High-speed 1-D and 2-D Raman scattering measurement for quantitative characterization of transient hydrogen jets

This paper reports the first quantitative characterization of transient hydrogen (H₂) jets issued from a single-hole injector into nitrogen (N₂), using high-speed 1-D and 2-D Raman scattering techniques. By employing the pulse-burst laser as the light source together with the low-noise EMCCD cameras running in subframe burst gating mode for signal collection, single-shot 1-D mole fraction of H₂ (x_H2) deduced from the ratio of H₂ and N₂ Raman scattering intensities, is obtained at repetition rate of 50 kHz. By changing the injection conditions, transient jet behaviors of both subsonic and highly under-expanded jets are well captured in single shots of 1-D, and the dynamic of the oblique shocks is visualized by local peaks in relative H₂ number density. Statistical information regarding the jet behaviors is also obtained by repeating the measurement at same injection conditions, and the mixing between H₂ and N₂ can be directly evaluated by the average x_H2. A coefficient of variance (COV) of 1.6 to 6 % is measured for all the conditions near the core region of the jets at late injection time, which demonstrates a good repeatability of the 1-D line measurements during the quasi-steady injection period. Using high-speed CMOS cameras, 2-D Raman imaging of transient H₂ jets is performed at repetition rate of 10 kHz. Results in single shots demonstrate the vortices formation at the start of injection leading to turbulent mixing layers at later stages. Due to the low SNR, the accuracy and precision of the 2-D measurements is limited, and a one-to-one comparison of statistical results shows a maximum deviation of approximately 15 % from the more accurate 1-D measurements.