Quenching of reaction in gas-sampling probes to measure scramjet engine performance

Abstract

The problem of chemical quenching in gas-sampling probes was studied as a reactive, Rayleigh, and Fanno flow. The typical lengths of friction, convective heat transfer, and reactions in the probes are evalnated under the testing conditions of ramjet and scramjet engines. A reduced chemical kinetic scheme was adopted, and the reaction times were evaluated from the ignition times derived by reflected shock wave experiments. Thereby, criteria for quenching in probes were obtained analytically. The critical Damköhler number ( Da ) for quenching in an H 2 -air reaction was found to be 23 (1500 K) to 10.5 (3000 K). A 2.6-fold greater Da was derived for CH 4 -air mixtures. Because reactions in CH 4 -air mixtures can be quenched even above 2200 K, combustion performance in a CH 4 -fueled ramjet combustor was measured from gas analysis. However, quenching of H 2 unburned gas in more difficult than quenching of CH 4 , Cooling with supersonic expansion is essential to freeze reactions in H 2 -air mixtures in scramjets tested under a total temperature higher than 2000 K. A constant Mach number expansion is recommended downstream of the sampling orifice. The contour of the expansion region and the decay of total pressure were determined taking into account friction and cooling effects. Reactions with unburned H 2 can be quenched if the sampling probe works as a supersonic diffuser operating in the supercritical mode.