Direct detonation initiation by an elliptical hotspot in hydrogen/oxygen/argon mixtures

Detonation is a fundamental phenomenon in propulsion technology and safety engineering. When initiated by a non-circular hotspot, its evolution and sustainability are significantly influenced by the resulting inhomogeneity. This study performs two-dimensional simulations with detailed chemistry in a stoichiometric hydrogen/oxygen/argon mixture to investigate direct detonation initiation from elliptical hotspots. The effects of hotspot pressure and aspect ratio are systematically examined. Results show that lower curvature along the major axis results in weaker compression and slower propagation compared to the minor-axis direction. Consequently, the detonation front progressively evolves from elliptical to circular and eventually to an ellipse with swapped principal axes. At intermediate hotspot pressures, an asymmetric propagation regime emerges: the detonation remains self-sustained along the minor axis while the reaction front decouples from the leading shock along the major axis, leading to local quenching. Similar to circular hotspots, successful initiation requires the hotspot pressure to exceed a critical threshold. Furthermore, this threshold increases with increasing ellipticity. These phenomena are primarily attributed to curvature-dependent flow divergence behind the leading shock, which enhances expansion cooling and suppresses chemical energy release in high-curvature regions.