional simulations. However, conventional simulation of the propagating thin flames is still subject to some uncertainty since the flame is much thinner than the grid resolution, and thus, cannot be resolved properly. This problem has been addressed in a new 'subgrid' flame propagation model that simulates the flame evolution within each of the grid cells. This new model has shown potential for reproducing many characteristics of the premixed flame structure. However, before such a model can be used for simulating realistic reacting flows it must be demonstrated that the small-scale statistics of the flame structure are reproduced correctly as seen in the direct simulations. Therefore, the statistical data obtained from the subgrid approach will be compared to momentum transport. Based on the preliminary results obtained so far, it is expected that the new subgrid approach will not only reproduce geometrical properties such as the flame curvature and the effect of strain, as seen in the direct simulations, but also will give additional higher order statistical information such as, the time evolution of the flame brush thickness and flame crossing frequency that is difficult to obtain using conventional direct simulations.
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.
1994
Thrust Area 4: Turbulent, Reacting Fluid Mechanics & Heat Transfer
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.
Effects of Turbulence Length-Scale Distribution on Scalar Mixing in Homogeneous Turbulent Flow
Cremer, M.A.; McMurtry, P.A. and Kerstein, A.R.
Physics of Fluids, 6:2143, 1994. Funded by ACERC and US Department of Energy.
The linear eddy-mixing model is used to study effects of the turbulence length-scale distribution on the transient evolution of a passive scalar in a statistically steady homogeneous turbulent flow. Model simulations are carried out using both wide-band length-scale distributions reflecting high-Reynolds-number scaling, and narrow-band (in effect, low-Reynolds-number) distributions. The two cases are found to exhibit qualitative differences in mixing behavior. These differences are interpreted mechanistically. The narrow-band case yields the best agreement with published direct numerical simulation results, suggesting that those results are, in effect, low-Reynolds, number results not readily extrapolated to high-Reynolds-number mixing.
Low-Wave-Number Statistics of Randomly Advected Passive Scalars
Kerstein, A.R. and McMurtry, P.A.
Physical Review E, 50:2057, 1993. Funded by ACERC, National Science Foundation and US Department of Energy.
A heuristic analysis of the decay of a passive scalar field, subject to statistically steady random advection, predicts two low-wave-number spectral scaling regimes analogous to the similarity states previously identified by Chasnov. Consequences of their predicted coexistence in a single flow are examined. The analysis is limited to the idealized case of narrow band advection. To complement the analysis, and to extend the predictions to physically more realistic advection processes, advection diffusion is simulated using a one-dimensional stochastic model. An experimental test of the predictions is proposed.
K-Distributions and Weighted-Sum-of-Gray-Gases - A Hybrid Model
Denison, M.K. and Webb, B.W.
Heat Transfer-1994, 2:19-24, 1994. (Also presented at the 10th International Heat Transfer Conference, Brighton, England, August, 1994.) Funded by ACERC.
The weighted-sum-of-gray-gases (WSGG) model is shown to be related to k-distributions resulting in a hybrid approach that draw on the strengths of each model. The WSGG weights are related to the cumulative k-distributions through an integration of Planck's function over wave number weighted by the cumulative d-distributions. This allows a single quadrature over the absorption cross-section (k-space) of each arbitrarily large band to account for non-gray boundaries or particulates. Predictions from the hybrid model compare well with line-by-line benchmarks.
The Spectral Line-Based Weighted-Sum-of-Gray-Gases Model in Non-Homogeneous Media
Denison, M.K. and Webb, B.W.
ASME Journal of Heat Transfer, 1994 (in press). Funded by ACERC.
An approach is developed to extend the previously developed spectral-line weighted-sum of gray gases (SLW) model to non-isothermal, non-homogeneous media. The distinguishing feature of the SLW gas property model is that it has been developed for use in arbitrary solution methods of the radiaive transfer equation (RTE). A spatial dependence results in significant improvement over the use of spatially uniform gray gas absorption cross-sections in comparisons with line-by-line benchmarks.
Development and Application of an Absorption-Line Blackbody Distribution for CO2
Denison, M.K. and Webb, B.W.
International Journal of Heat Mass Transfer, 1994 (in press). Funded by ACERC.
An absorption-line blackbody distribution function for CO2 is presented which permits efficient and accurate calculation of total heat transfer rates. The model allows the local absorption coefficient to be the basic radiative property permitting its use in arbitrary solution methods of the radiative transfer equation (RE). A mathematical correlation is presented to approximate the function for use in computer codes. Total emissivities calculated with the correlation agree well with Hottel data. Excellent agreement is also demonstrated with line-by-line solutions of the RTE.
The Spectral-Line Weighted-Sum-of-Gray-Gases Model for H2O/CO2 Mixtures
Denison, M.K. and Webb, B.W.
ASME Journal of Heat Transfer, 1994 (in press). Funded by ACERC.
