| 1. |
- Dahlkild, Anders
(författare)
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Finite wavelength selection for the linear instability of a suspension of settling spheroids
- 2011
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 689, s. 183-202
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Tidskriftsartikel (refereegranskat)abstract
- The instability of an initially homogeneous suspension of spheroids, settling due to gravity, is reconsidered. For non-spherical particles, previous studies in the literature report that normal-mode density perturbations of maximum growth rate are those of arbitrarily large, horizontal wavelength. Using the 'mixture theory' for two-phase flow we show that the maximum growth rate for horizontal density perturbations is obtained for a finite wavelength if the inertia of the bulk motion associated with the normal-mode density perturbation is accounted for. We find that for long wavelengths, lambda -> infinity, the growth rate approaches zero as lambda(-2/3). The maximum growth rate is obtained for lambda similar to d/root alpha(0)Re(L)(1/2), where d is the axis perpendicular to the axis of rotational symmetry of the spheroid, alpha(0) is the undisturbed volume fraction of particles and Re(L), typically << 1, is a Reynolds number of the bulk motion on a typical length scale L similar to d/p root alpha(0) and a velocity scale on the order of the undisturbed settling speed. The theoretical results for the wavelength selection agree qualitatively well with previous experimental results in the literature of measured correlation lengths of vertical streamers in settling fibre suspensions.
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| 2. |
- Dahlkild, Anders
(författare)
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Particle orientation distribution in a rotating, dilute suspension of rod-shaped particles
- 2023
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 954
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Tidskriftsartikel (refereegranskat)abstract
- We consider a theoretical model for the settling of rod-shaped particles of a dilute, initially homogeneous, suspension in rapid rotation. The particle Reynolds number and the particle Taylor number of the detailed flow around the particles are assumed small, representing a relevant limit for an industrial centrifugal separation process. By applying a statistical approach using the Fokker-Planck equation, and neglecting particle-particle interactions, we obtain an explicit, analytical solution for the time dependent, spatially uniform particle orientation distribution function. Not only does the volume fraction in the bulk of the suspension decrease with time due to the divergent centrifugal field, as similarly described in the literature for suspensions of spherical particles, the orientation of the rod particles also changes with time from an initially uniform distribution to one where the particles tend to align with a plane perpendicular to the axis of rotation. The corresponding particle trajectories, as also influenced by first-order effects from the Coriolis acceleration and gyroscopic effects, are obtained numerically for different initial particle orientation angles.
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| 3. |
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| 4. |
- Dahlkild, Åsa, 1976-
(författare)
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Phylogenetic and ecological studies in lichenology : including studies of both mycobiont and photobiont
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis different aspects in lichens are investigated, namely, the phylogeny of mycobiont and photobiont, and ecology in the form of species richness of certain habitats of the Uppland province, Sweden.Analyses of ITS sequences of the lichenized fungi genus Roccella (Roccellaceae) provide an intrageneric topology of the genus with emphasis on the European, Mediterranean and Macaronesian species. One species, Roccella elisabethae Tehler, is described and a key to the Macaronesian and European species is provided. Sequences from five Roccella species are new. One significant result is that the Macaronesian species constitute a monophyletic group.Newly designed algal-specific primers were used to amplify the ribosomal ITS region from 25 photobiont specimens from five lichenized fungal species of the family Physciaceae (Lecanorales). Trebouxia impressa was present in all mycobionts except Anaptychia ciliaris, which instead was associated with Trebouxia arboricola. A phylogeny for the photobiont, T. impressa, was compared to a phylogeny of the corresponding mycobionts. The general similarity of the mycobiont and photobiont trees may indicate a coevolutionary history. The ribosomal SSU region and the rbcL gene were sequenced from total DNA extracts from thalli of 8 species of Siphula (Icmadophilaceae) and 6 species of Parasiphula (Coccotremataceae). All specimens were found to have Elliptochloris bilobata in a wider sense as photobiont, indicating independent accession of photobionts for the two mycobiont genera.The epiphytic lichen flora on Fraxinus exelcior and Acer platanoides in lanes and two types of wooded meadows were investigated in Uppland, Sweden. Altogether 129 lichen species were found, five of which are listed in the Swedish Red List of threatened species. The species richness was affected by substrate, habitat, and distance to dust source. Lanes had the highest species richness, and Fraxinus excelsior had higher species richness than Acer platanoides.
