Drag coefficient of a dispersion of non-homogenously structured flocs in a flow field
Date Issued
2004
Date
2004
Author(s)
Li, Ming-Che
DOI
zh-TW
Abstract
The influences of the structure of floc and its concentration on the hydrodynamic drag acting on it are investigated theoretically. Due to the fact that floc formation involves various nonlinear, random processes, its structure is of complicated nature. Here, a two-layer model is adopted to simulate the behavior of a floc where various possible floc structures are simulated by varying the permeability of each layer, and a cell model is used to model the behavior of a concentrated floc dispersion. Experimental evidence reveals that the Reynolds number associated with the sedimentation of floc can be on the order of ten, that is, the Stokes law is inapplicable. In this study, a Navier-Stokes equation governing the flow field outside a floc and a Darcy-Brinkman model describing the motion of fluid inside it are solved numerically based on a finite scheme. The influence of the key parameters of the problem under consideration, including the swarm voidage ε, the Reynolds number Re, and the permeability ratio k1/k2, k1 and k2 being respectively the permeability of the outer layer and the inner layer of a floc, on the drag acting on a floc are discussed. The results of numerical simulation reveal that the more heterogeneous the structure of floc, the greater the drag is. Also, the more heterogeneous the structure of a floc the more serious the deviation of the variation of drag against Re curve from a Stokes-law-like relation. The drag on a floc decreases with the decrease in its concentration, and, due to the convective flow of fluid, the distortion of streamlines between flocs becomes more serious, and the deviation of the variation of drag against Re curve from a Stokes-law like relation is more significant.
Subjects
非均勻結構
膠羽濃度分布
細胞模型
兩層模型
拖曳係數
two-layer model
drag coefficient
cell model
Concentrated floc dispersion
nonuniform structure
Type
thesis
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