An empirical investigation on flow pattern, heat transfer, and pressure drop during flow boiling of R1234yf in an inclined plain tube

Abstract

In the present study, the flow pattern, boiling heat transfer, and pressure drop of R1234yf are experimentally studied inside a plain tube with an inner diameter of 8.3 mm, the outer diameter of 9.52 mm, and the length of 670 mm, in seven different inclination angles from −90° (downward vertical flow) to +90° (upward vertical flow). The experiments are conducted at four mass velocities of 80, 160, 240, and 320 kg m−2 s−1, mean vapor quality range of 0.1–0.77, saturation temperature of 31 °C, and four heat fluxes of 4.5, 9.0, 13.5, and 18.0 kW m−2. The effects of different parameters on the heat transfer coefficient and frictional pressure drop have been investigated. Based on the results, the tube inclination angle has a significant effect on the flow patterns and heat transfer coefficient, which is more prominent at low vapor qualities and low mass velocities. For all mass velocities, the highest and lowest heat transfer coefficients are attained in the tubes with inclination angles of +90° (vertical upward flow) and −90° (vertical downward flow), respectively. The frictional pressure drop is considerably affected by the tube inclination angle, and the effect is more noticeable at high vapor qualities. At low vapor quality regions, the heat flux strongly affects the heat transfer coefficient while it has a negligible effect on the frictional pressure drop. Finally, the experimental results are compared to the existing correlations, and modified correlations are proposed to predict the heat transfer coefficient and the frictional pressure gradient of R1234yf during flow boiling inside an inclined plain tube.