Effect of magnetic field on the viscosity of nanofluids

奈米流體為具有均勻分散且穩定懸浮奈米顆粒之懸浮液。根據過去的研究指出奈米流體的熱傳導係數會因為顆粒的聚結增加，但其黏滯係數也會隨之增加。因此若有辦法在需要時使顆粒聚結，不需要時讓顆粒恢復均勻分散之狀態，則奈米流體性質將可藉外在因素加以操控、而有助作更廣泛的熱流工程應用。 本文使用四氧化三鐵奈米顆粒和機油配製磁性奈米流體，並針對現有黏度計進行改裝，將金屬線繞纏在黏度計測量容器外，透過外加電場以在容器内産生磁場，如此便可使用原黏度計來測量在磁場效應下奈米流體的黏滯係數，以探究外加磁場此一變因是否能有效地操控流體的黏度。根據本文實驗之結果，我們得到以下結論: (1)黏度隨著體積分率的增加而上升、隨著溫度的增加而下降，但相對黏度卻不隨溫度的增加而改變。(2)隨著作用磁場的強度增加，奈米流體的相對黏度也會跟著上升。(3)當所施加之磁場較低時，每次切斷磁場後流體黏度皆可回復至初始狀態；但若施加較高磁場時，則會因為電熱轉換使待測流體的溫度改變、而使流體黏度無法回到初始狀態。(4)在磁場長時間作用下，奈米流體的黏度變化會出現兩種趨勢，分別由溫度主導及由粒子聚結主導。且在長時間磁場作用後，懸浮顆粒確實會發生沉澱之現象，其沉澱程度會隨磁場強度增加而增加。此外，當流體溫度較低時，受磁場作用之相對黏度增加會較為強烈。希望本文的研究有助了解奈米流體、及推廣奈米流體的應用。Nanofluid is a liquid (called base fluid) suspended uniformly and stably with nano particles. The thermal conductivity and viscosity of nanofluid could be substantially greater than that of the base fluid, because of the particle agglomeration according to the literature. Thus, it would be beneficial for the application if we could control the particle agglomeration via an external means, such as an applied magnetic or electric field, outside the fluidic system. Fe3O4-Engine oil nanofluid was prepared using the two-step method, together with the modification of an existing viscometer (Brookfield, DV2T model) for the experiment in the present study. Metal wire is coilled around the cylindrical test tube of the viscometer, so that we can generate a magnetic field along the axis of the tube via an applied electric field, for accessing the effect of magnetic field on the viscosity of nanofluids. According to the present experiments, we found: (1) the viscosity of nanofluid increases with volume fraction but decrease with temperature. However, the temperature ratio between the nanofluid and base fluid remains unchanged as the temperature varies. (2) The viscosity of nanofluid increases as the strength of the magnetic field increases. (3) The viscosity can recover to the original state when the applied magnetic field is cutoff for relative low applied magnetic field, but cannot for relative high magnetic field because of the temperature rise associated with Joule heating. (4) Two results may occur for a long-term application of a magnetic field: the viscosity decreases because of the increase of the temperature, and increases because of the particle agglomeration. The degree of precipitation increases as the magnetic field increases, and the magnetic effect on viscosity is more feasible at lower temperature. It is hoped that the present study is helpful for understand the physics, and for a boarder application of nanofluids