Numerical Simulation On Flow Boiling Heat Transfer Of R290 In Horizontal Copper Tubes

Faced with the problems of global warming and ozone layer depletion,the global refrigeration and air conditioning industry puts forward to some effective solutions,including accelerating the phase-out of R22 and reducing HFCs(hydrofluorocarbons)refrigerants.Therefore,it is urgent to find a green alternative refrigerant.R290,which is also called propane,is an HCs(hydrocarbon)refrigerant with zero ODP(ozone depletion potential)and extremely low GWP(greenhouse effect efficiency).Not only does R290 have outstanding environmental protection performance,but it also has the advantages of excellent thermodynamic properties and cycling performance,extensive sources and high practicability.However,due to its high flammability,the safety risk cannot be ignored when using in air conditioning system.Enhancing the flow boiling heat transfer of R290 in the heat exchange tube of evaporator can reduce the refrigerant charge in the system.It is one of the feasible safety measures to reduce the flammability of R290.Therefore,the flow boiling heat transfer characteristics of R290 in horizontal copper tubes were studied numerically in this paper.The effects of various factors on the enhancement of flow boiling heat transfer performance were proposed.The results provide a theoretical basis for the extensive application of R290 in refrigeration and air conditioning industry.Firstly,physics models of horizontal smooth tubes and micro-fin tubes under the outer diameter of 4?7 mm were set up using the ICEM software in this paper.In the ICEM software,the grid was established and the independence of grid was tested.The multiphase flow model and turbulent model were selected,and the boundary conditions and calculation method were set on using the FLUENT19.0 software.The numerical model was established and was validated with the experimental values.Secondly,the flow boiling heat transfer characteristics of R290 in the horizontal smooth copper tube and the horizontal micro-fin tube were simulated.Distributions of temperature field,velocity field and gas phase volume fraction in the horizontal copper tube were analyzed,and the effects of mass velocity,heat flux density,saturation temperature and pipe diameter on the flow boiling heat transfer coefficient were discussed.The results show that the flow boiling heat transfer coefficient increases with the increase of mass flux,heat flux and saturation temperature.The effect of mass flux is more obvious when the vapor quality is in the higher region.In the horizontal smooth copper tube,the flow boiling heat transfer coefficient increases with the decrease of pipe diameter.When the heat flux is high,the dry-out phenomenon occurs in the horizontal micro-fin copper tube with smaller outer diameter,and the flow boiling heat transfer coefficient decreases sharply in the middle and high vapor quality region.Finally,three evaluation methods of enhancement heat transfer in the micro-fin tube were introduced in this paper,including Lin Zonghu enhancement heat transfer,THE/PDI enhancement factor and the enhancement factor.Enhancement factor was used to evaluate the degree of enhancement heat transfer in micro-fin tube.It can be concluded that the mass flux has little effect on the enhancement factor at the initial stage of evaporation.The flow boiling heat transfer coefficient varied with the increase of vapor quality at different mass flux.When the mass velocity was greater than 180 kg/(m2·s),the flow boiling heat transfer coefficient increase with the increase of vapor quality constantly.The enhancement factor increases with the increasing heat flux and saturation temperature.The smaller the pipe diameter is,the greater the enhancement factor will be.However,when the heat flux is much higher,the enhancement effect of the micro-fin copper tube is greatly weakened due to the drying phenomenon in the micro-fin copper tube with the pipe diameter of 4mm and 5mm.