Boiling heat transfer via micro-nano composite gradient and grooved porous structures: Liquid film and vapor dynamics insights

• Micro-nano composite porous structures enhance heat transfer via liquid film and vapor motion optimization. • Gradient porous structures boost heat transfer by improving liquid film area and vapor motion frequency. • Grooved porous structures improve heat transfer by increasing vapor motion frequency inside the grooves. The distribution and dynamics of vapor-liquid phases within porous structures critically influence pool boiling heat transfer. Through high-resolution experimental visualization and quantitative characterization, this study identifies the liquid film area and vapor motion frequency as two key dynamic parameters governing heat transfer efficiency. Building upon this quantitative framework, we systematically investigate the mechanistic effects of micro-nano composite gradient porous structures and grooved porous architectures on boiling heat transfer enhancement. Results show that gradient microporous structures enhance heat transfer primarily through accelerated vapor expulsion, while exhibit minimal impact on micro-nano composite porous structures. Grooved porous structures achieve superior thermal performance by enabling a unique "short-cycle" vapor recirculation mode within the grooves. We further develop an optimized porous structure integrating grooves with variable pore sizes, which demonstrates superior heat transfer performance compared to conventional designs. This study advances the fundamental understanding of heat transfer mechanisms in micro-nano composite porous structures and expands the theoretical framework for designing multi-scale heat transfer structures.