Spontaneous combustion of high-pressure hydrogen leakage in the different sudden-expand tubes

• The influence of burst pressure and sudden expansion on the spontaneous combustion mechanism is studied. • Sudden expansion structure increases the likelihood of autoignition. • Autoignited flames intensity weakens with increasing sudden expansion length. • Increased duct length extends jet flame boundary development. • Jet flame length increases with extended sudden-expansion tube. The shock wave propagation characteristic and the hydrogen spontaneous combustion phenomenon in the sudden expansion tubes during the high-pressure hydrogen leakage are investigated. The influence of the burst pressure and the sudden expansion length on the leading shock wave, overpressure, and the spontaneous combustion mechanism is studied with four different lengths and types of sudden expansion tubes. The pressure histories captured by four positions are used to analyze the overpressure intensity, the shock wave intensity, and the overpressure decay rate. The photoelectric signals in four positions are used to determine the occurrence of hydrogen spontaneous combustion. Results show that the addition of a sudden expansion structure increases the likelihood of autoignition and advances the autoignition position as burst pressure rises, but the intensity of autoignited flames within the tube weakens with increasing length. As the total length of the sudden expansion section increases, shock wave velocity within the tube decreases more rapidly. At low pressures, when shock waves enter the first two types of sudden expansion tubes, their trend shows no obvious difference from that in ordinary straight tube. However, when entering the T3 tube with a longer sudden expansion section, a process of velocity decrease followed by stabilization is observed. Compared to straight tube, in the sudden expansion tube with a shorter sudden expansion section of the same total length, shock wave intensity inside the tube is notably reduced, and the overpressure decay rate increases. The T4 tube with a longer sudden expansion section exhibits particularly significant effects in reducing shock wave intensity, velocity, and overpressure decay rate, especially under high-pressure conditions.