This study conducts visualization research on the flow pattern of R290 flow boiling in horizontal micro-tubes. The main flow patterns and influence factors of flow boiling heat transfer under different pipe diameters were analyzed

in addition to the summary of the flow pattern transformation law by comparing to the theoretical flow pattern diagrams. The experimental conditions were as follows: heat flux (1–70 kW/m2)

mass flow rate (50–1 020 kg/(m2?s)

saturation temperature (-10–25 °C)

pipe diameter (1–3 mm)

and vapor quality (0–1). Eight types of flow patterns of R290 flow boiling heat transfer in micro-channels were observed

among which intermittent flow and wave flow were the main flow patterns in 3 mm-tubes

while slug flow and annular flow were the main heat transfer patterns in 1 mm-tubes. Bubble flow and mixed flow in 3 mm-tubes

bubble flow in 2 mm-tubes

and slug flow in 1 mm-tubes were consistent with the Damianides and Westwater flow pattern transition criterion. However

the discrete flow regions in 2 mm-tubes and 1 mm-tubes were poorly matched; the change in pipe diameter had an important influence on the flow pattern. A decrease in pipe diameter caused changes in the bubble shape

flow pattern

flow pattern distribution

and flow pattern transition curve

and the micro-scale effect of the pipe diameter can be observed.