| 1. |
- Birgersson, Erik, et al.
(författare)
-
A quantitative study of the effect of flow-distributor geometry in the cathode of a PEM fuel cell
- 2006
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753 .- 1873-2755. ; 153:1, s. 76-88
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Tidskriftsartikel (refereegranskat)abstract
- An isothermal three-dimensional model describing mass, momentum and species transfer in the cathode of a proton exchange membrane fuel cell has been used to study four different flow-distributors: interdigitated, coflow and counterflow channels, and a foam. A quantitative comparison of the results shows; that the interdigitated channels can sustain the highest current densities, followed in descending order by the foam, the counterflow and the coflow channels. The foam yields the most uniform current density distribution at higher currents, but care should be taken as to its permeability to avoid unreasonably high-pressure drops.
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| 2. |
- Birgersson, Erik, et al.
(författare)
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Analysis of a two-phase non-isothermal model for a PEFC
- 2005
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 152:5, s. A1021-A1034
-
Tidskriftsartikel (refereegranskat)abstract
- A non-isothermal, two-phase model for a polymer electrolyte fuel cell (PEFC) is presented, analyzed, and solved numerically under three different thermal, and two hydrodynamic, modeling assumptions; the consequences of these are then discussed in terms of thermal and water management and cell performance. The study is motivated by recent experimental results that suggest the presence of previously unreported, and thus unmodeled, thermal contact resistances between the components of PEFCs and the discrepancy in the value for the capillary pressure that is used by different authors when modeling the two-phase flow in PEFCs. For the three different thermal assumptions (assuming effective heat conductivities, isothermal flow, and interfacial and bulk conductivites), liquid saturations of around 10% are obtained at the cathode active layer for 1000 mA cm(-2) and a cell voltage of 0.6 V. When lowering the capillary pressure (hydrodynamic assumption), liquid saturations of almost 30% and locally up to 100% are observed at the active layer of the cathode. At this current density and voltage, temperature differences across the cell of around 9 degrees C are predicted. In addition, the effect of varying clamping pressure within the framework of the model is touched upon. The benefits of the scaling analysis conducted here, to predict correctly, prior to numerical computations, important characteristic cell performance quantities such as current density and temperature drop are also highlighted.
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| 3. |
- Birgersson, Erik, et al.
(författare)
-
Reduced two-phase model for analysis of the anode of a DMFC
- 2004
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 151:12, s. A2157-A2172
-
Tidskriftsartikel (refereegranskat)abstract
- An isothermal two-phase ternary mixture model that takes into account conservation of momentum, mass, and species in the anode of a direct methanol fuel cell (DMFC) is presented and analyzed. The slenderness of the anode allows a considerable reduction of the mathematical formulation, without sacrificing the essential physics. The reduced model is then verified and validated against data obtained from an experimental DMFC outfitted with a transparent end plate. Good agreement is achieved. The effect of mass-transfer resistances in the flow field and porous backing are highlighted. The presence of a gas phase is shown to improve the mass transfer of methanol at higher temperatures (>30 degreesC). It is also found that at a temperature of around 30 degreesC, a one-phase model predicts the same current density distribution as a more sophisticated two-phase model. Analysis of the results from the two-phase model, in combination with the experiments, results in a suggestion for an optimal flow field for the liquid-fed DMFC.
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| 4. |
- Brown, Shelley, et al.
(författare)
-
Impedance as a Tool for Investigating Aging in Lithium-Ion Porous Electrodes : II. Positive Electrode Examination
- 2008
-
Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 155:4, s. A320-A338
-
Tidskriftsartikel (refereegranskat)abstract
- High-power positive LixNi0.8Co0.15Al0.05O2 composite porous electrodes are known to be the main source of impedance increase in batteries based on GEN2 chemistry. The impedance of positive electrodes, both fresh and harvested from coin cells aged in an accelerated EUCAR hybrid electric vehicle lifetime matrix, was measured in a three-electrode setup and the results fitted with a physically based impedance model. A methodology for fitting the impedance data, including an optimization strategy incorporating a global genetic routine, was used to fit either fresh or aged positive electrodes simultaneously at different states of charge down to 0.5 mHz. The fresh electrodes had an exchange current density of approximately 1.0 A m(-2), a solid-phase diffusion coefficient of approximately 1.4 x 10(-1)5 m(2) s(-1), and a log-normal active particle size distribution with a mean radius of 0.25 mu m. Aged electrode impedance results were shown to be highly dependent on both the electrode state of charge and the pressure applied to the electrode surface. An aging scenario incorporating loss of active particles, coupled with an increase both in the local contact resistance between the active material and the conductive carbon and the resistance of a layer on the current collector, was shown to be adequate in describing the measured aged electrode impedance behavior.
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| 5. |
- Devine, K. M., et al.
(författare)
-
Analysis of a model for the formation of fold-type oscillation marks in the continuous casting of steel
- 2020
-
Ingår i: IMA Journal of Applied Mathematics. - : Oxford University Press (OUP). - 0272-4960 .- 1464-3634. ; 85:3, s. 385-420
-
Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the different possible behaviours of a recent asymptotic model for oscillation-mark formation in the continuous casting of steel, with particular focus on how the results obtained vary when the heat transfer coefficient (R-mf), the thermal resistance (mu(f)) and the dependence of the viscosity of the flux powder as a function of temperature, are changed. It turns out that three different outcomes are possible: (I) the flux remains in molten state and no solid flux ever forms; (II) both molten and solid flux are present, and the profile of the oscillation mark is continuous with respect to the space variable in the casting direction; (III) both molten and solid flux are present, and the profile of the oscillation mark is discontinuous with respect to the space variable in the casting direction. Although (I) gave good agreement with experimental data, it suffered the drawback that solid flux is typically observed during actual continuous casting; this has been rectified in this work via alternative (II). On the other hand, alternative (III) can occur as a result of hysteresis-type phenomenon that is encountered in other flows that involve temperature-dependent viscosity; in the present case, this manifests itself via the possibility of multiple states for the oscillation-mark profile at the instants in time when solid flux begins to form and when it ceases to form.
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| 6. |
- Florio, B. J., et al.
(författare)
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Mould-taper asymptotics and air gap formation in continuous casting
- 2015
-
Ingår i: Applied Mathematics and Computation. - : Elsevier BV. - 0096-3003 .- 1873-5649. ; 268, s. 1122-1139
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Tidskriftsartikel (refereegranskat)abstract
- We develop a coupled thermomechanical model, that includes mould taper, for the formation of air gaps in the vertical continuous casting of round billets. The system is very sensitive to the small width of the air gap. Mould tapers are used to mitigate the contraction of the solidified shell during cooling. We apply numerical and perturbation methods to show that a small mould taper significantly reduces the insulating effect of the air gap. The analysis is presented in a more transparent and less computationally expensive way than earlier, fully numerical models. We also consider a theoretical ideal taper, which eliminates the air gap altogether. The air gap is found to be quite robust; increasing the size of the taper does not constitute an equal reduction in the air gap size. Sample computations are carried out using parameters for the continuous casting of a pure metal (copper), although the framework can easily be extended to the continuous casting of alloys.
