Experimental Study Of Spontaneous Ignition And Subsequent Flame Propagation Of High-pressure Hydrogen Release

Due to the renewable, extensive sources, environment friendly of combustion products and high energy efficiency, hydrogen as an alternative energy carrier has obtained more and more attentions around the world. Because of the low density of hydrogen, high-pressure storge has become a key technique in the development of hydrogen energy. However, hydrogen has many unique hazardous properties. As a result, released of hydrogen during utilization can probably cause catastrophic fire and explosion accidents. In particular, spontaneous ignition can be induced without clearly identifiable ignition sources once high-pressure hydrogen is suddenly released into the air. The spontaneous ignition of hydrogen is very likely to develop into fire and/or explosion accidents, which is a huge potential risk for pressurized hydrogen storage. Therefore, it is urgent to to study the special combustion phenomena of spontaneous ignition of high-pressure hydrogen before the arrival of the hydrogen economy. Base on diffusion ignition theory, an investigation on shock wave propagation, spontaneous ignition dynamic mechanism and flame development is carried out by experimental and theoretical analysis methods.Firstly, generation and propagation of shock wave and microscopic development dynamic of hydrogen jet are examined, including propagation characteristics of shock wave in different structural tubes and microstructure variation of hydrogen jet flow field. The dynamic pressure transducer detects the instant pressure change in the tube. The high speed schlieren photography captures flow flield structure change in the vicinity of tube outlet. Results show that, a leading shock wave is generated and propagates downstream of the tube. Meanwhile, the intensity of shock wave increases with an increase of the propagation distance in the tube, and finally becomes stable. In a constant cross-section tube, the shock wave speed increases firstly, then decreases and finally keeps constant with increasing the distance to the diaphragm. The tube with a smaller diameter is conducive to the rapid formation of the stable shock wave. And the overpressure behind shock wave in the narrow tube is greater than the value in the larger diameter tube. However, in the tubes with varying cross-section, due to the presence of contraction or expansin structure, the strong multidimensional shock structures are produced, including shock refection, shock-shock interaction and shock focusing. It is also found as the leading shock wave exits from the tube, its velocity shows a trend of gradual decrease in the downstream propagation, and is eventually reduced to the speed of sound. Meanwhile, the schlieren images of the of the hydrogen jet in the vicinity of the tube exit clearly show various classical flow structures, i.e. hemisphere shock wave, Mach disk, reflected shock and shock triple point.Subsequently, the mechanisms of spontaneous ignition and flame growth, and the critical conditions for the occurrence of spontaneous ignition inside the tube are investigated using pressure records, flame detection. The concept of theoretical critical pressure of ignition was introduced. It is found that the theoretical critical pressure of hydrogen in constant-cross tube is significantly lower, only 1.63 MPa. A mathematical expression is established to predict whether the spontaneous ignition can occur when high-pressure hydrogen emits from a tank into air through a downstream tube. The influencing factors of auto-ignition are also discussed. And the influence of initial release pressure, tube length, diameter and varying cross-section geometries on the occurrence of ignition are the focus of analysis. The results show that the likelihood of spontaneous ignition increases with the increases of burst pressure and the ratio of tube length to diameter. Meanwhile, the position of initial ignition is closer to the upstream of the tube. In addition, the presence of the varying cross-section geometries can significantly promote the occurrence of spontaneous ignition. The most direct manifestation is that compared to the tube with constant cross-section area, the minimum release pressure needed for spontaneous ignition for the varying cross-section tubes is considerably lower. In particular, for the local contraction, spontaneous ignition is observed to occur at a burst pressure as low as 1.84 MPa. This is the lowest release pressure leading to spontaneous ignition compared with existing report of experimental results. Further, the possible positions of initial ignition inside the tube with local enlargement are discussed. It is found that the ignition model has an obvious difference for different initial ignition locations. The growth dynamic from spontaneous ignition to a strong flame are analyzed in detail. It is proposed that multistep rupture of the diaphragm can also lead to the occurrence and growth of spontaneous ignition. Before the completely open of diaphragm, a volume of hydrogen-air mixture is formed caused by a small crack leakage. With further break of the diaphragm, a shock wave can be generated and ignites the partial premixed combustible gas. Finally, the flame develops into the jet flame when it propagates toward the open space.Finally, the flame propagation outside the tube and behavior characteristics of jet fire are experimentally studied. Image analysis are used to examine flame propagation, deflagration development and the evolution of the shape of the jet flame. And the overpressure characteristics due to deflagration in the local confined space are analyzed. After the flame exits from the tube, a flame envelope is formed in the front of the hydrogen jet, which gradually splits into upstream and downstream regions. The upstream flame region propagates forward. However, the downstream flame region propagates toward the tube exit, which will be stabilized at the tube exit and grow gradually. At the beginning, the speed of the flame front is relatively small because of the effect of jet expansion. Then the velocity has a sharp increase, which is caused by the momentum transfer of the hydrogen jet in the open space. With the reduction of the hydrogen momentum transfer rate, the production rate of combustible mixtures slows down. As a result, the velocity of the flame decreases and oscillates with a stable value. Due to fast momentum transfer of hydrogen jet in the free space, a quantity of partially premixed combustible mixtures is produced in the vicinity of the nozzle. When the partially premixed combustible gas is ignited by the flame, a strong deflagration is appeared. A large number of pressure wave are generated due to deflagration, which results in the increase of the pressure in the chamber. The overpressure of the shock wave is significantly smaller than the maximum value of deflagration overpressure. In general, the shock wave and deflagration overpressure increase linearly with the initial release pressure. It is also found that the overpressure increases with the increase of the tube diameter. After the deflagration, a stable burning jet flame begins to establish. The overall shape of the jet flame consists of the horizontal jet fire of the momentum-controlled and the vertical fire plume controlled by buoyancy-driven flow. The evolution of jet flame morphology is mainly determined by the transient release pressure.