Pickup, critical and wind threshold velocities of particles

Abstract

This work presents experimental results on pickup velocity measurements for a variety of particulate solids in gases and in liquids. Based on our previously published experimental results for pickup in gas flow in pipes a three-zone master-curve is defined by establishing simple relationships between modified Reynolds and Archimedes numbers. The zones are distinguished by cohesive forces (van der Waals): Zone I represents negligible cohesion forces, Zone II represents considerable cohesion forces that increase the required pickup velocity of individual particles, and Zone III represents significant cohesion forces that cause pickup of agglomerates. Previously published experiments by others encompassing about 121 measurements for a wide range of particle sizes, shapes and densities picked up by liquids, were added to our master-curve with excellent agreement. The cohesive forces did not affect the critical velocity in case of liquid–particle systems. Therefore, these experiments extend the line fitting the pickup velocity of big dry particles. In most cases, the critical shear velocity (reported for liquid–particle systems) had to be converted to the average pickup velocity. Furthermore, additional 16 measurements of pickup velocities (in air) conducted in big wind tunnels were added to the master-curve with excellent agreement. We can conclude that our simple master-curve is appropriate for threshold velocities defined in three fluid–particle systems with a maximum error of only ± 30%.