Experimental investigation of flame-holding capability of hydrogen transverse jet in supersonic cross-flow

This paper describes an experimental effort to characterize the flame-holding process of a hydrogen jet injected into a high total enthalpy supersonic cross flow. An expansion tube is used to provide a correct simulation of true flight combustion chemistry, including ignition delay and reaction times. This approach permitted a number of unique experiments involving acceleration of radical-free air to high total enthalpies. The experiments were designed to map the near-field flow characteristics and autoignition process of an underexpanded transverse hydrogen jet injected into flight-Mach number 10 and 13 total enthalpy flow conditions. Flow visualization techniques included planar laser-induced fluorescence (PLIF) of OH and schlieren imaging applied simultaneously. Schlieren images show the shock structure around the jet and the periodically formed coherent structures in the jet-free-stream interface. Overlaid OH-PLIF and schlieren images allow characterization of the autoignition of a hydrogen jet in air cross flow for different jet-to-free-stream momentum flux ratios at both flow conditions. Transverse jet autoignition and flame-holding characteristics observed in both side view and top view images by OH-PLIF reveal differences with pre vious results in the literature. In the present experiments, the first OH signals are obtained in the recirculation region upstream of the jet exit and in the bow shock region, while in past experiments with similar geometry but lower total enthalpy conditions, no strong OH signal was observed within the first 10 jet diameters. The OH-PLIF results for Mach 10 conditions also show that the OH signal level decreases significantly as the mixture expands around the jet flow field, indicating a partial quenching of the ignition. This indicates that combustion of hydrogen and air in these high total enthalpy conditions is a mixing-limited process. It is evident from the results that improved injection schemes will be required for practical applications in scramjet engines.