Experimental And Numerical Study On The Mechanism Of Spontaneous Ignition During High-pressure Hydrogen Release Into A Tube

In current society,the excessive exploitation and the use of fossil fuels pose serious environmental and energy problems.As a clean and efficient alternative energy source,hydrogen is attracting more and more attention from all over the world including China because of its wide range of sources,high energy density,pollution-free and renewable prperties.At present,high-pressure hydrogen storage is the most ideal hydrogen storage means.However,the chemical properties of high-pressure hydrogen are very active.Under certain conditions,hydrogen can react with various metals and cause hydrogen embrittlement problems.As a result,the hydrogen storage container and the pipeline are broken,which eventually causes release of the entire hydrogen storage system.Once the high-pressure hydrogen gas releases,it is prone to spontaneous ignition in the pipeline,causing combustion and explosion,resulting in unpredictable casualties and property losses.The spontaneous ignition of high-pressure hydrogen is a major safety hazard for the safe use of hydrogen energy in the future.However,current researches on the mechanism,micro-kinetics and related influencing factors of spontaneous ignition of high-pressure hydrogen release is not deep enough.The critical conditions,spontaneous ignition characteristics,critical burst pressure,shock wave generation and propagation characteristics inside the tube,and the formation mechanism and propagation characteristics of the hydrogen jet flame outside the tube have not been fully revealed.Therefore,based on the previous studies,this paper uses the experimental method to study the effect of the tube geometry,the opening ratio of the burst disc and the mixing gas in the tube on the spontaneous ignitionof high-pressure hydrogen.The numerical simulation method is used as well to simulate the micro?kinetics in the tube and the spontaneous ignition mechanism in detail.A similitude analysis method was used to establish a prediction model of hydrogen spontaneous ignition based on the mechanism of spontaneous ignition of high-pressure hydrogen.Firstly,the pressure sensors,photodiodes and high-speed camera were used to study the effect of the tube geometry,the opening ratio of the burst disc and the mixing gas in the tube on the spontaneous ignition of high-pressure hydrogen.It is found that when high pressure hydrogen is released into a straight tube,the pressure in the high-pressure tank does not decrease rapidly.In the initial stage,as the leakage port area gradually increases,the hydrogen leak rate gradually increases;then the pressure in the high-pressure storage tank begins to decrease.During the entire release process,the pressure depletion rate in the high-pressure storage tank increases firstly and then decreases.In addition,as the burst pressure increases,the possibility of spontaneous ignition of hydrogen gradually increases,and the ignition delay time becomes shorter.When a straight pipe with a length of 360 mm and a diameter of 15 mm is used,the critical burst pressure of hydrogen spontaneous ignition is defined as 4.09 MPa.When the flame propagates into the exhaust chamber,it gradually sperates into two parts,one part is stable near the nozzle and the other part continues to propagate downstream.However,when a U-shaped tube is used,a second significant increase of shock pressure is detected in the tube,meaning that a reflected shock wave is formed in the tube.The reflected shock then propagates into the high-pressure storage tank and causes an increase in pressure.Because of the shock reflection in the tube,the shock pressure and shock intensity in the tube increase significantly.Therefore,the tube geometry has a very significant effect on the spontaneous ignition of high-pressure hydrogen.The critical burst pressure of spontaneous ignition of the high-pressure hydrogen is significantly reduced to 2.19 MPa.The ignition delay time and ignition position are closer to the upstream of the tube than straight pipes.The propagating behavior of the hydrogen flame in the exhaust chamber is also significantly different.Under the condition of lower burst pressure,the flame does not separate but moves downstream as a whole part.When the tube with different angles is used,obvious shock reflection phenomenon also occurs in the tube,causing a significant increase in the shock pressure.Moreover,it is found that the smaller the tube angle,the more obvious the shock reflection,and the higher the shock pressure inside the tube.The average speed of the shock wave decreases as it passes through the corner of the tube.The smaller the tube angle,the smaller the critical burst pressure of spontaneous ignition of hydrogen.Although there is a significant increase in the shock pressure inside the tube,the shock overpressure in the exhaust chamber is significantly reduced.Similar to the U-shaped tube,the flame outside the pipe does not separate at a lower burst pressure.In addition,the opening ratio of the burst disk is another important factor affecting the intensity of the shock inside the tube and the spontaneous ignition of hydrogen.The smaller the opening ratio of the burst disc,the smaller the average speed of the shock wave in the tube and the smaller the shock pressure.This is mainly because when the opening ratio is less than 1,the burst disc cannot open completely during the hydrogen release process,forming a convergent nozzle.When the supersonic flow passes through this structure,a significant speed reduction occurs,resulting in a decrease in shock pressure.In the case where the shock pressure is significantly reduced,the possibility of spontaneous ignition of high-pressure hydrogen is greatly reduced.It is found that when the opening ratio of the burst disk is?1/2,the spontaneous ignition cannot be initated even though the burst pressure is as high as 9.0 MPa.When the opening ratio=2/3,the critical burst pressure increases to 6.41 MPa.The shock overpressure outside the tube also decreases with the reduction of the burst disc opening rate,which greatly reduces the damage to surrounding facilities and personnel.When hydrogen is mixed with air inside the tube,the shock speed in the tube increases as the hydrogen concentration increases.One?dimensional normal