Simulation And Optimization Of Gas Replacement Process In LNG Tank

The growth of the world's population and economy has increased the world's demand for energy.Natural gas has not only abundant supply,high calorific value,but also cheaper and environmental protection.As a result,China's natural gas consumption has increased significantly in the past decade,and the number and size of China's LNG-building vessels are also rapidly expanding.Drying and replacement of LNG cryogenic storage tanks is a key step in the production of LNG tanks before they are put into production.It is to prevent LNG from being mixed with air when it enters the tank for the first time.Larger air intake rates and higher air intake temperatures increase the convective diffusion effect of nitrogen and cabin air,resulting in wasted nitrogen;smaller air intake speeds increase the replacement time,and lower air intake temperatures can cause The waste of cold energy.Therefore,it is of great significance to comprehensively and thoroughly study the law of convective diffusion in the gas displacement process,understand the distribution characteristics of the gas concentration field,and the convective diffusion modes in different intake stages in order to accurately control the gas replacement and reduce the production cost and investment.In this paper,the main influencing factors of the gas convection diffusion in different time periods of different inerting schemes are determined by studying the simulation and dynamics characteristics of the flow field in the inerting process of LNG ship tanks.Finally,an optimization scheme is proposed based on the influencing mechanism and experiment verification.The specific research content of the paper is as follows:Firstly,taking LNG ship tank as a prototype,the research object is low temperature nitrogen gas and normal temperature air.The calculation model is established according to the differential equations of mass conservation,energy conservation and momentum conservation,and a three-dimensional full-scale numerical model is established by numerical simulation.Secondly,based on the selected numerical model,the experimental data of the aircraft fuel tank is used to verify the accuracy and reliability of the numericalsimulation method.On this basis,the Froude number was used to study the flow field dynamics.Combining the changes in flow terms and diffusion terms for each monitoring,the phenomenon of gas migration in tanks is described in stages.Thirdly,by setting the reference group and simulation calculations,explore the influence of different air intake rate and temperature on the replacement efficiency.On this basis,an optimization plan is proposed and verified.In this paper,the gas transport in different regions of the LNG ship's tank replacement process is analyzed in stages to study the mixing mechanism of air and nitrogen in the replacement process,which has guiding significance for the inert gas safety operation and pollution control of LNG ships.