Effect of flow field heterogeneity in coagulators on aggregate size and structure

Abstract

Aggregate size and structure were investigated under turbulent conditions in stirred tank (ST) and Taylor-Couette-type (TC-type) devices. Root-mean-square radius of gyration, hRgi, and zero-angle intensity of scattered light, I(0), were acquired as a function of stirring intensity, characterized by an experimentally obtained average hydrodynamic stress, hsi exp , determined by torque measurements. Evaluating aggregate images revealed that aggregate structure and shape are independent of the device type. However, in TC-type devices, the aggregates grow to three to four times larger sizes than inside ST, although the same hsi exp was used in both coagulators. As confirmed by computational fluid dynamics, this can be attributed to the differences in the maximum hydrodynamic stress in ST compared with those in TC-type devices. In contrast, the power-law scaling of hRgi and I(0) with hsi exp is preserved for all investigated devices, with an exponent approximately equal to À0.5 and À0.7, respectively.

Fractal Dimension of Cohesive Sediment Flocs at Steady State under Seven Shear Flow Conditions

“…This median size vs. time behavior depicts the flocculation process of type II, which is also composed of the three following phases: fast floc growth (referred to as type II-phase 1 herein), breakage and restructuring (type II-phase 2), and a steady state (type II-phase 3), as described in Section 1 of this paper. The type II-phase 1 presented in the figure could be approximately described as a simple linear increase process, as shown in several previous studies [9,11,20,21,55], rather than the exponential process reported by many authors [10,18,20,56]. Ehrl et al [17] attributed the difference between the linear and exponential trends of the fast aggregate growth phase to different particle sizes for which the relative importance of Brownian motion and shear-induced aggregations changed correspondingly [9].…”

“…Among various systems that could be used to generate a shear flow, such as an impeller mixer, an oscillating grid and the Couette apparatus [9,12,14,27,29], the Couette-flow system was used in this study because it can generate a more isotropic shear flow than any other devices and avoids the need to account for the impacts of turbulence heterogeneity on the flocculation process [18,23,30]. The Couette device consists of an inner cylinder with a radius of 150 mm and an outer cylinder with a radius of 236 mm and 682 mm tall.…”

“…In the first trend, the median size distribution of the flocs rapidly increases with time at the beginning of flocculation development and then continually slows down until a steady state is approached [7][8][9][10][11][12][13][14][15][16][17][18][19]. This type of size-time profile is henceforth referred to as type I.…”

Determination of maximum turbulent energy dissipation rate generated by a rushton impeller through large eddy simulation

“…[10][11][12][13][14][15][16][17][18][19][20] In parallel to the experimental activity, mathematical modeling or theoretical considerations have been used to describe the breakup of aggregates. 2,3,9,[21][22][23][24][25][26][27][28] The result of these studies is a scaling of the maximum stable size of formed aggregates as a function of the applied stress, or corresponding shear rate or energy dissipation rate e. The critical stress can be precisely evaluated in uniform flow fields under laminar conditions, e.g., couette flow or contracting nozzle.…”

Direct numerical simulations of aggregation of monosized spherical particles in homogeneous isotropic turbulence

“…Ehrl et al 50 observed in their experiments (with g K a [ [1) that the radius of gyration R g scaled with the energy dissipation rate e according to R g ! To determine these, of large ensembles of aggregates, size n agg , and radius of gyration R g were determined, and the relationship n agg ¼ a R g a d f was fitted.…”