The Study On Condensation Heat Transfer Of Low GWP Refrigerants In A Horizontal Small Tube

At present, the refrigerant substitution has become an inevitable trend, due to thewidely used refrigerants have destructive effect on ozone layer and generatesignificant greenhouse effect. However, the productive technology of newenvironmental friendly refrigerants is not proficient and just in the stage of researchand development. On the current situation, R32is the preferred alternative refrigerantfor its favorable thermodynamic properties and environmental performance. On theother hand, the electronic devices and mechanical systems were towardmicrominiaturization with the development of science and technology and theincrease of market demand, promoting the heat exchanger toward more efficient andmore compact. Therefore, it is necessary to experimental study on the condensationflow pattern and heat transfer characteristics of R32in small channels, whichcontributes to the refrigerant substitution as well as rational design and optimizingoperation for compact heat exchanger.The possible flow patterns and characteristics of various flow patterns in smallchannels were analyzed. In addition, eleven kinds of friction pressure drop modelsand nine kinds of heat transfer models were summarized. Based on the researchachievements of condensation in small channels for the past few years, databases offriction pressure drop and heat transfer coefficients in small channels were built. Thecalculation results of the prediction models were compared with databases, found thatZhang&Webb’s friction pressure drop model, Koyama et al.’s and Baird et al.’s heattransfer model are the models with more extensive scope of application.A set of experimental system was designed and built which can be used toobserve the condensation flow pattern and measure pressure drop and heat transfercoefficients in small channels. The experimental contents and methods wereintroduced in detail. Experimental study on condensation flow pattern, pressure dropand heat transfer coefficients was conducted at mass flux of100~500kg/m~2s,saturation temperature of40~50°C and heat flux of10~38kW/m~2.The experimental results were analyzed, found that the heat flux has little effecton pressure drop and heat transfer coefficients, the friction pressure drop and heat transfer coefficients increased with quality, however, above a certain quality, theincrement rate for friction pressure drop and heat transfer coefficients decreased andincreased respectively. The friction pressure drop and heat transfer coefficientsincreased with the increase of the mass flux but decreased with the increased ofsaturation temperature and the influences of saturation temperature and mass flux onheat transfer coefficients were more significant at higher quality. It was also showedthat the shear stress is the dominate force at annular flow, and the heat transferperformance of annular flow is better than that of intermittent flow. Thus, it wassuggested that the flow pattern of refrigerant to be annular flow as possible during theflowing through heat exchanger when designed a compact heat exchanger.Compared the experimental data with prediction models, found that bothChisholm’s friction pressure drop model and Müller-Steinhagen&Heck’s frictionpressure were in good agreement with the experimental friction pressure drop, andWang et al.’s heat transfer model was found to agree well with heat transfercoefficients.