Visualization and experimental investigation of two-phase flow in S-shaped microchannel heat sink

An experimental visualization was used to explore the flow pattern and bubble behavior in S-shaped microchannel heat sink. The micro-channel and micro-fin width is 0.5 mm and 0.35 mm, respectively, and the micro-fin height is 0.29 mm. The heat flux over 180 W/cm² can be dissipated by a relatively low inlet flow rate of 15 mL/min using HFE7100 as the coolant. PDMS was used as the visual test assembly and the bubble behavior was observed by a high-speed camera. Results show that the two-phase flow patterns in the heat sink are as follows: bubbly flow, slug flow, churn flow, annular flow and local irregular bubbles by extrusion. Small bubbles re-perform the bubble behavior in the S-shaped microchannel: bubble formation - bubble growth- bubble deformation - bubble assemblage - bubble breakage - bubble growth. The main reasons for the instability of the two-phase flow are the flow mal-distribution, the flow pattern transition, the bubble behavior and periodic local drying. Moreover, the heat transfer performance was investigated and results show that the S-shaped microchannel heat sink with 3 inlets and 4 outlets has the maximum effective heat transfer coefficient of 1.1 × 10⁵ W/m²⋅K and two-phase heat transfer coefficient of 9.9 × 10⁵ W/m²⋅K when the heat flux is 61.91 W/cm² with a pressure drop of 18.3 kPa.