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  • 1.
    Birgersson, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet.
    Noponen, M.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet.
    Analysis of a two-phase non-isothermal model for a PEFC2005Inngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, nr 5, s. A1021-A1034Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 2.
    Birgersson, Erik
    et al.
    KTH, Tidigare Institutioner, Mekanik.
    Nordlund, Joakim
    KTH, Tidigare Institutioner, Kemiteknik.
    Vynnycky, Michael
    KTH, Tidigare Institutioner, Mekanik.
    Picard, Cyril
    KTH, Tidigare Institutioner, Mekanik.
    Lindbergh, Göran
    KTH, Tidigare Institutioner, Kemiteknik.
    Reduced two-phase model for analysis of the anode of a DMFC2004Inngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 151, nr 12, s. A2157-A2172Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 3.
    Birgersson, Erik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet.
    A quantitative study of the effect of flow-distributor geometry in the cathode of a PEM fuel cell2006Inngår i: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 153, nr 1, s. 76-88Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 4.
    Brown, Shelley
    et al.
    KTH, Skolan för kemivetenskap (CHE), Kemiteknik.
    Mellgren, Niklas
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindbergh, Göran
    KTH, Skolan för kemivetenskap (CHE), Kemiteknik.
    Impedance as a Tool for Investigating Aging in Lithium-Ion Porous Electrodes: II. Positive Electrode Examination2008Inngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 155, nr 4, s. A320-A338Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 5. Florio, B. J.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Mitchell, S. L.
    O'Brien, S. B. G.
    Mould-taper asymptotics and air gap formation in continuous casting2015Inngår i: Applied Mathematics and Computation, ISSN 0096-3003, E-ISSN 1873-5649, Vol. 268, s. 1122-1139Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 6. Florio, B. J.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Mitchell, S. L.
    O'Brien, S. B. G.
    On the interactive effects of mould taper and superheat on air gaps in continuous casting2017Inngår i: Acta Mechanica, ISSN 0001-5970, E-ISSN 1619-6937, Vol. 228, nr 1, s. 233-254Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 7. Fowler, A. C.
    et al.
    Rust, Alison C.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning. MACSI, University of Limerick, Ireland.
    The formation of vesicular cylinders in pahoehoe lava flows2015Inngår i: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 109, nr 1, s. 39-61Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 8.
    Ipek, Nulifer
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Tidigare Institutioner                               , Mekanik.
    Cornell, Ann
    KTH, Tidigare Institutioner                               , Kemiteknik.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Tidigare Institutioner                               , Mekanik.
    Single- and two-phase modelling of the electrochemical pickling of steelManuskript (preprint) (Annet vitenskapelig)
  • 9.
    Ipek, Nulifer
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lior, Noam
    University of Pennsylvania, Dept. Mech. Eng. and Appl. Mechanics.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Bark, Fritz H.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Processteknisk strömningsmekanik.
    Numerical and experimental study of the effect of gas evolution in electrolytic pickling2006Inngår i: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 36, nr 12, s. 1367-1379Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 10. Khaleque, Tania S.
    et al.
    Fowler, A. C.
    Howell, P. D.
    Vynnycky, Mikael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Numerical studies of thermal convection with temperature-and pressure-dependent viscosity at extreme viscosity contrasts2015Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 27, nr 7, artikkel-id 076603Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Motivated by convection of planetary mantles, we consider a mathematical model for Rayleigh-Benard convection in a basally heated layer of a fluid whose viscosity depends strongly on temperature and pressure, defined in an Arrhenius form. The model is solved numerically for extremely large viscosity variations across a unit aspect ratio cell, and steady solutions for temperature, isotherms, and streamlines are obtained. To improve the efficiency of numerical computation, we introduce a modified viscosity law with a low temperature cutoff. We demonstrate that this simplification results in markedly improved numerical convergence without compromising accuracy. Continued numerical experiments suggest that narrow cells are preferred at extreme viscosity contrasts, and this conclusion is supported by a linear stability analysis.

