| Popis: |
Hopper design codes are currently based on empirical knowledge which is limited to the conditions tested. There is also a lack of accurate analytical tools for the prediction of stresses and flow patterns in granular materials. Most methods are based on the continuum technique which does not explicitly include particle properties such as shape and size variation. With the recent advance in computer power the Distinct Element modelling technique which starts from the particle properties directly becomes a potentially powerful tool to aid understanding of granular physics. This thesis investigates how such a model can be applied to granular flow in hoppers and the accuracy of the results. This dissertation is the first systematic study of Distinct Element simulations of the filling and discharge of non-cohesive discs and spheres in 2-dimensional (2D) and 3-dimensional (3D) hoppers, respectively, under gravity. The model includes normal elastic forces between contacting particles using either the Continuous Interaction curve from molecular dynamics, the Hertz contact mechanics equations, or a Hooke linear spring constant. The effect of these different interactions is compared. The model also includes friction, contact damping, gravity, wall forces and air drag. The bulk of the work in this project has been aimed at establishing confidence in the simulation technique by comparing the results with experiment directly or with current established knowledge of granular flow in silos. Good agreement has been obtained for discharge rate, wall stresses (static and dynamic), velocity profiles, internal stress patterns, voidage profiles and rupture zones. Some of these have been compared with data in the literature or from laboratory scale experiments in the project involving 2D photographic and 3D gamma-ray tomography. The model has also been extended to include air drag on the particles for air-assisted flow and air-retarded flow. There is a strong Indication that the results are more accurate from this model than from current continuum approaches, because of the microstructural detail included. |
