纳米流体液滴碰撞壁面铺展动力学特性及碳纳米管分布取向机制的研究

The nanofluid is a class of fluids with high thermal conductivity and non-Newtonian flow behavior. The orientation of CNTs and the spreading characteristics of a nanofluid droplet impinging on the solid surface are key factors to efficient heat and mass transfer in related processes based on the deposition of droplets. However, the dynamic behaviors and characteristics of the nanofluid droplet haven’t been fully understood since the presence of the non-Newtonian fluid behavior and the interaction between the microstructure of CNTs and micro-flow field in the droplet which complicates the spreading process. In this project, we intend to obtain the evolution of droplet morphology and critical parameters of dynamics parameters during the droplet spreading by means of high-speed photography. By utilizing the light-curing technology and SEM scan, we will observe the morphology of CNTs in the droplet and analysis their distribution and orientation by probability statistics method. By analogizing the short fibers in suspension, we plan to derive a model to predict the orientation of CNTs in flow field and a constitutive equation to describe the rheological behavior of the nanofluid. With the numerical simulation, we intend to reveal the effects of non-Newtonian rheological properties and the initial impact conditions on the dynamic characteristics of the spreading. From a comprehensive analysis, we will elucidate the mechanism of coupling effect between the dynamic behaviors of micro-flow field and orientation of CNTs in the droplet. The results of this project will provide theoretical basis and specific guidance on designing enhanced heat transfer and cooling technology, as well as manufacturing micro-devices with high thermal-conductivity, which are based on the deposition of nanofluid droplet.

纳米流体是一类新型高导热非牛顿流体。纳米流体液滴碰撞固体壁面的铺展特性及液滴内碳纳米管的分布取向，是影响基于液滴沉积实现高效传热传质过程的关键因素。然而由于纳米流体的非牛顿流变特性及液滴内微流动与碳纳米管的耦合作用，目前对纳米流体液滴碰撞固壁铺展的动力学行为及碳纳米管分布取向缺乏足够的认识。本项目拟通过高速摄影的手段，获得纳米流体液滴碰撞壁面的铺展沉积形态及重要动力学特性参数的演变规律。通过液滴光固化技术及电镜扫描观测，结合统计学方法获得碳纳米管在液滴内的分布取向特征。基于短纤维悬浮体系理论建立碳纳米管在流场内取向预测模型及纳米流体流变学本构关系。结合数值模拟,揭示非牛顿流变特性及初始碰撞条件对液滴动力学特性的影响，认识并阐明纳米流体液滴内微流动流场与碳纳米管取向分布的相互作用机制。相关研究成果将为基于纳米流体液滴沉积的增益传热冷却技术及高导热微型电子器件制造提供理论依据和具体指导。

液滴撞击固体壁面的现象广泛存在于动力机械、喷雾冷却、薄膜材料沉积制备等工业领域。但是对于纳米流体液滴撞击固体壁面的研究尚少，纳米颗粒影响液滴撞击壁面行为的机制尚不明确。本项目利用超声波技术分别将碳纳米管、石墨烯和纳米石墨粉分散到高粘度的环氧树脂中，得到了均匀稳定的纳米流体。基于高速摄像技术捕捉了液滴撞击固体壁面的动态过程，利用图像后处理技术获得液滴撞击固体壁面过程中的关键实验参数，理论阐明了惯性力、壁面浸润性和纳米颗粒对液滴撞击壁面行为的影响机制。研究结果发现纳米颗粒的加入使牛顿流体的粘度增大并表现出剪切变稀特性，同时纳米颗粒会显著抑制液滴的铺展和回缩行为。基于有限元仿真模拟平台，采用水平集方法捕捉相界面的移动，利用幂律模型耦合纳米颗粒带来的非牛顿特性，建立了非牛顿流体液滴撞击固体壁面的数值模型。研究结果表明，对于采用幂律模型描述的非牛顿剪切变稀流体，随着幂律指数m的减小，液滴铺展过程中的粘性耗散减小，液滴铺展过程中的变化范围显著区别于牛顿流体液滴。基于流动诱导短纤维取向模型和取向张量的闭合近似理论，考虑碳纳米管间的相互作用，建立了预测碳纳米管随流动取向的数值模型，揭示了流动诱导的碳纳米管取向机制。以纳米二氧化硅颗粒为微结构材料制备了超疏水壁面，配制了表现出非牛顿剪切变稀特性的黄原胶溶液。研究结果阐明了黄原胶液滴撞击超疏水壁面后回弹，卷吸气体，以及液滴回缩过程中液体滞留壁面等现象的产生机制。本项目的研究结果为基于纳米流体液滴沉积的喷雾冷却及增材制造技术提供了重要的理论基础。

内容获取失败，请点击重试