Experimental visualization of sphere-induced oblique detonation in a non-uniform mixture

Abstract

To the best of our knowledge, this study is the first to adopt an experimental approach for addressing the theoretical properties of sphere-induced oblique detonation in a non-uniform mixture. A stratified H2/O2–3Ar mixture was supplied with a concentration gradient normal to the launching direction of a two-stage light gas gun. High-speed schlieren images were taken to study the morphologies of oblique detonation. The waiting time after the H2 injection until the launch was varied to investigate the effect of the concentration gradient. Oblique detonations were successfully formed in every condition considered with curved wave fronts and Chapman-Jouguet detonations were partially achieved. However, stronger concentration gradients induced several critical structures, such as local quenching and a Straw-Hat structure. Also interestingly, a part of the detonation front in the region where the Chapman-Jouguet solution does not exist propagated relative to the projectile, whereas the rest remained steady. Moreover, analysis of several relevant physical length scales indicated that the nondimensional diameter, which is critical for initiating detonation in a uniform mixture, could be a valid criterion near the projectile, but not far from it. Instead, the wave curvature determined the local structures of oblique detonation, which quantitatively explained the velocity deficit and emergence of the critical structures. We believe the findings of this study will benefit the understanding of non-uniformly premixed detonation in a propagating form owing to its analogy to an oblique detonation.