四氧化三鐵奈米流體在磁場作用下之黏度實驗探討

Abstract

奈米流體為具有均勻分散且穩定懸浮奈米粒子的懸浮液。近二十年來最廣被探討的應用為增加流體的傳熱效果，近日研究也轉向其他方面的應用發展。過去的文獻指出奈米流體會因為粒子的聚結，而使熱傳導係數和黏滯係數大幅增加。本論文以實驗方式探討外加磁場對奈米流體黏度的影響、並進而研究是否可對黏度以磁場作出操控。 本文選用四氧化三鐵奈米粒子和兩種黏度高的基底流體，機油(engine oil，簡稱EO)與真空泵油(vacuum pump oil，簡稱VPO)，在沒有添加表面活化劑下，以二步法來配制磁性奈米流體；並在商用轉子式黏度計量測套筒外纏繞金屬線圈以電磁方式產生所需磁場來進行實驗。但文獻顯示此一裝置在施加磁場約100高斯時，其所產生的熱效應所降低的奈米流體黏度已和因磁場效應所增加的流體黏度相當。針對此點本文設計及製作成功一項冷卻系統可將熱效應去除、而能探討純屬磁場效應對黏度的影響。針對磁性奈米流體本文所得到的結論如下：(1)因基底流體黏度較高，Fe3O4-EO奈米流體較Fe3O4-VPO奈米流體穩定。 (2)無磁場作用下，奈米流體黏度會隨著溫度的增加而下降、隨著體積分率的增加而上升。(3)磁性奈米流體的黏度在低粒子體積分率(約1%)及中度電場強度(約100 Gauss)下會較其基底液體的黏度高出一倍。(4)奈米流體受到磁場作用後黏度會升高，關閉磁場後仍然可以回復到原來的黏度，且反應靈敏，因此奈米流體黏度可用磁場此一非接觸方式作有效的調控。(5)磁場讓粒子聚集會加速沉澱狀況。(6)奈米流體在施加磁場後剪切稀化的非牛頓流體效應會益趨明顯。

Nanofluid is a liquid suspended uniformly and stably with nanoparticles. Its application in heat transfer enhancement was studied extensively for the past two decades, and the focus has been shifted recently to other applications. Both the thermal conductivity and viscosity of nanofluid can be increased significantly because of the particle agglomeration. The work of this thesis is to study experimentally the effect of an applied magnetic field on the viscosity of nanofluid, and investigate the possibility of manipulation of viscosity via the magnetic field. The nanofluids in this study were synthesized using two-step method, with Fe3O4 nano particles dispersed in two viscous base liquids, engine oil (EO) and vacuum pump oil (VPO), without using surfactants. The magnetic field was applied electrically through a coil around the cylindrical test chamber of a commercial Searle viscometer. However, it was found that Joule heating is significant for such a device even for a moderate magnetic field around 100 Gauss. The decrease of viscosity associated with the Joule heating is of the same order as the increase of viscosity associated with the magnetic field. A cooling system was thus designed and fabricated successfully in this study for removing the heat, so that the sole effect of magnetic field on viscosity can be studied. The findings for magnetic nanofluids in this study are as follows. (1) Fe3O4-EO nanofluid is more stable than Fe3O4-VPO nanofluid because of the higher viscosity of engine oil. (2) The viscosity of nanofluid increases with volume fraction of the suspended particles and temperature of the fluid in the absence of magnetic field. (3) The viscosity of nanofluid can be doubled by adding a small amount of nano particles (1 % volume fraction) at a moderate applied magnetic field (100 Gauss). (4) The viscosity of nanofluid responses sharply to the applied magnetic field. It can be increased by applying a magnetic field, and recovers after the field has been shut down, suggesting that the viscosity of nanofluid can be controlled effectively via the magnetic field. (5) The agglomeration of particles, and thus the settling, can be enhanced by the applied magnetic field. (6) The shear thinning effect associated with non-Newtonian behavior for the present nanofluids is more obvious because of the applied magnetic field.