| 1. |
- Rabiee, Navid, et al.
(författare)
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Green Biomaterials : fundamental principles
- 2023
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Ingår i: Green Biomaterials. - : Taylor & Francis. - 2993-4168. ; 1:1, s. 1-4
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Hassanzadeh, Ahmad, et al.
(författare)
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Estimation of flotation rate constant and particle-bubble interactions considering key hydrodynamic parameters and their interrelations
- 2019
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Ingår i: Minerals Engineering. - : Elsevier BV. - 0892-6875. ; 141
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Tidskriftsartikel (refereegranskat)abstract
- Particle-bubble sub-processes cannot be directly and physically obtained in froth flotation due to the complexity of the process as well as numerous and dynamic interactions of particles and bubbles in an extremely intensive turbulent condition. Therefore, over the last three decades, two fundamental model configurations have been used as an only solution for prediction of particle-bubble collection efficiencies (Ecoll). Additionally, the relative intensity of the main flotation parameters on flotation rate constant, particle–bubble interactions together with their interrelations is not adequately addressed in the literature. The present study attempts in two separate phases to overcome these difficulties. In the first stage, prediction and evaluation of particle-bubble sub-processes are critically discussed by categorizing them in two configurations. The analytical models (approach I) commonly applied generalized Sutherland equation (EcGSE), modified Dobby–Finch (EaDF) and modified Schulze stability (EsSC) models. The second approach, numerical models, utilized Yoon–Luttrell (EcYL), Yoon–Luttrell (intermediate) (EaYL) and modified Schulze stability (EsSC) models. In the second stage, relative intensity and interrelation of key effective hydrodynamic parameters on the probability of particle–bubble encounter (Ec) and flotation rate constant (k) are obtained and optimized by means of the response surface modeling (RSM) based on central composite design (CCD). Five key factors including particle size (1–100 µm), particle density (1.3–4.1 kg/m3), bubble size (0.05–0.10 cm) and bubble velocity (10–30 cm/s) together with turbulence dissipation rate (18–30 m2/s3) are considered in order to maximize the responses including the k and Ec. The results obtained show that the Ecoll calculated by numerical techniques (configuration (II)) is greater than that of analytical approaches (configuration (I)) due to assumptions involved in using Yoon–Luttrell collision and attachment models. It is also found that under the conditions studied, particle size and bubble velocity are the most effective factors on Ec and k, respectively. Furthermore, not only the relative significance of factors on Ec and k but also the interrelation of cell turbulence and bubble size as well as bubble velocity and turbulence are shown to be inconsistent in the literature and thus require further studies. We briefly reported the main long-standing challenges in flotation kinetic modeling and emphasized on a serious need for fulfilling lack of physical observations. Finally, the presented analyses with respect to three-zone model offer a new concept for the extension of common flotation modeling approach using analytical and numerical techniques.
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| 3. |
- Karimi, Mohsen, 1983, et al.
(författare)
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An exploratory study on fluid particles breakup rate models for the entire spectrum of turbulent energy
- 2018
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 192, s. 850-863
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the role of turbulence on the formulation of breakup kernels for fluid particles in dispersed multiphase flows is demonstrated. Two pieces of information for model derivations including the turbulent energy spectrum and the second-order structure function are chosen as the main candidates to extend the models for the entire turbulent spectrum, contrary to the original expressions that are limited for the inertial subrange. Further, a two-step validation procedure is proposed to incorporate the effect of turbulence for model validations, that is, the original and the extended models are compared and validated against two sets of experimental data for breakup rates. The first set covers the inertial subrange, while the second set is a dataset that includes direct measurements toward the dissipation subrange of turbulence. The results show that the predictive abilities of breakup kernels can be enhanced by the model extension, when the droplet diameters are outside the inertial subrange of turbulence. This work, thus, answers how the entire spectrum of turbulence can improve the breakup kernels and proposes a validation method to include the effect of turbulence for the validation of breakup kernels.
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| 4. |
- Karimi, Mohsen, 1983, et al.
(författare)
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Dual mechanism model for fluid particle breakup in the entire turbulent spectrum
- 2019
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Ingår i: AICHE Journal. - : Wiley. - 1547-5905 .- 0001-1541. ; 65:8
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Tidskriftsartikel (refereegranskat)abstract
- This work provides an in-depth understanding of different breakup mechanisms for fluid particles in turbulent flows. All the disruptive and cohesive stresses are considered for the entire turbulent energy spectrum and their contributions to the breakup are evaluated. A new modeling framework is presented that bridges across turbulent subranges. The model entails different mechanisms for breakup by abandoning the classical limitation of inertial models. The predictions are validated with experiments encompassing both breakup regimes for droplets stabilized by internal viscosity and interfacial tension down to the micrometer length scale, which covers both the inertial and dissipation subranges. The model performance ensures the reliability of the framework, which involves different mechanisms. It retains the breakup rate for inertial models, improves the predictions for the transition region from inertia to dissipation, and bridges seamlessly to Kolmogorov-sized droplets.
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| 5. |
- Karimi, Mohsen, 1983, et al.
