Determination of the process characteristics

The functional dependency, Eq. (1.62), is all that dimensional analysis can offer here. It cannot provide any information about the form of the function /. This can only be obtained experimentally. These experiments can however be carried out in a considerably simpler manner, since not the individual parameter but the numerical value of the process number Re has to be varied. This can be achieved by varying the stirrer speed n, the stirrer diameter d or the kinematic viscosity v. It is simplest to vary the kinematic viscosity, which can be varied over orders of magnitude in liquid mixtures of water and glycerine or molasses. In this way measurements were carried out to determine the power characteristic Ne(Re) of a blade stirrer of fixed geometry in a tank, see Fig. 1.36. 

Several extremely interesting findings can be obtained from this graph  

1) In the range Re 20 the proportionality Ne x Re is found, thus resulting in the expression NeRe s R/(/rn d ) = const. Density is irrelevant here - we are dealing with the laminar flow region. 

2) In the range Re 50 (vessel with baffles) or Re 5 x 10 (unbaffled vessel) a constant Newton number Ne = P/(pn d ) is found. In this case, viscosity was irrelevant - we are dealing with a turbulent flow region. 

3) Understandably, the baffles do not influence the power characteristics within the laminar flow region. However, their influence is extremely strong at Re 5 X 10. Here, the installation of baffles under otherwise unchanged operating conditions increases the power consumption of the stirrer by a factor of 20  

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