Origin of self-ignition in transient release of pressurized hydrogen into a rectangular tube: Flow visualization and numerical research

Self-ignition events during the transient release of pressurized hydrogen into a length of tube have been reported. Understanding the underlying mechanism is crucial for both process control and risk mitigation. This study combines flow visualization and a three-dimensional numerical simulation to investigate the origin of self-ignition within a smooth rectangular tube. The experiments eliminate the influence of tube-wall discontinuities caused by sensor arrangements in previous studies, while the simulations implement a realistic multi-step diaphragm opening scheme. The present experimental study clearly captures the boundary layer behind the leading shock within the tube in the scope of self-ignition. This finding is significant as it provides direct experimental evidence that self-ignition originates from the boundary layer. The contact surface at the head of the jet is difficult to accumulate high temperatures due to flow divergence, and the wall center is filled with a high concentration of hydrogen, making them unlikely locations for ignition initiation. The contact surface and post-shock high temperatures are stretched along the wall corners of the rectangular tube. The low-velocity flow at the wall corner promotes the accumulation of oxidizer and the mixing of cold hydrogen and hot air. Self-ignition originates from the overlapping boundary layers at the wall corner. Thin flames propagate downstream along the wall corner under limited turbulent mixing and fuel supply.