Flame acceleration and detonation initiation in a non-uniform hydrogen–air mixture with a combination of fluid and solid obstacles

Flame acceleration and the deflagration-to-detonation transition (DDT) process in premixed combustible gases are complex phenomena involving both fluid dynamics and chemical reactions. However in practical scenarios, premixed combustible gases are often non-uniform. Based on the OpenFOAM platform, this numerical study examines the impact of combined fluid and solid obstacles on flame acceleration and DDT within various non-uniform concentration fields. The results indicate that in the initial stage of flame development, the absence of blockages on one side of the pipe and the presence of a higher concentration of hydrogen contribute to faster flame acceleration. Additionally, the narrow channel formed between obstacles and the wall, enhanced by the pressure gradient, produces a stronger suction effect, causing the flame to experience multiple zones of velocity enhancement. Furthermore, the detonation initiation can be categorized into two types: a) detonation initiated by the interaction between the flame surface and the reflected shock wave; b) detonation triggered by the coupling of the flame front with high-pressure regions. During the detonation wave's propagation, hydrogen levels below 12.7% cause detonation wave decoupling, affecting its shape based on hydrogen concentration distribution. From the perspective of flame evolution, fluid obstacles introduce more disturbances and vortices, promoting the formation of pressure gradients, which accelerates flame development and facilitates detonation initiation. The combination of fluid and solid obstacles can effectively reduce the initiation distance and time required for DDT. Also, a more uniform distribution of hydrogen concentration leads to faster changes in the flame state, enabling quicker detonation initiation.