Air Entrainment by Free Falling Streams of Particles

This paper describes a study of free falling streams of particles or “particle plumes”. Experiments in air were carried out to determine the volumetric flow rate of entrained air as a function of drop height, mass flow rate and particle properties. Flow visualisation observations of the particle plumes are presented that show how the stream of particles descends in a dilating “core” of relatively constant radius after an initial contraction immediately below the source. The void fraction of the falling particle stream appears to increase homogeneously with height to a given point, but then the stream breaks up into a train of distinct particle “clouds” that descend and disperse. A theoretical model of the flow in such particle plumes is described whereby both entrained air flow rate and particle velocities are predicted for the situation where the core dilates homogeneously. Introduction Powders and granulated solid handling systems are widely used in industry for bulk solid materials storage, handling and transportation. Industries relying heavily on these systems include agriculture, mining, chemical engineering, power plants, cement and food processing. There are many bulk solids handling processes that involve free fall of the particles (e.g. silo filling, conveyor transfers, powder mixing/processing). When a stream of particles free fall, the surrounding air is induced to flow with the particle stream as the bulk solid accelerates and expands. The entrained air forms a jet or boundary layer around the falling “core” of bulk material. The radius of this boundary layer grows with increasing drop height and may carry with it fine, fugitive dust particles. Relatively little fundamental research has been carried out on the fluid and particle mechanics of these “particle plumes” which have some similarities with other plume and jet flows such as bubble plumes and water sprays [1,2] in that they are two-phase, non-Boussinesq problems. The purpose of the present research is to develop predictive tools for designers of dust control systems and to further the understanding of an important fundamental fluid flow situation. Articles have appeared in the past literature on air entrainment by falling streams of particles, notable that by Hemeon [3] who predicted air entrainment rates by extrapolating from the case of air entrained by a single, isolated particle freefalling through quiescent ambient air. The theory of Hemeon was later applied and modified by Morrison [4] and Tooker [5]. More recently work in the US by Plinke et al. [6] and at the University of Wollongong has focussed on quantifying the practical situation where falling streams of common bulk materials entrain air and work on determination of the velocity profile of the entrained air has been previously reported [7]. In this paper results of recent experiments are presented and the development of a theoretical model of the behaviour of particle plumes is discussed. Experimental Apparatus The experimental rig was designed to permit the quantitative evaluation of the flowrate of air entrained by a falling stream of bulk material by allowing the stream to pass into a sealed enclosure through an aperture (Figure 1). Induced and displaced air was extracted from the enclosure and the extraction flow rate was adjusted so that there was no static pressure difference between the chamber and the ambient air. The experimental methodology was validated by first replicating the results of Ricou and Spalding [8] by measuring the induced air flowing through the aperture from an isothermal jet generated by a nozzle where the hopper is located in Figure 1 [9]. The mass flowrate of bulk materials was maintained approximately constant during each test by means of the double-hopper arrangement, the test bin being flooded by the storage bin. The entire hopper system was suspended from a frame so that the mass flow rate of material could be measured directly by means of load cells attached to the suspension cables.