  • 11.
    Kohlstädt, Sebastian
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Neubauer, Alexander
    Volkswagen AG, Div Components Mfg, Business Unit Casting, Dr Rudolf Leiding Pl 1, D-34225 Baunatal, Germany..
    Gebauer-Teichmann, Andreas
    Volkswagen AG, Div Components Mfg, Business Unit Casting, Dr Rudolf Leiding Pl 1, D-34225 Baunatal, Germany..
    Comparative RANS turbulence modelling of lost salt core viability in high pressure die casting2019Inngår i: Progress in Computational Fluid Dynamics, An International Journal, ISSN 1468-4349, E-ISSN 1741-5233, Vol. 19, nr 5, s. 316-327Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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).

  • 12.
    Lacaze, J.
    et al.
    Univ Toulouse, CIRIMAT, 4 Allee Monso, F-31030 Toulouse, France..
    Jacques, B.
    Univ Toulouse, CIRIMAT, 4 Allee Monso, F-31030 Toulouse, France.;Fives Cryo, 25 Bis Rue Ft,BP 87, F-88194 Golbey, France..
    Mazet, T.
    Fives Cryo, 25 Bis Rue Ft,BP 87, F-88194 Golbey, France..
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Numerical Simulation of Brazing Aluminium Alloys with Al-Si Alloys2018Inngår i: Transactions of the Indian Institute of Metals, ISSN 0972-2815, E-ISSN 0975-1645, Vol. 71, nr 11, s. 2623-2629Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 13. McKee, S.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Cuminato, J. A.
    An elementary diffusion problem, Laplace transforms and novel mathematical identities2019Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 353, s. 113-119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 14.
    Mellgren, Niklas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Brown, Shelley
    KTH, Skolan för kemivetenskap (CHE), Kemiteknik.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindbergh, Göran
    KTH, Skolan för kemivetenskap (CHE), Kemiteknik.
    Impedance as a Tool for Investigating Aging in Lithium-Ion Porous Electrodes: I. Physically Based Electrochemical Model2008Inngår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 155, nr 4, s. A304-A319Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 15. Mitchell, S. L.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    On the accurate numerical solution of a two-phase Stefan problem with phase formation and depletion2016Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 300, s. 259-274Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 16. Mitchell, S. L.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    The oxygen diffusion problem: Analysis and numerical solution2015Inngår i: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 39, nr 9, s. 2763-2776Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 17. Mitchell, S. L.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Verified reduction of a model for a continuous casting process2017Inngår i: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 48, s. 476-490Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 18.
    Nick, Arash S.
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    A Theoretical Analysis of the Interaction Between Pores and Inclusions During the Continuous Casting of Steel2016Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47, nr 6, s. 2985-2999Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 19.
    Nick, Arash Safavi
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Michael, Vynnycky
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Pores and clustering of inclusions in the continuous casting of steel: PrecipitationManuskript (preprint) (Annet vitenskapelig)
  • 20. Noponen, Matti
    et al.
    Birgersson, Erik
    KTH, Tidigare Institutioner, Mekanik.
    Ihonen, Jari
    KTH, Tidigare Institutioner, Kemiteknik.
    Vynnycky, Michael
    KTH, Tidigare Institutioner, Mekanik.
    Lundblad, Anders
    KTH, Tidigare Institutioner, Kemiteknik.
    Lindbergh, Göran
    KTH, Tidigare Institutioner, Kemiteknik.
    A two-phase non-isothermal PEFC model: Theory and validation2004Inngår i: Fuel Cells, ISSN 1615-6846, E-ISSN 1615-6854, Vol. 4, nr 4, s. 365-377Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 21.
    Nordlund, Joakim
    et al.
    KTH, Tidigare Institutioner, Kemiteknik.
    Picard, C
    Birgersson, Erik
    KTH, Tidigare Institutioner, Mekanik.
    Vynnycky, Michael
    KTH, Tidigare Institutioner, Mekanik.
    Lindbergh, Göran
    KTH, Tidigare Institutioner, Kemiteknik.
    The design and usage of a visual direct methanol fuel cell2004Inngår i: Journal of Applied Electrochemistry, ISSN 0021-891X, E-ISSN 1572-8838, Vol. 34, nr 8, s. 763-770Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 22.
    Reddy, G. M. M.
    et al.
    Univ Sao Paulo Sao Carlos, Dept Appl Math & Stat, Inst Math & Comp Sci, POB 668, BR-13560970 Sao Paulo, Brazil..
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Cuminato, J. A.
    Univ Sao Paulo Sao Carlos, Dept Appl Math & Stat, Inst Math & Comp Sci, POB 668, BR-13560970 Sao Paulo, Brazil..
    An efficient adaptive boundary algorithm to reconstruct Neumann boundary data in the MFS for the inverse Stefan problem2019Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 349, s. 21-40Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 23.
    Reddy, G. M. M.
    et al.
    Univ Sao Paulo Sao Carlos, Inst Math & Comp Sci, Dept Appl Math & Stat, Sao Carlos, SP, Brazil..
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Cuminato, J. A.
    Univ Sao Paulo Sao Carlos, Inst Math & Comp Sci, Dept Appl Math & Stat, Sao Carlos, SP, Brazil..
    On efficient reconstruction of boundary data with optimal placement of the source points in the MFS: application to inverse Stefan problems2018Inngår i: Inverse Problems in Science and Engineering, ISSN 1741-5977, E-ISSN 1741-5985, Vol. 26, nr 9, s. 1249-1279Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 24. Redouane, B.
    et al.
    Mansour, C.
    Bouhouch, S.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Boualem, N.
    Experimental, statistical and numerical studies of the continuous casting process based on temperature profiles - part I2016Inngår i: Journal of Chemical Technology and Metallurgy, ISSN 1314-7471, Vol. 51, nr 1, s. 121-132Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 25.
    Saleem, Saud
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    A Study of the Oscillation Marks’ Characteristics of Continuously Cast Incoloy Alloy 825 Blooms2016Inngår i: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 47, nr 8, s. 4068-4079Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 26.
    Saleem, Saud
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Effect of casting parameters on the formation of oscillation marks during continuous castingManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    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.

