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- 2019
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Tidskriftsartikel (refereegranskat)
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- Andersson, M., et al.
(författare)
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Coupling of lattice boltzmann and volume of fluid approaches to study the droplet behavior at the gas diffusion layer/gas channel interface
- 2018. - 13
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-6737 .- 1938-5862. - 9781607688600 ; 86, s. 329-336
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Konferensbidrag (refereegranskat)abstract
- A typical polymer electrolyte fuel cell (PEFC) flow field consists of micro/minichannels. The continues removal of liquid water from the cathode channels is a critical topic, as water droplets forming in the channels may block the transport of gaseous oxygen to the active sites, which not only gives an uneven current distribution and substantial loss of performance, but also, increases degradation rates and unstable operation. Water generated by the electrochemical reactions condenses, depending on temperature mainly, into liquid form, potentially flooding various part of the PEFC. The aim of this work is to obtain an increased understanding of the droplet behavior at the gas diffusion layer (GDL) interface with the gas channels in PEFCs by the coupling of Lattice Boltzmann (LB) and Volume of Fluid (VOF) approaches. A multiscale environment is established with input parameters in the VOF model being extracted from in-house LB calculations. It is clear that the contact angle as well as the size of the liquid droplet vary with positions at the GDL surface, depending on the stochastic GDL geometry. A VOF model describing one straight channel with one gas inlet, one liquid inlet (at the GDL surface) and one two-phase outlet is employed.
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- Zhang, Shidong, et al.
(författare)
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Simple and complex polymer electrolyte fuel cell stack models : A comparison
- 2018. - 13
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-6737 .- 1938-5862. - 9781607685395 ; 86, s. 287-300
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Konferensbidrag (refereegranskat)abstract
- In this paper, two distinct polymer electrolyte fuel cell stack models are constructed: a detailed numerical model (DNM) employing a fine-scale computational mesh and a coarse-mesh approach based on a distributed resistance analogy (DRA) where diffusion terms in the transport equations are replaced by rate terms. Both methods are applied to a 5-cell, high-temperature polymer electrolyte fuel cell stack with an active area of 200 cm2 per cell. The polarization curve and local current density distributions from both the DRA and DNM are compared with experimental data, finding good agreement. Temperature, pressure, Nernst potential, and species distributions are also exhibited. The DNM displays details of fine-scale local extrema not captured by the DRA; however, the latter requires orders of magnitude less computer processor power and memory for execution. Both methods provide much finer-scale results than present experimental techniques.
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- Messa, Matteo, et al.
(författare)
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The young star cluster population of M51 with LEGUS - I. A comprehensive study of cluster formation and evolution
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 473:1, s. 996-1018
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Tidskriftsartikel (refereegranskat)abstract
- Recently acquired WFC3 UV (F275W and F336W) imaging mosaics under the Legacy Extragalactic UV Survey (LEGUS), combined with archival ACS data of M51, are used to study the young star cluster (YSC) population of this interacting system. Our newly extracted source catalogue contains 2834 cluster candidates, morphologically classified to be compact and uniform in colour, for which ages, masses and extinction are derived. In this first work we study the main properties of the YSC population of the whole galaxy, considering a mass-limited sample. Both luminosity and mass functions follow a power-law shape with slope -2, but at high luminosities and masses a dearth of sources is observed. The analysis of the mass function suggests that it is best fitted by a Schechter function with slope -2 and a truncation mass at 1.00 +/- 0.12 x 10(5) M-circle dot . Through Monte Carlo simulations, we confirm this result and link the shape of the luminosity function to the presence of a truncation in the mass function. A mass limited age function analysis, between 10 and 200 Myr, suggests that the cluster population is undergoing only moderate disruption. We observe little variation in the shape of the mass function at masses above 1 x 10(4) M-circle dot over this age range. The fraction of star formation happening in the form of bound clusters in M51 is similar to 20 per cent in the age range 10-100 Myr and little variation is observed over the whole range from 1 to 200 Myr.
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| 6. |
- Andersson, M., et al.
(författare)
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Modeling of droplet detachment using dynamic contact angles in polymer electrolyte fuel cell gas channels
- 2019
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 44:21, s. 11088-11096
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Tidskriftsartikel (refereegranskat)abstract
- Climate change, energy security and air pollution are all motivators for the further development of fuel cells. A volume of fluid approach was used to investigate the impact of dynamic contact angle boundary conditions (Kistler model), mainly at the gas diffusion layer surface but also at the channel wall, of a polymer electrolyte fuel cell gas channel. From this study, it is clear that a dynamic contact angle boundary condition, with advancing and receding contact angles, influences the droplet detachment characteristics, for example, the detachment time and droplet size. Implementing dynamic contact angle boundary conditions for a thin channel causes the droplet, after being reattached to the wall on the side opposite the GDL, to flow very slowly when attached to the wall, until it is merged with a second droplet and they exit the channel (but remain attached to the wall) fairly quickly. Similar phenomena are not observed while using a static contact angle.
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| 7. |
- Stewart, Joshua D., et al.
