Smith, TM
1994
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.
Smith, TM
1994
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.