Evaluation of parallel injector configurations for Mach 2 combustion

Abstract

An experimental study of wall-mounted parallel injector ramps has been conducted to explore techniques to enhance mixing in a scramjet combustor. Downstream parallel injection may be useful at high speeds to extract thrust from hydrogen that has been used to cool the engine and the airframe. The swept ramp fuel injector employed here should produce vortex shedding and local separation (like a rearward-facing step), which should enhance mixing. Perpendicular fuel injectors were added downstream of the swept ramps to determine if the vortical wake flow generated by the parallel injectors—with no fuel injection—is effective in enhancing the mixing of the transverse fuel jet. For performance comparisons, an unswept, but otherwise identical, parallel injector was also tested. The injector ramps were designed to yield a reflected shock wave from the duct top wall such that it passed just downstream of the barrel shock of the fuel injectors. The Mach number at the exit of the fuel injector was designed to be 1.7 to produce an underexpanded fuel flow under almost all operating conditions. Direct-connect tests were conducted with a vitiated heater over a total temperature range. Flow visualization tests with an unducted freejet configuration were conducted using shadowgraph and ultraviolet television camera systems for OH radical visualization. Three duct configurations were tested. Tests of the swept ramp with a 100-mm constant-area duct downstream of the injector block resulted in upstream interaction at almost any equivalence ratio. Increasing the duct expansion rate allowed the equivalence ratio to be increased beyond 1.5 with rapid mixing and combustion over a wide total temperature range. The unswept ramp design resulted in lower combustion efficiency and a sensitivity of efficiency to heater total temperature, or duct velocity.