- Title
- Bubble vibration behaviour and bubble-particle interactions under external excitation
- Creator
- Yasmin, Dilruba
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Flotation principle is widely used in the mineral processing industry wherein collisions among the air bubbles and mineral ore particles result in preferential collection of the valuable mineral particles based on the surface hydrophobicity. Over five decades, researches have been conducted to enhance minerals recovery using external perturbations such as ultrasonics. The reported studies in this area however largely focus onto the pre-conditioning of ores to remove contaminants to improve ore surface hydrophobicity. It is realised that studies on the effect of acoustic field directly in flotation are rather limited and in general there is a lack of a mechanistic description of the bubble-particle interaction in the presence of an external acoustic field. Consequently, how bubble-particle collision, particle attachment/collection and detachment phenomena are influenced due to acoustic field remain relatively unexplored. This study aims to contribute to this knowledge gap by developing a numerical model of bubble-particle interactions incorporating the effect of an externally applied acoustic field. Behaviour of the oscillating bubble (both free and shelled) was modelled based on a one-dimensional (radial direction) Rayleigh-Plesset equation (R-P) incorporating different damping factors (radiation, viscous, thermal and shell friction) and subject to both short duration (Gaussian envelope) and continuous (sinusoidal) acoustic field. The developed model was first validated based on the available simulation results on microbubbles from literature by comparing the radial oscillation behaviour. Effect of all damping parameters was investigated on the predicted behaviour of bubble radius, internal pressure and temperature. Next, oscillatory behaviour of a bubble of radius 900 µm typical in flotation operation was studied over a range of operating frequency of a continuous acoustic field. To ensure that bubble remains theoretically stable (without collapsing) within the operating range, a stability analysis was performed based on the Lyapunov exponent and bifurcation approach. It was found that within the spherical limit (0.1≤Bo≤0.5), in the below resonance frequency regime (f~35 to 79 Hz), bubble remains stable for corresponding acoustic field pressure range from 0 to 60 kPa. In the above resonance frequency regime (f~3.6 to 14.4 kHz), stability of bubble was also confirmed for the corresponding acoustic pressure field ranging from 0 to 19.2 kPa. Particle motion was modelled next based on the discrete element method (DEM) to simulate the bubble-particle interactions (bubble radius 900 µm and particle radius 66 µm) in absence of any acoustic field. Effect of relevant forces namely gravity, buoyancy, drag, contact force, hydrophobic force, capillary force (at the three-phase contact line) and pressure force were considered in the model. Temporal variation in the predicted particle trajectory was validated with the existing experimental data and good agreement was obtained. Effect of acoustic field was then studied in the previous bubble-particle system in the small amplitude perturbation limit ensuring spherical shape (Bond number (Bo) <1) of the bubble over a range of operating frequencies (below and above resonance) stated earlier. To simulate particle interactions with the oscillating bubble, R-P equation was one way coupled with the DEM model. The model predictions were validated with the reported experimental data on particle detachment from a stationary bubble-particle aggregate subject to low frequency vibration. Effect of liquid medium viscosity and particle surface hydrophobicity on the particle detachment behaviour was analysed and good agreement was obtained. Finally, interactions dynamics of a moving particle and an oscillating bubble were simulated by varying three major parameters independently namely bubble oscillation amplitude ratio (ε ≤ 0.1), acoustic field frequency (below and above resonance) and bubble-particle surface clearance (~1 to ~10.6% of bubble radius) in the previously mentioned Bond number range. Both bubble-particle collision and particle collection efficiency were determined based on a comprehensive set of simulations and regime maps were constructed to establish the suitable combinations of the three operating parameters. Although collision phenomenon was observed at all frequencies both below and above resonance regimes, attachment outcome was only predicted close to bubble resonance frequency (f=3.61 kHz) at low surface clearance in the Bond number range from 0.4 to 0.5. No attachment was predicted in the below resonance frequency regime while in the above-resonance regime when frequency was increased to 5.42 kHz, attachment efficiency decreased to 0.1.
- Subject
- bubble-particle interaction; acoustic field; bubble oscillation; Rayleigh-Plesset model; DEM; collision
- Identifier
- http://hdl.handle.net/1959.13/1421638
- Identifier
- uon:37753
- Rights
- Copyright 2020 Dilruba Yasmin
- Language
- eng
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| Thumbnail | File | Description | Size | Format | |||
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| View Details Download | ATTACHMENT01 | Thesis | 6 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 618 KB | Adobe Acrobat PDF | View Details Download |