Synergistic enhancement of flow boiling heat transfer by composite structure and gradient wettability

Microchannel flow boiling heat transfer has garnered significant attention in heat dissipation applications owing to its exceptional heat transfer capacity and compact configuration. However, microscale confinement effects often induce bubble blockage, leading to critical thermal issues such as localized dry-out, non-uniform wall temperature distribution, and vapor backflow, ultimately constraining improvements in heat transfer efficiency. The present work performs a systematic numerical investigation to explore two-phase flow behaviors within microchannels featuring innovative structural configurations and surface wettability modifications. We propose a composite microchannel design characterized by a radial gradient wettability, comprising an upper superhydrophobic region and a lower superhydrophilic region. Results revealed two distinct enhancement mechanisms: a cyclic rewetting mechanism induced by the superhydrophobic upper region, substantially improving thermal performance within the bubbly flow region by promoting liquid replenishment; and a vapor radial extraction mechanism resulting from synergistic structural and wettability interactions, effectively mitigating bubble blockage in the lower region. This dual mechanism notably suppressed localized dry-out and enhanced overall heat transfer performance. Comparative analyses against homogeneous wettability microchannels demonstrated that the gradient wettability composite structure achieved a maximum enhancement of 166.3 % in the Performance Evaluation Criterion, along with reductions of 64.2 % and 45.1 % in the average and exit bottom wall superheat, respectively, alongside improved temperature uniformity. These findings substantiate the significant potential of synergistic structural and wettability modifications for enhancing two-phase flow boiling performance, offering essential insights for advancing microchannel heat dissipation technologies.