Study On The Supercritical LNG Flow And Heat Transfer Characteristics In A Helically Coiled Tube Vaporizer

Due to the impact of climate change and low-carbon green energy trend,Liquid Natural Gas(LNG)as a safe,clean and economical low carbon energy has attracted more and more attention at home and abroad.In practical applications,LNG needs to be pressurized in condition over critical pressure and vaporized by the reception stations,then it can meet the needs of the downstream complex pipe network.The helically coiled tube vaporizer is the key equipment applied in the LNG gasification process.It has good heat transfer performance and can be used in high pressure condition.So the study of its internal flow and heat transfer has important practical guiding significance.This paper studies the following:Firstly,the heat transfer of LNG at supercritical pressure in the vertical coiled tube and the horizontal straight tube was numerically investigated and compared using the Reynolds Stress Model under the constant wall temperature.The reliability of the numerical model is verified by comparison with the experimental data.The physical parameters of LNG,flow field,temperature contours and change of heat transfer coefficient along the tube are analyzed.The effects of buoyancy force and centrifugal force on flow and heat transfer are discussed.The heat transfer mechanism of LNG under the supercritical pressure in the tube is clarifiedSecondly,according to analysis,the heat transfer coefficient of LNG under the supercritical pressure in the horizontal spiral tube is oscillating.Due to the particularity,the effects of the varying inlet pressure(Pin)the varying wall temperature(Tw)and the varying inlet mass flux(Gin)on the average velocity(Vb),bulk temperature(Tb),Reynolds number(Re),Richardson number(Ri)and heat transfer coefficient in a horizontal spiral tube are discussed in this paper.A new correlation was proposed for LNG at supercritical pressure convective heat transfer in the helical tube,which shows a good agreement with the present simulated results.Finally,based on the practical geometric parameters of the supercritical LNG vaporizer,a whole coupled heat transfer model is established.The reliability of the numerical model is verified by comparison with the experimental data.Based on the multi-objective genetic algorithm,the structure of the helically coiled tube(tube diameter d,winding diameter D,pitch b)in the vaporizer is optimized.Through sensitivity analysis,single factor analysis and three dimensional response surface analysis,the influence of structural parameters on the heat transfer performance of tube and drag performance of shell range is studied.The structural parameters are obtained by system optimization.In this paper,the heat transfer of LNG flow under the supercritical pressure in a horizontal tube,a vertical spiral tube and a horizontal spiral tube is compared by numerical simulation.The mechanism of heat transfer in a vertical spiral tube is clearly stated.The effect of operating parameters on the flow and heat transfer in a horizontal spiral tube is discussed.And the structure of the actual vaporizer model based on genetic algorithm is optimized;the reliability of the numerical model is verified by comparison with the experimental data.The research of this subject provides a theoretical basis for the design of the vaporizer.