Combustion Induced Choking and Unstart Initiation in a Circular Constant-Area Supersonic Flow

Incipient choking and unstart initiation mechanisms are investigated experimentally in an ethylene-fueled constant-area direct-connect supersonic combustor with a circular cross section (diameter of 28 mm) and full optical access. The experiments are performed in the ACT-II pulsed arc-heated hypersonic wind tunnel of the University of Illinois at Urbana-Champaign. The extended test time (up to 1 s) and the visualization capabilities allowed by the circular geometry represent major novelties as compared to previous studies. The combination of intrusive (pressure sensors, pitot tube, heat flux probe) and nonintrusive (planar laser-induced fluorescence of the hydroxyl radical) diagnostics provides unprecedented details on the flow behavior near the choking limit. Parallel tests with fuel and air injection are conducted to isolate the effects of combustion from mass injection. It is observed that combustion produces minimal heat addition in the core flow and that the induced pressure gradient is not sufficiently strong to separate the boundary layer according to the Korkegi criterion. Combustion is found to interact with the supersonic core flow mainly through the boundary layer, where most of the combustion and heat release occurs. Stagnation pressure losses across the combustor and Mach number at the exit are well correlated by Fanno flow theory, indicating a predominant role of irreversibilities on the reduction of the Mach number that ultimately leads to choking.