| 1. |
- Guío-Pérez, Diana Carolina, 1985, et al.
(författare)
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Effective drag on spheres immersed in a fluidized bed at minimum fluidization—Influence of bulk solids properties
- 2023
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Ingår i: Canadian Journal of Chemical Engineering. - : Wiley. - 1939-019X .- 0008-4034. ; 101:1, s. 210-226
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Tidskriftsartikel (refereegranskat)abstract
- The aims of this work are to elucidate the effects that bulk solids properties have on the effective drag experienced by large spheres immersed in an emulsion of group-B solids under minimum fluidization conditions and to analyze the ways in which the different suspensions react towards different applied shear rates. To investigate this, magnetic particle tracking was applied to resolve the trajectory of falling-sphere measurements in which the size, density, and sphericity of the bulk solids were varied as well as the size and density of the spherical tracers. The resulting experimental scope included both rising and sinking tracers as well as full segregation and in-bed stagnation of the tracers. The set-up provided highly resolved tracer trajectories, from which the drag experienced by the sphere can be calculated. For sinking tracers, the results showed that an increase in bulk solids size, angularity, and density reduced the terminal velocity of the sphere. This effect correlated well with the bed expansion and Hausner ratio, indicating that a reduced void space among the bulk solids is the main reason for the increase in motion resistance. At lower shear rates, namely, during the de-acceleration towards the stagnant state, beds of larger, more angular, or denser bulk solids yield lower levels of shear stress. The angle of repose of the bulk solids correlated with the rate at which the emulsion thins with increasing shear rate. For rising tracers, shear stress did not show any significant dependency on the properties of the bulk solids.
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| 2. |
- Köhler, Anna, 1989, et al.
(författare)
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Determination of the Apparent Viscosity of Dense Gas-Solids Emulsion by Magnetic Particle Tracking
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- When designing fluidised bed units a key to ensure efficient conversion is proper control of the mixing of the fuel in both lateral and axial directions in the bed. In order to mechanistically describe the mixing of fuel particles in a fluidised bed, there is a need to determine the apparent viscosity of the gas-solids emulsion, which determines the drag on the fuel particles. In this work the apparent viscosity of a bed of spherical glass beads and air at minimum fluidisation was determined by means of the falling sphere method. Hereto the drag of the bed on a single immersed object was obtained by measuring the velocity of a negatively buoyant tracer with magnetic particle tracking (MPT). MPT allows for highly temporally and spatially resolved trajectories (10-3 s and 10-3 m, respectively) in all 3-dimensions. The bed consisted of glass beads with a narrow size distribution (215 to 250 μm) and tracers with a size from 5 to 20 mm and densities from 4340 to 7500 kg/m3 were used. Hence, the literature, which typically covers data for velocities lying within or just above the Stoke flow regime (0.002 < Re < 2.0) could be expanded to Re numbers (53 to 152) well within the transition flow regime. The drag and apparent viscosity was compared to different fluid models and agreed well with the Newtonian model, when taking into account possible effects of the bed walls. Comparing the drag coefficient of data of free falling spheres and data of spheres falling with controlled velocities, the latter showed a dependence on the product of tracer diameter and falling velocity, dput, while the former was constant over dput. This indicates the method with controlled falling velocities to be intrusive and influencing the result of the apparent viscosity of the bed. Using the free falling sphere method this work obtained an apparent viscosity of 0.24 Pa s, which is consistent with values found in earlier literature for an emulsion of air and sand of similar size and density.
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| 3. |
- Köhler, Anna, 1989, et al.
(författare)
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Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A Magnetic Particle Tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74×0.74 m2 fluidized bed reactor operating at 800 °C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10-3 m and a time resolution of 10-3 s. For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in that the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle.
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| 4. |
- Köhler, Anna, 1989, et al.
(författare)
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Experimental characterization of axial fuel mixing in fluidized beds by magnetic particle tracking
- 2017
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Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 316:SI, s. 492-499
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Tidskriftsartikel (refereegranskat)abstract
- A magnetic particle tracking (MPT) system is applied to a bubbling fluidized bed to study how axial mixing and segregation of fuel are influenced by the fuel density and operational conditions (fluidization velocity, bed height and pressure drop across the gas distributor). The MPT system is used to determine the vertical distribution of the tracer particle in a fluid-dynamically down-scaled cold unit resembling a 0.74×0.74 m^2 fluidized bed reactor operating at 800°C. This work uses a tracer particle of 10 mm in diameter, corresponding to a fuel particle of 44 mm. Different tracer particles are applied with solids density representing biomass, biomass char and that of the average bulk. The MPT system yields a spatial accuracy in the order of 10^-3 m and a time resolution of 10^-3 s.For the operational range investigated, three fuel segregation regimes can be identified from the MPT measurements: 1) A flotsam regime which occurs at low fluidization velocities and for low density tracer particles, 2) A transition regime over which an increase in fluidization velocity results in the presence of fuel particles at the bed surface decreases rapidly, and 3) A fully developed mixing regime in which the presence of tracer particle at the bed surface and the splash zone remains constant with fluidization velocity. The transition velocities between the regimes depend on bed height and density of the tracer particle.
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| 5. |
- Köhler, Anna, 1989, et al.