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| 7. |
- Florio, B. J., et al.
(författare)
-
On the interactive effects of mould taper and superheat on air gaps in continuous casting
- 2017
-
Ingår i: Acta Mechanica. - : Springer. - 0001-5970 .- 1619-6937. ; 228:1, s. 233-254
-
Tidskriftsartikel (refereegranskat)abstract
- The formation of an air gap in continuous casting systems is detrimental to the process efficiency as it acts to thermally insulate the cast from the water-cooled mould. By tapering the mould wall, the thermal contraction of the cooling cast can be accommodated so that the thickness of the air gap is decreased. We consider a coupled thermomechanical model to investigate the effect of mould tapering on the formation and thickness of the air gap in an axisymmetric mould. Using asymptotic techniques, the model is reduced to allow analytic and inexpensive numerical investigations while maintaining the essential characteristics of the thermomechanical process. This work improves on previous models by including superheat, where the incoming molten metal is at a higher temperature than its melting point. The degree of superheating also affects the formation and thickness of the air gap and presents a viable alternative for control of the system. The efficacy of mould tapering in the presence of superheat is examined.
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| 8. |
- Fowler, A. C., et al.
(författare)
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The formation of vesicular cylinders in pahoehoe lava flows
- 2015
-
Ingår i: Geophysical and Astrophysical Fluid Dynamics. - : Informa UK Limited. - 0309-1929 .- 1029-0419. ; 109:1, s. 39-61
-
Tidskriftsartikel (refereegranskat)abstract
- Vertical cylinders of bubble-enriched, chemically evolved volcanic rock are found in many inflated pahoehoe lava flows. We provide a putative theoretical explanation for their formation, based on a description of a crystallising three-phase (liquid, solid, gas) crystal pile in which the water-saturated silicate melt exsolves steam and becomes more silica-rich as it crystallises anhydrous minerals. These cylinders resemble pipes that form in solidifying binary alloys as a result of sufficiently vigorous porous medium convection within the mush. A convection model with the addition of gas bubbles that provide the buoyancy source indicates that the effective Rayleigh number is too low for convection to occur in the mush of a basalt lava flow. However, the formation of gas bubbles during crystallisation means that the base state includes fluid migration up through the crystal mush even without convection. Stability considerations suggest that it is plausible to form a positive feedback where increased local porosity causes increased upwards fluid flow, which brings more silicic melt up and lowers the liquidus temperature, promoting locally higher porosity. Numerical solutions show that there are steady solutions in which cylinders form, and we conclude that this model provides a viable explanation for vesicular cylinder formation in inflated basalt lava flows.
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| 9. |
- Frosin, C., et al.
(författare)
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Exploring the potential of muon radiography for blast furnace assessments: advancements in non-invasive imaging and structural analysis
- 2024
-
Ingår i: Journal of Instrumentation. - : IOP Publishing. - 1748-0221. ; 19:2
-
Tidskriftsartikel (refereegranskat)abstract
- The BLEMAB European project (BLast furnace stack density Estimation through online Muon ABsorption measurements), the evolution of the previous Mu-Blast European project, is designed to investigate in detail the capability of muon radiography techniques applied to the imaging of the inner zone of a blast furnace. In particular, the goal of this collaboration is to characterize the internal region (so-called cohesive zone) where the slowly downward-moving material begins to soften and melt, which plays an important role in the performance of the blast furnace itself. In this contribution, we describe the state-of-the-art of the muon tracking system which is currently being developed and installed at a blast furnace on the ArcelorMittal site in Bremen (Germany). Moreover, we will present the GEANT4 simulation framework devised for this application together with the simulation results. Finally, we will show the possible contribution of multiple scattering effects to such peculiar applications.
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| 10. |
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| 11. |
- Ipek, Nulifer, et al.
(författare)
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Numerical and experimental study of the effect of gas evolution in electrolytic pickling
- 2006
-
Ingår i: Journal of Applied Electrochemistry. - : Springer Science and Business Media LLC. - 0021-891X .- 1572-8838. ; 36:12, s. 1367-1379
-
Tidskriftsartikel (refereegranskat)abstract
- As part of a progressive approach to model the electrolytic pickling process, this paper focuses on the important aspect of hydrogen and oxygen gas evolution on the electrodes and on the steel strip being pickled. The system considered consists of type 316 stainless steel pickled in aqueous sodium sulphate, with lead anodes and stainless steel cathodes. The mathematical model is two-dimensional steady-state, and includes the differential equations describing the effect of migration, giving the potential and current fields, and the Tafel kinetic rate expressions for hydrogen and oxygen gas generation. Experiments were conducted to obtain a better understanding of the process and for model validation. Good agreement between the experimental measurements of the global current efficiency and the model predictions was obtained.
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| 12. |
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| 13. |
- Khan, Fareed Ashraf, 1967-
(författare)
-
Some aspects of convection as well as graphite and carbide formations during casting.
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is an established fact that segregation during casting affects the physical properties of carbon-based ferro alloys; this motivates the study of the segregation behaviour of carbon and carbide-forming solute elements. In this context, this thesis investigates two different situations: the effects of cooling rate and turbulence on the precipitation behaviour of the graphite nodules in nodular cast iron; the nature of carbide precipitation in a bearing steel grade and the effects of subsequent soaking on these carbides.The structures of boiling water reactor inserts cast by the uphill and downhill casting of nodular cast iron were examined. The samples were taken from representative locations in the top, middle and bottom cross sections of the castings. It was observed that in uphill-cast inserts the nodules were larger but fewer in number the bottom section, whereas in downhill-cast inserts the nodules in the bottom section were smaller, but greater in number. Variation in volume fraction of the graphite nodules across the inserts was also observed. The probable cause of this variation was the difference in cooling rate in different sections of the insert. Between the steel tubes located at the central part of the casting, the fraction of graphite was lower, which could be the result of carburization of the steel tubes.To study the effects of melt stirring during the solidification of nodular cast iron, several experiments were conducted at variable cooling rates and for different stirring times. Examination of the microstructure was conducted using Light Optical Microscope (LOM) and Scanning Electron Microscope (SEM). It was observed that during stirring the melt oxidized and oxide nuclei were formed. The number of nucleation sites for the precipitation of graphite nodules increased, which raised the nodule count and the fraction of the graphite precipitated. The matrix transformed from pearlite to ferrite, which could be due to the fact that more carbon had diffused out of the matrix.The segregation behaviour in hypereutectoid bearing steel produced by ingot casting was also studied. The effects of soaking on micro and macro segregation was investigated in samples taken from as cast and soaked ingots; emphasis was laid on the bulk matrix and A-segregation channels. Samples were also taken from ingots which were soaked and then hot worked. The micro and macro examination of the microstructure was conducted using LOM and SEM. Quantitative and qualitative composition analysis was performed using Energy-dispersive X-ray spectroscopy (EDX) and an electron micro probe analyzer (EMPA). It was observed that M3C, M2C and M6C had precipitated. The carbide morphology in the bulk matrix was different to that in the A-segregation channels. All the primary carbides in the bulk matrix were found to have dissolved after 4 hours of soaking at 1200oC.
