Experimental Study On Effect Of Pipe Geometry On Impact Mechanism For Spontaneous Ignition Of High-pressure Hydrogen Release

As a clean and efficient new energy,hydrogen energy has been highly focused all over the world,however,hydrogen possesses some unique dangerous properties compared to traditional energies.Particularly,spontaneous ignition could be produced without ignition source when high pressure hydrogen sudden leaks,and fire and explosion accidents could be further induced,which has become one of the major obstacles for further hydrogen energy application.Considering pipelines with various geometric shape in high pressure hydrogen application,it is of great significant to study the effect of various geometric pipelines on the high pressure hydrogen release,so as to reveal spontaneous mechanism and develop prevention and control technology.This paper focuses on shock wave propagation characteristic,dynamic mechanism of hydrogen spontaneous ignition,flame propagation and behavior evolution mechanism in different geometric release tube.First of all,the shock wave generation and propagation dynamics,shock wave induced spontaneous ignition and critical condition for spontaneous ignition through different geometric release tube are examined,including tubes with different cross-sections(square,pentagon and circular)and tubes with obstacles(metal ring obstacles and triangle obstacles).The experimental results shows that the intensity of shock wave in all tubes increases as propagating along the tube and finally keeps constant and the cross-section has no significant influence on shock wave propagation dynamics.And the cross-section has no noteworthy impact on the occurrence of spontaneous ignition,spontaneous ignition would induce once the pressure on tube wall reaches 0.9MPa.Moreover,the combine effect of tube length and tube diameter can be represented as parameter L/D and hydrogen critical pressure induce spontaneous ignition decreases as L/D increases,which means spontaneous ignition is more prone to occur in a large LID tube.The presence of obstacle shows significant influence on shock wave propagation,the encounter of the leading shock wave and obstacles(metal ring obstacles or triangle obstacles)gives rise to strong shock wave reflection.The reflected shock wave accelerates spontaneous ignition in tube with triangle obstacles,which makes the critical pressure for spontaneous ignition is less than that in the tube without obstacles.Furthermore,intensity of the leading shock wave decreases when passes through metal ring obstacles and then recovers gradually.In the next place,the dynamic characteristic of spontaneous ignition in different geometric tubes are studied with the help of pressure transducers,flame detectors and high speed camera,and the possible mechanisms for spontaneous ignition in different geometric tubes are discussed.The initial spontaneous ignition position in circular tube lies in boundary layers,and the flame kernel grows up along the boundary layers to form a cylindrical flame until spouting out of the tube exit.In addition,spontaneous ignition can be induced in the axis of the tube as well,and flame kernels both in boundary layers and axis of the tube grows rapidly and merges together until jetting out the tube exit.In certain circumstance,the flame along the axis of the tube would quench and the flame in boundary layers grows into a complete flame across the tube and propagates into air.Due to the tube corner in the square tube,multi spontaneous ignition would occur in the middle of the boundary layers and tube corners and flame kernels gradually come together to form a complete flame as propagating along the tube.No matter there is obstacle inside tube or not,the initial ignition time would get shorter and the initial ignition position would be closer to the burst disk with higher hydrogen burst pressure.When the hydrogen pressure is relatively low,the presence of triangle obstacles would accelerate occurrence of spontaneous ignition,and the closer distance between obstacles and the burst disk is,the acceleration is stronger.With fixed obstacle position,obstacles present no effect on spontaneous ignition with further increase of hydrogen pressure,because spontaneous ignition has been induced before obstacles.When the hydrogen pressure is not sufficient to induce spontaneous ignition inside tube without obstalces,the presence of triangle obstacles would improve spontaneous ignition,and the ignition position is prone to occure surrounding obstacles.When the hydrogen pressure is sufficient to induce spontaneous ignition inside tube without obstalces,the arrangement of obstacles also effects on the initial ignition position:The ignition position would move toward to the burst disk when arranges triangle obstacles before initial ignition position in the tube without obstacles.Whereas there is no influence on initial ignition position when places triangle obstacles behind initial ignition position in the tube without obstacles.Different kind of obstacles show different influences on the mechanism of spontaneous ignition:(1)there are three possible mechanism of spontaneous ignition in tube with metal ring obstacles.The first one is the same as tube without obstacles,where initial ignition occurs before obstacles and flame kernel gradually grows along the tube.The second one is initial ignition occurs before obstacles and then quenches,the presence of obstacles facilitates the formation of hydrogen/air mixture and then induces the second ignition behind obstacles.The third one is initial ignition occurs behind obstacles and small amount of hydrogen/air mixture before obstacles is ignited under the effect of the reflected shock wave.(2)The presence of triangle obstacles leads to three high temperature regions(before obstacles,axis of the tube and behind obstacles),which may induce multi spontaneous ignition along tube wall and around obstacles.Then the flame merges into a complete flame as growing and propagates along the tube.Finally,hydrogen flame propagation dynamics inside and outside tube from different geometric tubes are experimentally studied.The results show that tube corners in non-circular tube(such as square and pentagon tube)leads to extra vortices,which improve hydrogen/air mixing,subsequently,the intensity of hydrogen combustion(such as flame peak value and comprehensive flame intensity)is more intensify then that in the circular tube.And the hydrogen combustion is more violent in the corner with a smaller angle.Moreover,non-circular tube with a smaller angler corner promotes mixing of hydrogen jet and ambient air near tube exit,which leads to accelerate hydrogen jet fire propagation.The microstructure of hydrogen jet fire outside tube is influenced by the cross-section of tube.Second flame separation phenomena could be observed when flame jets from square tube or pentagon tube.The effect of metal ring obstacles on critical pressure has no significant influence,nevertheless,the presence of metal ring obstacles would temporarily accelerate hydrogen combustion velocity.The velocity of hydrogen combustion would drop back once the increment of hydrogen/air caused by obstacles is consumed.The reflected shock wave induced by metal ring obstacles promotes hydrogen combustion as well,and the probability of the combustion enhancement by the reflected shock wave increases as the increasing distance between burst disk and obstacle location.