Menon, S
1994
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.
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.
1993
Linear Eddy Modeling of Turbulent Combustion
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.
A Linear Eddy Subgrid Model for Turbulent Combustion: Application to Premixed Combustion
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.
A Linear Eddy Mixing Model for a Large Eddy Simulation of Turbulent Combustion
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
Linear Eddy Mixing of Complex Engineering and Geophysical Flows, 1992 (in press). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
To improve predictive capabilities for modern combustion processes, sophisticated models that faithfully represent the physics of turbulent mixing and reaction are required. Models that are presently used to study combustion systems, such as those based on gradient diffusion assumptions, omit important aspects of the subgrid mixing process. More sophisticated methods based on probability density function (PDF) transport equations treat the reaction process exactly, but the molecular mixing submodels that are employed are not fully satisfactory. Although the utility of these models should not be overlooked, there is clearly a need to develop alternative methods for calculating turbulent mixing processes.
In modeling the scalar mixing process, one encounters difficulties not present in modeling momentum transport. These difficulties can be primarily attributed to the interactions among turbulent stirring, molecular diffusion, and chemical reaction at the smallest scales of the flow. A reliable model of subgrid mixing and reaction should therefore include and distinguish among these distinctly different physical processes. To accomplish this, it appears that a comprehensive description of the scalar microfield is needed. This is fundamentally different from momentum transport modeling, in which the main influence of the small scales is to provide dissipation for the large-scale structures. Thus, while the effect of subgrid stresses on the momentum transport can be reasonably treated with various eddy viscosity models, a similar characterization of the subgrid scalar field in terms of an eddy diffusivity is neither sufficient nor correct, since an accurate description of the small-scale dynamics is critical to the overall mixing and the combustion process. As a result of these features of the turbulent mixing process, progress in the development of subgrid mixing models for us in large eddy simulation (LES) has been limited.
These fundamental issues are relevant to both the solution of the Reynolds-averaged flow field (where only the mean motion is of interest) and the solution of the unsteady flow field simulated using LES techniques. In this chapter, the development of a subgrid modeling approach specifically directed at scalar mixing is addressed. The model is based on Kerstein's (1988, 1991) linear eddy model. The emphasis here is placed on premixed combustion applications, although the general formulation has also been applied to turbulent diffusion flames (McMurtry et al., 1991, 1992).
A New Unsteady Mixing Model to Predict NOx Production During Rapid Mixing in a Dual-Stage Combustor
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
AAIA Journal of Jet Propulsion and Power, 1992 (in press). (Also presented at the AIAA Annual Aerospace Sciences Meeting, Reno, NV, January 1992). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A dual-stage combustor with the potential for minimizing pollutants such as NOx emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary stage product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the secondary stage. Numerical design studies using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used in a stand-alone mode to study mixing and combustion in hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparison of the computed results with experimental data shows good agreement thereby providing validation of the mixing model. This mixing model has been developed so that it can also be implemented within steady-state prediction codes and thus, may eventually provide an improved engineering design analysis tool.
A Linear Eddy Subgrid Model for Turbulent Reacting Flows: Application to Hydrogen-Air Combustion
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, Sydney, Australia, July 1992. (Also presented at the APS Division of Fluid Dynamics, Tallahasee, Fl, November 1992 and the AIAA Annual Aerospace Sciences Meeting, Reno, NV, Janaury 1992.) Funded by U.S. Department of Energy, NASA Lewis Research Center and ACERC.
A new sub-grid mixing model for use in large eddy simulations of turbulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "splicing" events by which linear eddy elements are copied to and from neighboring grid cells based on the grid-resolved velocity field.
The model is first used to predict the mixing of a conserved scalar in a turbulent shear flow. The model correctly predicts the behavior of the pdf of the scalar field. In particular, it displays a non-marching peak at the preferred mixture fraction as the shear layer is traversed. It is then illustrated how a reduced chemical mechanism can be implemented within the linear eddy subgrid model formulation. The model is used to predict NO formation in a hydrogen-air diffusion flame using a reduced chemical mechanism involving nine reactive scalars.
1992
A Linear Eddy Mixing Model for a large Eddy Simulation of Turbulent Combustion
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
Linear Eddy Mixing of Complex Engineering and Geophysical Flows, 1992 (in press). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
To improve predictive capabilities for modern combustion processes, sophisticated models that faithfully represent the physics of turbulent mixing and reaction are required. Models that are presently used to study combustion systems, such as those based on gradient diffusion assumptions, omit important aspects of the subgrid mixing process. More sophisticated methods based on probability density function (PDF) transport equations treat the reaction process exactly, but the molecular mixing submodels that are employed are not fully satisfactory. Although the utility of these models should not be overlooked, there is clearly a need to develop alternative methods for calculating turbulent mixing processes.