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| 14. |
- Kohlstädt, Sebastian, et al.
(författare)
-
Comparative RANS turbulence modelling of lost salt core viability in high pressure die casting
- 2019
-
Ingår i: Progress in Computational Fluid Dynamics, An International Journal. - : INDERSCIENCE ENTERPRISES LTD. - 1468-4349 .- 1741-5233. ; 19:5, s. 316-327
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, the implementation of three turbulence models inside the open source C++ computational fluid dynamics (CFD) library OpenFOAM were tested in 2D and 3D to determine the viability of salt cores in high pressure die casting. A finite-volume and volume of fluid approach was used to model the two-phase flow of molten metal and air, with the latter being treated as compressible. Encouragingly, it is found that, although the choice of turbulence model seems to affect the dispersion of the two-phase interface, the force acting at the surface of the salt core depends only very weakly on the turbulence model used. The results were also compared against those obtained using the commercially available and widely-used casting software MAGMA(5).
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| 15. |
- Kohlstädt, Sebastian, et al.
(författare)
-
On determining the critical velocity in the shot sleeve of a high-pressure die casting machine using open source cfd
- 2021
-
Ingår i: Fluids. - : MDPI AG. - 2311-5521. ; 6:11
-
Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the critical plunger velocity in high-pressure die casting during the slow phase of the piston motion and how it can be determined with computational fluid dynamics (CFD) in open source software. The melt-air system is modelled via an Eulerian volume-of-fluid approach, treating the air as a compressible perfect gas. The turbulence is treated via a Reynolds-averaged Navier Stokes (RANS) approach that uses the Menter SST k-ω model. Two different strategies for mesh motion are presented and compared against each other. The solver is validated via analytical models and empirical data. A method is then presented to determine the optimal velocity using a two-dimensional (2D) mesh. As a second step, it is then discussed how the results are in line with those obtained for an actual, industrially relevant, three-dimensional (3D) geometry that also includes the ingate system of the die.
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| 16. |
- Kohlstädt, Sebastian, et al.
(författare)
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On the CFD Modelling of Slamming of the Metal Melt in High-Pressure Die Casting Involving Lost Cores
- 2021
-
Ingår i: Metals. - : MDPI. - 2075-4701. ; 11:1
-
Tidskriftsartikel (refereegranskat)abstract
- This paper uses computational fluid dynamics (CFD), in the form of the OpenFOAM software package, to investigate the forces on the salt core in high-pressure die casting (HPDC) when being exposed to the impact of the inflowing melt in the die filling stage, with particular respect to the moment of first impact-commonly known as slamming. The melt-air system is modelled via an Eulerian volume-of-fluid approach, treating the air as a compressible perfect gas. The turbulence is treated via a Reynolds-averaged Navier Stokes (RANS) approach. The RNG k-epsilon and the Menter SST k-omega models are both evaluated, with the use of the latter ultimately being adopted for batch computations. A study of the effect of the Courant number, with a view to establishing mesh independence, indicates that meshes which are finer, and time steps that are smaller, than those previously employed for HPDC simulations are required to capture the effect of slamming on the core properly, with respect to existing analytical models and empirical measurements. As a second step, it is then discussed what response should be expected when this force, with its spike-like morphology and small force-time integral, impacts the core. It is found that the displacement of the core due to the spike in the force is so small that, even though the force is high in value, the bending stress inside the core remains below the critical limit for fracture. It can therefore be concluded that, when assuming homogeneous crack-free material conditions, the spike in the force is not failure-critical.
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| 17. |
- Kohlstädt, Sebastian, et al.
(författare)
-
Towards the modelling of fluid-structure interactive lost core deformation in high-pressure die casting
- 2020
-
Ingår i: Applied Mathematical Modelling. - : Elsevier Inc.. - 0307-904X .- 1872-8480. ; 80, s. 319-333
-
Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the fluid-structure interaction (FSI) that would be expected to occur when a lost core deforms in high-pressure die casting. A two-phase compressible Volume of Fluid approach is used to model the fluid. The turbulence contribution to the Navier-Stokes equations is accounted for by using the Reynolds-averaged Navier Stokes (RANS) Menter SST k−ω model, whilst an isotropic linear elastic model is assumed for the core material itself. The computed results for the core deformation were compared to those obtained for test bodies manufactured by high-pressure die casting, and good agreement was found. An interesting and surprising feature of both the experimental and theoretical results was that the core was found to bend in the direction opposite to that expected from intuition and to that obtained by an earlier model that did not use FSI.
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| 18. |
- Lacaze, J., et al.
(författare)
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Numerical Simulation of Brazing Aluminium Alloys with Al-Si Alloys
- 2018
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Ingår i: Transactions of the Indian Institute of Metals. - : SPRINGER INDIA. - 0972-2815 .- 0975-1645. ; 71:11, s. 2623-2629
-
Tidskriftsartikel (refereegranskat)abstract
- Joining parts using low-melting temperature alloys has long been used for manufacturing complex components such as heat exchangers made of aluminium alloys. Investigations of the process have shown that core/clad interaction during heating and brazing can lead to a significant decrease in the amount of liquid available for joint formation. This study presents a transient one-dimensional model for the process that takes into account the diffusion of silicon and the movement of the core/clad interface, with the model equations being implemented in the finite element software COMSOL Multiphysics;the results are compared to literature experimental data. Silicon profiles in the core are well described, while there appears a significant difference between predicted and experimental values of remaining clads which suggest a strong effect of silicon diffusion and liquid penetration at core grain boundaries.
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| 19. |
- McKee, S., et al.
(författare)
-
An elementary diffusion problem, Laplace transforms and novel mathematical identities
- 2019
-
Ingår i: Journal of Computational and Applied Mathematics. - : Elsevier. - 0377-0427 .- 1879-1778. ; 353, s. 113-119
-
Tidskriftsartikel (refereegranskat)abstract
- While studying a problem in biomedical research a simple diffusion problem arose which admitted a solution by Fourier transforms. It was natural to ask if the same problem could be solved by Laplace transforms. In this note, we provide three solution techniques using Laplace transforms, with the last leading to a number of novel mathematical identities.
