Mode Transition in Cavity Based Dual Mode Scramjet Combustor

2D RANS and uRANS simulations using an SST - k-ω , density based implicit solver with finite rate/eddy dissipation chemistry, single step reaction are used to simulate combustion and flow characteristics in a cavity-stabilized dual-mode SCRAMJET combustor. Fuel flow rate of gaseous hydrogen injected transversely from a sonic orifice upstream of the cavity is altered by varying the pressure at the fuel inlet while holding flow parameters at the Mach 2.01 vitiated air inlet unchanged. Effect of fuel flow rate on combustor pressure rise and on the isolator shock train is studied. A saturation process is noted in which an increase in fuel flow rate causes an increase in heat release rate which causes an increase in combustor pressure near the cavity which causes an increase in the extent and intensity of the shock train in the isolator. The leading shock in the isolator causes a separation of the boundary layer which causes a large recirculation region upstream of the fuel injector which increases fuel penetration thereby increasing heat release. The processes seem to reinforce each other but do not lead to an oscillatory behavior. In unsteady simulations the effects of suddenly transitioning between a high and a low fuel flow rate on factors such as settling time and hysteresis are studied. The motion of shockwaves in the combustor and the isolator and the shock-boundary-layer interaction between the isolator shock system and the isolator boundary layer which critically influences fuel penetration through the formation of a separation region upstream of the fuel injector are studied.