表面纳米结构和润湿性对二氧化碳成核沸腾的影响:分子动力学研究

Yongfang Huang, Xianqiang Deng, Yongxiang Duan, Chao Liu, Xiaoxiao Xu
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摘要

纳米结构管在增强制冷/热泵系统传热方面具有巨大潜力。因此,研究纳米结构表面特性对制冷剂沸腾传热的影响至关重要。本文利用分子动力学模拟了二氧化碳在纳米结构表面的成核沸腾行为。研究了纳米结构尺寸和表面润湿性对 CO2 气泡成核和生长的影响机理。首先,模拟了光滑表面和具有三种不同宽槽的纳米结构表面的成核沸腾过程。结果表明,局部热聚集效应是纳米结构促进二氧化碳气泡成核的关键。在具有 5 nm 宽凹槽的纳米结构表面上,气泡成核效率最高。结果表明,亲水性壁面增强了固液传热和液体内部原子碰撞,从而提高了二氧化碳与壁面之间的沸腾传热能力。液相中的平均温度、平均热通量和临界热通量也得到了改善。在亲水壁上,二氧化碳液体层间存在明显的温度梯度,在这种情况下,分子间作用力和分子平流在传热机制中占主导地位。与此相反,在疏水壁上,分子间作用力主导了传热过程。
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Effects of surface nanostructure and wettability on CO2 nucleation boiling: A molecular dynamics study

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.

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