Experimental Study of Vitiation Effects on Flameholding in a Cavity Flameholder

Wind-tunnel tests were performed to determine the effect of vitiation on flameholding limits in a hydrocarbon-fueled dual-mode scramjet flameholder. The test air was generated both clean and with 6.7% water by mole to examine the effect of hydrogen combustion vitiation on flameholding. The tests were performed in a direct-connect configuration with a Mach 2 inlet, a constant area isolator, a directly fueled cavity flameholder, and a rectangular divergent combustor. Ethylene was chosen as the test fuel. An air throttle was used as an independent backpressure source to hold the position of the shock train constant, minimizing the variation of the flowfield upstream of the cavity so that the sensitivity of flameholding to chemical kinetics could be better isolated. Both the test medium total temperature and fuel equivalence ratio, in separate tests, were lowered until blowout to determine the lower limits of flight Mach number and lean fuel operation, respectively. The measurements showed no distinguishable difference between the clean-air and vitiated tests. This indicates that, when evaluating combustion heated wind-tunnel data on flameholding limits in directly fueled cavity flameholders, it is unnecessary to compensate for vitiation effects when combustor pressure is held constant.