Visualization of R1234yf, R1233zd(E), and R1336mzz(Z) flow in microchannel tube with emphasis on the velocity of vapor plugs

Visualization of the two-phase flow in micro-scale helps to reveal the thermophysical behavior of the fluids. This paper presents and discusses the flow patterns of two-phase R1234yf, R1233zd(E), and R1336mzz(Z) in a 0.643 mm microchannel tube. The condition covers mass flux from about 25 to 250 kg-m⁻² s⁻¹. The inlet saturation temperature is 30 °C for R1234yf and R1233zd(E), and 40–60 °C for R1336mzz(Z). As vapor quality increases, the flow is firstly in plug/slug flow, then transitional flow, and finally annular flow at high quality. When mass flux is 50 kg-m⁻² s⁻¹, no transitional flow is observed. The annular flow starts at high quality. The transitional flow in the low mass flux was absent. When mass flux is 100 kg-m⁻² s⁻¹, the transitional flow covers quality from 0.8 to 0.9. The superficial velocities follow a power function for flow pattern transitioning. The velocity is measured by a video processing method proposed in this paper. The vapor plug velocity is calculated by measuring the velocity of the vapor plug head in the frames and corrected by the video information (frame per second and meter per pixel). The velocity is slightly higher than the velocity calculated based on the homogeneous assumption when it is higher than 1 m-s⁻¹. A new correlation for predicting the vapor plug velocity is proposed. The new correlation has a small MAE (5.42%) compared to the results and can be used for Capillary number less than 0.14.