| 1. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Breakup of individual colloidal aggregates in turbulent flow investigated by 3D particle tracking velocimetry
- 2018
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Ingår i: Multiphase Flow Phenomena and Applications: Memorial Volume in Honor of Gad Hetsroni. - : World Scientific Publishing Co. Pte. Ltd.. - 9789813227392 - 9789813227385 ; , s. 83-96
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence where their breakup is recorded by three-dimensional particle tracking velocimetry (3D-PTV). The aggregates have an open structure with fractal dimension around 2.2, and their size varies from 0.9 to 3.1 mm which is large compared to the Kolmogorov length scale η = 0.15 mm. 3D-PTV allows for the simultaneous measurement of aggregate trajectories and the full velocity gradient tensor along their pathlines which enables us to access the Lagrangian stress history of individual breakup events. The analysis suggests that aggregates are mostly broken due to accumulation of drag stress over a time interval of order Kolmogorov time scale, O(τη). This finding is explained by the fact that the aggregates are large, which gives their motion inertia and which increases the time for stress propagation inside the aggregate.
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| 2. |
- Bäbler, Matthäus, 1977-, et al.
(författare)
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Rate of breakup of small inertial aggregates in homogeneous turbulence
- 2015
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Ingår i: Proceedings - 15th European Turbulence Conference, ETC 2015. - : TU Delft.
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Konferensbidrag (refereegranskat)abstract
- The hydrodynamic breakup of small inertial aggregates in homogenous and isotropic turbulence is studied through numerical simulations. Small inertial aggregates are subject to shear stress caused by the local velocity gradient and drag stress caused by the relative velocity of the aggregate and the fluid flow. In our simulations, we follow aggregates moving through the flow and record the total stress acting on them. Breakup is assumed to occur when the total stress overcomes a predefined threshold representing the aggregate strength. By determining how long it takes for an aggregate to reach a stress exceeding its strength for the first time, we are able to derive a breakup rate. It is found that with increasing aggregate inertia, the drag stress rapidly becomes the dominant stress resulting in an increase of the breakup rate with increasing the aggregate inertia.
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| 3. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
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Effect of Stefan flow on drag coefficient of reactive spherical particles in gas flow
- 2018
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Ingår i: Turbulence Heat And Mass Transfer 9 (THMT-18). - : Begell House. ; , s. 1089-1092, s. 1089-1092
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Konferensbidrag (refereegranskat)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle or deposition at the surface can generate additional flow phenomena that affect the heat, mass and momentum transfer between the particle and bulk flow. This work aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow. Fully resolved 3D simulations were carried out for particle Reynolds numbers based on the free stream velocity ranging from 0.5 to 3. Simulations are carried out in foam-extend CFD software, using the Immersed Boundary(IB) method for treating fluid-solid interactions. The simulations were validated against data for particles without reactive flow, and against the analytical solution for Stefan flow around a particle in a quiescent fluid. We found that in the considered range of Reynolds number the drag coefficient decreases linearly with in increase in Stefan flow velocity.
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| 4. |
- Jayawickrama, Thamali Rajika, et al.
(författare)
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The effect of Stefan flow on the drag coefficient of spherical particles in a gas flow
- 2019
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 117, s. 130-137
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Tidskriftsartikel (refereegranskat)abstract
- Particle laden flows with reactive particles are common in industrial applications. Chemical reactions inside the particle can generate a Stefan flow that affects heat, mass and momentum transfer between the particle and the bulk flow. This study aims at investigating the effect of Stefan flow on the drag coefficient of a spherical particle immersed in a uniform flow under isothermal conditions. Fully resolved simulations were carried out for particle Reynolds numbers ranging from 0.2 to 14 and Stefan flow Reynolds numbers from (-1) to 3, using the immersed boundary method for treating fluid-solid interactions. Results showed that the drag coefficient decreased with an increase of the outward Stefan flow. The main reason was the change in viscous force by the expansion of the boundary layer surrounding the particle. A simple model was developed based on this physical interpretation. With only one fitting parameter, the performance of the model to describe the simulation data were comparable to previous empirical models.
