Abstract
Unexpected self-ignition flame resulting from accidental release of pressurized hydrogen can induce a jet flame after it flows into the unconfined space. The flame evolution in the near-field region of the tube exit is important for the formation of jet flame. This paper presents a study on the physics for the evolution of spontaneously combusting hydrogen flame near the tube exit. Effects of release pressure and tube length are explored. Results show that the flame evolution is controlled by both the shock wave and vortex formed in the near-field region around the nozzle. Although two different types of flame evolution can be formed, both of them undergo the same four stages: initial flame, flame under the effect of shock wave, flame under the effect of vortex and stable combustion. The initial location of the vortex in the two types is different, which is the reason for the presence or absence of the flame separation phenomenon. If the vortex is formed inside the flame, flame separation is induced. In addition, two types of flame can transform into each other depending on the tube length. A hybrid flame can be clearly identified in the transition region in which the tube length varies from 300 mm to 700 mm. It has also been found that the flame in the transition region has features of both types and is easy to be extinguished.