(författare)
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Spatial ecology and conservation of Manta birostris in the Indo-Pacific
- 2016
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Ingår i: Biological Conservation. - : Elsevier BV. - 0006-3207 .- 1873-2917. ; 200, s. 178-183
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Tidskriftsartikel (refereegranskat)abstract
- Information on the movements and population connectivity of the oceanic manta ray (Manta birostris) is scarce. The species has been anecdotally classified as a highly migratory species based on the pelagic habitats it often occupies, and migratory behavior exhibited by similar species. As a result, in the absence of ecological data, population declines in oceanic manta have been addressed primarily with international-scale management and conservation efforts. Using a combination of satellite telemetry, stable isotope and genetic analyses we demonstrate that, contrary to previous assumptions, the species appears to exhibit restricted movements and fine-scale population structure. M. birostris tagged at four sites in the Indo-Pacific exhibited no long-range migratory movements and had non-overlapping geographic ranges. Using genetic and isotopic analysis, we demonstrate that the observed movements and population structure persist on multi-year and generational time scales. These data provide the first insights into the long-term movements and population structure of oceanic manta rays, and suggest that bottom-up, local or regional approaches to managing oceanic mantas could prove more effective than existing, international-scale management strategies. This case study highlights the importance of matching the scales at which management and relevant ecological processes occur to facilitate the effective conservation of threatened species.
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| 8. |
- Andersson, M, et al.
(författare)
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Dynamic Contact Angle Modeling of Droplet Reattachment at the Gas Channel Wall in Polymer Electrolyte Fuel Cells
- 2019
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Ingår i: eTransportation. - : Elsevier BV. - 2590-1168. ; , s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- Energy security, climate change and air pollution are all motivations for further development of fuel cells. Still, technical problems relating to water management, continue to hinder the marketability of polymer electrolyte fuel cells (PEFCs). The impact of dynamic contact angle (CA) boundary conditions, according to the Kistler model, is evaluated in this paper with the VOF approach, focusing on droplet reattachment at the gas channel wall. From this, it is clear that dynamic CA boundary conditions, compared to static CA boundary conditions, significantly influence the droplet reattachment characteristics, for example for the standard case with a gas velocity of 10 m/s, the first droplet awaits attachment to the channel wall on the side opposite the gas diffusion layer surface for a second droplet before merging and then moving out of the channel together, attached to the channel wall. The impact from dynamic CAs becomes even bigger for lower velocities (5 m/s in this case), where the droplet residence time increases significantly. It is clear that the channel dimensions, gas inlet velocity and value of CAs, as well as if a static or dynamic CA model is used, all have a significant impact on the droplet characteristics in PEFC GCs.
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| 9. |
- Andersson, M., et al.
(författare)
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Interface resolving two-phase flow simulations in gas channels relevant for polymer electrolyte fuel cells using the volume of fluid approach
- 2018
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 43:5, s. 2961-2976
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Tidskriftsartikel (refereegranskat)abstract
- With the increased concern about energy security, air pollution and global warming, the possibility of using polymer electrolyte fuel cells (PEFCs) in future sustainable and renewable energy systems has achieved considerable momentum. A computational fluid dynamic model describing a straight channel, relevant for water removal inside a PEFC, is devised. A volume of fluid (VOF) approach is employed to investigate the interface resolved two-phase flow behavior inside the gas channel including the gas diffusion layer (GDL) surface. From this study, it is clear that the impact on the two-phase flow pattern for different hydrophobic/hydrophilic characteristics, i.e., contact angles, at the walls and at the GDL surface is significant, compared to a situation where the walls and the interface are neither hydrophobic nor hydrophilic (i.e., 90° contact angle at the walls and also at the GDL surface). A location of the GDL surface liquid inlet in the middle of the gas channel gives droplet formation, while a location at the side of the channel gives corner flow with a convex surface shape (having hydrophilic walls and a hydrophobic GDL interface). Droplet formation only observed when the GDL surface liquid inlet is located in the middle of the channel. The droplet detachment location (along the main flow direction) and the shape of the droplet until detachment are strongly dependent on the size of the liquid inlet at the GDL surface. A smaller liquid inlet at the GDL surface (keeping the mass flow rates constant) gives smaller droplets.
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| 10. |
- Andersson, M., et al.
(författare)
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Modeling and synchrotron imaging of droplet detachment in gas channels of polymer electrolyte fuel cells
- 2018
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 404, s. 159-171
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Tidskriftsartikel (refereegranskat)abstract
- A computational fluid dynamics model of a channel (one liquid inlet, one liquid inlet and one two-phase outlet), applicable for PEFC gas channel water transport, is developed. A volume of fluid approach is used to study the two-phase flow behavior (interface-resolved) inside the gas channel, including the surface of GDL (gas diffusion layer), which is verified by experimental results from synchrotron based X-ray radiography and tomography imaging. A reasonably good agreement is found between the model and the measurements in terms of droplet dynamics, shape, and size. The channel height strongly influences the droplet transport behavior, with the droplet being attached to the GDL surface, as well as to the wall on the opposite side to the GDL at the same time for the shallowest channel (150 μm). The GDL contact angle influences the droplet size, with an increased GDL contact angle creating smaller droplets.
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