梯度表面驱动液弹脉动及脉动热管传热微观机理的研究

Oscillating heat pipes (OHPs) have many applications in high heat flux cooling systems such as microcomputer. Research results show that the heat transfer in OHPs mainly depends on sensible heat of the oscillating and circulation movement of liquid slugs. Most of the investigations focus on the experimental research and simulation analysis of macro performance of OHPs. However, the microscopic phenomena, such as liquid-vapor interface evolution and internal flow field distribution during the movement of the slug is limited. In order to explore the effect of microscopic phenomena on the macro heat transfer performance of OHP, this project will experimentally and theoretically investigate the liquid slug movement, liquid-vapor interface evolution, internal flow field distribution in the OHPs based on the liquid slug movement induced by wettability gradient surface. The circulation flow of the liquid slugs will be promoted through the ‘check valve’ effect by gradient surface in order to enhance the heat transfer performance of the OHPs with few turns especially in the horizontal operating mode. Visualization and the wall temperature data will be collected at the same time in the experiments and the simulations contain the wall temperature fluctuation with wavelet analysis and flow field distribution with CFD. The dimensionless correlation of the OHPs will be defined based on the surface wettability parameters. The innovative strategies of regulate liquid slug movement and strengthen the heat transfer process will be put forward on the base of the liquid slug regulation mechanism influenced by the gradient surface.

脉动热管在微型计算机等高热流密度散热系统方面具有重要的应用。脉动热管的传热方式主要为液弹脉动的显热传热，研究液弹的振荡运动和循环流动规律对脉动热管传热机理研究具有重要的意义。目前研究结果多为宏观性能的实验研究和模拟分析，对脉动热管内部液弹两端气-液界面形状、液弹内部流场分布等微观现象的研究报道较少。本项目基于梯度表面定向驱动液弹脉动、界面形状调控液弹运动等原理，分析梯度表面脉动热管液弹运动规律、气-液界面形状、内部流场等微观现象和宏观传热性能的影响规律。通过梯度表面“单向阀”效应驱动液弹的定向迁移和促进液弹循环流动，解决低弯管数脉动热管在水平操作模式不能运行的问题及传热性能的强化。可视化图像和壁面温度数据同时采集的实验研究和壁面温度波动的小波分析、液弹内部流场分布的CFD模拟相结合。并基于实验数据建立表面润湿性能等参数拟合的脉动热管无因次关系式，得出界面调控液弹运动强化脉动热管传热的准则。

脉动热管在微型计算机等高热流密度散热系统方面具有重要的应用，研究工作介质和脉动热管壁面的润湿性能对液弹运动、壁面温度波动和总传热性能的影响规律对小型脉动热管水平操作的稳态运动和脉动热管传热性能强化具有重要的意义。本项目提出通过梯度润湿表面调控液弹运动，强化脉动热管传热性能的新型调控策略。完成了梯度润湿表面、不同粗糙结构的微纳功能表面和与壁面具有不同润湿性能的离子液体、液态金属、乙醇和丙酮等工质的脉动热管性能研究。首先制备具有梯度润湿表面的脉动热管，润湿性能从脉动热管冷凝段向蒸发段逐渐提高，接触角从83°均匀变化至0°，促进工作介质向脉动热管蒸发段定向运动，解决了低弯管数脉动热管（6弯管）工作介质不易回流、蒸发段“烧干”等问题，与紫铜脉动热管相比，梯度润湿脉动热管的传热性能提高了30%。其次制备超亲水、亲水、紫铜、疏水和超疏水表面脉动热管，利用微纳结构表面粗糙结构效应和毛细抽吸效应，增加蒸发和沸腾的核化中心并形成薄膜蒸发，提高脉动热管的启动和传热性能。与普通紫铜脉动热管相比，加热功率100W时，亲水脉动热管的启动时间减少了100s。且具有微纳结构的超亲水和亲水脉动热管壁面温度波动的振幅、液弹脉动的振幅和频率均显著高于普通紫铜脉动热管，液弹振荡运动的振幅提高了0-57%，液弹振荡运动的频率提高了0-100%，脉动热管的传热性能提高了0-67%。最后研究了与壁面接触角低的离子液体、乙醇、丙酮和接触角高的液态金属工质等工质对脉动热管性能的影响，利用微溶于水且与水形成分层的低表面张力离子液体增加工质的不平衡力效应，提高脉动热管的启动性能，离子液体和水混合工质脉动热管的启动温度降低了30W，启动温度降低了10℃。利用热导率是水几十倍的液态金属镓工质提高工质的传热能力，液态金属和水混合工质脉动热管的传热性能提高了10-20%。本项目通过壁面润湿性能调控液弹的振荡运动、得出了界面调控液弹运动强化脉动热管传热性能的准则。