Experimental study on spontaneous ignition and subsequent flame development caused by high-pressure hydrogen release: Coupled effects of tube dimensions and burst pressure

Combined effects of tube dimensions and burst pressure on the spontaneous ignition caused by high-pressure hydrogen release into a semi-confined space are investigated experimentally. An important finding is that the influence of tube diameter on spontaneous ignition shows complex behavior. For tubes with different diameters, the minimum burst pressure for spontaneous ignition depends on not only the strength of the shock wave but also the mixing of hydrogen and air. A dimensionless parameter of tubes, L/D-which is defined as the ratio of the tube length to the tube diameter-is introduced to describe the effect of tube size. The results show that the possibility of spontaneous ignition increases with increasing L/D. Under appropriate conditions, spontaneous ignition leads to flame development. As the flame propagates into a semi-confined space, it first forms an envelope structure in front of the hydrogen jet. Since some amount of a partially premixed combustible mixture, created by the hydrogen jet, exists in the semi-confined space, the flame subsequently undergoes deflagration. The overpressure caused by deflagration is significantly greater than that caused by the leading shock wave. In addition, both the deflagration overpressure and the shock-wave overpressure increase with increasing tube diameter and initial release pressure.