Numerical Simulation On Jet Characteristics Of LNG Vapor Leakage With High Pressure From Orifice Near Ground

As the proportion of LNG in Chinese energy structure continues to increase,it is necessary to attach more importance to the risk of leakage during storage and transportation.Based the study of dangerous area of flammable gas,the stretching effect of ground on the vapor jet is particularly analyzed,and the boundary effect of ground on the jet is explored.The numerical simulations were carried out for three sets of cases of different ambient temperatures,different storage pressure and different leakage heights.The cloud maps of methane volume fraction were drawn.The temperature,velocity and methane volume fraction on the axis of jet were gotten.The results show that storage of LNG in the low-temperature and normal-pressure is less risky than pressurized storage in the event of a gaseous LNG leak.The lower the location of the leak,the longer the distance of the flammable vapor cloud.The Flammable clouds are more likely to be exposed to ignition sources near the ground,so the risk of secondary accidents caused by jets close to the ground is greater.Based on the conservation of mass,energy,momentum in the development of the jet and equivalent exit theory,the changes of the state parameters on the jet axis are obtained through the method of non-dimensional analysis.It reveals the influence mechanism of the ground on its nearby jet.The results of non-dimensional analysis show that each state parameter has the characteristic of piecewise linear distribution,and the jet flow near the ground can be divided into the area affected by the ground boundary layer and the area without boundary layer.When the jet enters the affected area of the boundary layer from the area without boundary layer influence,it will accelerate and deviate to the ground.At the same time,the jet velocity decays rapidly,the decay rate of methane volume fraction suddenly drops,and the heating rate increases.In addition,in the boundary layer affected zone,the air entrained by the jet is blocked by the ground.The boundary layer formed by the jet close to the ground not only reduces the horizontal velocity of the jet,but also suppresses the turbulence of gas diffusion,which makes the methane decay rate slower and the length of the dangerous area increase.Based on the analysis of the stretching influence mechanism of the near-ground jet,the evolution characteristics of the jet under different injection angles are further studied.Through the analysis of the length and width of the dangerous area of the combustible cloud,the comprehensive effect of jet angle and leakage height on vapor cloud diffusion is studied.The results show that when the jet does not touch the ground,the greater the elevation angle,the smaller the length of the dangerous area,and the width change characteristics are similar to those of a horizontal jet;when the jet is at a depression angle,the transformation of the kinetic energy and potential energy of the jet in the impact area makes LNG vapor diffuses and flows around,and the width of the dangerous range of the vapor cloud is greatly increased.The smaller the leakage angle,the more vertical kinetic energy is converted,and the wider the dangerous range is.The lower the jet height,the greater the kinetic energy when it impacts near the ground,the greater the velocity of the gas entering the wall jet area,and the greater the width of the dangerous area.It can be seen that the elevation angle injection is often safer than the depression angle injection.Therefore,when a high-pressure LNG vapor leak occurs near the ground,the jet can be directed to spray upward at an elevation angle to prevent the jet from contacting the ground and expand the dangerous area.