Performance Simulation Of Flow Boiling Heat Transfer Of R1234ze(E)/R152a In Horizontal Smooth Tubes

The ozone layer depletion and global warming have become the global environmental issues concerned by all countries nowadays.As a typical HCFCs refrigerant,R22,has been gradually restricted;at present,the third-generation refrigerants HFCs for R22 replacement,can only be used as transitional substitutes because of their high GWP.Therefore,a new near-azeotropic refrigerant mixture R1234ze(E)/R152a(0.4/0.6 by mass)(named NCUR01)with excellent performances and low GWP has been proposed as a long-term substitute for R22.The boiling heat transfer performances of the refrigerant in evaporator are directly related to the practicability of the refrigerant,and it has become one of the hot topics in the research and development of new environmental refrigerants in recent years,due to the complexity of boiling heat transfer mechanism in tube,most scholars prefer experimental research.In this paper,the boiling heat transfer characteristics,and gasliquid two-phase flow pattern transformation law of NCUR01 in horizontal smooth tubes were researched by numerical simulation,to further understand the flow boiling heat transfer mechanism of the NCUR01.The physical models of horizontal smooth tubes were constructed by the ICEM CFD software,based on the VOF model and the UDF module in FLUENT software,a three-dimensional unsteady numerical model of flow boiling heat transfer process of NCUR01 was established.Firstly,the numerical simulations and experimental study were carried out in horizontal smooth tubes of 6mm inner diameter,under different simulation conditions,the distribution characteristics of gas-liquid two-phase and temperature in the tube were analyzed,and the experimental values of boiling heat transfer coefficient were compared with the simulated values to verify the reliability of the model.Secondly,based on the model,the effects of heat flux density,mass flux density,saturation temperature and mean vapor quality on the boiling heat transfer coefficient of the refrigerant mixture in the horizontal smooth tube were studied.Finally,at the same working conditions and the same tube diameter(the tube inner diameter is8.72mm),the effect of different tube types(smooth tube and micro-fin tube)on the flow boiling heat transfer characteristics of NCUR01 was analyzed;and the flow pattern distribution characteristics of the NCUR01 in a horizontal microchannel of1 mm inner diameter were discussed,in order to further explore the effect of tube diameter(the tube inner diameters are 8.72 mm,6mm,and 1mm)on the boiling heat transfer performances of the NCUR01 under the same conditions.The simulation results show,when the phase change factor r is 0.1,the evolution process of bubbles can be completely displayed,five typical flow patterns appear in the tube: bubble flow,plug flow,stratified flow,wave flow and slug flow;with the heat flux and saturation temperature increase,the flow pattern at the same position of the tube will advance and the temperature will rise gradually,while the increase of mass flux will cause the flow pattern to delay and the temperature will drop gradually;at the same time,the change trend of the boiling heat transfer coefficients is basically consistent with the simulated value under all working conditions,and the mean relative deviation between them is less than 14%,so the calculation method can be feasible.Next,it is found that the dry-out phenomenon occurred in the flow boiling process,the boiling heat transfer coefficients in 6mm tube increase with the heat flux and saturation temperature,decrease and then increase with the mass flux;and the boiling heat transfer coefficients have overall downward trend with the mean vapor quality because of the mass transfer resistance.And compared with the smooth tube,the boiling heat transfer coefficients of NCUR01 in 8.72 mm micro-fin tube increase by about 20%;in addition,it turns out that,the bubbles grow faster and the phase change process is more intense in 1mm microchannel,and bubble flow,restricted bubble flow,slug flow,churn flow and mist flow appear successively,and since the relative effect of the surface tension is greater than the buoyancy,the bubble motion process mostly occurs on the tube axis,the boiling heat transfer coefficients of the NCUR01 are significantly increased with the decrease of tube diameter.