Characterization of Shock Train Structures inside Constant-Area Isolators of Model Scramjet Combustors

Abstract

Shock train structures inside constant-area isolators were investigated experimentally and numerically. Two heat- sink isolators, one with a rectangular configuration and one with a round configuration, and one ceramic-paneled rectangular isolator with an identical cross-sectional area a length of 25 were back pressurized using a throttle valve. Both heat-sink isolators were coated with a thermal barrier, which has a relatively rough surface. Pressure profiles inside the shock train and flow properties at the isolator exit plane were measured, using wall static pressures, Pitot pressures, and stagnation temperatures, at various back pressures, in Mach 1.8 and 2.2 flows. It was found that the round isolator can sustain a higher back pressure before unstarting the flow than the rectangular isolator. The rectangular isolator with smooth ceramic panels exhibits better shock holding capability than the heat- sink rectangular isolator. The shock train location is very sensitive to back pressure variation when the shock is stationed near the isolator entrance. As the shock train is pushed toward the isolator entrance by increasing back pressure, much more uniform distributions of flow properties are created at the isolator exit plane, where the centerline flow Mach number decreases linearly with the pressure ratio along the entire isolator length. The parameter group M0 2 ((Ps-Ps´)/P0)/(Pb/P1) was found to be able to bring the spatially shifted pressure profiles of the shock trains into a narrowly-distributed similarity profile with minor dependences on isolator configuration and flow Mach number. NOMENCLATURE D = characteristic length L1.0 = shock train length M = Mach number M0 = nominal Mach number for facility nozzle P = pressure Ppitot = Pitot pressure PR = pressure ration across the isolator, Pb/P1 Reθ = Reynolds number based on boundary layer momentum thickness T = temperature x = free stream direction xs = shock train leading edge x' = freestream distance from shock leading edge, x'=x-xs y = transverse direction θ = boundary layer momentum thickness Superscript ' = condition at shock leading edge