Effects of surface nanostructure and wettability on CO2 nucleation boiling: A molecular dynamics study

Abstract

Nanostructured tubes hold great potential for enhancing heat transfer in refrigeration/heat pump systems. Therefore, it is essential to study the effects of nanostructured surface characteristics on refrigerant boiling heat transfer. In this paper, the nucleation boiling behavior of CO₂ on the nanostructured surface is simulated using molecular dynamics. The effect mechanism of nanostructure size and surface wettability on CO₂ bubbles nucleation and growth is investigated. At first, the nucleation boiling processes of both smooth surfaces and nanostructured surfaces featuring three different wide grooves are simulated. The results show that the local thermal aggregation effect is the key for nanostructures to promote CO₂ bubble nucleation. The bubble nucleation efficiency is highest on the nanostructured surface with 5 ​nm wide groove. Then, based on a 5 ​nm wide nanostructured wall surface, the wettability effect on nucleation boiling is investigated by adjusting the potential energy factor α. The results show that the hydrophilic walls enhance the solid-liquid heat transfer and the collision of atoms within the liquid, resulting in boiling heat transfer capacity improvement between CO₂ and the walls. The average temperature, average heat flux and critical heat flux in the liquid phase are also improved. A significant temperature gradient between the layers of CO₂ liquid is noted on hydrophilic wall, where intermolecular forces and molecular advection dominate the heat transfer mechanism. In contrast, on hydrophobic wall, intermolecular forces dominate the heat transfer process.