剪力顆粒流中密度分離效應對顆粒環沉降的影響

The effect of density segregation on the sinking of particle ring in a shear granular flow

胡育明(Yu-ming Hu)

指導教授 ： 蕭述三

國立中央大學/工學院/機械工程學系/碩士(2011年)

摘要

本文主要是探討不同密度之顆粒體於類二維剪力槽系統中，因密度分離效應所造成之顆粒環的沉降行為。並以實驗的方式，分析剪力槽內底盤速度之快慢、粒子體積佔有比之大小以及浮力效應對顆粒環沉降行為的影響。其中浮力效應意指不同密度之顆粒體因重力效應的影響，而形成密度大之顆粒向下擠壓，密度小之顆粒向上堆積的顆粒分層現象。另外，為了更清楚地分析及比較不同條件下顆粒環沉降的狀況，故本文特別定義了無因次沉降深度及沉降速率兩個參數，分別針對顆粒環位置的變化及速率進行討論。
本研究的實驗結果顯示，無因次沉降深度與沉降速率兩者，皆會隨著粒子體積佔有比與底盤轉速的增加而提升，且兩者之間為正比的關係。另一方面，於相同實驗配置下，無因次沉降深度的值會隨著密度比的增加而上升。而於不同的實驗配置下，顆粒環的沉降行為則是會受到追蹤粒子整體重量的大小以及顆粒之間力鏈(Force Chain)結構的影響。此外，當密度比較大時，沉降速率也會隨之增加，這是由於不同密度之顆粒所受重力效應的差異較大的原故。最後本實驗也發現，當密度比小於1.57時，顆粒環會因為密度分離效應的不足而產生崩散，使得系統內之輕重顆粒產生均勻混合的狀態。

關鍵字

剪力槽 ； 浮力效應 ； 沉降速率 ； 無因次沉降深度 ； 粒子流 ； 分離現象

並列摘要

This study investigates the sinking behavior of particle ring due to the density segregation effect in a quasi-2D Couette shear cell device. The influences of bottom wall velocity, solid fraction of granular material and buoyancy effect are studied experimentally. Here the “Buoyancy effect” means the heavier particles sink to lower levels in the flowing layer while lighter ones rise due to the effects of gravity. Additionally, the parameters of the dimensionless sinking depth and sinking rate are defined to describe the change of particle ring’s position and quantify the sinking speed of the particles respectively.
The experimental results show that both the dimensionless sinking depth and the sinking rate increase with increasing the bottom wall velocity and solid fraction, and the linear relation is also observed between the dimensionless sinking depth and the sinking rate. On the other hand, in the case of the same experimental configuration, the dimensionless sinking depth will increase as the density ratio increases. However, the sinking behavior of particle ring will be affected by the overall weight of tracking particles and the force chains inter particles in different experimental configuration. The result also show that the sinking rate increase with increasing the density ratio due to the gravity effect. Finally, we found that the particle ring structure cannot be maintained due to the weak density segregation effect when density ratio is less than 1.57, and the binary mixture becomes the homogeneous mixing state in the granular system.

並列關鍵字

Granular flow ； Segregation ； Shear cell ； Sinking rate ； Dimensionless sinking depth

參考文獻

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