Chapter 29 - An experimental method for obtaining particle impact frequencies and velocities on impeller blades

Abstract

This chapter describes a method for in-situ measurement of particle-blade impact velocities and frequencies in a stirred vessel, using a thin coating of plasticine to record craters formed by particle impacts. The impact of particles on impeller blades is a common feature of solid-liquid mixing processes. Fragile particles may undergo breakup or abrasion by collision with the blade; with hard particles, the blade itself may be eroded. In crystallization, secondary nucleation sites can be formed by attrition of asperities from parent crystals on impact with a blade. This hydrodynamic phenomenon has a significant effect on the nucleation rate and the crystal size distribution. This chapter also describes a model for the plastic deformation of the target material and proposes an iterative method for calculating impact velocities from the dimensions of the craters. For a Rushton impeller blade, the impact velocities are close to the linear speed of the blade at the impact radius; the largest velocities therefore occur near the blade tip. The highest impact rates are also at the blade tip and below the disc of the Rushton turbine. In contrast, for a downward pumping 45 ° pitched blade turbine, the most frequent impacts occur at small radii, on the lower half of the blade, where the lowest impact velocities are measured.