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| 5. |
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| 6. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Mechanisms behind overshoots in mean cluster size profiles in aggregation-breakup processes
- 2018
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Ingår i: Journal of Colloid and Interface Science. - : Academic Press. - 0021-9797 .- 1095-7103. ; 528, s. 336-348
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Tidskriftsartikel (refereegranskat)abstract
- Aggregation and breakup of small particles in stirred suspensions often shows an overshoot in the time evolution of the mean cluster size: Starting from a suspension of primary particles the mean cluster size first increases before going through a maximum beyond which a slow relaxation sets in. Such behavior was observed in various systems, including polymeric latices, inorganic colloids, asphaltenes, proteins, and, as shown by independent experiments in this work, in the flocculation of microalgae. This work aims at investigating possible mechanism to explain this phenomenon using detailed population balance modeling that incorporates refined rate models for aggregation and breakup of small particles in turbulence. Four mechanisms are considered: (1) restructuring, (2) decay of aggregate strength, (3) deposition of large clusters, and (4) primary particle aggregation where only aggregation events between clusters and primary particles are permitted. We show that all four mechanisms can lead to an overshoot in the mean size profile, while in contrast, aggregation and breakup alone lead to a monotonic, "S" shaped size evolution profile. In order to distinguish between the different mechanisms simple protocols based on variations of the shear rate during the aggregation-breakup process are proposed.
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| 7. |
- Sadegh-Vaziri, Ramiar
(författare)
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Modeling in Biomass Harvesting, Biomass Pyrolysis and Producer Gas Cleaning
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biomass is a viable alternative in order to mitigate the environmental effectscaused by the aggressive use of fossil feedstock during the last century.This thesis builds around the idea of a biofuel production process that iscomprised of biomass production, biomass gasication, gas cleaning andfuel production. Biomass production includes microalgae synthesis followedby harvesting to supply microalgae to the gasication process. In the gasi-cation process, the prepared microalgae is dried, pyrolyzed and gasiedto obtain a producer gas containing syngas and impurities. The producergas is cleaned from the impurities and fed to a fuel production unit, e.g. amethanation process.This thesis investigates three distinct aspects related to this processscheme, namely the occulation of microalgae as part of harvesting, biomasspyrolysis as a part of gasication, and sulfur removal from the producer gasas a part of gas cleaning.The investigation of occulation focuses on secondary phenomena thataccompany the aggregation and breakup of the suspended particles, namelyrestructuring, decay of oc strength and settling. For the study of theconsidered phenomena, a population balance model is developed.Slow pyrolysis of biomass is studied on both the reactor scale and thepellet scale. A model for a rotary drum reactor, using principles of isoconversionalanalysis, is developed for the study of dierent biomass feedstock.The proposed model allows for deriving a preliminary reactor design withminimal experimental input data. A one-dimensional nite volume schemeis developed for the investigation of pyrolysis on the pellet scale. The proposedscheme accounts for convective and diusive heat and mass transfer,and is tested against analytical solutions and commercial software packages.Sulfur removal by metal oxides in a packed bed is studied on both thesystem level and the process level. Criteria for the selection of metal oxidesand the design of packed bed units are derived. A detailed analysis isundertaken to study the reaction of H2S with ZnO in a packed bed, wherethe nano-particles of ZnO experience void formation and outward growth.
