Rushton et al. (1950) investigated the relationship between power consumption and operating variables in baffled, agitated vessels using the technique of dimensional analysis. They demonstrated that power absorption during agitation of non-gassed Newtonian liquids could be represented by a dimensionless group termed the power number, defined by the expression:
is the power number, is the external power from the agitator, is the liquid density, is the impeller rotational speed, is the impeller diameter, the power number is the ratio of external force exerted (P) to the inertial force imparted ( pN3D5) to the liquid. The motion of liquids in an agitated vessel may be described by another dimensionless number known as the Reynolds number which is a ratio of inertial to viscous forces:
np where Np
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