| Hydrogen energy is a kind of clean and efficient energy which is recognized by many countries because of its advantages.In order to improve the utilization rate of hydrogen energy,high-pressure hydrogen storage method is mature and has been widely used in hydrogen storage system.However,self-ignition may occur in the absence of obvious ignition source after hydrogen embrittleness of storage vessel or failure of pressure relief device,thus leading to serious fire and explosion accidents,which is a potential threat to the healthy development of the hydrogen energy industry.At present,the special phenomenon of self-ignition during pressurized hydrogen leakage has not been fully revealed,and the characteristics of shock wave propagation,self-ignition process and subsequent flame evolution has not been in-depth enough.In the process of industrial hydrogen utilization,hydrogen produced by different methods usually contains a small amount of impurity gas.In addition,there are often random obstacles in the hydrogen transport pipeline.Aiming at the practical application of hydrogen energy,the influence law and mechanism of impurity gas in hydrogen and obstacles in the tube on hdyrogne self-ignition is studied experimentally in detial in this paper.Besides,the prediction model of high-pressure hydrogen self-ignition is established by using similarity analysis method.The visualization method utilizing the measurements of pressure and light sensors,the high-speed direct photography combined with flame identification methods are applied to investigate the effect of impurities(methane,carbon-monoxide,and nitrogen)in hydrogen on self-ignition mechanism,including characteristics of shock wave and self-ignition,and flame microscopic dynamics.The results show that the addition of impurity gas in hydrogen significantly affects the shock wave intensity and the possibility of self-ignition.Specifically,the larger the molecular weight and concentration of impurities,i.e.,the greater the relative molecular weight of the mixture in high-pressure region,the lower the intensity of induced shock wave inside the tube,resulting in a lower air temperature in shock-affected region,and the less likely selfignition to occur.The minimum release pressure required for self-ignition inside the tube and jet fire formation outside the tube increase obviously with the molecular weight of mixture in high-pressure region.Besides,there exists a certain critical static pressure threshold(1.00MPa)at the boundary layer of self-ignition in the tube,independent of the type and concentration of the impurity gas.This indicates that the decrease of air temperature after shock wave and the weakening of hydrogen-air diffusion mixing caused by the reduced shock wave are the main factor affecting the self-ignition onset.In terms of flame propagation,pure hydrogen flame develops rapidly along the axial and radial direction of the tube,and forms a complete flame across the tube cross-section.After the addition of low-concentration impurity gas to hydrogen,the self-ignited flame only develops along the boundary layer.After the addition of high-concentration impurity gas,flame is quicikly extinguished in situ without spreading downstream.When the flame spouts out the tube exit,the impurityweakened flame barely survives during the flow divergence and expansion cooling.Experiments of pressurized hydrogen leakage into obstructed tubes with different obstacle positions and numbers are carried out to study the influence mechanism of obstacles distribution on shock wave and self-ignition characteristics,flame structure and propagation behavior during self-ignition process of hydrogen leakage.After the leading shock wave meets the obstacle,part of the leading shock wave is reflected,and the overpressure of the reflected shock wave is 1.5-2 times that of the leading shock wave.The leading shock wave intensity decreases when passing through the obstacles due to flow divergence and momentum loss,and then gradually recovers.The more obstacle numbers in the tube,the stronger attenuation effect of obstacles on the leading shock wave.Results show that the presence of obstacles in the tube can obviously promote the occurrence of self-ignition.With a further position of a single group of obstacles away from the burst disk,the critical release pressure of self-ignition decreases first and then increases.While,there is no obvious difference on critical pressure of self-ignition under different obstacle numbers.The self-ignition onset mainly depends on the position of obstacles closest to the burst disk.The initial selfignition of hydrogen usually occurs near the inclined obstacle walls,tube centerline and tube sidewalls.This is mainly due to the generation,focusing and reflection of the reflected shock wave induced by the obstacle,resulting in a diversified location of selfignition onset.After passing through obstacles,hydrogen flame undergoes behavioral characteristics such as a sharp increase in flame area,the presence of unburned areas at flame tail,and flame separation.The rapid rise of flame area behind the obstacle leads to the acceleration of flame front propagation,indicating that the obstacle has an accelerating effect on flame propagation.Moreover,the peaks of flame area growth rate and flame front velocity increase with the rise of release pressure and distance between obstacle and burst disk.The coupling effects of impurities and obstacles on self-ignition of pressurized hydrogen leakage are investigated.The results show that the coupling inhibition effect of impurity gas and obstacle on the leading shock wave in the tube is more significant than that of a single factor.The leading shock overpressure at the end of the tube decreases as the number of obstacles increases.When 95%H2-5%CO leaks into a tube with three pairs of obstacles,the leading shock ovpressure at the tube end is only 33%47%of that when pure hydrogen leaks into a smooth tube.Under the coupling effect of impurity gas and obstacle,the maximum overpressure in the tube lies between the single effect of obstacle and the single effect of impurity gas.For all impurity gas and obstacle coupling conditions,the minimum release pressure required for self-ignition is higher than the critical pressure for pure hydrogen self-ignition in smooth tube.It is shown that when the two are coupled,the inhibition effect of impurity gases on hydrogen selfignition is more significant than the promotion effect of obstacles,that is,impurity gas plays a dominant role in changing self-ignition possibility.By adding the impurity gas,the accelerative effect of the obstacle on flame can be inhibited.Only under high release pressure(≥9MPa)can the flame be accerated after passing through the obstacle.On the basis of the above research,the macroscopic factors affecting the selfignition possibility of high-pressure hydrogen leakage are analyzed.It is found that there exists a linear positive correlation between the minimum release pressure required for self-ignition and the relative molecular weight of the gas in high-pressure region.The characteristic parameters affecting the critical pressure Pcr of self-ignition are further defined,including tube length L,tube diameter D,diaphragm opening time t,volume flow rate of hydrogen leakage Q,air pressure Pa in release tube and gas relative molecular weight M in high-pressure region.By using dimensional analysis method,dimensionless parameters describing the critical pressure of self-ignition are introduced.Based on a large number of experimental datas,a prediction model of self-ignition of high-pressure hydrogen leakage is obtained by fitting:Pcr=145.21(±8.21)[Pa(L/D)0.58M0.5(Qt/D3)-0.1]+3.12(±0.22).It can provide a theoretical basis for the prediction and prevention of self-ignition of high-pressure hydrogen leakage. |
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