以暫態熱面法量測奈米流體的熱傳導係數

Abstract

奈米流體是液體中穩定地懸浮有奈米粒子(1-100nm)的懸浮液，許多文獻指出在一定體積濃度(或稱體積分率)之下，熱傳導係數隨著體積濃度增加呈正相關，且根據其實驗結果皆高於傳統理論預測值，因此奈米流體的熱傳導機制以及此尺度之下電雙層、群聚效應、布朗運動的研究是相當重要的。本研究14nm、21nm、<50nm、<100nm四種粒子配置的奈米流體，以暫態平面熱源法量測其隨體積分率增加之熱傳係數與整理文獻結果相比較，發現其粒徑並無明顯影響關係，而與粒子的組成成分礦物來源晶形略有影響。另透過加入KCl及NaOH改變奈米流體導電度以及pH值，本文同時量測熱傳導係數以及黏度，兩者均會隨著導電度上升，但會趨於一定值；就pH的變化而言當pH約等於6(等電位點)附近時有峰值，黏度甚至上升至相對基底流體的幾十倍值，此現象乃是pH約等於6值時粒子間的電雙層斥力弱化甚至消失使得粒子間相互吸引至聚結。老化現象在低體積分率的奈米流體效應不明顯，而高體積分率的奈米流體相對於低的來得明顯，其熱傳導係數及黏滯度稍有下降。上述結果與之文獻定性相符，但本文使用了不同的熱傳導系數量測方法，且同時作了更多參數的量測。

Nanofluid is a liquid suspended stably with nano sized particles. It was found in the literature that the thermal conductivity of nanofluid increases as the volume fraction of particles increases, and the increase is substantially greater than that predicted by the classical theory. A large amount of effort was thus spent in studying the behind physics and chemistry, and various effects, including the electric double layer, particle agglomeration, and Brownian motion, were studied. The thermal conductivity of nanofluids with different monometer diameters (14, 22, <50 and <100nm) and volume fractions were measured in the present study using transient hot disk method. It was found that the effect of monomer diameter is small, and the detailed constituent of particles could have effort on thermal conductivity. Effect of electric conductivity and pH were also studied by adding different amount of KCl and NaOH into the nanofluids. The thermal conductivity increases as the electric conductivity increases, but tends to fltten out at large value. As for the effect of pH, the thermal conductivity attains its maximum around pH=6 (the isoelectric point), which can explained by the particle agglomeration effect associated with the weakening of the repulsive double layer force, and was supported by a viscosity measurement ; the nanofluid viscosity increases 2 folds in comparing with that of the base fluid at pH=6. The aging effect is insignificant when the volume fraction is low and is minor at large volume fractions. The above results are qualitatively similar to those in literature, but here we employed a different method for measuring the thermal conductivity, and provided results for a boarder ranges of parameters.