Experimental investigation of R-134a and R-245fa two-phase flow in microchannels for different flow conditions

An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. The tests were run in 0.5 mm and 0.8 mm diameter glass channels using R-134a and R-245fa at 26, 30 and 35 °C saturation temperatures. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were detected. The two-phase flow pattern transitions observed with R-134a did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. No significant influence of the inlet subcooling (2, 3, 5 °C) nor the saturation pressure has been observed on the flow pattern transitions. Furthermore, changing the heated length from 70 to 30 mm before the glass tube did not influence the location of the transition lines in the flow pattern map. The 0.8 mm diameter also did not show any significant difference to the 0.5 mm channel, although bubbly/slug flow was present over a wider range of mass flux. The transitions between flow regimes are less influenced by the mass flux for R-245fa than for R-134a; thus for low mass fluxes the difference is significant and the flow regimes appear earlier in terms of vapor quality.