Observation of the Processes of Ignition and Combustion Flowfield Formation in a Supersonic Combustor with Presence of Stream wise Vortices

Abstract

Quite different processes of ignition and combustion flowfield formation in a Mach 3 supersonic combustor depending on fuel injection and mixing schemes are observed by high speed schlieren video and wall pressure measurement. Experiments are conducted at a Mach 8 simulated flight condition using the High Enthalpy Shock Tunnel of JAXA, Japan. Parallel (12°) injection scheme of gaseous hydrogen fuel with generation of streamwise vortices by Hypermixer (HM) injector is examined and compared with the two typical normal (perpendicular) injection schemes, which are with and without backward facing step. Equivalence ratio is varied from Φ = 0.3 to 1.5. In the case of HM injector, the induction time increases as increasing Φ, to gradually push the ignition point far downstream of the injector. In larger Φ of 1.0 and 1.5, transient processes are observed, where a strong pressure wave generated by the explosive combustion of the well premixed gas at the combustor exit propagates upstream and decays around the injector to form a new quasi‐steady flowfield of a supersonic combustion mode downstream of the injector. In this new flowfield, combustion starts near downstream of the injector showing rather the features of diffusion flame combustion. The flow is choked upstream near the combustor exit and is accelerated to supersonic again in the nozzle. The properties of the observed pressure wave agreed well with those of Chapman‐Jouguet detonation wave on the basis of 1D‐flow analysis indicating that this pressure wave is a kind of detonation wave. For normal injectors, similar upstream propagation of the pressure wave is observed, but it propagates far upstream of the injector to form subsonic combustion mode. Comparing the characteristics of the pressure wave propagation for the injectors, some considerations are made on the mechanism of the transient processes, focusing on the roles of streamwise vortices to effectively operate the supersonic combustion.Quite different processes of ignition and combustion flowfield formation in a Mach 3 supersonic combustor depending on fuel injection and mixing schemes are observed by high speed schlieren video and wall pressure measurement. Experiments are conducted at a Mach 8 simulated flight condition using the High Enthalpy Shock Tunnel of JAXA, Japan. Parallel (12°) injection scheme of gaseous hydrogen fuel with generation of streamwise vortices by Hypermixer (HM) injector is examined and compared with the two typical normal (perpendicular) injection schemes, which are with and without backward facing step. Equivalence ratio is varied from Φ = 0.3 to 1.5. In the case of HM injector, the induction time increases as increasing Φ, to gradually push the ignition point far downstream of the injector. In larger Φ of 1.0 and 1.5, transient processes are observed, where a strong pressure wave generated by the explosive combustion of the well premixed gas at the combustor exit propagates upstream and decays around the inje...