Visualized flow boiling regime evolution and diverging flow paths enhancement in manifold microchannel heat sinks

The rapid advancement of the electronic chip industry has introduced increasingly severe thermal management challenges. Among emerging solutions, manifold microchannel heat sinks (MMCHSs) utilizing flow boiling are considered particularly promising for high–heat flux cooling. In this study, a full-scale visualized MMCHS was developed using HFE-7000 as the working fluid to investigate subcooled flow boiling characteristics. The experimental campaign elucidated the evolution of flow regimes and the mechanisms behind heat transfer degradation in conventional MMCHSs, and further assessed the effectiveness of diverging flow-path optimization strategy. Results revealed a heat-flux-driven transition from localized boiling to fully developed annular flow, accompanied by bubble retrograde growth phenomena. Under high heat fluxes, uniform-channel MMCHSs experienced pronounced vapor backflow and blockage, which impeded upstream liquid replenishment and induced intermittent dryout, ultimately triggering critical heat flux (CHF). To mitigate these limitations, a diverging flow-path configuration was proposed. By promoting forward liquid advection, the design effectively suppressed vapor blockage and backflow, thereby enhancing thermal-hydraulic performance. The optimized DD-MMCHS, which integrates two-level divergence in both the microchannel and manifold layout, achieved significant performance gains under identical conditions: an 18.8 °C reduction in heater temperature, a 40.7% decrease in pressure drop, and a 13% increase in CHF. These findings provide mechanistic insights and practical guidance for future studies on the thermal-hydraulic behavior and structural optimization of MMCHSs.