Numerical study on flame acceleration and deflagration-to-detonation transition in a closed annular channel with a temperature-gradient jet flow

This study employs large eddy simulation to conduct a comprehensive numerical investigation into flame acceleration and deflagration-to-detonation transition (DDT) processes affected by temperature-gradient jets in a confined annular channel. The findings reveal that both maintaining a constant-temperature jet and artificially introducing a temperature gradient present distinct advantages and limitations. Specifically, a constant-temperature jet promotes more stable combustion, which facilitates energy accumulation and leads to a higher peak pressure of the detonation wave. Under such conditions, the jet induces the formation of larger and denser vortex structures, further enhancing the flame acceleration process. When a temperature gradient is imposed on the jet, the resulting complex temperature field intensifies the leading shock wave and decreases its distance to the flame tip, thereby enhancing the shock–flame interactions and shortening the distance of detonation onset. Notably, increasing the average jet temperature does not necessarily favor the DDT process. On the contrary, the present study demonstrates that, irrespective of the presence of a temperature gradient, elevating the average jet temperature tends to delay detonation onset. Although the current outcomes exhibit sensitivity to the jetting frequency, the implementation of a temperature-gradient strategy indeed enhances detonation performance. Regarding detonation initiation, all DDT processes observed in this study align with the Zel'dovich gradient mechanism. The identified detonation initiation modes can be broadly categorized into three types: (i) detonation resulting from shock–flame interaction; (ii) detonation induced by shock wave reflection; and (iii) detonation triggered by shock wave focusing.