Evolutions of combusting flow in rocketramjet transition regime

Numerical analysis is performed on the transition of the Integral Rocket Ramjet using the Reynolds-Averaged Navier-Stokes equations with low Reynolds number k-e turbulence model and finite rate chemistry. Two-dimensional axisymmetric coaxial ramjet is considered from the boost end phase to the steady ramjet operating mode. The mode transition accompanies a certain amount of complex phenomena such as fuel spreading, mixing, ignition and chemical reaction. The detailed transient sequence will be described in the paper. For a fixed total fuel mass flow rate, the unsteady/unstable combusting flow field is driven when the fuel/air mixture ratio is locally changed at the flame holding region. By the investigation of pressure oscillation, heat release rate fluctuation and vortex shedding, it is numerically demonstrated that unsteady vortex shedding is one of the possible reasons of the low frequency combustion instability in ramjet.