Cavity flameholding in an optical axisymmetric scramjet in Mach 4.5 flows

The dynamics of an optical axisymmetric scramjet, preferred for generic combustion and flameholding studies without the corner boundary-layer effects that distort the freestream and flame propagation in rectangular scramjets, is investigated at Mach 4.5. Ethylene fuel is injected into supersonic flow via sixteen sonic fuel nozzles equally spaced in a circumferential direction and inclined at 45 degrees to the freestream. The gaseous fuel is auto-ignited by high-enthalpy flows that are compressed and decelerated by shockwaves and boundary layers while passing through the scramjet inlet and isolator. An axisymmetric cavity located downstream of fuel injectors achieves flame holding by providing slow flow recirculation regions and inducing shockwaves at cavity leading edge and ramp to redirect and recompress flows. Flow and flame behaviors are characterized by high speed flame chemiluminescence imaging and static pressure measurement, while the detailed flame structures are resolved by instantaneous ground-state hydroxyl radical (OH) distributions using planar laser-induced fluorescence (PLIF). Numerical simulation is employed to aid the inlet/isolator design to avoid unexpected unstart introduced by shock-boundary layer interactions in the shock train region. The Mach number profiles in the radial direction are measured using a Pitot probe at the combustor exit. We demonstrate supersonic flameholding with the presence of a cavity at ethylene fueled conditions where stable combustion is achieved without scramjet unstart under both mass and heat loading. The heat addition from the cavity-stabilized flame mainly reduces the flow Mach number in the near-wall flow region. In the core flow region, the flow Mach number is decreased by the jet- and cavity-induced shockwaves and minimally affected by heat addition. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.