Supercritical Gas Cooling and Near-Critical-Pressure Condensation of Refrigerant Blends in Microchannels
Andresen, Ulf Christian
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A study of heat transfer and pressure drop in zero ozone-depletion-potential (ODP) refrigerant blends in small diameter tubes was conducted. The azeotropic refrigerant blend R410A (equal parts of R32 and R125 by mass) has zero ODP and has properties similar to R22, and is therefore of interest for vapor compression cycles in high-temperature-lift space-conditioning and water heating applications. Smaller tubes lead to higher heat transfer coefficients and are better suited for high operating pressures. Heat transfer coefficients and pressure drops for R410A were determined experimentally during condensation across the entire vapor-liquid dome at 0.8, 0.9xPcritical and gas cooling at 1.0, 1.1, 1.2xPcritical in three different round tubes (D = 3.05, 1.52, 0.76 mm) over a mass flux range of 200 < G < 800 kg/m2-s. A thermal amplification technique was used to accurately determine the heat duty for condensation in small quality increments or supercritical cooling across small temperature changes while ensuring low uncertainties in the refrigerant heat transfer coefficients. The data from this study were used in conjunction with data obtained under similar operating conditions for refrigerants R404A and R410A in tubes of diameter 6.22 and 9.40 mm to develop models to predict heat transfer and pressure drop in tubes with diameters ranging from 0.76 to 9.40 mm during condensation. Similarly, in the supercritical states, heat transfer and pressure drop models were developed to account for the sharp variations in the thermophysical properties near the critical point. The physical understanding and models resulting from this investigation provide the information necessary for designing and optimizing new components that utilize R410A for air-conditioning and heat pumping applications.