Flow pattern visualization and heat transfer characteristics of R-134a during evaporation inside a smooth tube with different tube inclinations

Flow pattern and heat transfer during evaporation in an 8.9 mm diameter smooth tube are presented. The experimental facility makes the tube capable of having different inclination angles, α, with respect to horizon. The experiments were performed for seven different tube inclinations, α, in a range of − 90° to + 90° and five refrigerant mass velocities in a range of 53 to 170 kg/m2 s for each tube inclination angle. The experimental results indicate that the tube inclination noticeably influenced the vapor and liquid distribution as well as the evaporation heat transfer coefficient. Seven different flow regimes are observed during evaporation inside a smooth tube with different tube inclinations. In the case of evaporation heat transfer, for all refrigerant mass velocities, the highest heat transfer coefficient is attained at an inclination angle of α = + 90°. The effect of tube inclination angle on heat transfer coefficient is more prominent at low vapor quality and mass velocity. A correlation has also been developed to predict the heat transfer coefficient during flow boiling inside a smooth tube with different tube inclinations.