Experimental mixing study of a yield stress fluid in a laminar stirred tank

Batch mixing in the laminar flow regime was studied using single- and doubl e-helical-ribbon impellers and a series of yield stress fluids prepared fro m the polymer Carbopol 940, water, and sodium hydroxide. Power draw measure ments for these non-Newtonian fluids were compared to the values obtained f or comparably high viscosity Newtonian fluids. Circulation times in the mix er as a function of impeller speed were measured using the thermal response technique. The dimensionless circulation times were found to be independen t of pumping direction and fluid rheology for the double-ribbon impeller bu t were a function of both these factors for the single ribbon. For the high est viscosity fluid, the single-ribbon impeller with downward pumping gave very high values of dimensionless circulation time. For the single-ribbon i mpeller, the dimensionless circulation time with upward pumping was less th an 2/3 of the value previously observed for Newtonian fluids.