Investigation On The Microdynamic And Prediction Model Of Spontaneous Ignition During Highpressure Hydrogen Release Inside A Tube

Hydrogen is expected to become the mainstream energy form of human society in the future because of its advantages of renewable,pollution-free and high energy density.However,the hydrogen safety is drawing more attention due to the fact that a series of fire and explosion accidents have happened.For example,the spontaneous ignition of high-pressure hydrogen is easy to occur after accidental release,which has become one of the major safety risks in hydrogen energy application.Based on this background,this paper investigated systematically the microdynamic process of spontaneous ignition of high-pressure hydrogen inside a tube by numerical simulation and visualization experiment combined with theoretical analysis.The aim of this investigation is to enrich and improve the microdynamic theory of spontaneous ignition of high-pressure hydrogen during its release and provide theoretical basis for safe utilization of hydrogen energy.Firstly,the investigation on microdynamic process of spontaneous ignition of high-pressure hydrogen inside the straight tube is carried out.The numerical result indicates that the shock-affected and hydrogen/air diffusion time increase with the increase of tube length,which results in the generation of more high-temperature combustibles.The thicker diameter,the longer critical tube length forming Mach disk and normal shock wave.The increase of tube diameter declines the shock wave intensity and the temperature in the mixed diffusion layer.Based on the visualization experiment,four ignition conditions are observed with the increase of burst pressure:(a)no ignition;(b)failed ignition with one ignition position;(c)failed ignition with two ignition positions;(d)successful ignition.The shorter distance between ignition position and leading shock wave and contact surface is key factor causing successful ignition.Furthermore,the numerical investigation is conducted to study the microdynamic process of spontaneous ignition of high-pressure hydrogen inside two typical complex tubes during hydrogen engineering application:L-shaped and local contraction tube.It is found that the reflected-shock-affected and energy conversion regions are generated at the corner of L-shaped tube due to the shock wave reflection,and the hydrogen/air mixing process will be enhanced.There are three spontaneous ignition mechanisms at the corner of L-shaped tube:the spontaneous ignition is initiated by(a)the normal shock wave;(b)the combined effect of reflected shock wave and energy conversion;(c)shock reflection.The local contraction of tube causes the more complex shock wave interaction such as reflection,intersection,separation and coupling,which forming the reflected-shock-affected region and intersected-shock-affected region and promoting hydrogen/air mixed diffusion.There are two spontaneous ignition mechanisms at the contraction part:(a)reflected shock wave triggers ignition near the vertical wall;(b)shock wave intersection triggers ignition at the tube center.Finally,on the basis of diffusion ignition theory and one-dimensional shock wave theory,the prediction model of spontaneous ignition is studied by dimensional analysis and data fitting.According to hydrogen diffusion time(?_D),chemical reaction time(?_C),shock wave triggering time(?_I)and temperature accumulation time(?_T),the qualitative criterion based on the time scale is proposed for the spontaneous ignition inside the straight tube.Furthermore,two dimensionless parameters,?_i/?_s and L_i/L,are proposed to quantitatively characterize critical time and distance of shock wave affection,respectively.The quantitative prediction model on time and space characteristics of ignition in experimental cases is proposed by data fitting.