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The self-ignition of hydrogen released from a high-pressure tank using extension tubes (2200 mm) with different diameters was studied. The processes of flame transition at a nozzle and jet flame development were characterized using a high-speed camera. The results indicated that the intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube than that in a 15-mm-diameter tube. The pressure in a 15-mm-diameter tube was weaker than that in the 10-mm-diameter tube at the initial stage; however, it became higher in the later stage. Spontaneous ignition was more likely to happen in a 15-mm-diameter tube. The formation of a stabilized flame at the tube exit and Mach disk were observed during the transition of the flame to a jet fire. The stabilized flame showed a triangular shape because of the influence of a Prandtl–Meyer flow when a hydrogen jet entered a suddenly expanding environment. The formation and separation of a spherical flame were recorded during jet flame development. Large vortexes were formed in front of the flame because of the Kelvin–Helmholtz instability, which resulted in the separation of the spherical flame. The vortexes stopped rotating until the separated flame disappeared. • The intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube. • Spontaneous ignition is more likely to happen in a 15-mm-diameter tube. • The stabilized flame at the tube exit is affected by the Prandtl-Meyer flow. • The formed large vortexes have a relationship with the separation of spherical flames. [ABSTRACT FROM AUTHOR]