(författare)
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Stochastic simulation of droplet breakup in turbulence
- 2020
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 380
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates single droplet breakup from a theoretical perspective and addresses whether breakup in turbulent flows can be studied using highly-resolved simulations. Transient and three-dimensional turbulent flow simulations are performed to investigate if the apparent stochastic outcome from the droplet breakup can be predicted. For a given turbulent dissipation rate the breakup events were simulated for various detailed turbulence realizations. For this purpose, a well-characterized system widely used for kernel development is utilized to validate the simulations with respect to the key characteristics of stochastic breakup, including droplet deformation time, the number of fragments, and the specific breakup rate. The statistical validations show very good agreement with all the quantitative properties relevant to the breakup dynamics. Necklace breakup is also observed in line with patterns found in experiments. Evidence is found that the rate of energy transfer is positively correlated with higher order fragmentation. This can allow development of more accurate breakup kernels compared to the ones that only relies on the maximum amount of energy transfer. It is concluded that the simulation method provides new data on the stochastic characteristics of breakup. The method also provides a means to extract more details than experimentally possible since the analysis allows better spatial and temporal resolutions, and 3D analysis of energy transfer which provides better accuracy compared to experimental 2D data.
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| 6. |
- Karimi, Mohsen, 1983, et al.
(författare)
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Towards enhancement of gas–liquid mass transfer in bioelectrochemical systems: Validation of a robust CFD model
- 2021
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Ingår i: Biotechnology and Bioengineering. - : Wiley. - 0006-3592 .- 1097-0290. ; 118:10, s. 3953-3961
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Tidskriftsartikel (refereegranskat)abstract
- Mass transfer has been identified as a major bottleneck in gas fermentation and microbial conversion of carbon dioxide to chemicals. We present a pragmatic and validated Computational Fluid Dynamics (CFD) model for mass transfer in bioelectrochemical systems. Experiments were conducted to measure mixing times and mass transfer in a Duran bottle and an H-cell. An Eulerian–Eulerian framework with a simplified model for the bubble size distribution (BSD) was developed that utilized only one additional equation for the bubble number density while including the breakup and coalescence. Validations of the CFD model for mixing times showed that the predictions were within the confidence intervals of the measurements, verifying the model's capability in simulating the hydrodynamics. Further validations were performed using constant and varying bubble diameters for the mass transfer. The results showed the benefits of a simplified BSD model, as it yielded improvements of seven and four times in accuracy when assessed against the experimental data for the Duran bottle and H-cell, respectively. Modeling of the H-cell predicted that a lower stirring rate improves mass transfer compared with higher stirring rates, which is of great importance when designing microbial cultivation processes. The model offers a feasible framework for advanced modeling of gas fermentation and microbial electrosynthesis.
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| 7. |
- 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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| 8. |
- Mwandawande, I, et al.
(författare)
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Investigation of the mixing characteristics of industrial flotation columns using computational fluid dynamics
- 2022
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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. ; 122:5, s. 245-257
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Tidskriftsartikel (refereegranskat)abstract
- The mixing characteristics of industrial flotation columns were investigated using computational fluid dynamics (CFD). Particular emphasis was placed on the clarification of the relationship between the liquid and solids mixing parameters such as the mean residence time and axial dispersion coefficients. The effects of particle size and bubble size on liquid dispersion in the column were also studied. An Eulerian-Eulerian method was applied to simulate the multiphase flow, while additional scalar transport equations were introduced to predict the liquid residence time distribution (RTD) and particle age distribution inside the column. The results obtained show that particle residence time decreases with increasing particle size. The residence time of the coarser particles (112.5 ??m) was found to be at least 60% of the liquid residence time, while the finer particles (19 ??m) had a residence time similar to the liquid. The results also show that an increase in the particle size of the solids results in a decrease in the liquid vessel dispersion number, while a decrease in the bubble size increases liquid axial mixing. Finally, the simulated axial velocity profiles confirm the similarity between the liquid and solids axial dispersion coefficients in column flotation.
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| 9. |
- Mwandawande, I, et al.
(författare)
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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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| 10. |
- Naghavi, H., et al.
(författare)
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The effects of froth depth and impeller speed on gas dispersion properties and metallurgical performance of an industrial self-aerated flotation machine
- 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. ; 119:7, s. 661-669
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Tidskriftsartikel (refereegranskat)abstract
- In self-aerated flotation machines, the gas rate depends on operational variables (e.g. froth depth and impeller speed), pulp properties (e.g. solid content and viscosity), and reagent addition (e.g. type and concentration of frother). The gas rate has a strong correlation with the flotation performance by influencing the gas dispersion properties and froth retention time. A factorial experimental design was used to study how the gas dispersion properties, the froth retention time, and the flotation performance respond to changes in froth depth and impeller speed (as the most common operational variables). An in-depth understanding of the effects of impeller speed and froth depth on the gas dispersion properties, especially the bubble surface area flux and froth retention time, is necessary to improve operating strategies for self-aerated flotation machines. All experiments were carried out in a 50 m(3) self-aerated flotation cell in an iron ore processing plant. The results showed that the froth depth affected the metallurgical performance mostly via changing the froth retention time. The impeller speed had two important impacts on the metallurgical performance via varying both the froth retention time and the bubble surface area flux in the froth and pulp zones, respectively. The interaction effects of the froth depth and impeller speed were also established. This allowed us to develop a strategy for operating self-aerated flotation machines based on varying the froth depth and impeller speed with regard to the cell duty.
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