Research On Heat Transfer Performance And Its Optimization For Ambient Air Vaporizers Of Liquefied Natural Gas Under Forced Convection Conditions

With the continuous increase of China’s energy demand and the steady implementation of energy conservation and environmental protection policies,the Ambient Air Vaporizer(AAV)is widely used in liquefied natural vaporization stations due to its clean,efficient,energy-saving and environmental protection characteristics.However,as the frost on the surface of the finned tube seriously reduces the AAV heat transfer efficiency,Based on the solar heat pump-based LNG vaporization system(DX-SAHPNV system),the heat transfer performance of AAV finned tube and tube bundle under the condition of forced air supply was researched,the heat transfer performance of the single finned tube and tube bundle of forced draft AAV in the system was simulated and analysed,which can optimize the design of DX-SAHPNV.Firstly,in this paper,a heat and mass transfer model of finned tube surface considering condensation and frost layer growth was established.The dynamic changes of physical properties of frost layer and liquid film under specific environmental parameters and cold wall temperature were investigated,and the accuracy of the model is verified referring to the experimental data.And the influence of air temperature,air humidity,air velocity and cold wall temperature on the condensation,frosting and heat transfer performance were analyzed by using control variable method.The results show that the increase of air temperature,air humidity,cold wall temperature and air velocity can restrain frosting and improve the heat transfer coefficient.Secondly,this paper improved and analyzed the LNG vaporization system based on the Direct Expansion Solar Heat Pump(DX-SAHPNV system).Based on the Computational Fluid Dynamics(CFD)theories,the User defined function(UDF)programs of heat and mass transfer model in the tube and the UDF programs of forced convection heat transfer model outside the tube were explored and written.The vaporization and heat transfer performance of DF-AAV tube and the environmental factors such as air temperature and air velocity were simulated and analyzed.The results show that forced convection condition can significantly enhance the heat transfer performance of AAV single pipe and increase the air supply temperature,the air supply speed can enhance the heat transfer performance of finned pipe.The outlet temperature of natural gas increases by 3.40%,1.78%,1.57%,1.34% and 1.30% for every 10 K of air supply temperature between 273-323 K,and the outlet temperature increases by 0.97%,1.23%,2.07% and 0.45% for every 1 m/s of air velocity between 2-6 m/s.Finally,this paper takes 4×4 star finned tube bundle as an example to analysed the structure of AAV in DX-SAHPNV system.And the factors affecting the heat transfer performance of AAV finned tube bundle,such as forced air supply mode and AAV hood size,were simulated and analyzed.The results show that the air supply mode of side delivery back up can disperse the cold fog between finned tubes,inhibit the dew/frost of tube bundles,and enhance the heat transfer performance of AAV finned tube bundles;at the same time,reducing the size of the wind cover can improve the wind field between tube bundles and enhance the heat transfer efficiency of AAV finned tube bundles.