非牛顿纳米流体的平行剪切流及其稳定性研究

Nanofluids obtained from dispersing nanoparticles in host fluids can enhance thermal properties significantly and have great application potentials in several fields, such as aerospace, energy, electronics, chemical industry, etc. As the heat transfer in nanofluids is closely related to the flow states, the control of flow stability is a key technology in the application of nanofluids. Nowadays, the heat transfer and non-Newtonian behaviour of nanofluids have received much attention from Chinese and foreign scholars. However, the parallel shear flows and flow stability for nanofluids have not been studied thoroughly. In the present project, the parallel shear flows and flow stability for non-Newtonian nanofluids will be investigated by analytical and numerical methods: 1. the nonhomogeneous equilibrium model will be used to describe the coupling of nanoparticle motion and heat and mass transfer; 2. non-Newtonian fluid models will be used to describe rheological behaviours, and the viscosity stratification due to temperature variation will be considered; 3. linear, nonlinear stability analysis and direct numerical simulation will be used to determine the long-term, transient and nonlinear evolution of perturbations, the instability mechanism will be clarified, and the way to control the flow stability and heat transfer will be obtained. The results will deepen our knowledge of flow stability for nanofluids, and theoretically support their application techniques, which will have important academic and practical value.

将纳米颗粒加入基液中形成的纳米流体能显著提高流体的传热性能，在航空航天、能源、电子、化工等领域有广阔的应用前景。由于纳米流体的传热效果与其流动状态密切相关，掌控其流动稳定性是纳米流体应用中的关键技术。当前国内外学者高度关注纳米流体的传热及其非牛顿流体特性，但是对工程中广泛存在的各种平行剪切流及其流动稳定性尚未开展深入研究。本项目拟采用理论分析结合数值计算的方法，分析非牛顿纳米流体的平行剪切流及其稳定性：1、用非均匀平衡流体介质模型描述纳米颗粒运动与热质传输的耦合；2、用非牛顿流体模型描述流体的流变特性，并考虑温度分布导致的黏性分层；3、用线性和非线性稳定性理论结合直接数值模拟，分析扰动的长期、瞬态及非线性演化的情况，揭示流动失稳的物理机制，获得控制流动稳定性和传热效果的有效方法。研究结果有望深化对纳米流体流动稳定性的认识，为提升其应用技术水平提供理论支持，具有重要的学术意义和工程实用价值。

本项目采用理论分析结合数值计算的方法，研究了纳米流体在管道和界面流动中的热质传输和流动稳定性规律。我们以纳米流体管道流动的对流换热为应用背景，建立了管壁温度沿流向线性变化的纳米流体平面Poiseuille流动模型，发现：加入纳米颗粒会造成显著的粘性及导热系数分层；在对流冷却中纳米颗粒会在管道截面内呈正态分布；在对流加热中纳米颗粒会显著降低流速，并对温度场及传热效果产生重要影响。在界面流动方面，我们考虑了液面温度梯度引起的热毛细对流、界面上颗粒浓度不均引起的溶质毛细流动和电场驱动的电毛细对流。在热毛细对流中，我们考虑了多种非牛顿流体的双自由面液层以及附壁液滴的热毛细迁移，结果显示重力效应的加强会明显抑制表面波的出现，但几乎不影响对流失稳；弱弹性只改变临界数，强弹性会使得最优模态从对流失稳变为表面波。在硅油当中添加纳米颗粒形成的弱导电液可以在外电场驱动下产生电毛细对流，模态分析表明流场是线性稳定的；非模态方法显示流场对初始和外加扰动都存在显著放大。本项研究的成果对于理解纳米流体流动的热质传输规律和流动失稳机理具有重要的理论价值，对于发展微电子、电动汽车等领域的高效热管理技术具有重要的应用价值。

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