Experimental and Numerical Study of Rock breakage by Pulsed Water Jets

Sevda Dehkhoda (2011). Experimental and Numerical Study of Rock breakage by Pulsed Water Jets PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author Sevda Dehkhoda
Thesis Title Experimental and Numerical Study of Rock breakage by Pulsed Water Jets
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2011-12
Thesis type PhD Thesis
Supervisor Prof. Michael Hood
A/Prof. Habib Alehossein
Prof. David Buttsworth
Total pages 353
Total colour pages 66
Total black and white pages 287
Language eng
Subjects 091402 Geomechanics and Resources Geotechnical Engineering
091503 Engineering Practice
091405 Mining Engineering
Abstract/Summary Rock breakage is, arguably, the most important step in the mining process. Poor blasting practice produces large boulders that are difficult and expensive to handle. The presence of oversized boulders in draw-points can stop production in underground caving mines. Blasting and heavy impact hammers are the main techniques used in mines to fracture oversized rocks, but both impose undesirable extra costs as a secondary rock-breakage operation. Blasting is time-consuming and can damage the infrastructure, whilst impact hammers are cumbersome and cannot be used where boulders are not readily accessible. It is important, therefore, to develop more effective secondary rock-breakage techniques. Preliminary research demonstrated that pulsed water jets are routinely able to break large (approximately 1m3) boulders of competent hard rocks in less than a minute. Therefore, they can potentially be a more effective method for secondary rock-breakage. They can deliver high power intensity with a low reaction force meaning that a lightweight, flexible apparatus can be employed. The breakage mechanism of rocks under the impact of water pulses is, however, not yet clear. A pulsed water jet consists of a series of discrete, large water drops travelling at high velocity. These apply cyclical impact forces of short duration onto the target material. The water impacts generate stress waves inside the rock samples and these waves can, potentially, influence the rock- breakage process. The contribution of the stagnation water pressure on the target after impact may also play a crucial role in fracture initiation and propagation in the rock. The geometrical properties of the jet are controlled and influenced by the method that is used to generate the water pulses. The original contribution of this research is the determination of the processes of water pulse formation and the discovery of key factors that influence rock fracture initiation and propagation. A straightforward method was employed to generate a single water pulse using a hammer that stroke a piston, rested on top of a water-filled chamber. This impacting action pressurised the water, causing it to be expelled at high velocity through a nozzle that was mounted on the chamber axis opposite the piston. A theoretical investigation was undertaken to improve the understanding of this system for generating water pulses. A computational model, on the basis of continuity and momentum equations for a compressible viscous flow, was developed to simulate the pressure dynamics in the chamber. This model was used to optimise the relative sizes of the hammer, the piston, and the height of the water column in order to produce the largest and the highest-velocity water pulses. The model was validated experimentally using a purpose-built apparatus. Research was conducted to measure the impact loading of the water pulses on a target material. The main challenge was to measure this load for an event that lasted for less than one microsecond. This obviously required an extremely fast-response sensing system to capture the induced stress wave from the water hammer pressure before it reduced to the stagnation pressure. Customised sensing and data acquisition equipment was developed for these measurements. PVDF (Polyvinylidene fluoride) shock gauges were selected as the most appropriate sensors because of their unique rapid response, large stress range, and large signal to noise ratio. PVDF polymer films are piezoelectric material and generate electrical charge in response to applied stress. A current-mode measurement method was chosen because of the high-speed nature of the phenomenon of interest. The derivative of the stress was measured and the signal was then integrated numerically. This measurement, in conjunction with the high-speed photography of the water pulses, was applied to a study of the coherence of the generated water pulses. In the final stage of the research, experiments were conducted to examine the damage caused to confined rock specimens by sequences of water pulses of various pulse lengths and pulsation frequencies. These experiments were undertaken for different rock types. The observed rock damage was then used to construct an explanatory model of the mechanisms of rock fracture. The breakage mechanism was found to be controlled by the number of water pulse impacts and by the duration of stagnation water-pressure on the target. The successive high-energy impacts of the water pulses were found to create localised fracture zones in the vicinity of the impact surface. These impacts also initiated fatigue in the target, introducing micro-structural damages. The initial impacts of water pulses created a damaged area at the point of impact. The stagnation pressure from the water flow supplied the crack opening pressure, which controlled the crack opening process and thus affected the development of the failure zone. However, crack growth could be interrupted by energy dissipation and by toughening mechanisms. It was found that effective rock-breakage depended upon the physical properties of the target rock, in particular the brittleness, and that, ideally, the length and frequency of the water pulses should be tuned to accommodate these rock properties to optimise the rock-fracture process.
Keyword Pulsed water jet, rock breakage, experimental study,
computational analysis, pulsation frequency,
pulse length, and impact pressure measurement
Additional Notes colour: 24-25, 32, 57-59, 80-83, 99, 102-103, 109-112, 133, 135, 137, 142-144, 149-150, 155-158, 163, 166, 170-173, 177, 180, 184, 187-203, 206, 211-212, 290-295, 297, 299; landscape: 346-353

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Created: Mon, 02 Apr 2012, 14:54:44 EST by Ms Sevda Dehkhoda on behalf of Library - Information Access Service