Numerical study of bubble growth and flow boiling characteristics in open microchannels with different gap height

During flow boiling in parallel microchannels, flow instability caused by restricted bubble growth poses a significant challenge to achieving efficient boiling heat dissipation. This study employs numerical simulations to investigate the flow and heat transfer characteristics of a single bubble undergoing restricted growth in two adjacent open rectangular microchannels (ORM), focusing on the effects of top gap height and mass flux. The simulations are conducted using the Volume of Fluid (VOF) method, R-G phase change model, and fluid-solid coupled heat transfer in ANSYS Fluent 2020 R2. The results show that when bubble growth is constrained by the channel walls, the inlet liquid tends to flow into adjacent channels, while the bubble head preferentially enters neighboring channels through the top gap, suppressing reverse flow. Bubbles extending upstream merge and grow with bubbles in adjacent channels within the top gap. The presence of the top gap provides additional space for bubble expansion, enabling earlier evaporation heat transfer in adjacent channels. Increasing the top gap height and mass flux effectively suppresses bubble reverse flow. When the top gap height increases to 20 mu m, the thermal resistance is reduced by 28.24 %. At a top gap height of 100 mu m, the second peak pressure drop is reduced by 57.53 %. However, with increasing mass flux, the evaporation contacts time and area between the bubbles and channel walls decrease, reducing the enhancement of heat transfer by flow boiling. To balance heat transfer performance and flow stability in ORM, an optimal top gap height should be carefully designed.