Survey of Turbulent Combustion Models for Large-Eddy Simulations of Propulsive Flowfields

Abstract

A general review of turbulent combustion modeling closures applicable to large eddy simulations (LES) is provided. The focus is on “regime-independent” models able to provide turbulent combustion closures ranging from purely premixed to purely non-premixed and all regimes between these two limits. Special emphasis is placed on primary propulsion applications, including liquid rocket engines, diesel engines, gas turbines, and scramjets. These applications span a large range of physical phenomena including both idealand real-gas behavior, single-phase and multi-phase combustion, relatively low Mach number to supersonic and hypersonic combustion, and relatively simple geometries to highly complex geometries. Three classes of models are identified as possibly providing such broad based modeling closures: flamelet-library/presumed probability density function (PDF) models, linear eddy based models (LEM), and transported PDF or filtered density function (FDF) based models. This review focuses both on fundamental physical assumptions that apply across all of the models and assumptions that apply to each of the models individually. Namely, assumptions regarding the presumed size of the large dimensional turbulent scalar manifold apply to all of the models; however, flamelet models almost always presume only a few dimensions are necessary to yield adequate representation of the larger, turbulent manifold. In contrast, LEM and FDF models are not, in theory, bound by any manifold size assumptions (i.e. direct calculation of the turbulent scalar manifold is possible); however, due to current computational limitations, these models often employ manifold reduction techniques which are usually not as restrictive as those used by flamelet models. Individual assumptions associated with the specific formulation of each model are also analyzed. From these discussions, additional novel results testing some of the fundamental physical assumptions associated with each model are provided from a unique database of DNS of high pressure turbulent reacting temporally developing shear flames. The DNS database includes simulations of H2/O2, H2/Air, and Kerosene/Air flames with both detailed and reduced chemistry. The DNS include real property models, a real-gas equation of state, and generalized heat and mass diffusion derived from non-equilibrium thermodynamics. The simulations span a large range of Reynolds numbers and pressures (up to 125 atm), with resolutions approaching 1 billion grid points. Finally, some general comments towards the future challenges related to LES combustion modeling are offered.