直渠道顆粒流之顆粒密度分離效應

本論文以實驗的方式研究不同密度相同尺寸(3 mm)之球形顆粒體，於傾斜直渠道中流動所造成的顆粒密度分離現象，並改變渠道傾斜角及顆粒密度比等實驗控制參數，初始狀態為兩種顆粒等體積均勻混合狀態，隨著重力驅動所造成顆粒崩塌流場，重顆粒向渠道底部擠壓，輕顆粒向上堆積至自由表面，最後達成輕重顆粒之完全分離狀態。本實驗針對渠道側面拍攝連續影像，量測流場中輕重顆粒二維濃度分布，並以體積權重的方式計算出輕重顆粒之速度及顆粒溫度場，以觀測輕重顆粒從混合狀態至分離狀態之運動行為以及相關物理機制。　　實驗結果顯示輕重顆粒分離程度，會隨著密度比增加而上升，且流場之傾斜角越低分離程度越佳，顆粒濃度純層(pure layer)沿著流動方向越來越厚，兩種顆粒之水平速度差隨著流動方向越來越大；垂直速度差在特定位置之前隨著流動方向越來越小，輕顆粒之顆粒溫度略大於重顆粒。在低平均福祿數時流場中產生靜止層(fixed layer)，我們也探討此現象對分離效應的影響。

關鍵字

密度分離；顆粒流；崩塌流場；二維濃度；體積權重速度

並列摘要

The aim of this work is to experimentally investigate how a binary granular mixture made up of spherical glass beads behave when flowing down a straight chute. The mixture was composed by equal sized (3 mm) particles but with two different densities. Four different inclinations combining with two different density ratios are tested. Initially, a binary granular mixture is uniformly mixed. The heavier particles sink to lower levels in the flowing layer while the lighter ones rise toward the top due to the effects of gravity and buoyancy. Finally, the flow becomes completely separated. Processing the sidewall, we are able to measure the evolution of the 2-D concentration and velocity profiles. The volume averaged velocity field of two particles, and granular temperature field are calculated. They help to understand the physical mechanism and dynamic behavior of particles form mixture to segregation. The results show that the intensity of segregation increases with increasing the density ratio of particles, and decrease with increasing the inclinations. The pure-layer, composed of a single constitution, thickens along the direction of flow. Streamwise velocity difference of two particles increases along the direction of flow. And, transverse velocity difference of two particles, before a specific location, decreases along the direction of flow. Granular temperature of lighter particles is slightly larger than heavier ones. Moreover, fixed layer generates if the average Froude number is small. The segregation effect in this state also discussed.

並列關鍵字

Density segregation ； Granular flow ； Avalanche ； 2-D Concentration ； Volume weighted velocity

參考文獻

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國際替代計量

直渠道顆粒流之顆粒密度分離效應

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