Initiation process of non-premixed continuous rotating detonation wave through Schlieren visualization

High-speed schlieren visualization on the initiation process of non-premixed air and hydrogen continuous rotating detonation (CRD) is carried out in a well-designed rounded-rectangle hollow combustor in this study. The CRD initiation processes of pre-detonator ignition and spark plug ignition are comprehensively revealed by synchronous high-speed schlieren images and high-frequency dynamic pressure results. The results show that the CRD undergoes three processes after ignition, including initiation process, adjustment process and stable propagation process. The CRD initiation process of pre-detonator ignition involves the upstream-propagation phase of leading shock wave and reaction zone and the coupling phase of circumferentially-propagating shock wave and reaction zone, while that of spark plug ignition includes the upstream-propagation phase of reaction zone, and the coupling phase of circumferentially-propagating pressure wave and reaction zone. The formation of deflagration combustion flowfield and re-establishment of combustible mixture layer after ignition are important prerequisites for CRD initiation. The strong interaction between circumferentially-propagating shock wave and post-shock wave reaction conduces to the rapid initiation of CRD by pre-detonator ignition, while the pressure wave gradually interacts with reaction leading to the detonation initiation by spark plug ignition. The CRD initiation time and adjustment time of pre-detonator ignition are shorter than those of spark plug ignition. This study provides detailed insight into the physics of CRD initiation process of pre-detonator and spark plug ignition, which can deepen the understanding of the initiation mechanism and provide practical guidance for the optimal design of CRDE ignitor.