R513A flow boiling heat transfer inside horizontal smooth tube and microfin tube

•R513A flow boiling heat transfer is studied inside smooth tube and microfin tube.•Thermal performances of smooth and microfin tubes are compared between each other.•R513A experimental heat transfer coefficients are compared against predicted values. This paper presents experimental results for R513A flow boiling inside a 3.5 mm ID horizontal smooth tube and inside a 3.4 mm ID horizontal microfin tube. R513A is an azeotropic mixture made of R1234yf and R134a (56% and 44% in mass), and it is proposed as a direct substitute of the common R134a. The flow boiling heat transfer coefficients are measured for mass velocity ranging between 150 and 800 kg m−2 s−1, for heat fluxes from 12 to 60 kW m−2, at a saturation temperature at the inlet of the test section of 20°C. The effects of mass velocity, heat flux and vapor quality on the thermal performance are analyzed both for the smooth tube and for the microfin tube. The heat transfer enhancement of the microfin tube compared to the smooth tube is also presented. The comparison reveals that the microfin tube considerably augments the heat transfer coefficient especially at low mass velocity, whereas at high mass velocity microfin tube and smooth tube show almost the same heat transfer coefficient. Finally, the collected experimental heat transfer coefficients are compared against values predicted by empirical correlations available in the open literature.