Study On Heat Transfer Performance Of Air-heated Finned-tube Vaporize

The air-heated finned-tube vaporize is mainly used for converting the low temperature liquid into the suitable temperature gas and transported to the target user terminal.It is widely used in the gasification of liquid nitrogen,liquid oxygen,liquefied natural gas and other cold fluid,and it is the most common heat exchanger in low temperature engineering.The air-heated finned-tube vaporize has a simple appearance,which consists mainly of fins and core tubes,but the multiphase phase transition coupled heat transfer process is very complicated.At present,there are few studies on the numerical simulation of the coupling heat transfer between the fluid domain,the fin and the core solid domain and the outer air domain in the air-heated finned-tube vaporize.In this paper,the air-heated finned-tube vaporize,which is a practical application in Hengshui City,Hebei Province,is used as the research object.Liquid nitrogen as a cold medium,to analyze the flow,gasification and thermal process of air-heated finned-tube vaporize.The three-dimensional model of the air-heated finned-tube vaporize was established by using Fluent software.The model consisted of three parts:the cold fluid domain in the tube,the solid domain of the fin and core,and the outer air domain.For the phase transition heat transfer process,a three-phase flow mixing model and an evaporative condensation model are used.Combined with the calculation and analysis of the dimensionless constant,the standard-turbulence model is selected,the finite volume method is chosen to discrete the differential equation,the discrete approach is the first order upper wind scheme.Finally,based on the implicit solution of pressure,SIMPLE algorithm is used to simulate the transient.Analyzed and statistic the temperature of Hengshui City,Hebei Province in 2015.The air inlet velocity data of the air-heated finned-tube vaporize were investigated,the liquid nitrogen inlet speed of air-heated finned-tube vaporize were studied,and combined with the experimental data of liquid nitrogen inlet velocity,as the basis for setting the boundary conditions in the CFD simulation process.Simulation and experimental comparison of the working conditions of liquid nitrogen inlet boundary is 0.04 m·s-1,77 K,air boundary is 1atm,311 K,the fin surface temperature at 15 mm from the fin edge.It is found that the simulation is in good agreement with the experiment,and the change trend is basically the same,the simulation results are correct and reliable.The simulation results show that,under different air temperature conditions,the distribution of the wall temperature of the vaporizer fins and the core tube along the pipe length is not linearly related,and the surface temperature of fins has obvious curve stratification.Z-axis pressure drop decreases with increasing temperature,and the slope gradually decreases,so it can be concluded that the lower the air temperature,the greater the energy loss during the thermal process.The gasification rate at the center of the core tube(X = 0)is better than that in the X-axis direction at 1mm to 8mm,which is closely related to the flow,gasification and reflux in the core tube.The temperature changes at the boundary of the fin and air domain(X=77 mm),therefore,the direction of heat transfer performance optimization is focused on the heat transfer boundary between fins and air.The simulation results show that,under different nitrogen inlet velocity,the wall temperature of the fins and the wall temperature of the core tube decrease with the increase of the inlet nitrogen velocity,which is negative correlation.At Z = 0 to Z 1.0 m,the fin and core wall temperature trend is more variable,which is the actual operation need to be considered.With the increase of liquid nitrogen inlet speed,the difference of gasification degree in X-axis horizontal direction of pipeline increased obviously.1 mol liquid nitrogen of 77 K and 2.3 MPa gasification to simulation results export status,the amount of heat needed is 6256.81 J.In addition,the linear relationship between Z-pressure drop and liquid nitrogen inlet velocity is obtained,and the relationship between f-Re and Nu-Re was obtained by dimensionless analysis,which can provide reference for the optimal design of empty tube finned tube vaporizer.