The bubble merger in rectangular microchannels during boiling processes based on conjugate heat transfer

Abstract

In recent years, boiling heat transfer in microchannels has become an effective solution for managing high heat flux in electronic devices. Numerical simulations offer details on flow characteristics that experimental methods cannot capture, deepening our understanding of boiling mechanisms within microchannels. In this study, we developed a self-programmed solver, “interMultiRegionFoam,” based on the volume of fluid (VoF) method, phase-change mass transfer models, and conjugate heat transfer. Analyzing the force acting on individual bubbles, a model for two bubbles' merger was constructed, encompassing three stages: bubble bridging, bubble retraction, and bubble contraction. For isolated bubbles, during the restricted growth stage, the dry patch length increases by only 25% with an increase in the Capillary number, while in the elongation growth stage, it grows fivefold. Furthermore, the investigation into four typical bubble merger cases within microchannels demonstrated that the bubble merger model effectively describes the process. During the restricted and free growth stages, bubble merger was observed to enhance heat transfer, and the heat transfer coefficient in the bottom region of the macrolayer increased by a maximum of 16%. However, when bubbles merge in the elongation growth stage, the maximum increase in the heat transfer coefficient at the bottom of the macrolayer is only 7%. Several factors contribute to the enhanced heat transfer during bubble merger, including the evaporation and rewetting of macrolayer liquid, as well as the replenishment of the microlayer. Moreover, the disturbance caused by bubble merger and transient conduction arising from bubble retraction and contraction contributes to the enhancement in heat transfer. With an increase in the contact angle, the peak heat flux gradually decreases, with a maximum reduction of 27%. As the heat flux increases, the Nusselt number gradually increases, reaching a maximum peak increase of 52%. These studies provide theoretical guidance for the design of wall modifications and nucleation in microchannels.