Includes bibliographical references. / Industry often requires the sorting of one material from another. Although the detection of desired (or undesired) elements is well advanced, the mechanical ejection or removal of particles is fairly underdeveloped. Agriculture and mining applications have used air jets and water jets to eject particles ranging in weight from a few grams to several hundred. With the current trends in mechanization leading towards higher processing speeds, these traditional methods have been found to be unsuitable: they have slow turn-on and turn-off response times, leading to a high volume of material being ejected with the target. Higher processing speeds will lead to even greater amounts of waste material being ejected thus producing even lower yield concentrations. Thus the need for a quick response time, repetitive, impulse ejection mechanism in the sorting industry is apparent. A kinematic analysis of the required ejection mechanism blast strength shows that the required force depends on the target mass, the required deflection angle the force application angle and the force duration. Acoustical techniques in air are unsuitable as ejection force mechanisms. A water jet is proposed to meets these requirements. This water jet is caused by an electrical discharge in a liquid cavity. This produces a weak shock wave which is focused by the cavity to a nozzle where a slug of water is emitted. The cavity is an elliptical cavity of height h, with the electrodes mounted end on at the first focus and a reflecting cone and nozzle at the second focus. The propagation of weak shock waves in the elliptical cavity is studied theoretically and numerically - using a finite difference simulation program. The reflected converging wave is shown to depend on the cavity eccentricity and the wall admittance. The resulting converging shock wave has an asymmetrical pressure distribution. This analysis is used in the design of a prototype water jet generator. The electrical discharge circuit used for the production of shock waves in the cavity is analysed and the physical discharge process of electrical to shock energy conversion reviewed. Conditions for the maximisation of this transfer correspond to large water gap resistances, high voltages and low circuit inductances. Experiments on the prototype generator show that the transient water jet slug energy is relatively low. High speed photographic techniques reveal that the jet velocity is of the order of 30 m/s. Published results show much higher jets speeds are possible. The operation of the electrical discharge circuit is found to critically influence the water jet performance - electrical measurements show that the circuit is a sub-optimum, underdamped RLC circuit. The cone / nozzle operation is also shown to have a marked effect on performance. The nozzle in particular requires optimisation. The prototype in its present form is not suitable for use in an ejection system. Although the pulse length, rise time and channel spread of the device are suitable, the blast strength is not sufficient for deflection of the heaviest range of particles. Optimisation of the electrical circuit and increased energies will increase the blast strength.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/9637 |
| Date | January 1991 |
| Creators | Mortimer, Bruce John Peter |
| Contributors | Jongens, A W D |
| Publisher | University of Cape Town, Faculty of Engineering and the Built Environment, Department of Electrical Engineering |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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