Turbulent flow shear aggregation

The dynamics of reactor flow is also important for its effect on the crystal agglomeration, since the intensity of turbulent shear dominates the orthoki-netic mechanism for both processes of aggregation and disruption. The mean shear rate is estimated as (see Harnby etai, 1992)... 

Wang, L., Vigil, R. D. Fox, R. O. 2005b CFD simulation of shear-induced aggregation and breakage in turbulent Taylor-Couette flow. Journal of Colloid and Interface Science 285, 167-178. 

These surface vortices provide an efficient means for microfluidic mixing, as shown in Fig. 10 where a dye is rapidly mixed within several seconds. The mixing can be enhanced by inducing the vortex instabilities wherein turbulent-like mixing efficiencies are observed [18]. In addition, particles dispersed in the flow are also observed to be drawn into the vortices due to positive dielectrophoresis towards a point on the interface closest to the needle where the field is most intense. Once a sufficient particle concentration is achieved within the vortex, shear-induced migration leads to cross-streamline transport such that the interior of the vortex is populated [18], as shown in Rg. 11a. Upon relaxation of the electric field and hence termination of the flow, we observe the particle aggregate to remain intact, possibly due to van der Waals attraction (Fig. 1 lb). This therefore provides a mechanism for particle trapping and concentration. 

Particle and aggregate settling speeds are typically larger than the actual deposition rates of these particles onto bottom sediments. The total particle deposition flux to the bed J (g/m /s) (see Equation 10.7) is equivalent to the particulate matter settling flux modified by a probability of deposition that is almost always less than one. Turbulence at the interface may act as a barrier to the attachment of settling material. As shown in Figures 10.3 and 10.4, the probability of deposition is a function of shear stress. There is a range of bottom shear stresses for which the probability of deposition is less than one those particles that do not deposit will remain entrained or suspended in the flow. 

Instead of assigning different shear rates, he employed different breakage rate expressions for the two zones. The problem of coupling population balance models with fluid flow models has received some attention recently and coupled PB-CFD models have been developed for a wide variety of processes such as fluidization [70], gas-liquid reactions in bubble columns [71] and nanoparticle synthesis in flame aerosol reactors [72]. Complete description of aggregation in turbulent environments requires simultaneous solution of basic balance equations for mass, momentum, energy and concentration of species present along with population balances for particles/aggregates of different size classes. 

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