In modeling the scalar mixing process, one encounters difficulties not present in modeling momentum transport. These difficulties can be primarily attributed to the interactions among turbulent stirring, molecular diffusion, and chemical reaction at the smallest scales of the flow. A reliable model of subgrid mixing and reaction should therefore include and distinguish among these distinctly different physical processes. To accomplish this, it appears that a comprehensive description of the scalar microfield is needed. This is fundamentally different from momentum transport modeling, in which the main influence of the small scales is to provide dissipation for the large-scale structures. Thus, while the effect of subgrid stresses on the momentum transport can be reasonably treated with various eddy viscosity models, a similar characterization of the subgrid scalar field in terms of an eddy diffusivity is neither sufficient nor correct, since an accurate description of the small-scale dynamics is critical to the overall mixing and the combustion process. As a result of these features of the turbulent mixing process, progress in the development of subgrid mixing models for us in large eddy simulation (LES) has been limited.
These fundamental issues are relevant to both the solution of the Reynolds-averaged flow field (where only the mean motion is of interest) and the solution of the unsteady flow field simulated using LES techniques. In this chapter, the development of a subgrid modeling approach specifically directed at scalar mixing is addressed. The model is based on Kerstein's (1988, 1991) linear eddy model. The emphasis here is placed on premixed combustion applications, although the general formulation has also been applied to turbulent diffusion flames (McMurtry et al., 1991, 1992).
A New Unsteady Mixing Model to Predict NOx Production During Rapid Mixing in a Dual-Stage Combustor
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
AAIA Journal of Jet Propulsion and Power, 1992 (in press). (Also presented at the AIAA Annual Aerospace Sciences Meeting, Reno, NV, January 1992). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A dual-stage combustor with the potential for minimizing pollutants such as NOx emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary stage product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the secondary stage. Numerical design studies using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used in a stand-alone mode to study mixing and combustion in hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparison of the computed results with experimental data shows good agreement thereby providing validation of the mixing model. This mixing model has been developed so that it can also be implemented within steady-state prediction codes and thus, may eventually provide an improved engineering design analysis tool.
A Linear Eddy Subgrid Model for Turbulent Reacting Flows: Application to Hydrogen-Air Combustion
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, Sydney, Australia, July 1992. (Also presented at the APS Division of Fluid Dynamics, Tallahasee, Fl, November 1992 and the AIAA Annual Aerospace Sciences Meeting, Reno, NV, Janaury 1992.) Funded by U.S. Department of Energy, NASA Lewis Research Center and ACERC.
A new sub-grid mixing model for use in large eddy simulations of turbulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "splicing" events by which linear eddy elements are copied to and from neighboring grid cells based on the grid-resolved velocity field.
The model is first used to predict the mixing of a conserved scalar in a turbulent shear flow. The model correctly predicts the behavior of the pdf of the scalar field. In particular, it displays a non-marching peak at the preferred mixture fraction as the shear layer is traversed. It is then illustrated how a reduced chemical mechanism can be implemented within the linear eddy subgrid model formulation. The model is used to predict NO formation in a hydrogen-air diffusion flame using a reduced chemical mechanism involving nine reactive scalars.
1991
A Linear Eddy Subgrid Model for Turbulent Mixing and Reaction
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Fourth International Conference on Numerical Combustion, St. Petersburg, FL, November 1991. Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
A new subgrid modeling technique is described for use in large eddy simulations of turbulent mixing and reaction. In particular, a model for mixing and chemical reactions at the subgrid level is developed based on the Kerstein's linear eddy approach. A unique feature of this model is the separate treatment of turbulent convection and molecular diffusion over all length-scales of the flow. This distinction, which is not retained in existing mixing models, is critical for predicting the small scale mixing process and capturing Schmidt number dependency and the effects of finite rate reaction.
Menon, S
1994
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.
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.
1993
Linear Eddy Modeling of Turbulent Combustion
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.
A Linear Eddy Subgrid Model for Turbulent Combustion: Application to Premixed Combustion
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.
A Linear Eddy Mixing Model for a Large Eddy Simulation of Turbulent Combustion
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
Linear Eddy Mixing of Complex Engineering and Geophysical Flows, 1992 (in press). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
To improve predictive capabilities for modern combustion processes, sophisticated models that faithfully represent the physics of turbulent mixing and reaction are required. Models that are presently used to study combustion systems, such as those based on gradient diffusion assumptions, omit important aspects of the subgrid mixing process. More sophisticated methods based on probability density function (PDF) transport equations treat the reaction process exactly, but the molecular mixing submodels that are employed are not fully satisfactory. Although the utility of these models should not be overlooked, there is clearly a need to develop alternative methods for calculating turbulent mixing processes.