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| 20. |
- Mellgren, Niklas, et al.
(författare)
-
Impedance as a Tool for Investigating Aging in Lithium-Ion Porous Electrodes : I. Physically Based Electrochemical Model
- 2008
-
Ingår i: Journal of the Electrochemical Society. - PENNINGTON : ELECTROCHEMICAL SOC INC. - 0013-4651 .- 1945-7111. ; 155:4, s. A304-A319
-
Tidskriftsartikel (refereegranskat)abstract
- Electrochemical impedance spectroscopy is potentially a powerful diagnostic tool for the investigation of the effects of aging in porous electrodes. A physically based three-electrode model was developed for a LixNi0.8Co0.15Al0.05O2 composite porous electrode with three porous separators and a reference electrode between a current collector and a plane electrode. Two effects of aging were modeled for this particular electrode chemistry, namely, a resistive corrosion layer on the current collector and a contact resistance between the electronic conductor and the active material of the porous electrode. The derivation of an analytical solution for the impedances between each pair of electrodes in this model yielded a computationally fast, versatile, and modular formulation. The solution was used to study the impact of selected components of the physical model on the impedance spectrum of the porous electrode for a physically relevant base case. Approximating the active material particles as spherical or flake-shaped particles, lognormally or Dirac distributed in size, revealed that the distribution has a negligible impact while the shape makes a noticeable difference. The main aging-related parameters were shown to have quite distinct effects on the impedance spectrum, which is essential for the regression of experimental data and the study of aging hypotheses.
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| 21. |
- Mellgren, Niklas, 1977-, et al.
(författare)
-
Porous Electrode and Nonequilibrium Water Transport Modelling in Polymer Electrolyte Fuel Cells
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Recent years have seen the appearance of numerous modelling studies of the polymer electrolyte fuel cell. However, in spite of observations in different studies that a model for a cell operating under single-phase conditions must include nonequilibrium water transport and must spatially resolve the porous electrodes in order to capture the behaviour of the cell correctly, there are only very few models in the literature that simultaneously do both. This paper, however, formulates such a model and considers a one-dimensional version of it in a parameter study. In future work, the model will be used to calibrate model parameters against experiments and study the operation of cells in higher dimensions.
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| 22. |
- Mellgren, Niklas, 1977-
(författare)
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Validated Modelling of Electrochemical Energy Storage Devices
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells. A model is formulated to describe an experimental cell setup consisting of a LixNi0.8Co0.15Al0.05O2 composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged LixNi0.8Co0.15Al0.05O2 composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario. Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.
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| 23. |
- Mitchell, S. L., et al.
(författare)
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On the accurate numerical solution of a two-phase Stefan problem with phase formation and depletion
- 2016
-
Ingår i: Journal of Computational and Applied Mathematics. - : Elsevier. - 0377-0427 .- 1879-1778. ; 300, s. 259-274
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, the Keller box finite-difference scheme is employed in tandem with the so-called boundary immobilization method for the purposes of solving a two-phase Stefan problem that has both phase formation and phase depletion. An important component of the work is the use of variable transformations that must be built into the numerical algorithm in order to resolve the boundary-condition discontinuities that are associated with the onset of phase formation and depletion. In particular, this allows the depletion time to be determined, and the solution to be computed after depletion. The method gives second-order accuracy in both time and space for all variables throughout the entire computation.
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| 24. |
- Mitchell, S. L., et al.
(författare)
-
The oxygen diffusion problem : Analysis and numerical solution
- 2015
-
Ingår i: Applied Mathematical Modelling. - : Elsevier BV. - 0307-904X .- 1872-8480. ; 39:9, s. 2763-2776
-
Tidskriftsartikel (refereegranskat)abstract
- A recently derived numerical algorithm for one-dimensional time-dependent Stefan problems is applied to the classical moving boundary problem that arises from the diffusion of oxygen in absorbing tissue; in tandem with the Keller box finite-difference scheme, the so-called boundary immobilization method is used. New insights are obtained into three aspects of the problem: the numerical accuracy of the scheme used; the calculation of oxygen depletion time; and the behaviour of the moving boundary as the oxygen is depleted.
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| 25. |
- Mitchell, S. L., et al.
(författare)
-
Verified reduction of a model for a continuous casting process
- 2017
-
Ingår i: Applied Mathematical Modelling. - : Elsevier. - 0307-904X .- 1872-8480. ; 48, s. 476-490
-
Tidskriftsartikel (refereegranskat)abstract
- The casting of metals is known to involve the complex interaction of turbulent momentum and heat transfer in the presence of solidification, and it is believed that computational fluid dynamical (CFD) techniques are required to model it correctly. Here, using asymptotic methods, we demonstrate that the key quantities obtained in an earlier CFD model for a particular continuous casting process – ostensibly for a pure metal, but equally for an alloy of eutectic composition – can be recovered using a much simpler model that takes into account just the heat transfer, requiring the numerical solution of a two-phase Stefan problem. Combining this with a more recent asymptotic thermomechanical model for the same continuous casting process, we postulate that it should be possible, with the additional help of algebraic manipulation, to reduce a model that takes into account turbulent momentum and heat transfer in the melt and the thermomechanics in the solid shell to one formulated in terms of only heat transfer, without adversely affecting model predictions.
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| 26. |
- Muhmond, Haji Muhammad, 1984-
(författare)
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On the Inoculation and Graphite Morphologies of Cast Iron
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research was conducted to understand the nucleation mechanism of graphite flakes and nodules and the effect of various elements on the growth morphology of graphite. Investigations were started on flake graphite cast iron. The effect of S on the graphite morphology and MnS was experimentally analysed. The influence of various type of oxides on the growth morphology of MnS and their relation with the graphite growth morphology was studied carefully through series of experiments, and results were related to the thermodynamic calculations, performed to predict the nucleation of oxides and sulfides at any temperature. The calculations were supported by EDX analysis on particles in the samples, showing sequential nucleation of flake graphite. The austenite volume fraction and the dendrite growth orientations were affected by the addition of Al to the melt. The effect of aluminium oxide and sulfur was studied on the austenite growth morphology with the help of colour etching technique. Thermal analyses were performed to study the effect of sulfur content on austenite volume fraction and growth morphology using DSC. The latent heat of solidification and the rate of austenite formation was studied and related to the sulfur content in the alloy.To study the effect of O and S on the graphite morphology, pure Fe-C alloy was prepared and strong de-oxidizers and de-sulfurizer (Mg, Ca and Sr) were tested. With the help of ion etching and polishing, we were able to reveal the graphite growth morphology. The morphological changes in the graphite were explained by relating it to the composition of the melt in regard of dissolved S and O in the melt. The effect of other trace elements such as S, F, O, N, P, B and Se, was also investigated by testing them on an ultra-pure Fe-C alloy. The possibility of substituting a foreign atom in the basal plane of graphite was analysed based on the bonding energy between C and the foreign atoms. The resulting change in the structure of the basal plane was studied using Avogadro software and the resulting model were compared with the experimental results. The basal plane of nodular, flake and undercooled graphite was studied with the help of HRTEM, which showed differences in the lattice spacing between different graphite morphologies. The latent heat measurement showed that flake graphite have higher energy than the undercooled or vermicular graphite. This research made us enabled to tell about the growth along c-plane in graphite and the reason behind the transition of graphite morphology.The effect of inoculation temperature and inoculation sequence on the nodularity was studied. EDX analysis were performed and the results were compared to the thermodynamic calculations. It was found that, the melt composition is controlling the graphite growth morphologies. Strong de-oxidizers and de-sulfurizer are necessary to reduce O and S to a level where it does not influence the graphite growth morphology. The effect of various elements on the activity of oxygen in the melt is calculated. High Al cast iron was also studied. The major problem associated with this type of alloy was that, the Mg was not effective in producing nodular graphite. Experiments showed that de-oxidizers stronger than Mg are required to increase the nodularity. Thermodynamic calculations showed that the activity of oxygen in the melt was changing strongly by increasing the Al content in the alloy. This was increasing the required oxygen level to nucleate MgO.
