Evaluation of Correlations of Flow Boiling Heat Transfer of R600a in a Flat-Plate Solar Collector/Evaporator

Abstract

In the current research work, the behavior of the boiling heat transfer coefficient in a flat-plate solar collector/evaporator, component of a direct-expansion solar-assisted heat pump (DX-SAHP) was studied using a hydrocarbon refrigerant with zero Ozone Depletion Potential (ODP) and low Global Warming Potential (GWP). The main dimensions of the collector/evaporator are 0.8, 3.8, and 1000 mm of the fin thickness, internal diameter, and length, respectively. Five experimental tests were realized at different times of the day for obtaining the results, with mass velocities varying between 197.59 and 267.26 kg·m-2·s-1, and the heat flux having values between 72.83 and 488.27 W·m-2. The operating values in tests, such as refrigerant pressure and temperature, were taken in a built prototype. The numerical analysis was carried out considering different correlations proposed by Chen, Wojtan, and Kattan. The Wojtan mathematical model offered the best projection of the heat transfer effect for the different transition zones of a two-phase flow along the pipeline. The boiling heat transfer coefficients had approximate maximum values of 8.2, 8.5, 7.8, 6.7, and 5.8 kW·m-2·K-1 for the A, B, C, D, and E tests prediction by Wojtan. Moreover, the boiling heat transfer coefficients increased as the mass velocity enhanced and the rise of vapor quality as the mass velocity was fixed. In this study, the effect of solar radiation, vapor quality on the measured heat transfer coefficient was analyzed.