Flow patterns and plug/slug flow characteristic of R134a in a 0.643 mm microchannel tube

This paper presents visualization of the two-phase flow of R134a in a 24-port microchannel tube with the average hydraulic diameter of 0.643 mm. The experiment is conducted on the same facility in the previous work for R32 (Li and Hrnjak, 2019). Mass flux covers the range from 50 to 250 kg-m⁻²s⁻¹. The two-phase flow is generated by adding heat in several steps to originally subcooled refrigerant at the very entrance. When mass flux is 50 kg-m⁻²s⁻¹, no annular flow is observed in the tube. The annular flow starts at x = 0.9 when mass flux is 100 kg-m⁻²s⁻¹ and x = 0.6 when 250 kg-m⁻²s⁻¹. The transitional flow starts at x = 0.25 (G = 50 kg-m⁻²s⁻¹) and x = 0.8 (250 kg-m⁻²s⁻¹). Revellin and Thome (2007), Ong and Thome (2011), and Zhao and Hu (2000) correlations are plotted against the measurements. None of them fit our data since the experiment is in lower mass flux than the database of their correlations’. The interface velocity and vapor plug length fraction (close to void fraction) are measured from the high speed videos. The velocity and vapor fraction agree to results based on the homogeneous assumption. In plug/slug flow, both the vapor plug and liquid slug lengths are uneven. The length distribution of plug and slug seems to follow Beta distribution. When the quality is low, the quantity of short vapor plugs is larger than long plugs. At fixed mass flux, when quality increases, length of vapor plugs increases and length of liquid slugs decreases.