| 1. |
- Mwandawande, I., et al.
(författare)
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Investigation of flow regime transition in a column flotation cell using CFD
- 2019
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Ingår i: Journal of the Southern African Institute of Mining and Metallurgy. - : Academy of Science of South Africa. - 2225-6253 .- 2411-9717. ; 119:2, s. 173-186
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Tidskriftsartikel (refereegranskat)abstract
- Flotation columns are normally operated at optimal superficial gas velocities to maintain bubbly flow conditions. However, with increasing superficial gas velocity, loss of bubbly flow may occur with adverse effects on column performance. It is therefore important to identify the maximum superficial gas velocity above which loss of bubbly flow occurs. The maximum superficial gas velocity is usually obtained from a gas holdup versus superficial gas velocity plot in which the linear portion of the graph represents bubbly flow while deviation from the linear relationship indicates a change from the bubbly flow to the churn-turbulent regime. However, this method is difficult to use when the transition from bubbly flow to churn-turbulent flow is gradual, as happens in the presence of frothers. We present two alternative methods in which the flow regime in the column is distinguished by means of radial gas holdup profiles and gas holdup versus time graphs obtained from CFD simulations. Bubbly flow was characterized by saddle-shaped profiles with three distinct peaks, or saddle-shaped profiles with two near-wall peaks and a central minimum, or flat profiles with intermediate features between saddle and parabolic gas holdup profiles. The transition regime was gradual and characterized by flat to parabolic gas holdup profiles that become steeper with increasing superficial gas velocity. The churn-turbulent flow was distinguished by steep parabolic radial gas holdup profiles. Gas holdup versus time graphs were also used to define flow regimes with a constant gas holdup indicating bubbly flow, while wide gas holdup variations indicate churn-turbulent flow.
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| 2. |
- Mwandawande, I, et al.
(författare)
-
Prediction of gas holdup in a column flotation cell using computational fluid dynamics (CFD)
- 2019
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Ingår i: Journal of the Southern African Institute of Mining and Metallurgy. - : Academy of Science of South Africa. - 2225-6253 .- 2411-9717. ; 119:1, s. 81-95
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Tidskriftsartikel (refereegranskat)abstract
- Computational fluid dynamics (CFD) was applied to predict the average gas holdup and the axial gas holdup variation in a 13.5 m high cylindrical column 0.91 m diameter. The column was operating in batch mode. A Eulerian-Eulerian multiphase approach with appropriate interphase momentum exchange terms was applied to simulate the gas-liquid flow inside the column. Turbulence in the continuous phase was modelled using the k-epsilon realizable turbulence model. The predicted average gas holdup values were in good agreement with experimental data. The axial gas holdup prediction was generally good for the middle and top parts of the column, but was over-predicted for the bottom part of the column. Bubble velocity profiles were observed in which the axial velocity of the air bubbles decreased with height in the column. This may be related to the upward increase in gas holdup in the column. Simulations were also conducted to compare the gas holdup predicted with the universal, the Schiller-Naumann, and the Morsi-Alexander drag models. The gas holdup predictions for the three drag models were not significantly different.
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| 3. |
- Struzzi, C., et al.
(författare)
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Exploiting sensor geometry for enhanced gas sensing properties of fluorinated carbon nanotubes under humid environment
- 2019
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Ingår i: Sensors and Actuators B: Chemical. - : Elsevier BV. - 0925-4005. ; 281, s. 945-952
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Tidskriftsartikel (refereegranskat)abstract
- Modification of the surface electronic properties of vertically aligned and randomly distributed carbon nanotubes and the hydrophobic character after exposure to Ar:F2 and CF4 plasma are exploited to optimize the sensing characteristics of these materials. The sensing properties of fluorinated carbon nanotubes are disclosed by probing their stability and responsiveness towards the detection of two selected pollutants such as nitrogen dioxide and ammonia (NO2 and NH3). The effects of both humidity level and geometry of the sensing layer are assessed. It is demonstrated that fluorination, by increasing the surface hydrophobicity, results in increased response reproducibility and enhanced sensor response towards NH3 when using vertically aligned carbon nanotubes.
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