Unsteady simulations of turbulent premixed reacting flows

In this study Large-Eddy Simulations of premixed turbulent combustion in the flamelet regime are used to study flame/turbulence interactions. Statistics of turbulent flame structures from simulations of three-dimensional ramjet combustors and simulations of freely propagating flames in spatially evolving turbulence demonstrate that the large-scale flamelet propagation is correctly characterized using the flamelet model approach.;A numerical model for unsteady two-dimensional simulations of reacting stagnation point flow geometries has been developed. Using this numerical model, simulations of premixed flames have been conducted. Good qualitative and good quantitative agreement with experimental data has been demonstrated despite the simulations being two-dimensional.;The major results suggest that heat release play a dominant role in determining the flow field and flame structure at low-turbulence intensity but plays a diminishing role as the intensity increases. Results presented in the form of time averaged statistics show significant gradient and counter-gradient diffusion of the turbulent scalar flux.;The structure and propagation characteristics of turbulent premixed flames have been investigated using one-dimensional simulations based on the Linear-Eddy Model LEM of Kerstein. Extensions to an earlier LEM model were carried out to include finite-rate kinetics, thermo-diffusive and heat release effects. LEM predictions of turbulent flame speed are in good agreement with Yakhot's RNG model over a wide range in turbulence intensity.;The LEM shows the onset of distributed combustion in agreement with the Borghi phase diagram. This demonstrates that the LEM formulation has the capability to simulate combustion in different regimes without requiring any changes to the model.;Comparisons with two-dimensional DNS of freely propagating flames for different Lewis numbers were in good agreement. The LEM model correctly predicts the change in turbulent flame speed for different Lewis numbers.;A subgrid model for premixed combustion in the flamelet regime based on the Linear-Eddy Model has been developed for Large-Eddy Simulations. Several improvements to the existing splicing algorithm that are specific to the flamelet model have been developed. These include the addition of a LEM time line, that makes the subgrid scalar spatial distribution and temporal distribution independent, inter-LES flame propagation. which allows flames to propagate across LES cell interfaces and artificial flame reduction and time line inversion, which eliminate spurious burning. In addition, a straight forward procedure utilizing the LEM stand-alone model, has been developed for determining this calibration constant.;Results from unsteady simulations of stagnation point flames using the LES-LEM model have demonstrated the propagation of relatively thin flame fronts. Results have also shown that the subgrid combustion model plays an important role in the overall burning rate. The subgrid model showed correct turbulent flame speed scaling with turbulence intensity. (Abstract shortened by UMI.).