In modeling the scalar mixing process, one encounters difficulties not present in modeling momentum transport. These difficulties can be primarily attributed to the interactions among turbulent stirring, molecular diffusion, and chemical reaction at the smallest scales of the flow. A reliable model of subgrid mixing and reaction should therefore include and distinguish among these distinctly different physical processes. To accomplish this, it appears that a comprehensive description of the scalar microfield is needed. This is fundamentally different from momentum transport modeling, in which the main influence of the small scales is to provide dissipation for the large-scale structures. Thus, while the effect of subgrid stresses on the momentum transport can be reasonably treated with various eddy viscosity models, a similar characterization of the subgrid scalar field in terms of an eddy diffusivity is neither sufficient nor correct, since an accurate description of the small-scale dynamics is critical to the overall mixing and the combustion process. As a result of these features of the turbulent mixing process, progress in the development of subgrid mixing models for us in large eddy simulation (LES) has been limited.
These fundamental issues are relevant to both the solution of the Reynolds-averaged flow field (where only the mean motion is of interest) and the solution of the unsteady flow field simulated using LES techniques. In this chapter, the development of a subgrid modeling approach specifically directed at scalar mixing is addressed. The model is based on Kerstein's (1988, 1991) linear eddy model. The emphasis here is placed on premixed combustion applications, although the general formulation has also been applied to turbulent diffusion flames (McMurtry et al., 1991, 1992).
A New Unsteady Mixing Model to Predict NOx Production During Rapid Mixing in a Dual-Stage Combustor
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
AAIA Journal of Jet Propulsion and Power, 1992 (in press). (Also presented at the AIAA Annual Aerospace Sciences Meeting, Reno, NV, January 1992). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A dual-stage combustor with the potential for minimizing pollutants such as NOx emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary stage product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the secondary stage. Numerical design studies using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used in a stand-alone mode to study mixing and combustion in hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparison of the computed results with experimental data shows good agreement thereby providing validation of the mixing model. This mixing model has been developed so that it can also be implemented within steady-state prediction codes and thus, may eventually provide an improved engineering design analysis tool.
A Linear Eddy Subgrid Model for Turbulent Reacting Flows: Application to Hydrogen-Air Combustion
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, Sydney, Australia, July 1992. (Also presented at the APS Division of Fluid Dynamics, Tallahasee, Fl, November 1992 and the AIAA Annual Aerospace Sciences Meeting, Reno, NV, Janaury 1992.) Funded by U.S. Department of Energy, NASA Lewis Research Center and ACERC.
A new sub-grid mixing model for use in large eddy simulations of turbulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "splicing" events by which linear eddy elements are copied to and from neighboring grid cells based on the grid-resolved velocity field.
The model is first used to predict the mixing of a conserved scalar in a turbulent shear flow. The model correctly predicts the behavior of the pdf of the scalar field. In particular, it displays a non-marching peak at the preferred mixture fraction as the shear layer is traversed. It is then illustrated how a reduced chemical mechanism can be implemented within the linear eddy subgrid model formulation. The model is used to predict NO formation in a hydrogen-air diffusion flame using a reduced chemical mechanism involving nine reactive scalars.
1992
A Linear Eddy Mixing Model for a large Eddy Simulation of Turbulent Combustion
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
Linear Eddy Mixing of Complex Engineering and Geophysical Flows, 1992 (in press). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
To improve predictive capabilities for modern combustion processes, sophisticated models that faithfully represent the physics of turbulent mixing and reaction are required. Models that are presently used to study combustion systems, such as those based on gradient diffusion assumptions, omit important aspects of the subgrid mixing process. More sophisticated methods based on probability density function (PDF) transport equations treat the reaction process exactly, but the molecular mixing submodels that are employed are not fully satisfactory. Although the utility of these models should not be overlooked, there is clearly a need to develop alternative methods for calculating turbulent mixing processes.
In modeling the scalar mixing process, one encounters difficulties not present in modeling momentum transport. These difficulties can be primarily attributed to the interactions among turbulent stirring, molecular diffusion, and chemical reaction at the smallest scales of the flow. A reliable model of subgrid mixing and reaction should therefore include and distinguish among these distinctly different physical processes. To accomplish this, it appears that a comprehensive description of the scalar microfield is needed. This is fundamentally different from momentum transport modeling, in which the main influence of the small scales is to provide dissipation for the large-scale structures. Thus, while the effect of subgrid stresses on the momentum transport can be reasonably treated with various eddy viscosity models, a similar characterization of the subgrid scalar field in terms of an eddy diffusivity is neither sufficient nor correct, since an accurate description of the small-scale dynamics is critical to the overall mixing and the combustion process. As a result of these features of the turbulent mixing process, progress in the development of subgrid mixing models for us in large eddy simulation (LES) has been limited.
