SUMMARY

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, 0.9xP_critical and gas ‎cooling at 1.0, 1.1, 1.2xP_critical in three different round tubes (D = 3.048, 1.524, 0.762 ‎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 6.2 and 9,4 ‎mm to develop models to predict heat transfer and pressure drop in tubes with diameters ‎ranging from 0.7 to 9.4 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.‎