Numerical study on the effects of obstacle shape and thickness on deflagration-to-detonation transition in hydrogen–air mixtures with a transverse concentration gradient

This study explores the deflagration-to-detonation transition (DDT) in a 30 % hydrogen–air mixture with a transverse concentration gradient through numerical simulation. The study aims to analyze the impact of obstacle shapes and thicknesses on DDT mechanisms in the inhomogeneous mixture. The combustion chamber, a rectangular channel with both ends closed, contains seven obstacles with a blockage ratio (BR) of 0.6. The numerical results demonstrate significant variations in flame and flow dynamics depending on whether rectangular or semicircular obstacles are used. Rectangular obstacles cause the flame to collide more directly with their edges, producing stronger, more concentrated vortices and a jet-like flow that accelerates the flame front more effectively. Two primary detonation initiation mechanisms are identified through the comparison of these obstacle shapes: (1) collision of the shock wave reflected from the obstacle with the flame, and (2) focusing of pressure waves near the flame front. Furthermore, semicircular obstacles facilitate a more controlled DDT process compared to rectangular ones. This control is achieved because the round shape is less favorable for flame stretching or convolution, producing smaller recirculation zones. Semicircular obstacles lead to smoother flame interactions, generating less intense vortices and resulting in slower flame propagation and delayed DDT, thereby lowering the risk of detonation occurring and reducing potential economic losses. Varying the thickness of rectangular obstacles emphasizes the significance of shock–flame interactions and the role of the Mach stem formed near the lower wall in DDT mechanisms.