  • 27.
    Saleem, Saud
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Formation of the tin rich layer and inverse-segregation in phosphor bronzes during continuous casting2015Inngår i: Advances in the Science and Engineering of Casting Solidification: An MPMD Symposium Honoring Doru Michael Stefanescu, The Minerals, Metals, and Materials Society, 2015, s. 15-22Konferansepaper (Fagfellevurdert)
    Abstract [en]

    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.

  • 28.
    Saleem, Saud
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    The Influence of Peritectic Reaction/Transformation on Crack Susceptibility in the Continuous Casting of Steels2017Inngår i: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 48, nr 3, s. 1625-1635Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 29. Sharma, A. K.
    et al.
    Ling, C. Y.
    Birgersson, E.
    Vynnycky, Mikael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Han, M.
    Verified reduction of dimensionality for an all-vanadium redox flow battery model2015Inngår i: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 279, s. 345-350Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The computational cost for all-vanadium redox flow batteries (VRFB) models that seek to capture the transport phenomena usually increases with the number of spatial dimensions considered. In this context, we carry out scale analysis to derive a reduced zero-dimensional model. Two nondimensional numbers and their limits to support the model reduction are identified. We verify the reduced model by comparing its charge discharge curve predictions with that of a full two-dimensional model. The proposed analysis leading to reduction in dimensionality is generic and can be employed for other types of redox flow batteries.

