Effect of diffusion time on the mechanism of deflagration to detonation transition in an inhomogeneous mixture of hydrogen-air

Abstract

This study aimed to numerically investigate the effect of diffusion time between H2 injection and ignition on mechanisms behind deflagration to detonation transition (DDT) in an inhomogeneous H2/Air mixture. The combustion chamber is a closed, rectangular tube with seven obstructions and a blockage ratio of 30%. From the top wall of the air channel, hydrogen is injected with diffusion times of 3, 5, 7.5, and 10 s. As a result, the inhomogeneous mixtures with different concentrations gradients are formed in the vertical direction of flame propagation. The solution method is the finite-volume method with k−ωSST turbulence model and the Weller flame wrinkling combustion model. The current findings reveal that shortening the diffusion time between H2 injection and ignition, or increasing the mixture's inhomogeneity, causes DDT to occur faster for a lean inhomogeneous H2-Air mixture of 20%. The onset of detonation in a lean mixture is caused by shock waves focusing ahead of the flame front at the channel's top wall, where hydrogen concentration is higher. For a rich inhomogeneous H2/Air mixture of 35%, the concentration gradient weakens flame acceleration and delays the initiation of detonation. In the rich mixture, three different regimes of onset of detonation appear by changing diffusion time. In the first regime, a strong curved incident shock wave forms a Mach stem. Behind this Mach stem, chemical reactions are being activated. Finally, the flame front and the reaction region behind the Mach stem collide, causing DDT to happen. In the second regime, Mach stem does not form; however, DDT occurs in the direct vicinity of the flame where reflected shock waves from the obstacles collide flame front. In the third regime, neither the Mach stem nor the reflected shock waves play a significant role in detonation initiation.