The Effect of Tube Internal Geometry on the Propensity to Spontaneous Ignition in Pressurized Hydrogen Release

Over recent years, the spontaneous ignition from compressed hydrogen release has attracted considerable attention. Owing to the ultra fast evolution and intrinsic complexities of the flow structures, most previous experimental studies focused on phenomenal observations. Therefore, high resolution CFD approach is often employed to investigate the spontaneous ignition mechanism. Most previous studies on the topic considered compressed hydrogen release via a length of straight tube. However, according to Dryer?s [2] experimental findings, the internal geometry of the release tube plays an important role in the occurrence of the spontaneous ignition. In the present study, spontaneous ignition of compressed hydrogen release through a length of tube with different internal geometries is numerically investigated using our previously developed model [6-8, 10-12]. Four types of internal geometry are considered: local contraction, local enlargement, abrupt contraction and abrupt enlargement. It is found that the presence of internal geometries can significantly increase the propensity to spontaneous ignition. Shock reflections from the surfaces of the internal geometries and the subsequent shock interactions further increase the temperature of the combustible mixture at the contact region. The presence of the internal geometry also stimulates turbulence enhanced mixing between the shock-heated air and the escaping hydrogen, resulting in the formation of more flammable mixture. It is also revealed in this study in comparison with backward-facing vertical planes, forward- facing vertical planes are more likely to cause spontaneous ignition by producing the highest heating to the flammable mixture. Keywords: Spontaneous ignition; Shock reflection; Numerical simulation.