Visualization study of flow boiling characteristics in open microchannels with different wettability

Abstract

Open microchannels heat sink can reduce the pressure drop and mitigate the flow instability, but the influence of wettability on the two-phase flow boiling for open microchannels configuration is unclear. In this study, open microchannels with superhydrophilic (SHPi), hydrophilic (HPi) and superhydrophobic (SHPo) properties were fabricated, respectively. Experiments were conducted to explore the heat transfer and pressure drop characteristic and visualization was carried out to analyze the connection between flow pattern transition and two-phase flow boiling performance. Tests were performed with the inlet subcooling of 75°C at efficient heat flux of 261.8–597.9 kW/m2 under various mass flow ranging from 172 to 382 kg/m2•s. Local dryout was captured during stratified flow for HPi microchannels, yet was absent for SHPi ones. A novel trapped bubble was observed in SHPo microchannels, which led to trapped vapor film and slug film that limited areas to be rewetted at high heat flux. SHPo microchannels with the largest quantity of nucleating sites achieved the highest heat transfer coefficient at low to medium high heat flux (222.8–446.6 kW/m2 at G = 172 kg/m2•s, 296.3–728.5 kW/m2 at G = 278 kg/m2•s, 331.7–949.8 kW/m2 at G = 382 kg/m2•s), however SHPi ones yielded a better performance at the highest heat flux owing to the excellent rewetting capacity. The two-phase pressure drop of open microchannels was found to be dominated by vapor acceleration loss that was related to the vaporization efficiency, thus the SHPo microchannels possessed the highest pressure drop and the SHPi ones exhibited the lowest value.