Experiments were carried out to investigate the saturated flow boiling heat transfer and visualize the two-phase flow patterns for deionized water in a micro-pin-fin heat sink with cylindrical fins with a height of 500 μm and diameter of 500 μm. The test parameters were as follows: a mass flow rate G = 341–598.3 kg/(m2?s)

heat flux q = 20–160 W/cm2

and steam dryness X = 0–0.2. The results show that the local boiling surface heat transfer coefficient decreases monotonously with an increase in heat flux. The heat transfer coefficient of the local boiling surface increases with the increase in the mass flow rate

and decreases with an increase in steam dryness within a low dryness region. With the influence of sub-cooled boiling bubbles

the lower the working fluid inlet temperature

the greater the local boiling surface heat transfer coefficient. The flow pattern of bubbling in the micro-channels shows a bubbly flow and an annular flow with an increase in the heat flux density. The local boiling surface heat transfer coefficient of a bubbly flow is significantly higher than that of an annular flow.