Effect of Obstacle Length Variation on Hydrogen Deflagration in a Confined Space Based on Large Eddy Simulations

In the field of hydrogen safety and combustion, the effect of obstacles on hydrogen deflagration is a topic of general interest to scholars. In previous studies, scholars usually used uniform obstacles under various operating conditions and obtained conclusions by changing their number and positions. However, in practice, the shapes of obstacles at an accident site are often not the same and regular. In this paper, a series of obstacles with variations in length were investigated, and the effects of the obstacles on hydrogen deflagration under different working conditions were analyzed. The configuration of the obstacles with gradually increasing lengths amplified the vortices in the flow field so that the propagation direction of the flame front surface was reversed after passing three obstacles. The variations in the lengths of the obstacles had a significant stretching effect on the propagation of the flame and a considerable acceleration effect on the propagation speed of the flame. The main reason for the acceleration was the rapid propagation of the flame achieved by the vortex when rupture occurred. The change in the pressure gradient that occurred at the center of rotation caused rapid movement of the combustion gases, which ultimately led to an increase in the flame propagation speed. A configuration with gradually increasing lengths of the obstacles promoted the overpressure. A configuration with gradually decreasing lengths of the obstacles suppressed the overpressure. The reason for the formation of the local high-pressure area was that unburned gas was accumulated there by pressure waves and the obstacle walls, and then the thermal expansion formed a high pressure. The Rayleigh–Taylor and Kelvin–Helmholtz instabilities caused the overpressure to rise further. The results can provide a theoretical basis for hydrogen transportation, storage, and safety.