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| 27. |
- Nick, Arash S., et al.
(författare)
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A Theoretical Analysis of the Interaction Between Pores and Inclusions During the Continuous Casting of Steel
- 2016
-
Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 47:6, s. 2985-2999
-
Tidskriftsartikel (refereegranskat)abstract
- A mathematical model is derived to predict the trajectories of pores and inclusions that are nucleated in the interdendritic region during the continuous casting of steel. Using basic fluid mechanics and heat transfer, scaling analysis, and asymptotic methods, the model accounts for the possible lateral drift of the pores as a result of the dependence of the surface tension on temperature and sulfur concentration. Moreover, the soluto–thermocapillary drift of such pores prior to final solidification, coupled to the fact that any inclusions present can only have a vertical trajectory, can help interpret recent experimental observations of pore-inclusion clusters in solidified steel castings.
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| 28. |
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| 29. |
- Noponen, Matti, et al.
(författare)
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A two-phase non-isothermal PEFC model : Theory and validation
- 2004
-
Ingår i: Fuel Cells. - : Wiley. - 1615-6846 .- 1615-6854. ; 4:4, s. 365-377
-
Tidskriftsartikel (refereegranskat)abstract
- A two-dimensional, non-isothermal, two-phase model of a polymer electrolyte fuel cell (PEFC) is presented. The model is developed for conditions where variations in the stream-wise direction are negligible. In addition, experiments were conducted with a segmented cell comprised of net flow fields. The, experimentally obtained, current distributions were used to validate the PEFC model developed. The PEFC model includes species transport and the phase change of water, coupled with conservation of momentum and mass, in the porous backing of the cathode, and conservation of charge and heat throughout the fuel cell. The current density in the active layer at the cathode is modelled with an agglomerate model, and the contact resistance for heat transfer over the material boundaries is taken into account. Good agreement was obtained between the modelled and experimental polarization curves. A temperature difference of 6°C between the bipolar plate and active layer on the cathode, and a liquid saturation of 6% at the active layer in the cathode were observed at 1 A cm-2.
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| 30. |
- Nordlund, Joakim, et al.
(författare)
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The design and usage of a visual direct methanol fuel cell
- 2004
-
Ingår i: Journal of Applied Electrochemistry. - 0021-891X .- 1572-8838. ; 34:8, s. 763-770
-
Tidskriftsartikel (refereegranskat)abstract
- In order to better understand the influence of gas evolution on the performance of the direct methanol fuel cell ( DMFC) anode, a visual DMFC, comprising of a transparent anode and a cathode endplate with an integrated heat exchanger, and a picture analysis methodology were developed. The result was an inexpensive, but very powerful, tool for analyzing the role of two-phase flow. An important finding is that gas bubbles do not appear uniformly throughout the fluid flow matrix, but rather only at a few active sites. Another important finding is that the gas saturation ( volume fraction of gas/volume fraction of liquid) increases along the streamwise direction.
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| 31. |
- Reddy, Gujji Murali Mohan, et al.
(författare)
-
A Compact FEM Implementation for Parabolic Integro-Differential Equations in 2D
- 2020
-
Ingår i: Algorithms. - : MDPI. - 1999-4893. ; 13:10
-
Tidskriftsartikel (refereegranskat)abstract
- Although two-dimensional (2D) parabolic integro-differential equations (PIDEs) arise in many physical contexts, there is no generally available software that is able to solve them numerically. To remedy this situation, in this article, we provide a compact implementation for solving 2D PIDEs using the finite element method (FEM) on unstructured grids. Piecewise linear finite element spaces on triangles are used for the space discretization, whereas the time discretization is based on the backward-Euler and the Crank-Nicolson methods. The quadrature rules for discretizing the Volterra integral term are chosen so as to be consistent with the time-stepping schemes; a more efficient version of the implementation that uses a vectorization technique in the assembly process is also presented. The compactness of the approach is demonstrated using the software Matrix Laboratory (MATLAB). The efficiency is demonstrated via a numerical example on an L-shaped domain, for which a comparison is possible against the commercially available finite element software COMSOL Multiphysics. Moreover, further consideration indicates that COMSOL Multiphysics cannot be directly applied to 2D PIDEs containing more complex kernels in the Volterra integral term, whereas our method can. Consequently, the subroutines we present constitute a valuable open and validated resource for solving more general 2D PIDEs.
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| 32. |
- Reddy, G. M. M., et al.
(författare)
-
An efficient adaptive boundary algorithm to reconstruct Neumann boundary data in the MFS for the inverse Stefan problem
- 2019
-
Ingår i: Journal of Computational and Applied Mathematics. - : ELSEVIER SCIENCE BV. - 0377-0427 .- 1879-1778. ; 349, s. 21-40
-
Tidskriftsartikel (refereegranskat)abstract
- In this exposition, a simple practical adaptive algorithm is developed for efficient and accurate reconstruction of Neumann boundary data in the inverse Stefan problem, which is a highly nontrivial task. Primarily, this algorithm detects the satisfactory location of the source points from the boundary in reconstructing the boundary data in the inverse Stefan problem efficiently. To deal with the ill-conditioning of the matrix generated by the MFS, we use Tikhonov regularization and the algorithm is designed in such a way that the optimal regularization parameter is detected automatically without any use of traditional methods like the discrepancy principle, the L-curve criterion or the generalized cross-validation (GCV) technique. Furthermore, this algorithm can be thought of as an alternative to the concept of Beck's future temperatures for obtaining stable and accurate fluxes, but without it being necessary to specify data on any future time interval. A MATLAB code for the algorithm is discussed in more-than-usual detail. We have studied the effects of accuracy and measurement error (random noise) on both optimal location and number of source points. The effectiveness of the proposed algorithm is shown through several test problems, and numerical experiments indicate promising results.
