Experimental Investigation of Air Entrainment in Free-Falling Particle Plumes
Powders and granulated solids are widely used in industrial bulk solids storage, handling and transportation systems. Such bulk materials handling operations frequently involve a falling stream of material. During such a process, the surrounding air is induced to flow with the falling particle stream forming a particle-driven plume. Herein, experimental research results are reported on this fundamental problem, focussing in particular on the velocity profile of air entrained by the free-falling particles. This investigation shows that the velocity profile of the induced air can be modelled as a Gaussian distribution. The radius of the particle plume is found to increase linearly with increasing drop height and it also increases with increasing bulk solid mass flow rate. Comparisons are made with other entrainment flows, such as jets and plumes, and it was found that air entrainment and hence the angle of spread of the particle-driven plumes was much less than for the other entrainment flows. The angles of spread of the particle-driven plumes were found to be in the range 1.3 as compared to qs H 5.7 for miscible plumes arising from sources of heat, for example. In addition, the centreline velocity of the induced air in the particle plumes was found to increase significantly with increasing drop height. Results from high-speed digital video records show that the bulk material does not dilate in a uniform manner as it falls, and a series of distinct particle clouds form in the core of the particle-driven plume. These clouds eventually disperse over a sufficiently large drop height.