INFLUENCE OF THE SATURATION TEMPERATURE ON THE HEAT TRANSFER COEFFICIENT FOR FLOW BOILING INSIDE HORIZONTAL SMOOTH TUBES

This article presents experimental results of heat transfer from two different studies [1, 2] for horizontal flow boiling inside smooth copper tubes with different refrigerants. The main goal of this study was to verify the influence of the saturation temperature on the heat transfer coefficient. The experimental tests were carried for refrigerants R-134a, R-22 and R-404A, at saturation temperatures ranging from 5 to 20°C for two tubes of 12.7 and 13.84 mm ID and three heat fluxes of 5, 7.5 and 17.5 kW/m2. The vapor quality ranged from 5 to 90% and the mass velocity was 300 kg/s.m2. Under these conditions the heat transfer coefficient shows an interesting trend: it behaves differently, in function of the saturation temperature, in the nucleate and convective boiling dominated regions. In the nucleate boiling dominated region the heat transfer coefficient increases with the saturation temperature. In the convective boiling dominated region this phenomenon is inverted, that is, the heat transfer coefficient decreases with the saturation temperature. This phenomenon was verified through the concept of the thermal resistance of the liquid thickness. Since the heat transfer coefficient is proportional to the inverse of the thermal resistance of the liquid film, it increases with decreasing saturation temperature. The transition between nucleate and convective boiling dominated regions, denominated by [1] as a local minimum of the heat transfer coefficient, was also clearly observed with all the refrigerants.