Numerical investigation of H2/air combustion instability driven by large scale vortex in supersonic mixing layers

Abstract

Combustion instabilities of diluted hydrogen and air driven by the large scale vortex in supersonic reacting mixing layer flows have been numerically studied using detailed chemical reaction kinetics and accurate transport properties. The overall unburned, unstable and stable combustion modes are then observed under different air temperature conditions, corresponding to 925, 1100, and 1600 K. Very large pressure oscillations are observed for the reacting flow with the medium-temperature air, the amplitudes of which are several times larger than those of the unreactive flow or stable combustion case. The analyses show that the auto-ignition of fuel and oxidizer premixed mixtures are formed inside the turbulent shedding vortexes and causes the extremely high local pressures; hence, large amplitude pressure oscillations occur. The mixing layer is ignited very close to the inlet and in advance of the vortex shedding of the flow with the high-temperature air, and the diffusion combustion moves downstream. It is concluded that the auto-ignition of the fuel and oxidizer premixed mixtures induced by the turbulent coherent structures could lead to combustion instability and drive the transition from the partially premixed quasi-constant-volume combustion to the diffusion combustion depending on the increasing air stream temperature.