奈米流體中粒子的聚結及其對熱傳導係數的影響

Abstract

奈米流體乃是懸浮有奈米尺寸粒子的液體，文獻中對此流體進行研究其中目的之一是為了開發出具有更佳散熱效果的冷卻劑，而其中奈米粒子可為金屬或非金屬材料、且形狀不拘。然而隨著時間演變，奈米粒子會由於本身布朗運動(Brownian motion)的關係而不斷相互碰撞，聚結成更大顆的粒子，導致沉澱的現象發生而失去其原本的效用，故本論文旨在探討奈米流中粒子的聚結現象及其對熱傳導係數的影響，共進行以下三項研究：(1)粒子平均粒徑大小隨時間的演變行為，(2)聚結效應對奈米流體熱傳導係數的影響，(3)聚結過程中粒子粒徑分布的變化。就第(1)項，我們使用碎形模式(fractal model)來預測平均粒徑的變化、並對粒徑及界達電位(zeta potential)進行實驗量測，發現碎形模式理論無法很好地預測到實驗量測到的粒徑大小；但如採用Lei等人考量粒子微觀運動的聚結理論，則有較合理的結果。故我們將實際量測到的粒徑大小作為當前的粒徑，採用Prasher等人的理論就第(2)項奈米流體的熱傳導係數進行計算，對於二氧化鈦奈米流體在體積分率為1%、1.5%及2%時，理論計算與實驗結果相互吻合，並發現奈米流體中粒子適當的聚結對其散熱方面是有幫助。就第(3)項粒徑分布方面，我們考慮在粒子質量守恆下的一維通用動力方程式(general dynamic equation)，並利用粒子隨體積呈對數常態分佈(log-normal distribution)的關係，將其簡化成一常微分方程式，透過克蘭克-尼可森(Crank-Nicolson)法將其時間離散化進行求解。在上述理論模式的聚結項中，我們僅考慮由布朗運動引致的布朗聚結(Brownian coagulation)行為，並從數值計算與實驗結果得知，奈米流體的初始總粒數濃度、粒子間作用力及溶液離子強度等皆為影響粒子聚結的重要因素。 以上這些理論分析模式除了讓我們更了解奈米流體中粒子聚結背後的物理作動機制外，希望本論文對日後欲投入相關研究的學者或是奈米流體的應用方面上也有一定的幫助。

Nanofluid is a liquid suspended stably with nano size particles, and it attracts many research efforts because it has a potential to be an excellent coolant. The nano particles could be metal or non-metal, and of different geometric shapes. Because of the Brownian motion, the particles could collide one another and form agglomerates with time, and might precipitate. In such a situation, the nanofluid loses its ability of heat transfer enhancement. The goal of this thesis is to study the particle agglomeration and its effect on thermal conductivity of nanofluid, and focus on: (1) the time evolution of the particle agglomerates, (2) the effect of particle agglomeration on thermal conductivity, and (3) the size distribution of particles during agglomeration. For item (1), we employ the fractal model in literature for predicting the time evolution of average particle size, and measure the average diameter and zeta potential of the particles. The fractal model fails to predict the time evolution of particle diameter, but a recent model by Lei et al. based on the microscopic motion of particles does predict correctly the experimental findings. Thus the experimental particle diameters, instead of the diameters predicted using fractal model, were incorporated with the model of Prasher et al. for item (2) for predicting the thermal conductivity. The theoretical results agree nicely with the experiments of TiO2-water nanofluids for volume fraction from 1 – 2%. It was found that particle properly agglomerate in nanofluids, which is helpful for its heat transfer. For item (3), simulations were performed based on the one-dimensional general dynamic equation subject to Brownian coagulation, together with the log-normal distribution assumption for particles, and the Crank-Nicolson scheme. It was found that the initial particle number concentration, the interaction forces between particles, and the ionic strength of the nanofluid are crucial for particle agglomeration. It is hoped that the present study can provide us a better understand of the physics of particle agglomeration in nanofluids, which is helpful for further research and application.