Experiments of convective evaporation of refrigerant R513A in a horizontal stainless-steel tube

Refrigerant R513A represents an interesting solution for the retrofit of conventional high-GWP fluorinated gases, such as R134a, R401A, R401B and R409A for low and medium temperature applications. R513A is an azeotropic mixture (almost zero-temperature glide at any operating pressure) made up of R134a and R1234yf (44% and 56% in mass, respectively), allowing at the same time a very low GWP of 580 and favourable safety characteristics such as no flammability and no toxicity (A1 ASHRAE class). The boiling performance of this blend is scarcely explored and studied in scientific literature, especially in case of commercial tubes typically adopted for refrigeration purposes. For this reason, this paper presents two-phase flow boiling experiments of refrigerant R513A in a 6.00 mm horizontal stainless-steel tube. Heat is provided by means of Joule effect directly on the tube surface, and the peripheral average heat transfer coefficients are obtained by measuring the temperatures at four sides (top, bottom, left and right) of the channel. The effect of the operating conditions is experimented and discussed, by varying the mass flux between 150 and 300 kg/m2 s, saturation temperature between 20 and 50°C and imposed heat flux between 5 and 20 kW/m2. Also, a comparison with the boiling performance of refrigerant R134a is proposed within the same operating conditions. Finally, the assessment of well-known flow boiling prediction methods is presented and discussed.