  • 30. Sharma, A. K.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Ling, C. Y.
    Birgersson, E.
    Han, M.
    The quasi-steady state of all-vanadium redox flow batteries: A scale analysis2014Inngår i: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 147, s. 657-662Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 31.
    Shugai, Galina
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Tidigare Institutioner                               , Mekanik.
    Holmqvist, Claes
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Tidigare Institutioner                               , Mekanik.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet. KTH, Tidigare Institutioner                               , Mekanik.
    Analysis of a model for twin-wire forming2004Inngår i: PROGRESS IN INDUSTRIAL MATHEMATICS AT ECMI 2002    / [ed] Buikis A; Ciegis R; Fitt AD, 2004, Vol. 5, s. 369-373Konferansepaper (Fagfellevurdert)
  • 32. Stewart, I. W.
    et al.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    McKee, S.
    Tome, M. F.
    Boundary layers in pressure-driven flow in smectic a liquid crystals2015Inngår i: SIAM Journal on Applied Mathematics, ISSN 0036-1399, E-ISSN 1095-712X, Vol. 75, nr 4, s. 1817-1851Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 33. Tome, M. F.
    et al.
    Bertoco, J.
    Oishi, C. M.
    Araujo, M. S. B.
    Cruz, D.
    Pinho, F. T.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    A finite difference technique for solving a time strain separable K-BKZ constitutive equation for two-dimensional moving free surface flows2016Inngår i: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 311, s. 114-141Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 34.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning. University of Limerick, Ireland.
    An asymptotic model for the primary drying stage of vial lyophilization2016Inngår i: Journal of Engineering Mathematics, ISSN 0022-0833, E-ISSN 1573-2703, Vol. 96, nr 1, s. 175-200Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Asymptotic methods are employed to analyse a commonly used one-dimensional transient model for coupled heat and mass transfer in the primary drying stage of freeze-drying (lyophilization) in a vial. Mathematically, the problem constitutes a two-phase moving boundary problem, in which one of the phases is a frozen porous matrix that undergoes sublimation, and the other is a low-pressure binary gaseous mixture. Nondimensionalization yields a model with 19 dimensionless parameters, but a systematic separation of timescales leads to a reduced model consisting of just a second-order differential equation with two initial conditions for the location of a sublimation front; the temperature and gas partial pressures can be found a posteriori. The results of this asymptotic model are compared with those of earlier experimental and theoretical work. Most significantly, the current model would be a computationally efficient tool for predicting the onset of secondary drying.

  • 35.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Applied mathematical modelling of continuous casting processes: A review2018Inngår i: Metals, ISSN 2075-4701, Vol. 8, nr 11, artikkel-id 928Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    With readily available and ever-increasing computational resources, the modelling of continuous casting processes—mainly for steel, but also for copper and aluminium alloys—has predominantly focused on large-scale numerical simulation. Whilst there is certainly a need for this type of modelling, this paper highlights an alternative approach more grounded in applied mathematics, which lies between overly simplified analytical models and multi-dimensional simulations. In this approach, the governing equations are nondimensionalized and systematically simplified to obtain a formulation which is numerically much cheaper to compute, yet does not sacrifice any of the physics that was present in the original problem; in addition, the results should agree also quantitatively with those of the original model. This approach is well-suited to the modelling of continuous casting processes, which often involve the interaction of complex multiphysics. Recent examples involving mould taper, oscillation-mark formation, solidification shrinkage-induced macrosegregation and electromagnetic stirring are considered, as are the possibilities for the modelling of exudation, columnar-to-equiaxed transition, V-segregation, centreline porosity and mechanical soft reduction.

  • 36.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Univ Limerick, Dept Math & Stat, Limerick V94 T9PX, Ireland..
    Continuous Casting2019Inngår i: Metals, ISSN 2075-4701, Vol. 9, nr 6, artikkel-id 643Artikkel i tidsskrift (Fagfellevurdert)
  • 37.
    Vynnycky, Michael
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    On an Anomaly in the Modeling of Electromagnetic Stirring in Continuous Casting2018Inngår i: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Vol. 49, nr 1, s. 399-410Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Early, yet still often-cited, mathematical models for electromagnetic stirring (EMS) in continuous casting are re-examined and found to contain a surprising anomaly: the solutions obtained were not unique. Analysis for the case of a round billet under rotary EMS shows how to avoid this behavior, whilst still making use of the experimental data that motivated the original models. The relevance of this result for current-day modeling of EMS is highlighted, particularly in the context of modulated EMS.

