Impeller power draw during turbulent operation in solid-liquid suspensions

摘要

Experimental measurements with six impeller types in solid-liquid suspensions indicate that impeller power draw in the turbulent regime is approximately proportional to the solid-liquid suspension density when the solids are distributed throughout the liquid; however, the accuracy of this approach is limited and there are clear differences in the behaviours of the various impellers. In general, power draw increases are less than suspension density increases for impellers with large blade-trailing vortices, while power draw increases are equal to or greater than suspension density increases for impellers with smaller blade-trailing vortices. The power draw data is well-described using linear relations between the impeller power number and the density difference correlating parameter proposed by Micheletti et al.,([9]) with the slope of the relation being dependent on impeller type. More extensive testing with a pitched-blade turbine, using a greater variety of solids, found that the relation between the impeller power number and the density difference correlating parameter is independent of particle size for particles as large as 1 mm (1000 microns). For particles larger than 1.7 mm (1700 microns), in addition to suspension density, the solid volume fraction affects the pitched-blade turbine power number; however, it is difficult to determine if this effect exists at all scales or if it is a result of the large particle size relative to the impeller dimensions in the experimental system. For large particles, the power draw is increased by the addition of neutrally-buoyant particles that do not change the suspension density, with the magnitude of the increase being dependent on impeller type.