Characterization of H₂-air detonations with high-speed (2.5 MHz) Rayleigh scattering and single-shot planar laser-induced fluorescence of nitric oxide

This study characterizes a CJ hydrogen-air detonation using high-speed Rayleigh scattering and single-shot planar laser-induced fluorescence of nitric oxide (NO-PLIF). Firstly, we demonstrate that a Nd:YAG laser cluster can be employed to conduct high-speed Rayleigh scattering visualizations of hydrogen-air detonations at 25 kPa, achieving effective visualization frequencies of 2.5MHz in this study. This multipulse Rayleigh scattering diagnostic is complementary to the existing techniques: (i) it enables the characterization of any detonation, from CJ detonations in large aspect ratio channel to marginal detonations in narrow channels; (ii) it is significantly less expensive than typical burst-mode lasers; (iii) it has adjustable visualization frequency, which can go beyond 10MHz with the same energy per pulse. This makes this diagnostic even more suitable for high-pressure detonations or for undiluted conditions, where structures are smaller and detonations faster, respectively; (iv) it is easily applicable to higher initial pressures since the Rayleigh signal scales with number density. Secondly, we combine this multipulse Rayleigh scattering with NO-PLIF to complement the average speed and speed decay measurements with local induction zone length (δi) measurements. This five-beam configuration provides measurement capabilities similar to our previous three beam configuration, outside of the local speed decay. Furthermore, an experimental and numerical assessment reveals that δi measurements cannot be achieved from Rayleigh scattering, as they are significantly underestimated—by approximately 25%—compared to those from NO-PLIF. Hence, NO-PLIF is recommended over Rayleigh scattering for accurate δi measurements in future studies.