Water-Heated Pool Boiling of Different Refrigerants on the Outside Surface of a Horizontal Smooth Tube

Pool boiling heat transfer has been extensively studied over decades, but the effect of boundary heating conditions on boiling received little attention. In this work, heat transfer coefficients during pool boiling of five different refrigerants (R123, R245fa, R236fa, R134a, and R22) on the outside surface of a smooth copper tube were measured at the saturation temperature of 6.7 °C; water flows inside the tube and provides heat to the refrigerants to boil (thus, water-heated boiling). Measurements showed that the refrigerant of a higher vapor pressure has a higher heat transfer coefficient, with the exception that R22 performs nearly the same as R134a. A correlation previously developed for electrically-heated pool boiling on cylindrical tubes underpredicts by 30%–46% the heat transfer coefficients during water-heated boiling of the five refrigerants. Among the pool boiling correlations reviewed in this work, the Cooper correlation (for pool boiling on cylindrical tubes) predicts the boiling heat transfer coefficients of R22 and R245fa reasonably well (within ±8.5%), but not as well those of the other three refrigerants (underpredicts by nearly 30% for R134a and R236fa and overpredicts by nearly 40% for R123). It is found that the predicted boiling heat transfer coefficients of the five refrigerants by the modified Gorenflo correlation (simply adding a constant multiplier of 1.47 to the Gorenflo correlation) are in excellent agreement with their respective measurements.