Experimental Characterization of Hydrocarbon-Fueled, Axisymmetric, Scramjet Combustor Flowpaths

A summary is presented of recent experimental efforts at the Air Force Research Laboratory (AFRL/RZAS) related to the development, calibration, and evaluation of axisymmetric scramjet combustor flowpaths. The flowfields exiting the facility nozzles and isolators in the direct-connect rig were characterized using a radially traversing probe at conditions that simulated flight Mach numbers between 3 and 5. The performance characteristics of two isolator configurations were compared using a throttling, backpressure valve. The fully divergent isolator was found to sustain a slightly higher pressure ratio and was used for all of the subsequent combustor evaluations. Two cavity-based combustor configurations with equivalent exit-to-inlet area ratios were evaluated. One configuration was continuously divergent, whereas the other incorporated a step flameholder downstream of the cavity. Heated ethylene was used to fuel the combustors over an equivalence ratio range 0.2 <  < 1.0. The wall pressure distribution in the divergent combustor exhibited a single pressure peak whereas the step-combustor pressure profiles exhibited a secondary pressure rise downstream of the sudden expansion. The shape and magnitude of this secondary pressure rise was sensitive to the overall equivalence ratio and the fuel distribution between the primary and secondary injectors. The step combustor exhibited a wider operating margin and higher combustion efficiency because of the area relief and the second flameholding zone. The effects of varying fuel distribution between the primary and secondary fuel-injection sites were also explored. Stream thrust based on Pitotpressure measurements at the combustor exit compares very favorably with load-cell measurements, and an in-stream imaging probe provided qualitative information related to combustor operation and the effects of fuel distribution in the combustor.