  • 38.
    Vynnycky, Michael
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, FaxénLaboratoriet.
    On the modelling of two-phase flow in the cathode gas diffusion layer of a polymer electrolyte fuel cell2007Inngår i: Applied Mathematics and Computation, ISSN 0096-3003, E-ISSN 1873-5649, Vol. 189, nr 2, s. 1560-1575Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A front-tracking approach is derived for the numerical solution of the equations arising in the multi-fluid model for isothermal multiphase multicomponent flow in the gas diffusion layer of the cathode of a polymer electrolyte fuel cell under conditions of local thermodynamic equilibrium. The method is able to find the location of the one-phase/two-phase interface explicitly and without need for the artificial diffusion, smoothing and ad hoe source terms that are required in existing formulations. Also, the analysis indicates the presence of a previously unidentified integrable singularity, which can be removed provided that the dependent variables are chosen correctly. For quantitative comparison, a benchmark example is implemented using both approaches in the commercially available finite-element software Comsol Multiphysics.

  • 39.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Univ Limerick, Dept Math & Stat, Limerick V94 T9PX, Ireland..
    Assuncao, M.
    Univ Sao Paulo Sao Carlos, Inst Math & Comp Sci, Dept Appl Math & Stat, BR-13560970 Sao Carlos, SP, Brazil..
    THE VANADIUM REDOX FLOW BATTERY: AN ASYMPTOTIC PERSPECTIVE2019Inngår i: SIAM Journal on Applied Mathematics, ISSN 0036-1399, E-ISSN 1095-712X, Vol. 79, nr 4, s. 1147-1172Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Asymptotic methods are used to analyze a time-dependent two-dimensional (2D) model for the operation of a vanadium redox flow battery-an energy storage technology that has attracted much attention recently. The model takes into account mass, momentum, and charge conservation involving a total of seven ionic species in two porous electrodes that are separated by a proton exchange membrane and attached to external recirculating tanks. In particular, we demonstrate a self-consistent asymptotic reduction of the original model. From this, we identify the presence of concentration boundary layers in each porous electrode at its interface with the membrane, and are able to explain the linear evolution in time of the inlet concentrations of the reacting ionic species-an assumption used in earlier models but never justified. The results of the asymptotic model, which ultimately requires only the numerical solution of four coupled nonlinear ordinary differential equations, are found to compare favorably with those of the original 2D transient problem, which involves 11 coupled nonlinear partial differential equations and two algebraic relations. The solution of the fully reduced asymptotic model is found to require around 300 times less computational time than that of the original model.

  • 40.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning. University of Limerick, Ireland .
    Gordon, A. D.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning. University of Limerick, Ireland .
    Non-isothermal hydrophobicity-dependent two-phase flow in the porous cathode gas diffusion layer of a polymer electrolyte fuel cell2015Inngår i: Journal of Engineering Mathematics, ISSN 0022-0833, E-ISSN 1573-2703, Vol. 92, nr 1, s. 123-146Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, we extend a recent one-dimensional isothermal steady-state generalized Darcy model for two-phase flow in the porous cathode gas diffusion layer of a polymer electrolyte fuel cell, so as to include the effect of heat transfer. As for the isothermal case, we arrive at either a fixed- or free-boundary problem, depending on the main problem parameters: inlet temperature (), inlet water saturation (), inlet relative humidity (RH), porous medium hydrophobicity and cathode overpotential (). The inclusion of heat transfer is found to limit the range of values of and RH over which two-phase flow can occur, as compared to that predicted by the isothermal model. The ensuing non-isothermal two-phase flow model equations are then computed numerically, with particular care being required for the treatment of an integrably singular inter-phase mass transfer term.

  • 41.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning. University of Limerick, Ireland .
    Kanev, K.
    Mathematical Analysis of the Multisolution Phenomenon in the P3P Problem2015Inngår i: Journal of Mathematical Imaging and Vision, ISSN 0924-9907, E-ISSN 1573-7683, Vol. 51, nr 2, s. 326-337Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The perspective 3-point problem, also known as pose estimation, has its origins in camera calibration and is of importance in many fields: for example, computer animation, automation, image analysis and robotics. One line of activity involves formulating it mathematically in terms of finding the solution to a quartic equation. However, in general, the equation does not have a unique solution, and in some situations there are no solutions at all. Here, we present a new approach to the solution of the problem; this involves closer scrutiny of the coefficients of the polynomial, in order to understand how many solutions there will be for a given set of problem parameters. We find that, if the control points are equally spaced, there are four positive solutions to the problem at 25 % of all available spatial locations for the control-point combinations, and two positive solutions at the remaining 75 %.

