Nitrogen and hydrogen coherent anti-Stokes Raman scattering thermometry in a supersonic reactive mixing layer

A supersonic reactive mixing layer between a Mach 2 cylindrical jet of hydrogen and a Mach 2 reflow of air was studied experimentally, The hydrogen was injected parallel into the main air flow and ignited spontaneously. Temporally and spatially resolved temperature measurements were performed by coherent anti-Stokes Raman scattering (CARS) successively on the nitrogen and hydrogen molecules. The temperature field and its fluctuations are described and commented on in View of pressure wall distributions, velocity measurements by laser Doppler velocimetry, and visualizations of the OH radical by planar laser-induced fluorescence (PLIF). Autoignition length was evaluated for air total temperature ranging from 1850 down to 1600 K, at a static pressure of 0.08 MPa. For the highest temperature level obtained, no vitiation effect due to water vapor was seen The ignition occurred in intermittent structures taking place 10 injector diameters downstream from the injector. The flame was stabilized at 30 diameters downstream from the injection, leading to a moderate pressure rise, then limited by the divergence of the test section at a distance of 60 diameters. The combustion regime took place in large structures in the thin reactive layer, identified by the velocity, temperature, and PLIF signal averaged profiles. That feature was confirmed by the temperature probability density functions collected in rich and lean regions on H-2 and N-2 CARS signals, respectively. The degree of mixing, evaluated by the temperature rise in the hydrogen jet and the thickness of the thermal layer, remained too poor to provide good combustion efficiency