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| 33. |
- Reddy, G. M. M., et al.
(författare)
-
On efficient reconstruction of boundary data with optimal placement of the source points in the MFS : application to inverse Stefan problems
- 2018
-
Ingår i: Inverse Problems in Science and Engineering. - : Taylor & Francis. - 1741-5977 .- 1741-5985. ; 26:9, s. 1249-1279
-
Tidskriftsartikel (refereegranskat)abstract
- Current practice in the use of the method of fundamental solutions (MFS) for inverse Stefan problems typically involves setting the source and collocation points at some distance, h, from the boundaries of the domain in which the solution is required, and then varying their number, N, so that the obtained solution fulfils a desired tolerance, Tol, when a random noise level d is added to the boundary conditions. This leads to an open question: can h andN be chosen simultaneously so that N is minimized, thereby leading to a lower computational expense in the solution of the inverse problem? Here, we develop a novel, simple and practical algorithm to help answer this question. The algorithm is used to study the effect of Tol and d on both h andN. Its effectiveness is shown through three test problems and numerical experiments show promising results: for example, even with d as high as 5% and Tol as low as 10-3, we are able to find satisfactory solutions for N as low as 8.
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| 34. |
- Redouane, B., et al.
(författare)
-
Experimental, statistical and numerical studies of the continuous casting process based on temperature profiles - part I
- 2016
-
Ingår i: Journal of Chemical Technology and Metallurgy. - 1314-7471. ; 51:1, s. 121-132
-
Tidskriftsartikel (refereegranskat)abstract
- Statistical, experimental and numerical studies are carried out to investigate the frequencies of breakouts during solidification phenomenon in steel continuous casting process at Arcellor Mittal-Annaba plant (Algeria). These breakouts frequencies which have an impact on the management quality field in terms of the quality cost (CoQ) are statistically censused and experimentally investigated during the mould process. The molten steel fluctuation level is measured and the temperature is read during the solidification phenomenon using thermocouples at different locations in the mould connected to the data acquisition. The numerical model involves a generalized set of mass, momentum and heat equations that is valid for the solid, liquid and solidification interval in the mould. The melting and solidification model generated with the software package FLUENT is used to predict numerically the solidification behaviour during the mould process. The variation of the casting speed during the mould process, the molten steel level and the thermal behaviour denoted as temperature profiles are experimentally followed and compared with the statistical data. The effects of the components modifications of the mould, particularly the length, were investigated based on the predicted temperature profiles and field temperature distributions inside the mould.
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| 35. |
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| 36. |
- Safavi Nick, Arash, 1985-, et al.
(författare)
-
Analysis of a model for longitudinal electromagnetic stirring in the continuous casting of steel
- 2021
-
Ingår i: International journal of applied electromagnetics and mechanics. - : IOS Press. - 1383-5416 .- 1875-8800. ; 66:1, s. 35-61
-
Tidskriftsartikel (refereegranskat)abstract
- A recent three-dimensional (3D) model that revisited earlier theoretical work for longitudinal electromagnetic stirring in the continuous casting of steel blooms is analyzed further to explore how the bloom width interacts with the pole pitch of the stirrer to affect the magnetic flux density. Whereas the first work indicated the presence of a boundary layer in the steel near the interface with the stirrer, with all three components of the magnetic flux density vector being coupled to each other, in the analysis presented here we find that the component along the direction of the travelling wave decouples from those in the other two directions and can even be determined analytically in the form of a series solution. Moreover, it is found that the remaining two components can be found via a two-dimensional computation, but that it is not possible in general to determine these components without taking into account the surrounding air. The validity of the asymptotically reduced model solution is confirmed by comparing it with the results of 3D numerical computations. Moreover, the asymptotic approach provides a way to compute the time-averaged Lorentz force components that requires two orders of magnitude less computational time than the fully 3D approach.
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| 37. |
- Safavi Nick, Arash, 1985-, et al.
(författare)
-
On longitudinal electromagnetic stirringing the continuous casting of steel blooms
- 2020
-
Ingår i: Journal of Engineering Mathematics. - : Springer Science and Business Media LLC. - 0022-0833 .- 1573-2703.
-
Tidskriftsartikel (refereegranskat)abstract
- Recent work highlighting an anomaly in the modelling of rotary electromagnetic stirring (EMS) in the continuous casting of round steel billets is extended to the case of longitudinal stirring for rectangular blooms. An earlier, still often-cited, model forms the basis of the current analysis, which uses asymptotic methods on the three-dimensional (3D) Maxwell equations and demonstrates how the earlier result for the components of the Lorentz force is but a particular case of a more general form. Time-dependent 3D computations using finite-element methods are also performed to verify the validity of the asymptotic analysis, and the relevance of the results to modulated EMS is noted.
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| 38. |
- Safavi Nick, Arash, 1985-
(författare)
-
Pores, inclusions and electromagnetic stirring : Topics from the continuous casting of steel
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with two topics of relevance to the continuous casting of steel,in view of their importance as regards the quality of the final solidified structure.The first concerns the precipitation of gas pores and inclusions in the interden-dritic region of the solidifying metal. Motivated by experimental results thatindicate the formation of pore-inclusion clusters in the final cast structure, a the-oretical model is developed to describe how thus might occur; the model makesuse of the basic principles of fluid mechanics and heat transfer, with asymptoticmethods then being used in order to obtain solutions. In particular, it is foundthat soluto-thermocapillary drift in a direction perpendicular to the direction ofcasting, as a consequence of the dependence of surface tension at the pore-metalinterface on temperature and sulphur concentration, could explain cluster forma-tion. The second is a theoretical study concerning longitudinal electromagneticstirring (EMS), which is often used in the continuous casting of blooms in order toimprove product quality. Via an analysis of the three-dimensional (3D) Maxwellequations for the components of the magnetic flux density, a flaw is found inthe way that the components of the stirring Lorentz force have previously beencalculated; this is corrected and the new results are confirmed by comparison ofsolutions obtained from asymptotic analysis and time-dependent 3D computa-tions using finite-element methods. The analysis identifies the importance of theproduct of the bloom width and the wave vector of the applied field as a keydimensionless parameter.
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| 39. |
- Saleem, Saud, et al.
