Condensation Heat Transfer Characteristics Of R32and R1234ze In A Horizontal Small Tube

With the environmental issues becoming more prominent, people pay muchattention to refrigerant substitution, and the research about new refrigerants with lowGWP and zero ODP has become a hot topic. Consequently, the environmental-friendly refrigerants R32and R1234ze are tanken into consideration for their excellentthermodynamic and transport properties. Additionally, miniaturization of heatexchanger has become an inevitable trend for their advantages of compact size andhigh heat transfer coefficients. However, as the diameter decreases, the effects ofgravity, surface tension and shear stress on the heat exchanger are also significant,which causes the flow patterns as well as the heat transfer characteristics and pressuredrop performance in small sized tubes different form those of conventional channels.Whereas, the studies in heat transfer and friction performances in small channel arenot in good agreement and some of the results are discrepant with each other.Therefore, it is necessary to carry out more experiments to study the hydraulic andthermal characteristics in small sized channel.Experiments were conducted to study the condensation heat transfer coefficientsof R1234ze and R32in a single circular minitube of2mm inner diameter. Theexperiments were performed at the saturation temperature of35℃,40℃and45℃,mass flux of100~400kg/m2s and heat flux of4~28kW/m2, vapor quality of0~1.The test results shows that the heat transfer coefficients of R1234ze is1.5~8kW/m2K, and the heat transfer coefficients of R32is1~10kW/m2K. Furthermore, theheat transfer coefficients of R32and R1234ze increase with the mass flux, anddecrease with the increase of saturation temperature, while the effects of heat flux andinlet vapor quality are not very notable. In addition, the frictional pressure dropsincrease with the mass flux, but decrease as the saturation temperature increases, andthe greater the mass flow is, the more predominent the effect is. Next, the flowpatterns of R32were performed, and the plug, slug, annular-wavy and annular flowpatterns were observed. The results show that with increase of mass flux, the annularflow regime becomes large and the corresponding vapor quality becomes small whenthe annular flow of the fluid transits to intermittent flow. Compared the experimental results of R32and R1234ze condensing in tubes withdifferent diameters, it is found that the heat transfer coefficient increases as thediameter decreases. Compared the experimental results of R1234ze, R32, R1234yfand R134a, it revelas that the condensation heat transfer coefficient of R32is thehighest, significantly higher than the other three refrigerants, the results of R1234zeare in accordance with those of R1234yf, which is larger than those of R134a, butsmaller compared with those of R32.The experimental results were compared with several typical predictedcorrelations, which indicate the prediction values using Baird model are in goodagreement with the experimental data of heat transfer, and the calculation results byMüller-Heck model and Chisholm model can predict the experimental frictionalpressure drop with high precision, but the former is relatively better than the latter.These models can also predict the data in the literatures. The Yang-Shieh model canpredict the flow pattern well. Consequently, considering the dimensionless numbersof Xtt, Bd, Re, a new correlation for predicting the boundary of intermittent flow andannular wavy flow was also developed.