Two-dimensional visualization of induction zone in hydrogen detonations

The induction zone length is an important length scale for gaseous detonations, as it is correlated with other characteristic parameters such as the cell size. A good characterization of the induction zone can then lead to a better comprehension of the detonation structure. In this context, this study investigates two-dimensional (2D) visualizations of induction zones in stoichiometric hydrogen-air and hydrogen-oxygen-nitrogen-argon mixtures. The detonation speed for both mixtures is close to the Chapman Jouguet speed, and without wall losses. Planar laser-induced fluorescence (PLIF) technique was employed, by exciting nitric oxide (NO) molecules (i.e., NO-PLIF). In both mixtures, 1900 ppm of NO was added as a tracer to ensure a good signal-to-noise ratio in the von Neumann condition. The NO-PLIF images displayed a variation of the experimental induction length along the cellular cycle. Large and short induction lengths were observed for incident shock and Mach stem, respectively. A total of 2727 experimental induction lengths were extracted from the NO-PLIF images, among 79 individual experiments. Statistical analyses of these data showed that the mixture with lower activation energy exhibited less dispersion of the data compared to the mixture with higher activation energy. Non-equilibrium effects were not observed in the fluorescence profiles and induction length measurements.