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

• First systematic study of direct detonation initiation from elliptical hotspots • Moderate ellipticity significantly alters initiation energy and front evolution • Novel intermediate regime: detonation sustains on minor axis, quenches on major • Critical energy exceeds spherical/cylindrical limits, rises with aspect ratio • Elliptical fronts show transient circularization before swapped-axis relaxation 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 (100-350 times ambient pressure) and aspect ratio (1-2.24) 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 (e.g., 150 times ambient pressure for aspect ratio 1.4), a novel 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. Like 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. This work advances beyond prior studies assuming uniform curvature by quantifying ellipticity’s impact on initiation thresholds and uncovering asymmetric regimes, offering insights into realistic detonation initiation in propulsion and safety applications.