Characteristics of R513A evaporation heat transfer inside small-diameter smooth and microfin tubes

This paper investigates the characteristics of R513A during flow boiling. R513A is an azeotropic mixture made of R1234yf and R134a (0.56/0.44 by mass), and it has been proposed as a direct drop in of the common R134a. Experimental tests were run in a wide range of operative conditions: mass velocity in the range 200-800 kg m−2 s−1, heat flux in the range 12-60 kW m−2, for saturation temperatures of 15, 20 and 25°C. Two different mini tubes were tested: a smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. Heat transfer coefficients and frictional pressure drops were evaluated from the experimental measurements. The approximately same diameter of the two tubes permitted to highlight the effect of the microfins on the thermal and hydraulic behavior during R513A flow boiling. Finally, experimental heat transfer coefficients and frictional pressure drops were compared against values predicted by correlations available in the literature.