The weighted-sum-of-gray-gases model, first introduced by Hottel and Sarofim (1967) for expressing total gas emissivities and in e context of the zone method, has recently been extended to the general form of the radiative transfer equation (RTE). The fundamental radiative property of the model is the locally defined absorption coefficient that permits the use of arbitrary solution methods of the RTE. Denison and Webb developed a spectral line-based weighted-sum-of-gray-gases (SLW) model by constructing a histogram representation of the high-resolution spectra of H2O. Subsequently, a novel absorption-line blackbody distribution function was developed which easily allows the blackbody weights of aj of any desired number of gray gases to be determined by simple differencing rather than accessing detailed spectral line information. The distribution function also provides the means of incorporating a spatial dependence of the gray gas absorption cross-sections on temperature, pressure and species mole in non-isothermal, non-homogeneous problems.
The development of the SLW model in the above references has only considered individual species (H2O and CO2) independent of one another. In most practical gas flames a mixture of gases must be considered. Previous treatment of gas mixtures with the weighted-sum-of-gray-gases models has involved either determining a single set of absorption coefficients and blackbody weights from emissivities of the mixture or determining the weights of the mixture as a product of the weights of the individual species under the assumption of random positions of absorption-lines. In this paper the SLW model is formulated for H2O/CO2 mixtures.
Prediction of NOx Production in a Turbulent Hydrogen-Air Jet Flame
Menon, S.; McMurtry, P.A.; Kerstein, A.R. and Chen, J.
AIAA Journal of Propulsion and Power, 10:161-168, 1994. Funded by ACERC, NASA and US Department of Energy.
A major concern in the numerical study of turbulent nonpremixed flames is the accurate prediction of trace species. The production of pollutants such as NOx during unsteady combustion needs to be understood and predicted accurately so that the design of the next generation's combustion systems can meet the forthcoming stricter environmental restrictions. Numerical studies using steady-state methods cannot account for the unsteady phenomena in the mixing region, and therefore, fail to accurately predict the NOx production that could occur. A novel unsteady mixing model is demonstrated here that accounts for all the length scales associated with mixing and molecular diffusion processes. Finite-rate kinetics in the form of a reduced mechanism have been used to study hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparisons with experimental data and PDF calculations show good agreement, thereby, providing validation of the mixing model.
Mean-Field Theories of Random Advection
Kerstein, A.R. and McMurtry, P.A.
Physical Review, E, 49:474-482, 1994. Funded by ACERC, National Science Foundation and US Department of Energy.
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.
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.
Radiation Heat Transfer in a Laboratory-Scale, Pulverized Coal-Fired Reactor
Butler, B.W.; Denison, M.K. and Webb, B.W.
Experimental Thermal and Fluid Science, 9:69-79, 1994. Funded by ACERC and US Department of Energy.
This article 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.
The Structure of Premixed Flames in Isotropic and Shear Driven Turbulent Flows
Smith, T.M.; Menon, S. and McMurtry, P.A.
Proceedings of the 32nd Aerospace Sciences Meeting, Reno, NV, January 1994. Funded by ACERC and NASA.
The characteristic properties of constant density premixed flames in turbulent flows are analyzed using the results of direct simulations. A thin flame model is used to simulate the propagation of the flame sheet in both decaying isotropic turbulent flow and temporal mixing layers with three-dimensional instability. The study so far has focused on characterizing the geometry of the flame surface that evolves in different types of turbulent fields. The curvature of the flame surface and the strain field in the plane of the flame has been determined and the results show that the most probable shape of the scalar structure in three-dimensional flow fields is cylindrical (i.e. two-dimensional), in both isotropic turbulence and turbulent mixing layers. This agrees with the results of Ashurst, who showed that in constant energy systems, the flame sheet tends to align itself in the direction of the most compressive strain and attain a cylindrical shape. The preferential two-dimensional nature of the flame structure seen in different types of turbulent flows suggests that realistic premixed combustion studies could be carried out using two-dimensional simulations. However, conventional simulation of the propagating thin flames is still subject to some uncertainty since the flame is much thinner than the grid resolution, and thus, cannot be resolved properly. This problem has been addressed in a new 'subgrid' flame propagation model that simulates the flame evolution within each of the grid cells. This new model has shown potential for reproducing many characteristics of the premixed flame structure. However, before such a model can be used for simulating realistic reacting flows it must be demonstrated that the small-scale statistics of the flame structure are reproduced correctly as seen in the direct simulations. Therefore, the statistical data obtained from the subgrid approach will be compared to momentum transport. Based on the preliminary results obtained so far, it is expected that the new subgrid approach will not only reproduce geometrical properties such as the flame curvature and the effect of strain, as seen in the direct simulations, but also will give additional higher order statistical information such as, the time evolution of the flame brush thickness and flame crossing frequency that is difficult to obtain using conventional direct simulations.
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.