A study of the sedimentation of bimodal distributions of microspheres

The batch sedimentation of bimodal distributions of unflocculated microspheres was studied using a radioisotopic tracer to follow settling concentrations. Experimental data are provided for overall particle fractional concentrations between 0.05 and 0.3 which extend the bimodal settling data base to particles having a ratio of particle Reynolds number of 300:1. A theoretical correlation is proposed which modifies the Mirza-Richardson equation and adds a suspension viscosity correction term. The proposed correlation is compared with recent and new experimental data and contrasted to a study which made a density correction to the Stokes' law velocity term. The correlation was found to predict bimodal settling velocities with between 8 and 25% average error, the greatest deviations occurred with settling velocities outside the creeping flow range. Further work is suggested with more concentrated suspensions having concentration ratios of small to large particles of 4:1, where the proposed theory makes predictions of settling velocities of the order of 25% slower than the Selim theory.