Subcooled pool boiling heat transfer enhancement on hierarchically biomimetic laser-structured surfaces with gradient pores

In this work, an experimental study was conducted to investigate the pool boiling heat transfer characteristics of hierarchically micro/nano-structured surfaces with biomimetic honeycomb-like gradient pores. Working fluid was HFE-7100 dielectric fluid. The structured surface was fabricated on a silicon substate with femtosecond laser orthogonal scanning processing. A smooth surface and a nano-porous surface were also investigated to provide a comprehensive evaluation of the performance differences and to elucidate the main mechanisms for heat transfer enhancement. The hierarchically porous surface demonstrated a significant reduction in the inception temperature at a liquid subcooling of 35 K, from 16.2 K to 6.6 K, compared to a smooth surface. Additionally, the enhancements in the heat transfer coefficient and critical heat flux increased 310.9% and 247.3%, respectively. The findings showed that the hierarchical structure with gradient pores led to enhanced heat transfer performance, attributed to increased nucleate sites, reduced bubble departure diameter, higher departure frequency, improved rewetting ability, and the induction of micro-convection. Furthermore, an interesting phenomenon of vapor bubble-bubble penetration was observed during the growth stage, which was motivated by microbubble emission from the heated surface and can be considered as a contributing factor to the enhanced heat transfer on the hierarchical porous surface during subcooled pool boiling.