以煤油為基底磁性奈米流體磁黏效應的實驗探討

Abstract

本文設計並製造一項狹縫式黏度計，可成功地用於研究低黏度(1 cP量階)磁性奈米流體的磁黏效應，並以“四氧化三鐵-煤油”奈米流體進行實驗加以佐證。就煤油及奈米流體，此狹縫式黏度計所量測結果與商用轉子式黏度計所量結果相符。 主要成果如下：(i)本狹縫式黏度計其量測原理按矩形截面直管內全展流之分析解，在壓克力板上以精密加工製作出流道。操作時以定流量幫浦驅動，以毛細管液柱高度差量測壓力降，進而推算出黏度。在施加磁場方面，採用兩片永久磁鐵置於流道外上下或左右以產生垂直或平行於流場渦度的磁場，並藉調整磁鐵間距離以產生不同的磁場強度。(ii)以化學共沉澱法製造四氧化三鐵奈米粒子，並以油酸包覆，再分散至煤油中，以合成穩定的“四氧化三鐵-煤油”奈米流體。其中粒子單體粒徑約9 nm，粒子懸浮在煤油中的平均粒徑約30 nm。(iii)以磁化儀量測不同體積百分率(1 – 4%)奈米流體，在不同磁場強度(200 – 500 Gauss)下所相對應的磁化強度。其磁化強度隨磁場強度與體積百分率上升而上升。(iv)在磁黏效應方面，就不同體積百分率(1 – 4%)、磁場方向、磁場強度(200 – 500 Gauss)、流場剪變率(70 – 189 s-1)、流道截面寬高比(8或12)、及流體合成後的量測時間點等均進行了量測，所獲結果如下：(1)磁黏效應隨體積百分率及磁場強度增加而增加。在剪變率70 s1、體積百分率4%、磁場方向垂直於流場渦度、及磁場強度500 Gauss (中低強度磁場)下，因磁場效應引致之黏度增益約68%。(2)磁場方向垂直於流場渦度時黏度增益為磁場方向平行於流場渦度時的3 4倍。(3)剪切稀化效應明顯，黏度增益的降幅隨體積百分率及剪變率之增加而增加。就體積百分率4%之奈米流體，當磁場方向垂直於流場渦度時，其黏度增益因剪切稀化效應從剪應變率70 s1的59%降為剪應變率189 s1的33%。(4)在流體合成三週後黏度增益約有六成的降幅，顯示流體有一定程度的老化現象且流體在經過三週後達至穩態。(5)流道截面之寬高比對磁黏效應與剪切稀化效應影響不大。

A slit viscometer was designed and fabricated, which could be applied for studying the magneto-viscosity of low viscosity (about 1 cP) magnetic nanofluids. The validity of the measurement using the slit viscometer was checked against measurement using a commercial Brookfield viscometer with kerosene and Fe3O4-Kerosene nanofluids. The main findings are as follows. (i)The theory of the slit viscometer is based on the analytical solution of the steady fully-developed flow in a square straight channel. The channel was fabricated on an acrylic sheet using precision machining. The test fluid was pumped through the channel using a syringe pump. The viscosity was accessed through the pressure drop in the channel measured via a manometer. The magnetic field was applied through two permanent magnet plates sandwiching the channel from the top and bottom wall (for generating field perpendicular to the flow vorticity) or from both side walls (for generating field parallel to the flow vorticity); and the field strength was adjusted by varying the distance between those two magnets. (ii) Fe3O4 nano particles were generated using chemical co-precipitation method, coated with oleic acid, and then dispersed into kerosene for synthesizing stable nanofluids. The average diameter of the particle monomer was measured as 9 nm, and the average diameter of the suspended particles in nanofluid is 30 nm. (iii) Magnetization curves of nanofluids at different volume fractions (1 – 4%) were measured, and it was found the magnetization increases as both the volume fraction and the magnetic field strength increase. (iv)As for the magneto-viscosity, we have performed measurements using different volume fractions (1 – 4%), magnetic field directions, magnetic field strengths (200 – 500 Gauss), shear rates (70 – 189 s-1), aspect ratio of the channel (8 or 12)、and the times for measurement after the fluids were synthesized. We found: (1) Viscosity enhancement increases as both the volume fraction and the magnetic field strength were increased. For example, the viscosity increase was 68% for a shear rate 70 s1, a volume fraction 4%、and moderate field strength 500 Gauss applied perpendicular to the flow vorticity. (2) The viscosity increase when the magnetic field is perpendicular to the flow vorticity is about 3 – 4 times greater than that when the field is parallel to flow vorticity. (3) Shear thinning is obvious; the reduction of the viscosity increase increases as both the volume fraction and shear rate increase. For example, the 59% viscosity increase at a shear rate 70 s1 is reduced to 33% at 190 s1 for a fluid with volume fraction 4% under a field applied perpendicular to the vorticity at 500 Gauss. (4) Viscosity increase was reduced by about 60% and would be stable three weeks later after the fluid has been synthesized. (5) Effect of aspect ratio of the channel on magneto-viscosity and shear thinning are minor.