Power number significance

Figure 6-40 shows power number vs. impeller Reynolds number for a typical configuration. The similarity to the friction factor vs. Reynolds number behavior for pipe flow is significant. In laminar flow, the power number is inversely proportional to Reynolds number, reflecting the dominance of viscous forces over inertial forces. In turbulent flow, where inertial forces dominate, the power number is nearly constant. 

The shape, size, and baffling of a specific mixing vessel significantly influences the Reynolds number, flow, and power numbers. 

There are more than 400 operating nuclear reactors throughout the world. These reactors supply nearly 17% of the world s total electricity production capacity. The dependence on nuclear power varies significantly from country to country with some like Lithuania and France, which derive approximately 80% of their electricity from nuclear power to the United States, which while having the largest number of reactors at 104, only puUs 20% of its electricity from nuclear sources. 

For accuracy, in power number Np calculations, the power of the load on the impeller rather than the mixer motor should be used. Before attempting to use dimensional analysis, one has to measure/estimate power losses for empty bowl or dry stage mixing. Unlike power consumption of the impeller (based on torque measurements), the baseline for motor power consumption does not stay constant and changes significantly with load on the impeller, mixer condition, or motor efficiency. This may present inherent difficulties in using power meters instead of torque. Torque, of course, is directly proportional to power drawn by the impeller (the power number can be determined... 

The following conclusions can be drawn from the foregoing discussion (i) DT requires the highest power for solid suspension in three-phase systems not so much as due to high A but because of its significantly higher power number (ii) both DT and PTD suffer from serious instabilities in the presence of gas. These instabilities increase with increase in the gas flow rate, (iii) The six-blade upflow pitched turbine yields most stable operation and requires relatively low power inputs as compared to other impellers. Therefore, for duties that require solid suspension in the presence of gas, the six-blade upflow pitched turbine should be preferred (Nienow and Bujalski 2004). This advantage is also discussed and applied in Chapter 7B. 

The Rushton impeller is still the most commonly used radial mixing device for standard applications. New, more efficient impeller types were developed in the 1980s and 1990s (see Table 1.6). These developments improved mixing at significantly smaller power numbers. They are also favorable in the context of prevention of stirrer flooding. 

Hub diameter. Shafts in small vessels tend to be overdesigned compared with large vessels consequently, the hubs fitted to small impellers can be excessively large. This can sometimes be important, particularly when investigating the flow patterns generated by the impeller. However, due to the term in the power draw expression, the effect on the impeller power number may not be significant. 

The number of baffles (Nb) and their width (B) have a significant effect on Np. As the parameter NbB increases, Np increases (Figure 6-15) up to the power number of the conventional configuration, with four baffles having width equal to T/10. At higher NbB values, the power number is constant at a level which depends on D/T. 

The method of molecular dynamics (MD), described earlier in this book, is a powerful approach for simulating the dynamics and predicting the rates of chemical reactions. In the MD approach most commonly used, the potential of interaction is specified between atoms participating in the reaction, and the time evolution of their positions is obtained by solving Hamilton s equations for the classical motions of the nuclei. Because MD simulations of etching reactions must include a significant number of atoms from the substrate as well as the gaseous etchant species, the calculations become computationally intensive, and the time scale of the simulation is limited to the... 

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