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.

在本研究中，R1234yf 在内径为 8.3 mm、外径为 9.52 mm、长度为 670 mm 的平管内，在 7 种不同的管中进行了流动模式、沸腾传热和压降的实验研究。倾角从 -90°（向下垂直流）到 +90°（向上垂直流）。实验在80、160、240和 320 kg m ^-2  s ^-1四种质量速度、0.1-0.77 的平均蒸汽质量范围、31 °C 的饱和温度和 4.5、9.0、 13.5 和 18.0 kW m ^-2. 研究了不同参数对传热系数和摩擦压降的影响。根据结果​​，管倾角对流动模式和传热系数有显着影响，在低蒸汽质量和低质量速度下更为突出。对于所有质量速度，最高和最低的传热系数分别在倾角为 +90°（垂直向上流动）和 -90°（垂直向下流动）的管中获得。摩擦压降受管倾斜角的影响很大，在高蒸汽质量下效果更明显。在低蒸汽质量区域，热通量强烈影响传热系数，而对摩擦压降的影响可以忽略不计。最后，