Flow boiling heat transfer of R134a in multi microchannels

•Flow boiling patterns were observed and included bubbly, slug and annular flows.•The effect of heat and mass flux on the bubble departure diameter is presented.•Flow reversal in the micro channel heat sink was observed and discussed.•The dependence of the heat transfer coefficient on heat and mass flux is presented.•Comparison with correlations predicting the heat transfer coefficient is included. Experiments were conducted to investigate flow boiling heat transfer of R134a in a multi microchannel heat sink. The heat sink consisted of 25 rectangular microchannels with nominal dimensions of 300µm wide, 700µm deep (Dh=420µm) and 200µm separating wall thickness. The heat sink was made of oxygen free copper by CNC machining and was 20mm long and 15mm wide. The experiments were conducted at 6.5bar system pressure and covered a footprint area-based heat flux range 11.46–403.1kW/m2 and mass flux range 50–300kg/m2s. A high speed camera was used to capture the flow patterns simultaneously with heat transfer measurements. Three flow patterns were observed namely bubbly, slug and wavy-annular flow when the heat flux increased gradually. The heat transfer coefficient increased with heat flux and there was no mass flux effect. Assessing existing correlations indicated that the correlations of Mahmoud and Karayiannis (2013) and Cooper (1984) predict the data very well with a mean absolute error less than 20% compared to the other correlations.