The Mixing of Miscible Liquids with Large Differences in Density and Viscosity

Summary

The mixing of miscible fluids with significant density differences, but with similar viscosities, may lead to stratification, in which case buoyancy effects must be overcome to achieve a homogeneous mixture. Differences in viscosity between the added and the bulk fluid cause a reduction in the strain rate experienced by the more viscous fluid; furthermore, large viscosities damp the inertial effects which would cause entrainment and mixing. Stirred tanks have commonly been used to perform such batch mixing processes but they are not always the most effective devices.

Experimental studies were carried out to determine the conditions under which a viscous and dense fluid would be easily mixed into a low viscosity bulk, in fully turbulent flow (in all cases, the length scale of the viscous addition was larger than the integral velocity length scale for the flow, so that the bulk fluid appeared fully turbulent to the added material). The mixing time was determined experimentally from the concentration time-history of tracer fluid using conductivity microprobes. Preliminary studies of the mixing of non-Newtonian fluids showed the difficulty in determining an apparent viscosity at the strain rate actually experienced by the added fluid.

Dimensional analysis has been used to interpret these mixing time results in terms of the magnitude of the bulk inertia forces relative to the initial buoyancy forces and to viscous forces within the added fluid. A critical Reynolds number, which depends on the viscosity ratio of the two fluids, has been proposed as the criterion for the operating conditions under which the mixing rate is extremely slow and the mixing time is much longer than that required for mixing fluids with similar properties.