(författare)
-
A Study of the Oscillation Marks’ Characteristics of Continuously Cast Incoloy Alloy 825 Blooms
- 2016
-
Ingår i: Metallurgical and Materials Transactions. A. - : Springer Publishing Company. - 1073-5623 .- 1543-1940. ; 47:8, s. 4068-4079
-
Tidskriftsartikel (refereegranskat)abstract
- A comprehensive experimental study of oscillation mark (OM) formation and its characteristics during the solidification of Incoloy alloy 825 in the continuous casting of blooms is investigated by plant trials and metallographic study. The experiments involved two heats with the same casting and mold conditions and sampling at different locations across the strand. The metallographic study combined macro/micro-examinations of OMs and segregation analysis of Cr, Mn, Mo, Ni, and Si by microprobe analysis. The results show that OMs have widely different characteristics, such as mark type, depth, segregation, and accompanying microstructure. Furthermore, the mark pitch can vary considerably even for the similar casting conditions, leading to different conditions for the marks’ formation in relation to the mold’s cyclic movement. Finally, a mechanism for the OM formation is discussed and proposed. Possible solutions for minimizing the observed defects by optimizing the mold conditions are suggested.
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| 40. |
- Saleem, Saud, et al.
(författare)
-
Effect of casting parameters on the formation of oscillation marks during continuous casting
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In the present investigation, oscillations marks formed at the surfaces of two different steel grades are studied; this includes metallographic investigation. The characteristics of the marks are examined rigorously, and the analysis is performed serially. The statistical data is compared withanalytical relations and possible reasons for the deviations from the average values are discussed.From the ongoing analysis, it can be seen that the formation of the primary shell is an important parameter which can affect the depth of the depression. Moreover, the results show that theformation of the oscillation marks is a complex phenomenon and that there could be more than one explanation for their formation.
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| 41. |
- Saleem, Saud, et al.
(författare)
-
Formation of the tin rich layer and inverse-segregation in phosphor bronzes during continuous casting
- 2015
-
Ingår i: Advances in the Science and Engineering of Casting Solidification. - Hoboken, NJ, USA : The Minerals, Metals, and Materials Society. - 9781119082385 ; , s. 15-22
-
Konferensbidrag (refereegranskat)abstract
- Continuous casting of the phosphor bronzes has been investigated experimentally and analyzed with the help of a thermo-mechanical model. The microscopic investigation shows the spread of the tin rich liquid at the chill surface cause by the formation of flow channels underneath the chill surface. Precipitation of the secondary phases has also been observed under some casting conditions. The macro segregation profile along the solidification thickness predicts a strong casting parameter sensitive inverse segregation. The simulation results show high compressive stresses at the surface of the cast during solidification. The flow channels depth and thermal stress coupled with microsegregation calculations shows the possibility of the pressure driven flow of tin rich liquid towards the chill surface during solidification. The experimental observation and calculated results show that the inverse segregation can be homogenized and decreased by controlling the casting parameter that defines the liquid pool depth into the mould.
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| 42. |
- Saleem, Saud, 1985-
(författare)
-
On the surface quality of continuously cast steels and phosphor bronzes
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work concerns about the importance of the cast surfaces, surface phenomenon such as the formation of the oscillation marks and exudation and related defects including cracks and segregation that happened during the continuous casting. All of the investigated materials were collected during the plant trials while an in-depth analysis on these materials was performed at the laboratory scale with certain explanations supported by the schematic and theoretical models. The work consists on different material classes such as steels and phosphor bronzes with a focus on the surface defects and their improvements. In order to facilitate the theoretical analysis which could be capable of explaining the suggested phenomenon in the thesis, a reduced model is developed which required lesser computational resources with lesser convergence problems.
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| 43. |
- Saleem, Saud, 1985-, et al.
(författare)
-
The Influence of Peritectic Reaction/Transformation on Crack Susceptibility in the Continuous Casting of Steels
- 2017
-
Ingår i: Metallurgical and materials transactions. B, process metallurgy and materials processing science. - : Springer. - 1073-5615 .- 1543-1916. ; 48:3, s. 1625-1635
-
Tidskriftsartikel (refereegranskat)abstract
- The work presented here examines the surface cracks that can form during the continuous casting of near peritectic steels due to the volume changes during the peritectic reaction/transformation. The investigated samples were collected during plant trials from two different steel grades. The role and mode of the peritectic reaction/transformation are found to depend on the composition of the alloy, resulting in different types of surface cracks. The effect of the local variation in the cooling rate on the formation of the different types of cracks present in each steel grade, which can be due, for example, to the formation of oscillation marks, is demonstrated. The enhanced severity of the surface and internal oxidation, both of which depend on the alloy composition and consequent peritectic reaction, is highlighted. Experimental and theoretical studies show that different types of surface cracks can occur in peritectic steels depending upon the alloy composition and cooling rate, both of which define the fraction of the remaining liquid upon completion of the peritectic reaction/transformation.
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| 44. |
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| 45. |
- Sharma, A. K., et al.
(författare)
-
The quasi-steady state of all-vanadium redox flow batteries : A scale analysis
- 2014
-
Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 147, s. 657-662
-
Tidskriftsartikel (refereegranskat)abstract
- In general, mathematical models for all-vanadium redox flow batteries(VRFB) that seek to capture the transport phenomena are transient in nature. In this paper, we carry out scale analysis of VRFB operation and derive the conditions when it can be assumed to be quasi-steady state in nature, i.e., time-dependence only through a boundary condition. We find that it is true for typical tank volume and flow rate employed for VRFBs. The proposed analysisis generic and can also be employed for other types of redox flow batteries. (C) 2014 Elsevier Ltd. All rights reserved.
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| 46. |
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| 47. |
- Stewart, I. W., et al.
(författare)
-
Boundary layers in pressure-driven flow in smectic a liquid crystals
- 2015
-
Ingår i: SIAM Journal on Applied Mathematics. - : Society for Industrial & Applied Mathematics (SIAM). - 0036-1399 .- 1095-712X. ; 75:4, s. 1817-1851
-
Tidskriftsartikel (refereegranskat)abstract
- This article examines the steady flow of a smectic A liquid crystal sample that is initially aligned in a classical "bookshelf" geometry confined between parallel plates and is then subjected to a lateral pressure gradient which is perpendicular to the initial local smectic layer arrangement. The nonlinear dynamic equations are derived. These equations can be linearized and solved exactly to reveal two characteristic length scales that can be identified in terms of the material parameters and reflect the boundary layer behavior of the velocity and the director and smectic layer normal orientations. The asymptotic properties of the nonlinear equations are then investigated to find that these length scales apparently manifest themselves in various aspects of the solutions to the nonlinear steady state equations, especially in the separation between the orientations of the director and smectic layer normal. Non-Newtonian plug-like flow occurs and the solutions for the director profile and smectic layer normal share features identified elsewhere in static liquid crystal configurations. Comparisons with numerical solutions of the nonlinear equations are also made.
