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Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Newson, Timothy

2nd Supervisor

Ahmed, Aly

Co-Supervisor

Abstract

To understand the challenges on grain flow during handling in hoppers and silos require better understanding of the granular physical flow characteristics through experiments done at large scale. However, due to the complexity of the large, controlled tests, scaled models are often used to study the behaviour. Very few of these model scale studies had been done appropriately taking the effects of gravity on silo discharge into account. This subject has also been widely investigated in different areas of geotechnical engineering related to soil erosion, land slides, and dynamic liquefaction. Granular flow rate and the dynamics of the flow are also studied in fields like chemical, mining and material engineering. Two conditions for scaled model testing have been investigated in this study: the dynamics of the granular flow at natural gravity and enhanced gravities of 1.9, 3.8, 7.7 and 11.7 g conducted in a centrifuge. The granular outflow tests have covered the features of the granular shear flow, outflow rates and deformation patterns. The observed granular flow rates (W) for the novel half hopper model under natural and enhanced gravities were then estimated from the Beverloo et al. (1961) relations and were compared. Particle image velocimetry was used to study the vertical particle velocity profiles and the flow deformation patterns during the natural and enhanced gravity tests. The granular flow rate was found to be directly proportional to the ratio of orifice size and influenced by the soil particle size (W ∝ Do/d). However, this comparison was found to be more applicable for cohesionless soils compared to soils with small cohesions. The results in this study show that the flow mechanisms in a cylindrical hopper are quite complex and vary both temporally and spatially. Three distinct zones of behaviour were observed from the particle image velocimetry analysis: a) an upper inflow zone, b) a narrow vertical funnel flow zone and c) a near static stagnant zone. For the enhanced gravity tests, the mass flow direction inside and outside of the hopper was observed to be influenced by the resultant inclined vector of the enhanced gravity due to the combined effect of the centrifuge rotation and Earth’s gravity.

Summary for Lay Audience

To understand the challenges of the flow of particles during handling in hoppers and silos, we require better understanding of the physical flow characteristics through experiments done at full scale. However, due to the complexity of large-scale tests, reduced scale models are often used to study this behaviour. These scaled model tests of the outflow of particles can be completed using a geotechnical centrifuge. A centrifuge induces higher gravity levels than natural gravity thereby increases material self-weight and allowing small model tests to simulate large-scale prototype. Such behaviour is studied in fields like geotechnical, chemical, mining and material engineering. Two conditions for the scaled model hopper testing have been investigated in this study: the behaviour of particle flow at natural gravity and at enhanced gravities of 1.9, 3.8, 7.7 and 11.7 g. The centrifuge particle outflow tests covered the important features such as rate of outflow, internal deformation patterns and material outflow behaviour as it leaves the opening of the hopper. The observed granular flow rates (W) for the novel half hopper model under natural and enhanced gravities were then estimated using the Beverloo et al. (1961) relations and were compared. Particle image velocimetry was used to study the vertical particle velocity profiles and the flow deformation patterns during the tests. The flow rate (W) was found to be directly proportional to the ratio of hopper orifice size (Do) and influenced by the soil particle size (d). Three distinct zones of behaviour were observed from the particle image velocimetry analysis: a) an upper inflow zone, b) a narrow vertical funnel flow zone and c) a near static stagnant zone. For the enhanced gravity tests, the mass flow direction inside and outside of the hopper was observed to be influenced by the non-vertical resultant self-weight force of enhanced gravity due to the combined effect of the centrifuge rotation and Earth’s gravity.

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