Settling Velocity and Drag Coefficient

The magnitude of the free settling velocity has proven useful in characterizing solid suspension problems into easy, moderate, or difficult categories (see Table 10-2). It is also used in solid-liquid mixing correlations, as described below. 

Correlations for the free settling velocity have been derived for spherical particles. In Newtonian fluids, the free settling velocity, Vt, is calculated by the expression (Perry and Green, 1984). 

The correlation for Cd (Uke the friction factor and the impeller power number, Np) covers several hydrodynamic regimes. The corresponding ranges for Rep and the correlating expression for Cd are shown in Table 10-1 for three hydrodynamic regimes. 

When the expressions for Cd are substituted in eq. (10-1), the resulting expressions for the free settling velocity, Vt are, respectively  

1 Effect of Solids Particle Size and Distribution. Solids particles encountered in industrial applications usually have a distribution of sizes. Larger particles settle faster than smaller ones. Studies by Baldi et al. (1978) suggest that for a distribution of particle sizes, the appropriate particle diameter to use in the expressions above is the mass-mean diameter, (dp)43- This is calculated from size distribution data by 

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