以暫態熱線法量測奈米流體的熱傳導係數

Abstract

奈米流體是液體中穩定懸浮有奈米粒子(粒徑 1 - 100 nm)的懸浮液。前人研究指出奈米流體的熱傳導係數會因奈米粒子的加入而明顯高於原液體(稱基底流體)者，而文獻中最當採用量測奈米流體熱傳導係數的方法為暫態熱線法。本文主要的目的即為自行建立一套暫態熱線量測設備、並予以驗證，及探討不同狀况下的奈米流體的熱傳增益。 經與文獻中多項實驗結果的比較，本研究所發展的暫態熱線量測設備應能合理量測出奈米流體的熱傳導係數。再者，就裝置架設及量測過程方面，除文獻中所述外，本研究並獲致一些如下實作心得。實驗中的惠斯通電橋可使用低電阻進行平衡配置，可解決輸出阻抗過高而導致的雜訊問題；而使用細長的白金線可提高熱線電阻，對於輸入電壓的校正有莫大幫助；本實驗的訊號截取和雜訊處理也必須謹慎，使用DAQ介面卡時要維持原訊號不失真的情況下擷取數值，最好的方法是對原始訊號均值處理後再儲存。 就奈米流體熱傳導係數的研究上，本文針對不同奈米粒子及基底流體，於不同體積分率下進行熱傳導係數的量測、並與文獻相比較後，得到以下結論。無論基底溶液、奈米粒子為何，隨着體積分率增加，熱傳導係數上升；將二氧化鈦奈米粒子加入乙二醇中配置的奈米流體，其熱傳增益會小於二氧化鈦與離子水所配置的奈米流體者。另因設備為自製者，便於進行改裝以各項相關研究，據此本文亦在裝置試體的容器外加裝可通電線圈，以探究磁場效應對奈米流體熱傳導係數的影響，以Fe3O4-Engine Oil奈米流體進行的初步實驗顯示磁場效應確有助提升熱傳導係數。

Nanofluid is a liquid suspended stably with nanoparticles, whose sizes are from 1 – 100 nm. It was found in the literature that the thermal conductivity of nanofluid is substantially greater than that of the original liquid (called the base fluid), because of the introduction of nanoparticles. A common technique for accessing the thermal conductivity of nanofluid is the transient hot wire method. The goal of this thesis is to build up a transient hot wire device, and apply it to study the thermal conductivity gain of nanofluids under different situations. The transient hot wire device developed in this study was validated by comparing its measurements with several existing experimental data for wide ranges of experimental parameters。Furthermore, we found the followings which could be helpful for performing the experiments other than those stated in the literature. Low resistance resistors can be applied for the circuit of the Wheatstone bridge for avoiding the noise associated with high impedance output, long and thin platinum wire is helpful for calibrating the input voltage, and DAQ interface card for data acquisition can help to maintain the original signal undistorted. It is better to take the average of several data scanned before storage. For the thermal conductivity of nanofluids, we have performed measurements for different nanoparticles (TiO2, Al3O2, Fe3O4), base fluids (de-ionized water, ethylene glycol (EG), engine oil), and volume fractions for validation of the device. Based on the present measurements as well as the findings in the literature, we conclude: (1) the thermal conductivity of nanofluids increase with the volume fraction, irrespective of different nanoparticles and base fluids. The TiO2-water nanofluids perform better than TiO2-EG nanofluids, regarding the enhancement of thermal conductivity associated with the application of nanofluids. As the device is assembled in our laboratory, vary modification of the device can be easily implemented for different research. Electric wires were coiled around the test tube of the present-developed device, for accessing the magnetic effect on thermal conductivity of nanofluids. Preliminary experiments were performed using Fe3O4-Engine Oil nanofluids, and it was found that the thermal conductivity can indeed be increased via an applied magnetic field by passing electric current through the coil.