Heat Transfer Mechanisum And Coupled Heat Transfer Performance Of LNG Ambient Air Vaporizer

LNG plays important role in safe and stable gas supply system due to its advantages of high energy density and low storage space.LNG should be vaporized to natural gas using vaporizer before being transmitted to natural gas pipeline.Ambient air vaporizers which utilize ambient air to heat LNG in the fin tube are widely used in LNG gas terminals due to its advantages of energy saving and environment protection.However,the type selection and operation regulation of ambient air vaporizer are lack of theoretical guidance and basis because of the short of the investigation of the integrated heat transfer of ambient air vaporizer.This causes technical and safe hazard for the operation and regulation of LNG gas terminals.This paper focused on the integrated heat transfer of ambient air vaporizer based on the related heat transfer mechanisms during the operation of ambient air vaporizer,and the research method were the combination of the numerical simulation and experimental validation.The research program of the fluid solid coupled heat transfer problem of ambient air vaporizer was proposed.The optimal switch period and the maximum value of the vaporization flux of ambient air vaporizer were given,which were beneficial to the type selection and operation of ambient air vaporizer.The fluid solid coupled heat transfer of ambient air vaporizer was numerically investigated based on the whole-field solution method.The whole-field numerical model of LNG vaporization in single fin tube was established and solved.Based on the model,the distribution regulation of the wall heat flux along the two phase zone was obtained.The VOF multiphase model was used to numerically predict the flow pattern of the LNG vaporization in the fin tube of ambient air vaporizer.The flow pattern mainly contains the bubble flow,slug flow,churn flow and mist flow.LNG flow boiling heat transfer model in the vertical circular tube was proposed,which introduces two group interfacial area equations to the two fluid model and the wall boiling model.This model can mathematically describe the bubble flow,slug flow and churn flow.The experimental data of flow boiling of liquid methane and LNG in the vertical tube was used to validate the model.Based on the numerical solution of the model,the effect of the heat flux,pressure and volume fraction on the flow boiling heat transfer coefficient were analyzed.By the comparison of the model with the existing correlation,LNG flow boiling heat transfer correlation was proposed,which can be used for the heat transfer coefficient of LNG flow boiling in the ambient air vaporizer.A cryogenic frost formation model was established based on the mass and energy conservation during the heat and mass transfer of the vapor in the air on the cryogenic surface of the ambient air vaporizer.To validate the cryogenic frost formation model,an experiment of the frost layer thickness on the surface of the ambient air vaporizer was carried out in the LNG gas terminal.The effect of the thermal resistance of cryogenic frost layer was analyzed.The “peak thermal resistance temperature” was defined.The heat transfer model of the air side of ambient air vaporizer was established to analyze the effect of the frost formation on the heat transfer coefficient of the air side of ambient air vaporizer.The heat transfer characteristic of the fin tube bundle was investigated based on the whole-field fluid solid coupled model.The heat transfer characteristic of the air side of each fin tube in the bundle was analyzed.A fitting empirical correlation of the variation coefficient of natural heat transfer coefficient of each fin tube in the bundle was raised,which was validated by the experiment in the LNG gas terminal.The results showed that the empirical correlation of the variation coefficient is applicable in the calculation of the natural convection heat transfer coefficient of ambient air vaporizer when the relative temperature is from 0.2 to 0.46,and the relative vaporization flux is from 0 to 0.9.The integrated heat transfer characteristic of single tube of ambient air vaporizer was investigated based on the domain solution-boundary coupling method.The distribution rule of the thickness of the frost layer and heat transfer coefficient along the tube length direction were analyzed.The “delay effect” of the distribution rule of the heat transfer coefficient caused by the thermal resistance of the frost layer was revealed.The integrated heat transfer characteristic of fin tube decreases gradually as the operation lasts.The outlet temperature of the vaporized natural gas reduces as well,and the ambient air vaporizer should be switch to the backup when the outlet temperature cannot satisfy the requirement of the natural gas pipeline.The integrated heat transfer characteristics of the ambient air was investigated on the basis of the fluid solid coupled heat transfer model and variation coefficient of the heat transfer coefficient of the air side of the tube bundle.The effect of the air temperature,vaporization flux and methane content on the integrated heat transfer characteristics of ambient air vaporizer was analyzed.The switch period of the dependent ambient air vaporizer under different air temperature was given.The research of this study not only improve the design defect of LNG ambient air vaporizer using simple estimate method,but also provide theoretical foundation of the proper type selection and optimal operation of ambient air vaporizer.The theoretical outcomes can be applied to the heat transfer element containing the fluid solid coupled heat transfer problem.Therefore,this study has important theoretical guidance value.