Mach 2.5 experiments of reaction quenching in gas sampling for scramjet engines

In the measurement of combustion performance of scramjet engines using the gas sampling method, quenching of reactions consuming reactants is a prerequisite. This requirement becomes difficult under conditions of hypersonic flight, because the air temperature (T-a) greatly increases. To quantify the degree of quenching of reactions in the probes, we constructed four kinds of gas sampling probes, that is, freezing-oriented probes, in which the gas compositions were expected to be frozen in the sampling process, and reaction-oriented probes, in which unburned reactants might react as much as possible in the probes. We conducted sampling experiments using these probes and compared their combustion efficiencies using a supersonic combustor operating with a M2.5 airflow of T-a up to 2200 K. Autoignition was observed in the reaction-oriented probes for T-a > 910 K. This "yes or no" behavior of the probes supported our theoretical prediction about the thermal runaway in gas-sampling probes. On the other hand, the freezing-oriented probes yielded gas compositions consisting of partially burned H-2 and O-2, even at T-a = 2190 K. The combustion efficiency calculated from the gas compositions increased from 0% to 100% in the freezing-oriented probes as T-a increased. Pitot pressure is sensitive to the development of localized combustion in supersonic flow. The pitot pressure measurement indicated that the combustion actually took place in the supersonic combustor, not in the freezing-oriented probes. We concluded, by comparing with the static pressure-type probes, that the probes with a fine tip orifice of 0.3 mm indicated the correct combustion efficiency for the scramjet engine conditions.