脉动波对纳米颗粒运动及强化传热的作用机制

The influence mechanisms of pulsating waves on nanoparticle response frequency and pulsating amplitude inside nanofluid as well as on the mean square deviation of nanoparticle distribution are studied in a deep-going way through experiments with high-speed photography, micro-PIV and thermal imaging technologies, and through numerical simulations based on lattice Boltzmann method (LBM). In addition, the two phase heat transfer enhancement of nanofluid under pulsating waves is investigated in detail with special efforts to distinguish different pathways of heat transfer enhancement, and to quantify the contributions of each pathways. Then, the transfer functions correlate either the effective thermal conductivity or the effective viscosity of nanofluid and the nanoparticle movements are developed.As a result,the underlying physical mechanisms for the heat transfer enhancement of nanofluid by pulating waves are revealed through the analysis of above transfer functions. On the other hand, multicomponent Lattic Boltzmann Method (LBM) is employed to simulate nanofluid flow and heat transfer inside a laboratory scale of computational zone. Various types of forces between the carrier liquid and nanoparticles are taken into consideration, and large eddy simulation method is combined into the LBM model.The effect of turbulence on the heat transfer results of nanofluid flow is explored, and the large eddy simulation model is improved for the problem nanofluid flow and heat transfer under pulsating waves. The results obtained in this project provide empirical formulas and theoretical models for nanofluid heat transfer enchancement by pulsating waves, which is important and very useful in the design and the applications of nanofluid.

本项目通过高速摄影、Micro-PIV和热成像等实验手段以及格子波尔兹曼数值模拟方法，深入探索脉动波对纳米流体中纳米颗粒的响应频率、脉动振幅和纳米颗粒浓度分布均方差的作用机制；详细研究脉动波强化纳米流体两相传热的规律，区分其强化传热途径以及各个途径的贡献度，进而构建脉动波作用下纳米流体有效导热系数、有效黏度与纳米颗粒运动规律的传递函数，揭示脉动波作用下纳米流体强化传热的内在物理机理。此外，基于多组分格子玻尔兹曼方法，充分考虑纳米流体基液和纳米颗粒之间的各种相互作用力，同时结合大涡模拟方法，详细模拟实验室尺度的计算区域内的纳米流体在脉动波作用下的两相流动和传热特性，探索湍流效应对数值模拟结果的影响程度，修正脉动波作用下纳米流体两相流动和传热的大涡模型。研究结果为深入了解脉动波强化纳米流体传热提供经验公式和理论模型，对纳米流体的开发应用具有重要的指导意义。

提高换热器换热性能的核心理念是在固有的限制条件下最大化地提高换热系数，从而达到节省空间、节减材料和节约能源。本项目采用实验研究和数值模拟方法深入探索了脉动波对纳米颗粒运动及其强化传热的作用机制，获得纳米颗粒粒径、体积浓度、脉动振幅、脉动频率与对流传热性能的相互耦合规律。. （1）研究了纳米流体制备研究方法，制备了不同种类的纳米流体，研究发现采用超声手段分散纳米颗粒时，存在最佳连续超声时间。其次，研制了纳米流体导热系数测量装置，可高效准确地测量纳米流体的导热系数，发现热线温升及测试时间的选择是影响导热系数测量精度的重要因素。在最优的测量条件下，水和乙二醇导热系数的测量误差为1.2%和1.3%，与理论计算结果接近。（2）搭建了多通道扁平铝管内纳米流体对流换热性能的实验研究台架，实验发现对流换热系数随纳米粒子体积浓度和Re的增加而增大，此外还发现粘度对纳米流体传热性能的影响超过了导热性的影响。在施加脉动后，在相同的脉动波频率和振幅条件下，流量越小，总传热系数增强幅度越大。（3）纳米流体换热性能的数值模拟研究，解释了纳米流体强化传热的机理，发现必须考虑布朗运动效应，同时发现粒径是影响纳米流体传热和水力性能的重要因素。（4）开发了二维和三维LBM热流耦合程序，研究了脉动流的传热特性，发现粘性应力的改变与强化传热呈现出正相关性，与实验规律一致。（5）开展螺旋管内脉动波强化传热的数值模拟研究，发现脉存在一个最佳脉动波频率，且Re数越小，最佳脉动波频率越小。脉动波频率越小时，对流换热系数的脉动振幅越大。螺旋管内纳米流体脉动波强化传热主要归于螺旋管横截面中产生的二次流和轴向上形成的反向旋转涡流。. 自本项目立项以来，共发表期刊论文11篇，全部为SCI收录论文，其中2018版中科院大类分区1区论文1篇，2区论文3篇。以本基金号作为第1标注的期刊论文4篇，以本基金号作为第2标注的期刊论文7篇。