Impeller tip velocity

Vu2 = tangential component of the absolute velocity U2 = impeller tip velocity... 

Maximum shear rate, average shear rate and mixing time, impeller tip velocity, NDt. 

Scale-up calculation is based on constant shear forces, where shear is directly related to impeller tip velocity. Shear forces are defined as... 

The constant shear concept has been applied for bioreactor scale-up that utilises mycelia, where the fermentation process is shear sensitive and the broth is affected by shear rate of impeller tip velocity. For instance, in the production of novobicin, the yield of antibiotic production is dependent on impeller size and impeller tip velocity. 

Traditional scale-up rules Table 4.2 (These scale-up rules have been used for many types of equipment scale-ups, e.g., the stirred vessel for mixing of the fluids that have no reaction ability. It is clarified that the distribution of the dimensionless turbulent statistical values by making use of impeller tip velocity UT(= ND) becomes identical regardless of impeller rotational... 

Flow patterns in a mechanically agitated reactor with disk turbine, pitched-blade turbine, and propeller types of agitator are schematically illustrated by Joshi et al. (1982). The flow pattern in the presence of gas is described later in the section on slurry reactors. In each of these cases, the dimensionless velocity profile with respect to the impeller tip velocity has been found to be independent of the impeller speed and has shown slight dependence on the impeller diameter. 

In these runs, the presence of the Geiger-Mueller counter may affect the flow pattern significantly. Further, examination of the results presented shows that the radial velocities do not satisfy material balance requirements. Nevertheless, several of Aiba s conclusions are of interest. With baffles, the vertical circulation is large compared with the unbaffled case, but considerable tangential flow remains. In neither the baffled nor unbaffled case do the mean flow velocities approach the tip velocity of the impeller the ratio of fluid velocity to impeller-tip velocity is essentially independent of impeller rotation speed, varies... 

While this is dimensionally correct, there is not a great deal of published data to support it. We have seen that the mean fluid velocity near the impeller tip is related to, but less than, the impeller-tip velocity itself (Section II). Eqs. (49) and (50) together show that... 

The strong periodicity in the discharge flow velocity near the blade is caused by the trailing vortices shed by the impeller. In these vortices, the circumferential velocity is of the order of the impeller tip velocity, vt, and the... 

Abrasion is usually a major consideration in pump selection. Except for the mild chemical-type duties already referred to, slurry pumps are of special construction, essentially to resist abrasion to the pump and its shaft seals but often also to permit fast maintenance and liner or component replacement for very abrasive duties. Generally either replaceable rubber lining or specially hard abrasion-resistant alloys are used shaft seals are of special design, and are usually continually flushed with clean water. Impeller tip velocities are kept within proven limits for the application. Power transmission is often through V-belts rather than direct-drive couplings, to permit exactly the required speed to be obtained (rather than synchronous speeds) and to permit speed optimization in service (varying the impeller diameter is not so convenient). 

High-speed flows and vortices occur behind the impeller blades and remain coherent as the flow moves into the bulk. The velocities in these flows can be higher than the impeller tip velocity. Vortices are low-pressure regions that can coalesce lower-density materials and sometimes form gas pockets and cavities. Strong vortices are important for dispersion processes, while high velocity flows are desired for liquid blending and solids suspension. 

At high Reynolds numbers, the concepts of shear, mean shear, and the impeller rotational speed, N, become unimportant relative to the impeller tip velocity, ND. Viscosity is no longer the mechanism by which momentum is transferred. 

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