Model studies on mixers in the viscous flow region

Abstract The batch mixing of viscous liquids with rotating agitators was studied in vessels of 0·24 and 1·8-m dia. The quantities measured were power input and mixing time. For mixing time measurement a decolouration and a thermal-response technique were used, both yielding reproducible quantitative data. Seven types of mixers were tested: a flat turbine, inclined-blade paddles, a helical screw, a helical ribbon, a propeller and an anchor agitator. For some agitators, in the region of truly viscous flow, the mixing time was inversely proportion to stirrer speed. The mixers could be compared in terms of two dimensionless numbers based on vessel diameter, liquid viscosity and density, mixing time and power input. The power required to reach a certain mixing time was lowest for the helical screw in a draught tube, with inclined blades or propellers in a draught tube a close second. The turbine and anchor mixers were unsatisfactory for viscous mixing.