Effects of chamber geometry and ignition sequence on high-pressure pre-mixed H2/O2 flame propagation and deflagration-to-detonation transition

chemical kinetic mechanism consisting of 8 species and 19 elementary reactions, high-resolution simulations were conducted on flame development in two-dimensional axially symmetric combustion chambers. By altering geometric parameters such as chamber length and contraction angle, as well as implementing different multi-point ignition sequences, the study analyzed flame acceleration mechanisms, turbulent evolution patterns, and DDT (deflagration-to-detonation transition) triggering mechanisms. The results show that chamber contraction geometry modifies flame development by changing the available propagation space and reflected-shock trajectories. In the elongated contraction channel, the rightward-propagating flame is stretched into a finger-like structure and accelerates to approximately 1000 m/s before DDT initiation. The ignition sequence determines the shock-wave interference pattern; later-ignited flames experience pronounced deflection and compression due to pre-existing shock waves. Two dominant DDT triggering mechanisms are identified: hotspot-induced DDT caused by reflected-shock focusing and flame-stagnation-induced DDT caused by sustained shock-flame coupling in confined regions. Ultimately, detonation wave evolution is governed by both high acoustic impedance ratios and intense interface instability. This study clarifies the roles of contraction geometry and ignition sequence in controlling flame-shock coupling and DDT initiation in confined high-pressure hydrogen-oxygen combustion systems.