Pickup velocity from particle deposits

In order to optimize the design process of pneumatic or hydraulic conveying systems, all possible modes, cases, patterns etc. of the two-phase flow (fluid-solids) must be known and understood. One of the most significant parameters for designing conveying systems is the threshold velocity for picking up the initially at rest deposited particles. Numerous studies focus on the pickup velocity from a horizontal layer of particles. However, the pickup mechanism from hill-shaped deposits has gained much less attention. The main aim of this work is to investigate the air flow over the deposits of particles (including both horizontal layers and hill-shaped particles) and to determine the minimum air velocity for picking up the particles from the deposits. Based on our experimental results and an empirical analysis of seven different materials, covering a wide range of properties, we found that the pickup velocity strongly depends on the deposit height and shape. It was also found that there is a relationship between the pickup velocities from particle deposits and from a layer of particles. This relationship takes into account the ratio between the wind tunnel and the deposit height and the particulate solids angle of repose which defines the deposit shape.

The main objective of this paper is to investigate all those parameters that influence the pickup velocity from particle deposits and to propose a universal equation for a wide range of particulate solid properties. It should be noted that in additional to particle properties (size, density and shape) which influence the pickup mechanism from a layer of particles, the pickup from hill-shaped deposits is affected by properties such as fluid permeability and the hill's height and shape, therefore, the hill geometry and fluid permeability also needs to be taken into account.This work presents the experimental results and empirical analysis of pickup velocities from particle deposits in horizontal pneumatic conveying systems for a variety of particulate solids.It was found that the influence of vortex after the deposit is negligible. The influence of air permeability was found to be negligible for deposit heights lower than 70% of the wind tunnel height, however the permeability factor influenced the manner of the deposit erosion.It was also found that the deposit height and shape significantly influenced the pickup velocity. By increasing the deposit height the corrected pickup velocity first decreases until hi/D = 0.2 ÷ 0.3 and then increases.An empirical relationship between the corrected pickup velocity, the pickup velocity from a layer, the angle of repose and the non-dimensional deposit height is presented on the follow figure. The results calculated from this relationship for the pickup velocity are bounded within the limits of ± 15%.Figure optionsDownload as PowerPoint slide