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| 5. |
- Danielsson, Carl-Ola, et al.
(författare)
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A Model for the Enhanced Water Dissociation On Monopolar Membranes
- 2009
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 54:11, s. 2983-2991
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Tidskriftsartikel (refereegranskat)abstract
- A model for the enhanced water dissociation that takes place at the solution/membrane interface in electromembrane processes is presented. The mechanisms behind the enhanced water dissociation are poorly understood and therefore a semi-empirical approach is suggested. The enhanced water dissociation is introduced as a heterogeneous surface reaction similar to the well established Butler–Volmer law for electrode reactions. In the model there are two parameters that need to be determined through experiments. A 1D diffusion boundary layer problem is presented and solved in order to show that a sufficient rate of water dissociation can be obtained with the model. The advantage of the presented model is that it can easily be incorporated into simulations of electromembrane processes such as electrodialysis, electrodeionization and electropermutation. The influence of the enhanced water dissociation on these processes can then be studied.
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| 6. |
- Danielsson, Carl-Ola
(författare)
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Electropermutation assisted by ion-exchange textile : removal of nitrate from drinking water
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increased levels of nitrate in ground water have made many wells unsuitable as sources for drinking water. In this thesis an ion-exchang eassisted electromembrane process, suitable for nitrate removal, is investigated both theoretically and experimentally. An ion-exchange textile material is introduced as a conducting spacer in the feed compartment of an electropermutation cell. The sheet shaped structure of the textile makes it easy to incorporate into the cell. High permeability and fast ion-exchange kinetics, compared to ion-exchange resins, are other attractive features of the ion-exchange textile. A steady-state model based on the conservation of the ionic species is developed. The governing equations on the microscopic level are volume averaged to give macro-homogeneous equations. The model equations are analyzed and relevant simplifications are motivated and introduced. Dimensionless parameters governing the continuous electropermutation process are identified and their influence on the process are discussed. The mathematical model can be used as a tool when optimising the process parameters and designing equipment. An experimental study that aimed to show the positive influence of using the ion-exchange textile in the feed compartment of an continuous electropermutation process is presented. The incorporation of the ion-exchange textile significantly improves the nitrate removal rate at the same time as the power consumption is decreased. A superficial solution of sodium nitrate with a initial nitrate concentration of 105 ppm was treated. A product stream with less than 20 ppm nitrate could be obtained, in a single pass mode of operation. Its concluded from these experiments that continuous electropermutation using ion-exchange textile provides an interesting alternative for nitrate removal, in drinking water production. The predictions of the mathematical model are compared with experimental results and a good agreement is obtained. Enhanced water dissociation is known to take place at the surface of ion-exchange membranes in electromembrane processes operated above the limiting current density. A model for this enhanced water dissociation in presented in the thesis. The model makes it possible to incorporate the effect of water dissociation as a heterogeneous surface reaction. Results from simulations of electropermutation with and without ion-exchange textile incorporated are presented. The influence of the water dissociation is investigated with the developed model.
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| 7. |
- Danielsson, Carl-Ola, et al.
(författare)
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Modeling Continuous Electropermutation with Effects of Water Dissociation Included
- 2010
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Ingår i: AIChE Journal. - : Wiley. - 0001-1541 .- 1547-5905. ; 56:9, s. 2455-2467
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Tidskriftsartikel (refereegranskat)abstract
- The repeating unit consisting of a cell pair of one concentrate and one feed compartment of an electropermutation stack is modeled. Both the feed and the concentrate compartments are filled with an ion-exchange textile material. Enhanced water dissociation taking place at the surface of the membrane is included in the model as a hetrogeneous surface reaction. Results from simulations of nitrate removal for drinking water production are presented and comparisons with previous experimental results are made. The influence of both conductive and inert textile spacers on the process is investigated via simulations
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| 8. |
- Danielsson, Carl-Ola, et al.