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| 48. |
- Tadesse, Abel, 1983-
(författare)
-
On the Volume Changes during the Solidification of Cast Irons and Peritectic Steels
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis work deals with the volume changes during the solidification of cast irons and peritectic steels. The volume changes in casting metals are related to the expansion and/or contraction of the molten metal during solidification. Often, different types of shrinkage, namely macro- and micro-shrinkage, affect the casting quality. In addition to that, exposure of the metal casting to higher contraction or expansion during the solidification might also be related to internal strain development in samples, which eventually leads to surface crack propagation in some types of steel alloys during continuous casting. In consequence, a deep understanding of the mechanisms and control of the solidification will improve casting quality and production.All of the experiments during the entire work were carried out on laboratory scale samples. Displacement changes during solidification were measured with the help of a Linear Variable Displacement Transformer (LVDT). All of the LVDT experiments were performed on samples inside a sand mould. Simultaneously, the cooling curves of the respective samples during solidification were recorded with a thermocouple. By combining the displacement and cooling curves, the volume changes was evaluated and later used to explain the influence of inoculants, carbon and cooling rates on volume shrinkages of the casting. Hypoeutectic grey cast iron (GCI) and nodular cast iron (NCI) with hypo-, hyper- and eutectic carbon compositions were considered in the experiments from cast iron group. High nickel alloy steel (Sandvik Sanbar 64) was also used from peritectic steel type. These materials were melted inside an induction furnace and treated with different types of inoculants before and during pouring in order to modify the composition.Samples that were taken from the LVDT experiments were investigated using a number of different methods in order to support the observations from the displacement measurements: Differential Thermal Analysis (DTA), to evaluate the different phase present; Dilatometry, to see the effect of cooling rates on contraction for the various types of alloys; metallographic studies with optical microscopy; Backscattered electrons (BSE) analysis on SEM S-3700N, to investigate the different types of oxide and sulphide nuclei; and bulk density measurements by applying Archimedes' principle. Furthermore, the experimental volume expansion during solidification was compared with the theoretically calculated values for GCI and NCI.It was found that the casting shows hardly any shrinkage during early solidification in GCI, but in the eutectic region the casting expands until the end of solidification. The measured and the calculated volume changes are close to one another, but the former shows more expansion. The addition of MBZCAS (Si, Ca, Zr, Ba, Mn and Al) promotes more flake graphite, and ASSC (Si, Ca, Sr and Al) does not increase the number of eutectic cells by much. In addition to that, it lowers the primary austenite fraction, promotes more eutectic growth and decreases undercooled graphite and secondary dendritic arm spacing (SDAS). As a result, the volume expansion changes in the eutectic region. The expansion during the eutectic growth increase with an increase in the inoculant weight percentage. At the same time, the eutectic cells become smaller and increase in number. The effect of the inoculant and the superheat temperature shows a variation in the degree of expansion/contraction and the cooling rates for the experiments. Effective inoculation tends to homogenize the eutectic structure, reducing the undercooled and interdendritic graphite throughout the structure.In NCI experiments, it was found that the samples showed no expansion in the transversal direction due to higher micro-shrinkages in the centre, whereas in the longitudinal direction the samples shows expansion until solidification was complete. The theoretical and measured volume changes agreed with each other. The austenite fraction and number of micro-shrinkage pores decreased with increase in carbon content. The nodule count and distribution changes with carbon content. The thermal contractionof NCI is not influenced by the variation in carbon content at lower cooling rates. The structural analysis and solidification simulation results for NCI show that the nodule size and count distribution along the cross-sections at various locations are different due to the variation in cooling rates and carbon concentration. Finer nodule graphite appears in the thinner sections and close to the mold walls. A coarser structure is distributed mostly in the last solidified location. The simulation result indicates that finer nodules are associated with higher cooling rate and a lower degree of microsegregation, whereas the coarser nodules are related to lower cooling rate and a higher degree of microsegregation. As a result, this structural variation influences the micro-shrinkage in different parts.The displacement change measurements show that the peritectic steel expands and/or contracts during the solidification. The primary austenite precipitation during the solidification in the metastable region is accompanied by gradual expansion on the casting sides. Primary δ-ferrite precipitation under stable phase diagram is complemented by a severe contraction during solidification. The microstructural analysis reveals that the only difference between the samples is grain refinement with Ti addition. Moreover, the severe contraction in solidification region might be the source for the crack formation due to strain development, and further theoretical analysis is required in the future to verify this observation.
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| 49. |
- Tome, M. F., et al.
(författare)
-
A finite difference technique for solving a time strain separable K-BKZ constitutive equation for two-dimensional moving free surface flows
- 2016
-
Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 311, s. 114-141
-
Tidskriftsartikel (refereegranskat)abstract
- This work is concerned with the numerical solution of the K-BKZ integral constitutive equation for two-dimensional time-dependent free surface flows. The numerical method proposed herein is a finite difference technique for simulating flows possessing moving surfaces that can interact with solid walls. The main characteristics of the methodology employed are: the momentum and mass conservation equations are solved by an implicit method; the pressure boundary condition on the free surface is implicitly coupled with the Poisson equation for obtaining the pressure field from mass conservation; a novel scheme for defining the past times t' is employed; the Finger tensor is calculated by the deformation fields method and is advanced in time by a second-order Runge-Kutta method. This new technique is verified by solving shear and uniaxial elongational flows. Furthermore, an analytic solution for fully developed channel flow is obtained that is employed in the verification and assessment of convergence with mesh refinement of the numerical solution. For free surface flows, the assessment of convergence with mesh refinement relies on a jet impinging on a rigid surface and a comparison of the simulation of a extrudate swell problem studied by Mitsoulis (2010) [44] was performed. Finally, the new code is used to investigate in detail the jet buckling phenomenon of K-BKZ fluids.
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| 50. |
- Vynnycky, Michael, et al.
(författare)
-
An analytical and numerical study of coupled transient natural convection and solidification in a rectangular enclosure
- 2007
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 50:25-26, s. 5204-5214
-
Tidskriftsartikel (refereegranskat)abstract
- The transient process of the solidification of a pure liquid phase-change material in the presence of natural convection in a rectangular enclosure is considered both analytically and numerically. One vertical boundary is held at a temperature below the melting-point of the material, the other above; the horizontal boundaries are both assumed adiabatic. A nondimensional analysis of the problem, principally in terms of the Rayleigh (Ra) and Stefan (St) numbers, indicates that some asymptotic simplification is possible for materials often considered in the literature (water, gallium, lauric acid). This observation suggests a way to simplify the full problem when Ra >> 1 and St << 1, giving a conventional boundary value problem for the liquid phase and pointwise-in-space first-order ODEs for the evolution in time of the solidification front. The method is tested against full 2D finite-element-based transient numerical simulations of solidification. In addition, simpler approaches for determining the average thickness of the solid layer, based on boundary-layer and enclosure flow correlations, are also investigated.
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