Boiling heat transfer coefficient and pressure drop of R1234yf flow inside smooth flattened tubes: An experimental study

•Using flattened tubes enhances heat transfer coefficient up to 154%.•Flattening tubes increases pressure drop up to 270%.•Heat transfer and pressure drop increase with vapor quality and mass velocity.•Vapor quality has no major effect on the performance evaluation criterion (PEC).•The highest PEC is at high mass velocity for flattened tube with aspect ratio of 2. In this study, the boiling flow of R1234yf inside flattened tubes is experimentally investigated to determine the heat transfer coefficient and pressure drop. The experiments were carried out on a round tube of 8.2 inner diameter and three flattened tubes with internal heights of 6.2, 5.1 and 2.8 (aspect ratios of 1.5, 2, and 4), with mass velocities between 110 and 410 kg/m2s, and vapor qualities between 0.1 and 0.9. The experimental data were analyzed to determine the effect of vapor quality, mass velocity, and flattening the tubes on the heat transfer coefficient and pressure drop. Also, the performance evaluation criterion (PEC) was used to investigate the effect of flattening on the heat transfer coefficient and pressure drop simultaneously. The results showed that for the flattened tubes with 1.5, 2, and 4 aspect ratios, the heat transfer coefficient enhances up to 53%, 99%, and 154%; while the pressure drop increases up to 54%, 167%, and 270%, respectively, compared to those of the round tube. In addition, according to the results, the enhancement factor increases with the aspect ratio, and the tube with the aspect ratio of 2 showed the highest PEC at high mass velocities.