剪力流內重力對傳輸性質的影響

The influence of gravity force on transport properties in Sheared Granular Flows

李政勳(Jheng-Syun Li)

指導教授 ： 蕭述三

國立中央大學/工學院/機械工程學系/碩士(2008年)

摘要

本論文以操作實驗方法，使用二維直立式與水平式剪力槽裝置來研究重力對顆粒流動以及傳輸現象的影響。藉由使用粒子追蹤技術與影像處理方法來測量粒子水平方向與垂直方向的平均速度、擾動速度、粒子溫度、擴散係數等物理量。實驗設定為固定粒子佔有體積，改變四組轉盤速度，以及不同方位來拍攝顆粒流場：直立式的View 1、View 2、View 3，與水平式的horizontal來進行實驗。
本文中，重力對於顆粒流場十分重要。藉由使用直立式剪力槽，在固定整體粒子佔有體積的情況下，重力與粒子流動方向還是會導致三個觀測面的粒子堆積緊密度有些許不同：粒子由下往上流動的觀測面(View 1)的粒子堆積最緊密；粒子由上往下流動的觀測面(View 3)的粒子堆積最鬆散，粒子由右往左流動的觀測面(View 2)堆積程度則位於兩者之間。因此三個拍攝面的粒子會因為堆積緊密度的差異而導致相關物理量有差別，亦即View 3的平均速度、擴散係數會是最大，而擾動速度與粒子溫度最小，View 1則是和View 3相反，View 2介於兩者之間。
而在直立式與水平式剪力槽的研究中顯示，重力效應會影響顆粒運動與傳輸現象的強弱。直立式中，重力與傳輸方向垂直；水平式中，重力與傳輸方向平行且反向，導致水平式的運動與傳輸性質，如速度、粒子溫度、擴散係數等，皆小於直立式。由以上結果我們可得知，重力效應在粒子流的流變行為上，十分重要。

關鍵字

剪力槽 ； 重力效應 ； 顆粒流

並列摘要

Experiments were performed both in vertical and horizontal shear cell devices to investigate the influences of gravity force on the flow behavior and transport properties. The motions of the granular materials were recorded by a high-speed camera. By using image processing technology and particle tracking method, the average velocity and fluctuation velocity in the streamwise and the transverse directions could be measured and analyzed. We performed the experiments at four different wall velocities from four observational views (vertical : View 1、View 2、View 3 and horizontal : horizontal).
Even the bulk solid fraction is constant in the vertical shear cell, the local solid fraction is not homogenous due to the gravity force and flow direction. Therefore the velocity gradient and transport properties in the three observational views were different. Solid fraction in the View 1 is the highest where particles move upwards. Oppositely, View 3 has the smallest solid fraction and particles move downwards. In View 3, the average velocity and diffusion coefficient of particles are the greatest comparing with the other views. However the fluctuation velocity and granular temperature are the smallest in View 3.
Gravity force has a significant influence on transport properties. In the horizontal shear cell, the gravity force acts downward, opposite to the direction of transport properties, resulting in the reducing of the transport properties in the granular flows. However in the vertical shear the direction of gravity force action is perpendicular to the direction of the transport property, the velocity gradient and transport properties are smaller than those in the vertical shear cell.

並列關鍵字

gravity ； granular flow ； shear cell

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