The Numerical Simulation On Liquid Phase Leakage And Dispersion Process Of LNG Tank

Liquefied natural gas tank leakage accidents occur frequently,which has caused huge economic losses and casualties.Although predecessors have done a lot of relevant research work,it is still a hot topic in the academia.In this paper,the liquid phase leakage and dispersion process of LNG tank is studied by the means of numerical simulation,the evolution process and characteristics of LNG pool and vapor cloud are discussed.The main work and conclusions are as follows:Firstly,a three dimensional numerical model is established to describe the liquid phase leakage and dispersion process of LNG tank.Realizable k-? turbulent model and mixture model are adopted.The quality source model and energy source model are compiled by user defined function(UDF).The three dimensional numerical simulation of the physical processes such as liquid jet,pool spreading,heat and mass transfer,and vapor cloud dispersion is carried out.Secondly,the spreading and vaporization of the pool in the LNG tank liquid phase leakage and dispersion is studied.The pool size and average vaporization rate is influenced by the leakage rate.When the leakage rate is large,the pool size along the jet direction is bigger than the vertical direction pool size,the average vaporization rate increases with the increase of leakage time and then decreases.When the leakage rate is small,the pool size along the jet direction is smaller than the vertical direction pool size,the average vaporization rate increases with the increase of leakage time.Wind speed influences the pool spreading and vaporization,with the increase of wind speed,the vaporization rate increases and the pool radius decreases.Thirdly,the evolution process and fluid mechanics behavior of the vapor cloud dispersion in the LNG tank liquid phase leakage and dispersion are analyzed,and the change rules of frostbite temperature distribution and fire distance are obtained.The vapor cloud dispersion state changes from the heavy gas dispersion to the free dispersion.Due to the flow around the tank cylinder,a back-flow wind field forms in the vertical direction on the leeward side of the tank,causing the vapor cloud accumulate and climb along the wall of the tank,and increasing the risk of combustion and explosion.With the increase of time,the range of the frostbite temperature rises,and the required fire safety distance also increased.Fourthly,the effects of wind speed,barrier,terrain and leakage rate on the LNG vapor cloud are studied.When the wind speed is low,the air flow plays a dominant role in the transport of the vapor cloud.When the wind speed is high,the air flow plays a dominant role in the dilution of the vapor cloud.The vapor cloud gathers on the leeward side of the barrier forming a high concentration region which is difficult to disperse,and the blocking effect of the barrier reduces the danger zone.Uphill and downhill terrain forms the vortex,which enhances the extent of atmospheric turbulence,increases the temperature of the vapor cloud and diminishes the frostbite range.The bigger the leakage rate,the longer the duration of the hazard concentration(2.5% concentration and above)vapor cloud and the greater the potential hazard.Finally,the inhibition and blocking mechanism of the water curtain on the LNG vapor cloud dispersion is discussed.The effects of the pressure,the total mass flow rate and the momentum ratio of the water curtain on the dilution efficiency are analyzed.The porous effect of water curtain prevents the dispersion of the vapor cloud in horizontal direction,the water curtain injects down forming vortexs and having the lifting effect on vapor cloud between the leakage source and the water curtain,and at the same time,the ambient air entrained by the water curtain mixes with the vapor cloud,so as to achieve the dilution effect of the vapor cloud.When the ratio of the water curtain momentum and the vapor cloud momentum is 5,the water curtain dilution efficiency is the best,which can not only eliminate the accumulation of the vapor cloud along the tank,but also make the water curtain less consumption.