  • 42.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Kimura, S.
    Can natural convection alone explain the Mpemba effect?2015Inngår i: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 80, s. 243-255Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Mpemba effect is popularly summarized by the statement that "hot water can freeze faster than cold water", and has been observed experimentally since the time of Aristotle; however, there remains no definitive explanation for the effect. Here, we consider experimentally and theoretically the freezing of water in a rectangular vessel, with a view to investigating natural convection as a possible mechanism. The experimental and theoretical results are, in general, found to agree well; however, in combination, the results suggest that, whereas natural convection gives the correct general timescale for freezing, supercooling adjusts the actual time required. Moreover, the effect of supercooling leads to a spread in the experimental freezing times, giving results that constitute evidence of the occurrence of the Mpemba effect, even though the model results by themselves do not.

  • 43.
    Vynnycky, Michael
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Kimura, Shigeo
    An analytical and numerical study of coupled transient natural convection and solidification in a rectangular enclosure2007Inngår i: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 50, nr 25-26, s. 5204-5214Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

  • 44.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Univ Limerick, Dept Math & Stat, Limerick V94 T9PX, Ireland.
    Lacaze, J.
    Univ Toulouse, CIRIMAT, 4 Allee Emile Monso CS44362, F-31030 Toulouse 4, France..
    On the modelling of joint formation in dissolutive brazing processes2019Inngår i: Journal of Engineering Mathematics, ISSN 0022-0833, E-ISSN 1573-2703, Vol. 116, nr 1, s. 73-99Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, earlier dissolutive wetting models describing the dynamics of an axisymmetric alloy drop spreading on pure metal substrate are extended to describe reactive wetting and subsequent joint formation in brazing processes. A two-dimensional time-dependent problem is formulated, and the model equations are nondimensionalized, revealing the possibilities for asymptotic model reduction. Whilst the numerical solution of the time-dependent problem, which contains two moving contact lines and would not in general be amenable to lubrication theory, is relegated to future work, the steady-state problem is analyzed in detail. The analysis offers an arguably more transparent alternative to an earlier energy minimization approach for finding the location of the meniscus, which ultimately constitutes the joint. The results of the present model are found to compare favourably to those of earlier experimental and theoretical work.

  • 45.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Mitchell, S. L.
    On the numerical solution of a Stefan problem with finite extinction time2015Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 276, s. 98-109Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In many phase-change problems of practical interest, it is important to know when a phase is depleted, a quantity referred to as the extinction time; however, there are no numerical schemes that are able to compute this with any degree of rigour or formal accuracy. In this paper, we develop such a scheme for the one-dimensional time-dependent problem of an evaporating spherical droplet. The Keller box finite-difference scheme is used, in tandem with the so-called boundary immobilization method. An important component of the work is the careful use of variable transformations that must be built into the numerical algorithm in order to preserve second-order accuracy in both time and space, in particular as regards resolving a square-root singularity in the droplet radius as the extinction time is approached.

  • 46.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Reddy, G. M. M.
    Univ Sao Paulo Sao Carlos, Inst Math & Comp Sci, Dept Appl Math & Stat, POB 668, BR-13560970 Sao Carlos, SP, Brazil..
    On the Effect of Control-Point Spacing on the Multisolution Phenomenon in the P3P Problem2018Inngår i: Mathematical problems in engineering (Print), ISSN 1024-123X, E-ISSN 1563-5147, artikkel-id 5932508Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The perspective 3-point (P3P) problem, also known as pose estimation, has its origins in camera calibration and is of importance in many fields: for example, computer animation, automation, image analysis, and robotics. One possibility is to formulate it mathematically in terms of finding the solution to a quartic equation. However, there is yet no quantitative knowledge as to how control-point spacing affects the solution structure-in particular, the multisolution phenomenon. Here, we consider this problem through an algebraic analysis of the quartic's coefficients and its discriminant and find that there are significant variations in the likelihood of two or four solutions, depending on how the spacing is chosen. The analysis indicates that although it is never possible to remove the occurrence of the four-solution case completely, it could be possible to choose spacings that would maximize the occurrence of two real solutions. Moreover, control-point spacing is found to impact significantly on the reality conditions for the solution of the quartic equation.

