The main objective of this study is an insight into physical phenomena underlying spontaneous ignition of hydrogen at sudden release from high pressure storage and its transition into the sustained jet fire. This paper describes modelling and large eddy simulation (LES) of spontaneous ignition dynamics in a tube with a rupture disk separating high pressure hydrogen storage and the atmosphere. Numerical experiments carried out by a LES model have provided an insight into the physics of the spontaneous ignition phenomenon. It is demonstrated that a chemical reaction commences in a boundary layer within the tube, and propagates throughout the tube cross-section after that. Simulated by the LES model dynamics of flame formation outside the tube has reproduced experimental observation of combustion by high-speed photography, including vortex induced "flame separation". It is concluded that the model developed can be applied for hydrogen safety engineering, in particular for development of innovative pressure relief devices.