Flow visualization of R1234ze(E) in a 0.643 mm microchannel tube

This paper presents the discussion of regimes of two-phase flow of R1234ze(E) in a 24-port microchannel tube with average hydraulic diameter of 0.643 mm, obtained by visualization. The experiment is conducted in the same facility in the previous work for R32 (Li and Hrnjak, 2019). The conditions cover mass flux from 50 to 225 kg m−2 s−1. The inlet saturation temperature is 30 °C. The two-phase flow is generated by adding heat in several steps to refrigerant initially subcooled at the entrance of test line. As vapor quality increases at a fixed mass flux, the flow is firstly plug/slug, then transitional, and finally is annular flow regime at high quality. When mass flux is 50 kg m−2 s−1, no annular flow observed in the tube. The vapor quality of boundary between two flow patterns decreases as mass flux increases. The annular flow starts at x = 0.85 (G = 100 kg m−2 s−1) and x = 0.63 (225 kg m−2 s−1). The transitional flow starts at x = 0.8 (G = 50 kg m−2 s−1) and x = 0.3 (225 kg m−2 s−1). Comparing to R32 and R134a, flow patterns of R1234ze(E) transition at lower quality due to the lower vapor density and thus higher vapor velocity. Three flow pattern maps in literature are compared to the results and they have limited agreement with our observations. The plug/slug flow behaves as homogeneous. The velocity at interface between liquid slug and vapor plug is close to the homogeneous velocity. The vapor plug length fraction also agrees to the homogeneous void fraction. The length of vapor plug increases dramatically as vapor quality increases at fixed mass flux. The length of liquid slug first increases and then unchanged as quality increases. The video supports that there are liquid droplets formed from the liquid slug and liquid ring collision.