Turbulent Blending

Mersmann [3] evaluated the measurement results of various authors and determined a demarcation line for favorable conditions. It is characterized by the 

EKATO s own measurements show that this equation forms a good basis for design calculations for impellers with larger diameter ratios ( 2/ 1 0.5). The following 

This equation only applies for hi jdi = 1 and single-stage impellers. 

The circulation characteristic of impellers is used for comparison purposes, especially in the English-speaking world, for assessing the performance of agitator impellers. The blend time is then derived from the number of times it is necessary to circulate the vessel contents to achieve homogeneity  

The empirical value z = 4 is often used for the number of times the vessel contents are circulated. 

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Turbulent Blending. The length downstream of the mixer is often referred to as the tailpipe. In laminar flow, no further mixing occurs in this region. 

Etchells, A. W., and D. G. R. Short (1988). Pipeline mixing a user s view turbulent blending, Proc. 6th European Mixing Conference, Pavia, Italy, pp. 539-544. 

Dimensionless pumping number and blend time are independent of Reynolds number under fully turbulent conditions. The magnitude of concentration fluctuations from the final well-mixed value in batch mixing decays exponentially with time. 

Blend time and ehemieal produet distribution in turbulent agitated vessels ean be predieted with the aid of Computational Fluid Mixing... 

Operations such as blending, solids-suspension, dissolving, heat transfer and liquid-liquid extraction are typical of systems requiring high flow relative to turbulence, while gas-liquid reactions and some liquid-liquid contacting require high turbulence relative to flow. The case of (1) 100% of suspension—requires head to keep particles suspended and (2) 100% uniformity of distribution of particles—requires head for suspension plus flow for dis-tiibution. 

Blend one grade of oil (or gasoline) into another oil. Turbulent 1 six-element module... 

Liquid-Liquid blending to a homogenous product Turbulent Flow 4-6... 

With injection mixers (Figures 10.52b,c), in which the one fluid is introduced into the flowing stream of the other through a concentric pipe or an annular array of jets, mixing will take place by entrainment and turbulent diffusion. Such devices should be used where one flow is much lower than the other, and will give a satisfactory blend in about 80 pipe diameters. The inclusion of baffles or other flow restrictions will reduce the mixing length required. 

The (isotropic) eddy viscosity concept and the use of a k i model are known to be inappropriate in rotating and/or strongly 3-D flows (see, e.g., Wilcox, 1993). This issue will be addressed in more detail in Section IV. Some researchers prefer different models for the eddy viscosity, such as the k o> model (where o> denotes vorticity) that performs better in regions closer to walls. For this latter reason, the k-e model and the k-co model are often blended into the so-called Shear-Stress-Transport (SST) model (Menter, 1994) with the view of using these two models in those regions of the flow domain where they perform best. In spite of these objections, however, RANS simulations mostly exploit the eddy viscosity concept rather than the more delicate and time-consuming RSM turbulence model. They deliver simulation results of in many cases reasonable or sufficient accuracy in a cost-effective way. 

Mixing processes involved in the manufacture of disperse systems, whether suspensions or emulsions, are far more problematic than those employed in the blending of low-viscosity miscible liquids due to the multi-phasic character of the systems and deviations from Newtonian flow behavior. It is not uncommon for both laminar and turbulent flow to occur simultaneously in different regions of the system. In some regions, the flow regime may be in transition, i.e., neither laminar nor turbulent but somewhere in between. The implications of these flow regime variations for scale-up are considerable. Nonetheless, it should be noted that the mixing process is only completed when Brownian motion occurs sufficiently to achieve uniformity on a molecular scale. 

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