Investigation of supersonic combustion dynamics via 50 kHz CH* chemiluminescence imaging

Time-resolved CH* chemiluminescence imaging was performed at 50 kHz in order to study flame motion and stabilization in the axial flow direction in a dual-mode scramjet at the University of Virginia Supersonic Combustion Facility. The combustor was operated in a fully premixed mode at a global equivalence ratio of 0.40 using ethylene fuel and air with stagnation temperature and stagnation pressure of 1200 K and 300 kPa, respectively. From the high-speed CH* measurements, information regarding the flame anchoring position and flame spreading angle is collected, providing insight on the effect of cavity flame holding and flame penetration into the freestream flow. Statistics have been collected on the variation of flame brush width as a function of axial position (or along the flame length). The flame spreading angle of 9.5 degrees calculated from the CH* chemiluminescence imaging is comparable to flame angles derived from previous LES calculations. In addition, the combustor is noted to be highly dynamic, exhibiting significant variations in the integrated CH* chemiluminescence signal and the flame brush width as a function of time. A characteristic frequency of 340 Hz has been determined from the CH* imaging which governs the periodic oscillations of the CH* signal, flame brush width, and convective motion of the flame brush front. It is likely due to an instability in which acoustic waves are reflected and convected between the shock train and flame front. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.