Doctor of Philosophy
Faculty of Engineering
Liu, Ze Qin, Air entrainment in free falling bulk materials, Doctor of Philosophy thesis, Faculty of Engineering, University of Wollongong, 2003. http://ro.uow.edu.au/theses/1816
Bulk material handling operations involving a falling stream of particles are very common in industry throughout the world. In the process of bulk material transfer, fugitive dust may be generated and the surrounding air is entrained into the particle stream during free fall conditions. Understanding of how air entrainment by particles in free fall can assist in the design of effective dust control.
However, at the commencement of this project, research in the field of air entrainment in a free falling particle stream was limited due to the shortage of theoretical analysis and experimental data reported in the literature. For this reason, the purpose of this thesis was to undertake detailed experimental work and to formulate a theoretical model to predict the volume of air entrainment by free falling particles as a function of material properties and process parameters.
A series of experiments were performed to demonstrate the measurement methodology for air entrainment by free falling particles. To obtain detailed information on the characteristics of air entrainment and falling particle systems, a systematic experimental investigation was undertaken into the dynamics of a stream of particles and the associated entrained air flow. These experiments, were carried out to investigate the effects of material properties and material process parameters, such as mass flow rate of bulk material, particle size distribution, particle density, outlet size of the discharging hopper and particle drop height.
A significant finding in the experimental work was that the core of free falling particles contracted immediately after exit from the outlet of a hopper, and thereafter maintained an approximately constant radius for a considerable height. However, the dilation of the bulk material was not homogeneous, and the core of the particle stream appeared to break up into different sizes and shapes of individual clouds. Eventually, these particle clouds tended to dissipate and the falling particles spread out with drop height.
A mathematical model to predict the quantity of air entrainment in free falling particles was developed and numerical solutions obtained. Experiments were also conducted to determine the velocity profile of the entrained air flow.
In the experimental investigation, the air entrainment constant was found to be varied with different bulk materials. The comparison of the results of the present study on the air entrainment in the particle plume and previous research indicated that the entrainment capability of the particle plume is less than that of miscible plumes, bubble plumes and droplet-driven spray flows.