Combustion in Supersonic Flows and Scramjet Combustion Simulation

The scramjet (supersonic combustion ramjet) is an air-breathing engine with the supersonic flow at the combustor entrance, i.e., with essentially lower deceleration of flow in the inlet with respect to common ramjet. The scramjet is designed for hypersonic flight of vehicle with Mach number large than 5 or 6, where the efficiency of a subsonic ramjet decreases, because the deceleration of high-speed flow to small subsonic speeds leads to extremely high temperature at the entrance to combustor, that, in its turn, generates a series of effects, deteriorating the performance of classical ramjet. The scramjet is characterized by strong coupling of all its elements. Supersonic core from the inlet to the nozzle, essential subsonic zones in thick boundary layers and high losses caused by strong shock waves, by viscous effects, by dissociation and radiation result in a situation, when positive thrust may be reached only on the basis of joint optimization of the whole flowpath. In comparison with experimental investigations, which remain very challenging to conduct in such flow conditions, computational fluid dynamics is an attractive complementary tool for the study supersonic reactive flow in the scramjet flowpath. Understanding and prediction of the flow structure are necessary for achieving the stable and efficient combustion, high thrust, and thermostable construction of the scramjet. The first half of the chapter addresses fundamentals of turbulent supersonic combustion: physics of combustion in supersonic flows with regard to scramjets, Navier--Stokes equations for multispecies reacting gas flow, kinetic schemes for simulation of scramjets, RANS/URANS, and LES approaches, the closure problems for turbulent fluxes. Particular attention is paid to the discussion of the difficulties when resolving closure problems for reaction rates. The contemporary models to account for turbulence-chemistry interactions (TCI) are shortly presented. The second half of the chapter focuses on partially stirred reactor (PaSR) turbulent combustion models. Transported PaSR (TPaSR) and unsteady PaSR models are described in details, and experience of their application to simulation of experiments on supersonic combustion (within the framework of LES approach) is demonstrated. Finally, the problem of the selection of “correct” solution among multiple solutions of PaSR steady-state equations is considered.