重力驅動顆粒流體之流動行為研究;Study of granular flow behavior in gravity-driven flows

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Title:	重力驅動顆粒流體之流動行為研究;Study of granular flow behavior in gravity-driven flows
Authors:	鄒仕豪;Chou,Shih-hao
Contributors:	機械工程學系
Keywords:	顆粒流;重力驅動流;旋轉儀;滑道;granular flow;gravity-driven flow;rotating drum;inclination chute
Date:	2012-11-15
Issue Date:	2012-12-25 13:39:47 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文主要在研究與探討重力驅動的顆粒體之運動行為。其中，包括漿態旋轉儀中的顆粒之尺寸分離機制、顆粒的傳輸性質與所造成的不同流態。此外，並探討濕系統下顆粒之間因為液橋現象造成的內聚力對顆粒運動行為的影響。最後本論文也針對顆粒在傾斜滑道中高速流動下時，若遇到阻礙物時所產生的震波行為加以探討。在漿態旋轉儀中，若同時存在兩種不同尺寸大小的顆粒時，將產生因為滲透效應形成的核心分離情形。由實驗結果中我們可以發現，隨著間隙流體黏度的增加或是顆粒填充率的減少，都將導致分離指標的降低。其中，當間隙流體黏度超過一定值後，我們更可以發現到，在分離的過程中，分離指標將急劇的降到一個極低值再開始回升，此現象稱之為過混合行為(overmixing)。另外，在此研究中我們更定義了一個新的無因次流動參數，藉由這個參數，我們可以很清楚的用來區分兩種不同的顆粒流動形態。當此無因次流動參數低過一定值後，流態將從滾動型態(rolling regime)轉變至翻滾型態(cascading regime)。在漿態旋轉儀中顆粒傳輸性質與流態變化的研究方面，同一時間我們只採用單一性質的顆粒體在旋轉儀中。藉由幾個分析參數對間隙流體黏度的變化，我們可以定義出在漿態旋轉儀中有著四個不同的流態:滾動型態、翻滾型態、激流型態(cataracting regime)和懸浮型態(suspension regime)。從實驗結果中我們可以發現，在低黏度下，所有顆粒的平均重心與容器中心的距離和初始配置下的距離是很接近的，但是當間隙流體黏度增加後，將產生劇烈性的變化。此外，在系統中所有顆粒的平均速度隨著間隙流體黏度的變化方面，整體顆粒平均速度呈現先減速後加速的行為。最後我們更以間隙流體黏度和單位寬度下的顆粒流率為參數，借此畫出漿態系統中的流態分佈圖(regime map)。此外，藉由一系列添加微量液體於顆粒流場的實驗，我們可以用來探討濕粒子間液橋力對顆粒流動行為的影響。由實驗結果中指出，當添加的液體量很少時，因為液體將先填滿顆粒表面的凹凸不平處，因此在顆粒之間將沒有液橋的產生。當添加的液體量超過一定值後，每顆顆粒之間都將產生液橋。此外，隨著添加液體量的增加，顆粒運動過程中液橋的建立與斷裂所造成的能量損耗也會增加。最後本論文也利用離散元素法(DEM)來針對在傾斜滑道中顆粒撞擊阻礙物所產生的震波現象加以研究。我們發現藉由離散元素法所得到的模擬結果和傳統的斜震波理論有著相當好的一致性。另外，在此研究中我們更進一步的去探討實驗中較難得到的顆粒微觀性質，如堆積密度(packing density)和配位數(coordination number)。 The main topic in this thesis is to investigate granular dynamics under gravity-driven forces. The research topics include particle size segregation and flow regime map in a slurry rotating drum, cohesive force between particles in wet rotating drums, and shock behavior of rapid granular flow down an inclined chute. Firstly, in binary-mixture slurry granular systems, we find that the time revolution of the segregation index indicates that an increase in liquid viscosity and a decrease in filling degree will cause the segregation index to decrease. When the liquid viscosity is above a critical value, many systems rapidly decay to a local minimum after a short period of time (called “overmixing”). A new dimensionless flow variable is proposed to be used to distinguish the flow regimes. We also find that the flow regime changes from rolling to cascading when the dimensionless flow variable is below a critical value. Furthermore, the change of the segregation index occurs during the transition of the granular flow regime. In mono-disperse slurry-granular systems, the distance between the centroid of all particles and the center of the rotating tank is the same as in the initial configuration at lower rotation speed or liquid viscosity, but the distance decreases rapidly when the liquid viscosity is above a critical value. The mean flow velocity will decrease with an increase of the liquid viscosity. When the liquid viscosity is above a critical value, the mean flow velocity will increase, and the granular flow behavior will transform into a suspension regime. Furthermore, the experimental results indicate that the liquid viscosity and the flow rate per unit width have a significant influence on the dynamic properties and flow behavior of the immersed granular matter. To quantitatively determine the effect of the cohesive force on the dynamic properties of wet granular systems is one research topic in this thesis. A series of experiments is performed for wet granular matter in a rotating drum. The results indicate that when only very small amounts of liquid are added, no liquid bridges are formed. This is because the liquid is first trapped on the surface of the particles due to the particle roughness. When the volume fraction of the fluid becomes larger, liquid bridges formed on almost every particle. Once the liquid bridges are formed between all particles, the average energy dissipation due to the hysteretic formation and rupturing of the liquid bridges increases with an increase in the liquid content. When a rapid avalanche flow is deflected by an obstacle, this usually causes abrupt changes in the flow thickness and velocity and exhibits characteristics like oblique shock waves in the aerodynamic system or oblique hydraulic jumps in the gravity-driven granular flows. We use the Discrete Element Method (DEM) to simulate the motion of granular materials impinging on a wedge obstacle in an adjustable inclined chute. The results of the simulations are compared with the classical oblique shock theory. We note that there is good agreement between the theoretical calculations and the DEM simulation results. Moreover, the microdynamic variables, related to the flow structure, such as the packing density and coordination number, are also discussed in the present study
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