Investigation of scramjet flow path instability in a shock tunnel

Three-dimensional computational simulations of reactive flowfields within a hydrogen-fueled scramjet-like geometry experimentally tested in a free piston shock tunnel are presented. The experimental configuration (Odam and Paull, AIAA Paper 2003-5244) involves injection of hydrogen fuel into the scramjet inlet, followed by mixing, shock-induced ignition, and combustion. Results are compared with time-resolved and steady-state surface pressure distributions. The predictions for both fuel-off and fuel-on conditions were observed to be sensitive to the choice of the wall temperature boundary conditions, with the best comparisons with experimental data achieved through the coupling of a simplified conjugate heat transfer model for the wall temperature distribution with the flow solver. When integrated in a fully time-accurate manner, the fuel-on simulations showed a marked sensitivity to the modeled rate of air ingestion into the engine. Either steady combustion or a progression toward inlet unstart could be achieved, depending on this rate. The results also provide some support for a "radical-farming" hypothesis, proposed to explain the ability of the hydrogen-air mixture to auto-ignite at relatively low inlet contraction ratios