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| 8. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Modeling of slow pyrolysis of various biomass feedstock in a rotary drum using TGA data
- 2018
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Ingår i: Chemical Engineering and Processing. - : Elsevier. - 0255-2701 .- 1873-3204. ; 129, s. 95-102
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Tidskriftsartikel (refereegranskat)abstract
- Design and optimization of biomass gasification faces the challenge of feedstock variation. Specifically, design calculations require kinetic rate expressions for the given feedstock, whose rigorous determination is demanding and often exceeds available recourses in an early development stage. In this work, we model the slow pyrolysis of biomass for the production of biochar. The aim is to predict the conversion of raw biomass to biochar as a function of the process conditions. Here, we will show that TGA data processed with an isoconversional method is enough to obtain an effective rate expression which allows for predicting the behavior of the biomass at an arbitrary temperature evolution. Such rate expressions can then be used in the process model to simulate conversion of raw biomass to biochar. To illustrate the feasibility of this approach we consider four vastly different biomass, namely spruce wood, pulp, lignin and xylan–lignin, undergoing slow pyrolysis in an indirectly heated rotary kiln reactor. The results of our modeling are compared to experimental data obtained from a 500 kW pilot plant pyrolyzer and to a more detailed process model.
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| 9. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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PBE Modeling of Flocculation of Microalgae : Investigating the Overshoot in Mean Size Profiles
- 2017
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Ingår i: Proceedings of the 9th International Conference on Applied Energy. - : Elsevier. ; , s. 507-512
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Konferensbidrag (refereegranskat)abstract
- Microalgae is considered as a viable feedstock to biomass gasification. After synthesis in water medium, microalgae are separated and dried to a suitable degree to be fed to the gasification process. In order to achieve an efficient separation, a flocculation process is employed, in which microalgae primary particles aggregate and form larger clusters. Although flocculation is a well-established process, there are still some unknown issues related to it, that are worth further research. Experiments show that the mean size of clusters during flocculation goes through a maximum and then decreases with time. We refer to this pattern in the mean size profile as the overshoot. Studying this phenomenon is crucial since the size of clusters has a significant effect on the overall efficiency of the separation of microalgae from water. In this work, we aim at investigating the mechanisms behind the overshoot. The flocculation process is modeled as an aggregation-breakup system by using population balance equations (PBEs). The primary results show that the aggregation and breakup alone cannot lead to the overshoot in the mean size profile. Thus, we suggested three mechanisms that can lead to the overshoot: deposition of large clusters (DLC), restructuring of clusters (RC), and primary particle aggregation (PPA). These mechanisms were examined with numerical simulations and it was revealed that all three lead to the overshoot.
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| 10. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Providing sulfur free syngas to a fuel cell system
- 2019
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Ingår i: Energy Procedia. - : Elsevier Ltd. - 1876-6102. ; , s. 448-453
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Konferensbidrag (refereegranskat)abstract
- Fuel cells are viable alternatives as power backup systems for mini-grids. In this work a case is considered, where the hydrogen fuel to the fuel cells is supplied from biomass gasification. However, the producer gas obtained from biomass gasification needs to be cleaned of impurities and contaminants. In this work we examined the superiority of the hot producer gas cleaning, which results in a better thermal efficiency since the heat loss from the system is reduced. In order to have a viable hot cleaning process, sulfur should be removed at 800°C and this was shown possible by promising primary data from the experiments where H 2 S was removed down to an acceptable level.
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| 11. |
- Sadegh-Vaziri, Ramiar, et al.
(författare)
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Removal of hydrogen sulfide with metal oxides in packed bed reactors-A review from a modeling perspective with practical implications
- 2019
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Ingår i: Applied Sciences. - : MDPI AG. - 2076-3417. ; 9:24
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Forskningsöversikt (refereegranskat)abstract
- Sulfur, and in particular, H2S removal is of significant importance in gas cleaning processes in different applications, including biogas production and biomass gasification. H2S removal with metal oxides is one of the most viable alternatives to achieve deep desulfurization. This process is usually conducted in a packed bed configuration in order to provide a high solid surface area in contact with the gas stream per unit of volume. The operating temperature of the process could be as low as room temperature, which is the case in biogas production plants or as high as 900 °C suitable for gasification processes. Depending on the operating temperature and the cleaning requirement, different metal oxides can be used including oxides of Ca, Fe, Cu, Mn and Zn. In this review, the criteria for the design and scale-up of a packed bed units are reviewed and simple relations allowing for quick assessment of process designs and experimental data are presented. Furthermore, modeling methods for the numerical simulation of a packed bed adsorber are discussed.
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