On subgrid combustion modeling for large-eddy simulations

Development of accurate combustion models is needed to aid in the design of improved combustors in both the aircraft and automotive industries. Numerical models capable of providing the necessary information must be able to predict the highly unsteady behavior associated with turbulent-chemistry interactions. One promising approach to investigate flows of practical interest is large-eddy simulation (LES). For LES simulations of turbulent flames to be successful requires the specification of a subgrid mixing model and the specification of an appropriate chemical mechanism for the reactant species under consideration. The subgrid mixing model is required to characterize the coupled effects of small scale turbulent convection, molecular diffusion and chemical reactions at scales which are unresolved by the LES equations. An appropriate chemical mechanism must also be specified which captures the desired physics of a problem but is also computationally affordable. Detailed chemical mechanisms are often intractable for general application and must be modeled in reduced form through the use of simplifying assumptions.;Inaccuracies in either the subgrid mixing model or the reduced chemical mechanism employed in an LES simulation will lead to poor results. This study investigates both of these aspects of the application of LES to turbulent reacting flows. This study is divided into two parts. The first part focuses on the validity of assumptions commonly used in the development of reduced chemical mechanisms applied in turbulent flame simulations. These assumptions are assessed in the context of a jet diffusion flame by comparing simulation results which use both full and reduced chemical mechanisms. The second part of this study investigates the application of an innovative mixing model as a subgrid model for LES. This mixing model separately treats the physical process of molecular diffusion and turbulent stirring at the small scales so that an accurate picture of the interaction of turbulence and chemistry can be obtained. The new mixing model is applied to mixing and reaction in turbulent shear layers with and without heat release to demonstrate its capabilities. The simulation results are also used to expand insight into the physics of turbulence-chemistry interactions at high Reynolds number.