Hybrid air-water cooling of high-aspect-ratio mini-channels with suppressed thermal boundary-layer growth

The rapid increase in heat dissipation in AI processors, high-performance computing systems, and data centers has challenged the effectiveness of conventional cooling technologies in maintaining safe operating temperatures. To address this limitation, a novel hybrid air-water mini-channel heat sink is proposed, wherein two high-aspect-ratio rectangular mini-channels (aspect ratio = 28:1) replace four conventional circular channels, and plate fins on the top surface are exposed to turbulent impingement air flow. The key mechanism of the proposed design is the synergistic interaction between liquid cooling and air impingement, where the air-side flow compensates for the downstream degradation of water-side heat transfer caused by thermal boundary-layer growth. Three-dimensional conjugate heat transfer simulations were conducted to analyze the thermal and flow behavior, with both the water and air inlet temperatures fixed at 25°C. The results show that the rectangular water-cooled configuration reduces thermal resistance by 37.5% and increases the heat transfer coefficient by 71.7% compared with the circular four-channel design. The hybrid configuration further enhances the thermal performance, achieving a 52.5% reduction in thermal resistance compared to the circular design and a 24% reduction relative to the rectangular water-only configuration. Moreover, it exhibits a significantly lower axial decay in the heat transfer coefficient (19% versus 32%), indicating improved thermal uniformity along the flow direction. Statistical analysis confirms the superiority of the rectangular mini-channels configuration (p < 0.001). Multi-objective optimization identifies Re water = 800 and Re air = 4500 as the optimal operating conditions at 100,000 W/m². Taguchi analysis indicates that Re water predominantly governs the heat transfer coefficient, while Re air has the strongest influence on thermal resistance.