Managing high heat flux electronics of over 600 W/cm² by capillary evaporation in bi-porous wicks

In this work, uniform porous wicks and bi-porous wicks with a thickness of 1 mm are prepared. Additionally, deionized water serves as a working fluid, and the effects of factors such as the particle size (10 ∼ 200 μm), the content of a pore-forming agent (0 ∼ 20 wt%), and the subcooling degree (0 K and 40 K) on capillary evaporation are studied. The results show that for uniform porous wicks, as the particle diameter increases, the critical heat flux (CHF) gradually increases. For bi-porous wicks, the addition of the pore-forming agent reduces the flow resistance of liquid, and increases the space for vapor escape, thereby significantly improving the heat transfer performance by capillary evaporation. When the particle diameter is 200 μm and the content of the pore-forming agent is 20 wt% under subcooled conditions, the CHF reaches up to 621.5 W/cm², the average thermal resistance falls to 0.06 K/W, and the evaporation heat transfer coefficient (HTC) increases to 1.6 × 10⁵ W/(m²·K). In contrast to the uniform porous wick with the particle size of 10 μm, the CHF is increased by 46.5 %, the average thermal resistance decreases by 50 %, and the evaporation HTC increases by 400 %. Moreover, a critical heat flux of 573 W/cm² can be achieved while maintaining a wall superheat of 38°C under saturated conditions. Additionally, as the copper powder particle diameter becomes smaller, heat transfer by the addition of the pore-forming agent is further improved. Finally, based on the conservation of energy and mass, a theoretical model for predicting the CHF of both uniform porous and bi-porous wicks is proposed, with the predicting error of within ± 20 %. This article can provide fundamental data and theoretical support for the design of high heat flux passive phase-change cooling devices.