  • 47.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Univ Sao Paulo Sao Carlos, Dept Appl Math & Stat, Inst Math & Comp Sci, POB 668, BR-13560970 Sao Carlos, SP, Brazil.
    Saleem, Saud
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Kanthal, Sandvik Heating Technol, Sorkvarnsvagen 3, S-73427 Hallstahammar, Sweden.
    On the explicit resolution of the mushy zone in the modelling of the continuous casting of alloys2017Inngår i: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 50, s. 544-568Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In the continuous casting of alloys, it is well-known that the mushy zone is decisive for the final properties of the casting. Most numerical models' for the process use enthalpy based methods on fixed grids which determine the extent the mushy zone implicitly. Here, on the other hand, we develop a methodology for explicitly resolving the geometrical extent of the mushy zone; this involves a sharp-interface formulation to solve a dual moving boundary problem to locate the solidus and liquidus isotherms. The results compare favourably with those from enthalpy-based methods, and the advantages of our approach with respect to future multiphysics calculations are discussed. 

  • 48.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Saleem, Saud
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    On the resolution of thermal gradients in a model for the continuous casting of binary alloysInngår i: Applied Mathematics and Computation, ISSN 0096-3003Artikkel i tidsskrift (Annet vitenskapelig)
  • 49.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. University of São Paulo, Brasilia.
    Saleem, Saud
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap. Sandvik Heating Technology, Sweden.
    Devine, K. M.
    Florio, B. J.
    Mitchell, S. L.
    O’Brien, S. B. G.
    On the formation of fold-type oscillation marks in the continuous casting of steel2017Inngår i: Royal Society Open Science, E-ISSN 2054-5703, Vol. 4, nr 6, artikkel-id 170062Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Asymptotic methods are employed to revisit an earlier model for oscillation-mark formation in the continuous casting of steel. A systematic non-dimensionalization of the governing equations, which was not carried out previously, leads to a model with 12 dimensionless parameters. Analysis is provided in the same parameter regime as for the earlier model, and surprisingly simple analytical solutions are found for the oscillation-mark profiles; these are found to agree reasonably well with the numerical solution in the earlier model and very well with fold-type oscillation marks that have been obtained in more recent experimental work. The benefits of this approach, when compared with time-consuming numerical simulations, are discussed in the context of auxiliary models for macrosegregation and thermomechanical stresses and strains.

  • 50.
    Vynnycky, Michael
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Saleem, Saud
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    Fredriksson, Hasse
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Metallernas gjutning.
    An asymptotic approach to solidification shrinkage-induced macrosegregation in the continuous casting of binary alloys2018Inngår i: Applied Mathematical Modelling, ISSN 0307-904X, E-ISSN 1872-8480, Vol. 54, s. 605-626Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The modelling of macrosegregation in the continuous casting of alloys normally requires resource-intensive computational fluid dynamics (CFD). By contrast, here we develop an asymptotic framework for the case when macrosegregation is driven by solidification shrinkage; as a first step, a binary alloy is considered. Systematic asymptotic analysis of the steady-state two-dimensional mass, momentum, heat and solute conservation equations in terms of the shrinkage parameter indicates that the overall problem can be reduced to a hierarchy of decoupled problems: a leading-order problem that is non-linear, and a sequence of linear problems, with the actual macrosegregation of the solute then being determined by means of one-dimensional quadrature. A numerical method that solves this sequence is then developed and implemented, and yields realistic macrosegregation profiles at low computational cost.

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