Effects of combustion on flowfield in a model scramjet combustor

Abstract

The effects of combustion on the change of the flowfield in a model SCRamjet combustor with a backward step is investigated experimentally and numerically. The main airflow has a Mach number of 2.0. The total temperature is 1000 K for cold flow and 1800 K for hot flow. Hydrogen fuel is injected parallel to the main airflow through a slit on the backward face of the step. The combustion mode is categorized in two modes. One is a weak combustion (WC) mode that is not accompanied by a shock wave, where the flowfield is similar to that in cold flow. Another is an intensive combustion (IC) mode that is accompanied by a shock wave, where the flowfield is much different from that in cold flow. In IC mode, a large separation region is generated behind the step by the shock wave, and the vortex generated at the region rolls the fuel up. The main reacting region is in the shear layer just behind the shock wave, where the main airflow bumps the rolled-up fuel, and the temperature is relatively high due to the shock wave. The flowfield is then controlled by the rate of mixing, leading to fast heat release, which raises the pressure level in the combustor and supports the shock wave. This passive feedback works, and both the mixing efficiency and the combustion efficiency become high. On the other hand, in WC mode, the reacting region spreads over the shear layer downstream of the step, and its heat release rate is lower than that in IC mode. The flowfield is then controlled by the rate of chemical reaction, and the combustion efficiency remains low.