Numerical Simulation On Precooling Characteristics Of LNG Discharge Pipeline

The LNG discharge pipeline is operated in a low-temperature environment.If the lowtemperature fluid is directly introduced into the normal-temperature pipeline,a severe heat exchange phenomenon will occur,causing damage to the pipeline and affecting normal operation.Therefore,the pipeline needs to be precooled before normal operation.Therefore,this paper comprehensively considers the characteristics of radiant heat transfer,fluid-solid heat transfer and fluid flow,and conducts a multi-physics coupling simulation study on the precooling problem of LNG discharge pipe with π-shaped elbow.First,based on the thermodynamic properties of BOG and LNG,the theoretical formulas of convective heat transfer,solar radiation and heat transfer characteristics inside and outside the LNG discharge pipe are established,applied to the multiphysics simulation process,and the same scale model is established according to the literature data for accuracy Calculation,the results show that the model can accurately calculate the precooling characteristics.Secondly,based on the horizontal and vertical analysis methods,the flow and heat transfer characteristics of the single-phase and gas-liquid two-phase pre-cooling media in the pipeline are studied,and the low temperature,the dense fluid is mainly concentrated at the bottom of the pipe and the outside of the elbow.The wall temperature has a sudden drop at the elbow,and there is a large temperature difference between the gas side and the liquid side wall surface.Based on the numerical calculation method,the relationship between the convection state,the heat transfer situation and the characteristic number are studied.Finally,the influence of inflow temperature,inflow speed and pipe diameter on the precooling of the pipeline is studied by the control variable method.The results show that the faster the inflow speed and the lower the inflow temperature,the lower the pipeline temperature and the faster the temperature drop rate.Under the same mass flow rate,the smaller the pipe diameter,the higher the pipe speed,the lower the wall temperature,and the more intense the internal heat transfer.The heat exchange in the tube is mainly based on mixed convection heat transfer,and the buoyancy in the tube cannot be ignored.The flow heat transfer characteristics of the staged injection BOG precooling stage,LNG partial full-flow stage and full-flow stage studied in this paper are of certain significance to clarify the temperature change during the precooling process of the LNG discharge pipe.