The effect of fluid elasticity on power consumption and mixing times in stirred tanks

Results are presented to show the effects of fluid elasticity on the power required for mixing and the time necessary to obtain a certain degree of homogenization (i.e. mixing time) in the viscous regime. To separate the effects of elasticity from those of shear thinning viscosity, elastic fluids with constant viscosities are used. Experiments in baffled tanks with Rushton turbine impellers show that elastic fluids require more power than inelastic fluids having equal viscosities. A correlation for the increased torque required to agitate an elastic fluid is presented that has a form suggested by the analytical solution for the effect of elasticity on the torque required to spin a sphere in a second-order elastic fluid. Elasticity also increases mixing time. Mixing time measurements are made using three probes at different locations in the vessel. The probes, based on fiber-optic sensors, detect a fluorescent dye tracer at the parts per billion concentration level.