shock wave theory calculations show that the the Mach number,shock pressure and the post shock temperature decrease with increasing of hydrogen additions.When the burst pressure is 2 MPa,the hydrogen concentration increases to 20%,the shock pressure and the post shock temperature decrease by 5%and 3%,respectively;when the burst pressure is 6 MPa,the hydrogen concentration increases to 20%,and the shock pressure and post shock temperature decrease by 7%and 6%,respectively.The possibility of hydrogen spontaneous ignition increases with the increase of hydrogen additions.When the hydrogen concentration reaches 20%,the critical burst pressure is only 1.79 MPa.When CO2 is mixed with air inside the tube,there is no significant effect on the shock speed and shock pressure in the tube.However,the possibility of spontaneous ignition of hydrogen decreases with the increase of CO2 additions.When the CO2 concentration reaches 20%,the critical burst pressure is as high as 6.41 MPa,which is the 1.47 times.In addition,the average speed of flame,the maximum value and duration of the photodiode signal decrease with increasing CO2 additions;while the ignition delay time and ignition position increase with increasing CO2 additions.This is mainly because CO2 is highly susceptible to chemical reaction with H radicals at high temperatures to form CO,thereby suppressing the combustion reaction of hydrogen.Then,Fluent software,the LES turbulence model,EDC combustion model and the 18-step hydrogen-air detailed chemical reaction mechanism are used to study the microscopic kinetics and spontaneous ignition mechanism in the straight tube and numerical results are compared with the experimental results.It is found that after the high-pressure hydrogen is released,a hemispherical shock wave is first formed in the tube.When it propagates and hits the wall of the tube,it reflects and interacts with other reflected shock waves in the tube to form the Mach disk,shock triple point,a barrel shape and diamond-like cell structures;during this period,hemispherical shock wave gradually transformes into normal shock wave.With the gradual opening of the burst disc,the angle of the reflected shock wave gradually increases,the height of the Mach disc gradually decreases until it disappears,and other structures such as the barrel shock wave and the diamond-shaped cell structure disappear.Due to the formation of the Mach disk in the tube,the pressure in front of the Mach disk is significantly greater than the pressure behind the Mach disk.After the Mach disk disappears,the pressure after the Mach disk gradually increases.The pressure distribution of the high-voltage shock wave action region,the high-pressure Mach disk region,the low-pressure diamond-type cell structure,the high-pressure Mach disk region,and the low-pressure diamond-type cell structure is formed in the axial direction of the tube.A high-speed area is formed behind the Mach disk.As the Mach disk gradually disappears,the speed of this area gradually decreases.In addition,the velocity near the wall is much higher than the velocity inside the tube.And as the Mach disk gradually disappears,the range of the high-speed area near the wall gradually expands until it fuses together,causing a further increase in speed.The range of the low-speed area behind the second Mach disk does not decrease,forming an elliptical low-speed area inside the tube.In the axial direction,the velocity of hydrogen at initial stage in the center of the pipe is slightly larger than that near the wall,and the hydrogen jet is a forward convex shape.Subsequently,the velocity near the wall of the pipe is significantly larger than that inside the tube,and the hydrogen jet gradually changes into a shape that is recessed backward.Finally,the hydrogen/air mixture layer is formed near the tube wall and the center of the tube.In the initial stage,the internal temperature of the hydrogen jet is rapidly reduced,and the temperature before the Mach disk is higher than that after the Mach disk.As the Mach disk gradually disappears,the low temperature region near the wall gradually expands,the temperature in front of the Mach disk gradually decreases,and the temperature after the Mach disk gradually increases,and the three regions gradually merge into a region with a similar temperature.The temperature of the shock wave front end of the hydrogen jet gradually increases,and the range gradually increases.Due to viscous dissipation,the temperature of the hydrogen/air mixture neart the wall of the tube is higher than that inside the tube.The temperature of the hydrogen/air mixture near the wall of the tube gradually reaches the autoignition temperature.After maintaining for a period of time,spontaneous ignition occurs firstly near the wall of the tube and causes a rapid increase in temperature.Subsequently,the temperature in the front end of the hydrogen jet also rises rapidly and merges with the high temperature region of the wall of the pipe to form a high temperature region,and hydrogen spontaneous ignition occurs again inside the tbe.Subsequently,the two combustion zones gradually merge to form a complete combustion zone.In addition,the three cases of the numerical simulation successfully reproduced the occurrence of spontaneous combustion and the ignition position in the experiments.Finally,based on the experimental and numerical simulation studies,it is proposed that the spontaneous ignition of hydrogen requires two conditions:one is the high shock wave intensity,ie the shock pressure;the other is the sufficient time for the hydrogen/air mixture to maintain the high temperature.The similitude analysis method is used and the expression describing the dimensionless shock pressure in the tube is given:(PbIPa)(D/ust),the first part of which represents the ideal shock pressure without considering the opening time of the burst disc.The second part is the ratio of the time taken by the shock wave to propagate the characteristic length(ie,the pipe diameter D)to the opening time of the rupture disc,describing the effect of the burst disc opening time on the shock pressure in the tube.The time required for the hydrogen/air mixture to maintain high temperatures is described by L*=L/D.The fitting results show that the prediction model of the spontaneous ignition of high-pressure hydrogen is(Pb/Pa)(D/ust)=22.3(L/D)0.58.