Numerical Investigation On Condensation Heat Transfer Of R1234ze(E)/R152a In Smooth Copper Tubes

The heat transfer characteristics of the refrigerant in the phase change process directly affect the heat transfer area of the condenser and the thermodynamic parameters of the refrigerant at the outlet,which in turn affects the financial benefits and thermal performance of the refrigeration system.Thus,it is necessary to study the condensation heat transfer characteristics of new refrigerant in the field of refrigerant replacement.In the search for a long-term alternative substitute for R22,the research group previously proposed a new environment-friendly near-azeotropic mixture,NCUR01（a mixture of 40%R1234ze（E）and 60%R152a）,and experimentally studied its condensation heat transfer characteristics in horizontal tubes.However,the experimental conditions were narrow and the research results had limitations.Thus,in order to provide a more thorough and organized study conclusion,it is essential to conduct numerical simulation research on the condensation heat transfer characteristics of the near-azeotropic mixture based on experimental research.Firstly,the three-dimensional model of NCUR01 in the 6mm smooth tube condensed heat exchange was established,and the multi-phase flow model was determined as the VOF model,the fluid flow model as the SST k-w model and the phase change model as Lee model.The simulations were conducted at mass fluxes of 150 to 200 kg·m^-2·s^-1,heat fluxes of 8 to 10 k W·m^-2 and saturation temperature of308.15 to 313.15 K.Then compared the simulated values of the condensation heat transfer coefficient and the outlet vapor quality with the experimental data under the same conditions.The results show that the simulations are in good agreement with the experimental results,with a mean absolute relative deviation of 10.081%and10.044%,respectively and all points falling in the±20%deviation band.By comparing the simulated vapor-liquid two-phase flow pattern with the standard flow pattern,it is found that the two flow patterns are highly consistent.Secondly,the effect of mass flux,heat flux,saturation temperature and inlet vapor quality on the condensation heat transfer coefficient were analyzed.The results show that the flow rate and temperature of NCUR01 decrease along the length of the tube,while the liquid volume fraction increases along the length of the tube.And the condensation heat transfer coefficient increases with the increase of mass flux,heat flux and inlet vapor quality,and decreases with the increase of saturation temperature.When the mass flux increases from 200 to 400 kg·m^-2·s^-1,the condensation heat transfer coefficient increases by 55.97%;when the heat flux increases from 6 to 14 k W·m^-2,the condensation heat transfer coefficient increases by 64.87%;and when the saturation temperature increases from 303.15 to 318.15 K,the condensation heat transfer coefficient decreases by 28.87%.Then,comparing the condensation heat transfer characteristic of NCUR01 in horizontal smooth tubes with different diameters from 5 mm to 10.7 mm,it was found that as the diameter of the tube decreases,the effect of vapor-liquid shear force is strengthened,which will inhibit the increase of the liquid film thickness in the wall and enhance heat transfer.The reinforcement of condensation heat transfer is more obvious in the high vapor quality area.The condensation heat transfer coefficient in the tubes is increased 1,086 times when the tube diameter is reduced from 10.7 mm to 5 mm.It can be explained that the small diameter tube increases the shear stress,which reduced the liquid film.Besides,the strengthening effect of condensation heat transfer is more obvious at high vapor quality.Finally,the geometric model of the elliptic tube with a circumference of 6 and the ratio of long and short axes of 1,1.5,and 2.0 is established when the elliptic tube is placed horizontally（the long axis is the X-axis direction,the short axis is the Y-axis direction）.The effects of the ratio of the long and short axes of the elliptic tube and the different placement modes（the elliptic tube of the same size has the opposite placement of the short and short axes）on the flow condensation heat transfer characteristics in NCUR01 tube were compared.The results show that when the mass flux is 400 kg·m^-2·s^-1,the heat flux is 12 k W·m^-2,and the saturation temperature is 308.15 K,the condensation heat transfer coefficient of NCUR01 in elliptical tube increases with the increase of the ratio of long and short axes.When the ratio of long and short axes increases from 1.0 to 1.5 and 2.0 the average condensation heat transfer coefficient increases by 9.62%and 18.21%,respectively.According to the simulation results,the NCUR01 condensation heat transfer coefficient increased by 5.81%when the elliptic tube was positioned horizontally as opposed to vertically,while the outlet vapor quality was essentially unaffected.