These fundamental issues are relevant to both the solution of the Reynolds-averaged flow field (where only the mean motion is of interest) and the solution of the unsteady flow field simulated using LES techniques. In this chapter, the development of a subgrid modeling approach specifically directed at scalar mixing is addressed. The model is based on Kerstein's (1988, 1991) linear eddy model. The emphasis here is placed on premixed combustion applications, although the general formulation has also been applied to turbulent diffusion flames (McMurtry et al., 1991, 1992).
A New Unsteady Mixing Model to Predict NOx Production During Rapid Mixing in a Dual-Stage Combustor
Menon, S.; McMurtry, P.A. and Kerstein, A.R.
AAIA Journal of Jet Propulsion and Power, 1992 (in press). (Also presented at the AIAA Annual Aerospace Sciences Meeting, Reno, NV, January 1992). Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
An advanced gas turbine engine to power supersonic transport aircraft is currently under study. In addition to high combustion efficiency requirements, environmental concerns have placed stringent restrictions on the pollutant emissions from these engines. A dual-stage combustor with the potential for minimizing pollutants such as NOx emissions is undergoing experimental evaluation. A major technical issue in the design of this combustor is how to rapidly mix the hot, fuel-rich primary stage product with the secondary diluent air to obtain a fuel-lean mixture for combustion in the secondary stage. Numerical design studies using steady-state methods cannot account for the unsteady phenomena in the mixing region. Therefore, to evaluate the effect of unsteady mixing and combustion processes, a novel unsteady mixing model is demonstrated here. This model has been used in a stand-alone mode to study mixing and combustion in hydrogen-air nonpremixed jet flames. NOx production in these jet flames was also predicted. Comparison of the computed results with experimental data shows good agreement thereby providing validation of the mixing model. This mixing model has been developed so that it can also be implemented within steady-state prediction codes and thus, may eventually provide an improved engineering design analysis tool.
A Linear Eddy Subgrid Model for Turbulent Reacting Flows: Application to Hydrogen-Air Combustion
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Twenty-Fourth Symposium (International) on Combustion/The Combustion Institute, Sydney, Australia, July 1992. (Also presented at the APS Division of Fluid Dynamics, Tallahasee, Fl, November 1992 and the AIAA Annual Aerospace Sciences Meeting, Reno, NV, Janaury 1992.) Funded by U.S. Department of Energy, NASA Lewis Research Center and ACERC.
A new sub-grid mixing model for use in large eddy simulations of turbulent combustion is presented and applied to a hydrogen-air diffusion flame. The sub-grid model is based on Kerstein's Linear Eddy Model (Comb. Sci. Tech. 60, 391, 1988), which reduces the description of the scalar field to a locally one-dimensional representation. The formulation involves performing separate linear eddy calculations in each cell to describe the small-scale scalar mixing and reaction process. Convective transport across grid surfaces is accomplished by "splicing" events by which linear eddy elements are copied to and from neighboring grid cells based on the grid-resolved velocity field.
The model is first used to predict the mixing of a conserved scalar in a turbulent shear flow. The model correctly predicts the behavior of the pdf of the scalar field. In particular, it displays a non-marching peak at the preferred mixture fraction as the shear layer is traversed. It is then illustrated how a reduced chemical mechanism can be implemented within the linear eddy subgrid model formulation. The model is used to predict NO formation in a hydrogen-air diffusion flame using a reduced chemical mechanism involving nine reactive scalars.
1991
A Linear Eddy Subgrid Model for Turbulent Mixing and Reaction
McMurtry, P.A.; Menon, S. and Kerstein, A.R.
Fourth International Conference on Numerical Combustion, St. Petersburg, FL, November 1991. Funded by US Department of Energy, NASA Lewis Research Center and ACERC.
A new subgrid modeling technique is described for use in large eddy simulations of turbulent mixing and reaction. In particular, a model for mixing and chemical reactions at the subgrid level is developed based on the Kerstein's linear eddy approach. A unique feature of this model is the separate treatment of turbulent convection and molecular diffusion over all length-scales of the flow. This distinction, which is not retained in existing mixing models, is critical for predicting the small scale mixing process and capturing Schmidt number dependency and the effects of finite rate reaction.