Numerical simulation of supersonic mixing and combustion

Abstract

Problems of hydrogen mixing and combustion in supersonic air streams are numerically investigated. An implicit lower-upper (LU) scheme is used for solving the full, averaged, two-dimensional Navier-Stokes and species transport equations. For turbulence closure, a two-equation low-Reynolds-number q-ω turbulence model is employed. While the chemical source terms are treated fully coupled with the fluid motion, the conservation equations for q and ω are integrated in a decoupled way. This is done by the same LU algorithm as for the main flow equations. The combustion process in chemical nonequilibrium is based on a 20-step, 9-species finite-rate chemistry model. Because of the short residence times of the gas in supersonic combustors, the kind of hydrogen supply is very important for combustion process. This paper investigates hydrogen injections axial to the main stream at subsonic, sonic, and supersonic injection velocities. In addition to an optimal mixing that is strongly influenced by the shear layer behind the injector, it is important that the main stream remains supersonic. This results in a diverging geometry that compensates for the increase in boundary-layer thickness and rise in temperature due to combustion. There is also a strong interaction with shock waves, which often induce subsonic regions. The turbulent shear flow of hydrogen and air enables the gases to interpenetrate and can improve mixing.