(författare)
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Magnetic tracking of a fuel particle in a fluid-dynamically down-scaled fluidised bed
- 2017
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Ingår i: Fuel Processing Technology. - : Elsevier BV. - 0378-3820. ; 162, s. 147-156
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Tidskriftsartikel (refereegranskat)abstract
- The mixing of a fuel particle in a fluid-dynamically down-scaled bubbling fluidised bed was studied using magnetic particle tracking. Both the resulting steady-state fuel distributions and the underlying mixing dynamics (fuel velocity field) were investigated. The experimental set-up applied resembles the mixing of an anthracite coal particle in a bed with a cross-section of 0.85 × 0.85 m2 operated at 900 °C with fluidisation velocities in the range of 0.16–0.45 m/s and bed heights in the range of 0.25–0.35 m. Four different gas distributors with variable pressure drops and orifice configurations were investigated. For the cases studied, 7.5 min of sampling time at a sampling frequency of 20 Hz was found to be sufficient to resolve the spatial distribution of the tracer. However, to provide a reliable estimate of the mixing dynamics, a sampling frequency of at least 100 Hz was required, together with a sampling time of approximately 20 s. Results on axial mixing showed improved mixing with increasing fluidisation velocity and bed height. The lateral dispersion coefficients were in the order of 10− 3–10− 2 m2/s (on an up-scaled basis), increased with fluidisation velocity, and were only moderately influenced by the configuration of the gas distributor.
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| 6. |
- Köhler, Anna, 1989, et al.
(författare)
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Modeling Axial Mixing of Fuel Particles in the Dense Region of a Fluidized Bed
- 2020
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 34:3, s. 3294-3304
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Tidskriftsartikel (refereegranskat)abstract
- A semiempirical model for the axial mixing of fuel particles in the dense region of a fluidized bed is presented and validated against experimental magnetic particle tracking in a fluid-dynamically downscaled fluidized bed (Köhler et al. Powder Technol., 2017, 316, 492-499) that resembles hot, large-scale conditions. The model divides the bottom region into three mixing zones: a rising bubble wake solid zone, a zone with sinking emulsion solids, and the splash zone above the dense bed. In the emulsion zone, which is crucial for the mixing, the axial motion of the fuel particle is shown to be satisfactorily described by a force balance that applies experimental values from the literature and an apparent emulsion viscosity of Newtonian character. In contrast, the values derived from the literature for key model parameters related to the bubble wake zone (such as the upward velocity of the tracer), which are derived from measurements carried out under cold laboratory-scale conditions, are known to underestimate systematically the measurements relevant to hot large-scale conditions. When applying values measured in a fluid-dynamically downscaled fluidized bed (Köhler et al. Powder Technol., 2017, 316, 492-499), the modeled axial mixing of fuel tracers shows good agreement with the experimental data. © 2020 American Chemical Society.
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| 7. |
- Köhler, Anna, 1989, et al.
(författare)
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Modeling the motion of fuel particles in a fluidized bed
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 305
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Tidskriftsartikel (refereegranskat)abstract
- A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called ‘recirculation cells’, which are crucial for the lateral mixing. Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (covering a broad range of coefficients: 10−4–10−1 m2/s) reveal satisfactory agreement. The validated model was used to investigate how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section (which is typically uneven in industrial FB furnaces), as well as the size and density of the fuel particles.
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| 8. |
- Köhler, Anna, 1989, et al.
(författare)
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Modelling axial mixing of char - application to the dense bottom bed in CFB boilers
- 2017
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Ingår i: 12th International Conference on Fluidized Bed Technology, CFB 2017. - 9788362079162 ; , s. 397-404
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Konferensbidrag (refereegranskat)abstract
- This work presents a semi-empirical two-phase model for the axial mixing of a spherical tracer particle, aiming to represent a char particle, with application to conditions relevant for the dense bottom bed flow in CFB boilers. The velocity fields of both the bubble and emulsion phases are modelled with validated expressions from literature, while the velocity of the tracer particle in the emulsion phase is obtained from the equation of motion. A correlation for the drag force created by the bed and acting on the tracer particle is found with the help of experimental data from fluid-dynamically down-scaled tests (Köhler et al, 2017). The model is used to predict trends in axial mixing with operational parameters (fluidization velocity, dense bed height) and tracer properties (size and density), which agree well with experimental findings in literature.
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| 9. |
- Köhler, Anna, 1989, et al.
(författare)
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The gas-solid suspension drag on large particles in the transport zone of a circulating fluidized bed
- 2021
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Ingår i: CFB 2021 - Proceedings of the 13th International Conference on Fluidized Bed Technology. ; , s. 148-153
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Konferensbidrag (refereegranskat)abstract
- The effective drag of the gas-solid suspension on large particles representing fuel particles in fluidized beds, has a crucial effect on the mixing of the fuel particles. The suspension drag is therefore an important parameter in CFB reactor modeling, but little is known on its magnitude. This paper presents a method to experimentally determine the suspension drag by measuring the terminal velocity of large particles in the transport zone of a fluidized bed, which is compared with the terminal velocity in the absence of bed material. The drag coefficient of the particles suspended in the gas-solids flow and Reynolds numbers of the fluid flow are presented and compared to the standard drag curve of a single spherical particle. Finally, the effective viscosity of the gas-solid suspension is determined.
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| 10. |
- Sette, Erik, 1984, et al.
(författare)
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3-dimensional particle tracking in a fluid-dynamically downscaled fluidized bed using magneto resistive sensors
- 2015
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Ingår i: The 8th International Symposium on Coal Combustion.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents a measurement technique for continuous tracking of particles in 3-dimensional bubbling fluidized beds operated according to scaling laws. By applying Glicksman’s full set of scaling laws to both bulk solids and tracer particle the bed is assumed to be fluid-dynamically similar to a combustor operated at 900 °C with the tracer particle corresponding to a fuel particle with properties similar to anthracite coal. Two different gas distributors with varying pressure drop are used to investigate the influence of bed design on fuel mixing.Flow structures formed around rising gas bubbles, so called bubble paths, are identified and the tracer particle traverses the entire bed for a gas distributor yielding a high pressure drop. For a gas distributor yielding a low pressure drop flow structures are less pronounced and the tracer particle is not circulating the entire bed.
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