Dual-scale and hybrid micropillar architectures for enhanced boiling and capillary transport: A three-dimensional pore-scale study

Efficient thermal management of high-heat-flux systems requires boiling surfaces capable of sustaining liquid replenishment while limiting vapor accumulation near the heated surface. Conventional single-scale micropillar arrays are constrained by a permeability–capillarity trade-off, where enhanced capillary pumping can restrict liquid transport and vapor removal. In this study, hierarchical dual-scale micropillar architectures are proposed to partially decouple these competing mechanisms. Three-dimensional pore-scale volume of fluid simulations are conducted to investigate liquid–vapor transport, interfacial evolution, and phase-change behavior in monopillar, bipillar, and hybrid pillar configurations over heat fluxes ranging from 10 to 100 W/cm2. The bipillar architecture substantially increases permeability relative to the monopillar geometry while maintaining comparable capillary pressure. However, its larger inter-cluster pores promote localized vapor accumulation at elevated heat fluxes, reducing liquid rewetting and increasing wall superheat. To address this limitation, a hybrid architecture incorporating secondary micropillars within the large pores of the bipillar structure is introduced. The secondary pillars modify the local pore morphology and phase distribution, reducing vapor accumulation and persistent vapor pockets while largely preserving the enhanced liquid-transport characteristics of the bipillar geometry. Consequently, the hybrid structure exhibits lower wall superheat, reduced vapor saturation fluctuations, and more stable phase-change behavior across the investigated range. The predicted heat transfer coefficient increases from approximately 4.8 to 8.9 W/cm2 K between 10 and 100 W/cm2, with the hybrid configuration consistently outperforming the monopillar and bipillar structures. Overall, the results demonstrate that boiling performance in micropillar arrays is governed not only by capillary-driven liquid supply but also by pore-scale vapor management. Under the investigated conditions, the proposed hierarchical architecture suppresses vapor accumulation while preserving favorable hydraulic characteristics, highlighting the potential of multiscale pore designs for improved high-heat-flux boiling performance.