(författare)
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Nitrate Removal by Continuous Electropermutation Using Ion-Exchange Textile : II. Experimental Investigation
- 2006
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 153:4, s. D62-D67
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Tidskriftsartikel (refereegranskat)abstract
- Water with nitrate concentrations above 100 ppm has been treated with continuous electropermutation which partially substitutes the nitrate with chloride. The performance of a textile anion exchanger as conducting spacer in the feed compartment of an electropermutation cell was investigated. Experiments with and without textile are compared and the influence of the textile is discussed. The process could, using the textile, successfully treat feed water with 105 ppm nitrate to produce a water with less than 25 ppm nitrate. The importance of establishing a good contact between the membranes and the textile spacer was pointed out. The experimental results were compared to model predictions and a good agreement was found.
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| 9. |
- Danielsson, Carl-Ola, et al.
(författare)
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Nitrate Removal by Continuous Electropermutation Using Ion-Exchange Textile : I. Modeling
- 2006
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 153:4, s. D51-D61
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a steady-state model of the feed compartment of an electropermutation cell, used for nitrate removal, with ion exchange textiles incorporated as a conducting spacer. In the model the ion-exchange textile is treated as a porous medium and volume averaging is applied to obtain a macrohomogeneous two-phase model. The ion-exchange between the two phases is modeled assuming that the rate-determining step is the mass-transfer resistance on the liquid side of the phase interface. Analysis of the model equations reveals appropriate simplifications. The influence of the governing dimensionless numbers is investigated through simulations based on the model.
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| 10. |
- Fiusco, Francesco, 1994-
(författare)
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Computational modelling of blood flow in medical assist devices
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Extracorporeal membrane oxygenation (ECMO) is a life-saving support treat-ment in case of pulmonary and/or cardiac failure. An artificial extracorporealcircuit is used to offload the function of lungs and/or heart. Patient blood is drained through a drainage cannula, pumped with a centrifugal pump, oxygenated in a membrane lung and returned to the body through a reinfusion cannula. Tubing and connectors complete the circuit. However, its use canlead to thromboembolic and haemolytic complications, which are related to mechanical stresses arising in the flow of blood through its components. Numerical simulations of some of the pumps and cannulae used in the circuit were performed to investigate the flow structures developing in these components and their relation to measures of blood damage in the form of platelet activation state (PAS) and haemolysis index (HI). Simulations of two magnetically levitated centrifugal ECMO pumps were performed both in on- and off-label conditions with flow rates compatible with adult and neonatal use. The results showed that off-label low flow rate can be damaging due to an increase of residence time of the particles, which exposed them for longer to non-physiological stress. This held true for both passive tracers and inertial particles subjected to lift and drag. The neonatal pump showed a backflow structure with flow swirling back to the inlet tubing over its whole labelled range. Simulations of a lighthouse drainage cannula were undertaken to assess drainage characteristics at different haematocrits and flow rate ratios. The results indicated that the flow field was dominated by a jet in crossflow type of structure, with the most proximal holes draining the largest amount of fluid in all the studied cases and for all the considered haematocrits. The effects due to non-Newtonian behaviour of blood were less relevant in the drainage area, allowing to use a Reynolds number analogy to bridge between water and blood results.A lighthouse cannula in return configuration was also considered in both a centred and a tilted position. A characteristic confined jet configuration was found, with a backflow developing at the vessel wall, increasing residence time. In the tilted case, a group of small vortical structures developed at the holes close to the wall, which behaved as an obstacle to the vessel flow and increased both residence time and stress. This led to locally increased haemolysis which, however, did not impact haemolysis at large due to the low flow exposed to this area. The use of different viscosity models in this case led to small variations in the results, which were minor compared to the uncertainty introduced by the use of different model coefficients in the computation of the haemolysis index.
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