| 1. |
- Asako, Y., et al.
(författare)
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Mach Number on Outlet Plane of a Straight Micro-Tube
- 2014
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Ingår i: Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition. Volume 7A: Fluids Engineering Systems and Technologies. - 9780791856314 ; 7A, s. 07-020
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Konferensbidrag (refereegranskat)abstract
- The Mach number and pressure on the outlet plane of a straight micro-tube were investigated numerically for both laminar and turbulent flow cases. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The LB1 turbulence model was used for the turbulent flow case. The compressible momentum and energy equations with the assumption of the ideal gas were solved. The computational domain is extended to the downstream region from the micro-tube outlet. The back pressure was given to the outside of the downstream region. The computations were performed for a tube whose diameter ranges from 50 to 500 gm. The average Mach number on the outlet plane of the fully under-expanded flow depends on the tube diameter and ranges from 1.16 to 1.25. The flow characteristics of the under-expanded gas flow in a straight micro-tube were revealed.
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| 2. |
- Hamian, Sina, et al.
(författare)
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Finite element analysis of transient ballistic-diffusive phonon heat transport in two-dimensional domains
- 2015
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 80, s. 781-788
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Tidskriftsartikel (refereegranskat)abstract
- While sub-continuum heat conduction becomes more important as the size of micro/nanodevices keeps shrinking under the mean free path of heat carriers, its computation still remains challenging to the general engineering community due to the lack of easily accessible numerical simulation tools. To address this challenge, this article reports the finite element analysis (FEA) of transient ballistic-diffusive phonon heat transport in a two-dimensional domain using a commercial package (COMSOL Multiphysics). The Boltzmann transport equation under the gray relaxation-time approximation was numerically solved by discretizing the angular domain with the discrete ordinate method (DOM) and the spatial domain with the FEA. The DOM-FEA method was validated by comparing the results with different benchmark studies, such as the equation of phonon radiative transfer, the ballistic-diffusive equation, and the finite difference method of the phonon Boltzmann transport equation. The calculation of phonon heat transport for a 2-D square slab reveals that heat conduction becomes more ballistic with temperature jumps at boundaries as Knudsen number (Kn) increases. The ballistic nature also significantly affects transient thermal behaviors at high Kn numbers. The obtained results clearly demonstrate the capability of the DOM-FEA as a promising engineering tool for calculating sub-continuum phonon heat transport. (C) 2014 Elsevier Ltd. All rights reserved.
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| 3. |
- Johansson, Erik, et al.
(författare)
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Dissipative particle dynamics approach for nano-scale membrane structure reconstruction and water diffusion coefficient estimation
- 2015
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 1879-3487 .- 0360-3199. ; 40:4, s. 1800-1808
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Tidskriftsartikel (refereegranskat)abstract
- The membrane is often considered as the heart of the proton exchange membrane fuel cell (PEMFC), and it is of pivotal importance for the involved transport phenomena and performance of the entire PEMFC. The Nafion(R) membrane is the most commonly used type of membrane in PEMFCs. In this paper, the nano-scale structure of the Nafion(R) membrane is reconstructed using dissipative particle dynamics. The average water pore sizes of the equilibrated membranes are estimated by employing the radial distribution function, and the reconstructed nano-scale membranes are then used to evaluate water diffusion within the membrane by calculating the mean square displacement. The tortuosity of the membrane is evaluated by comparing the diffusivity of the membrane to that of a limiting case of cylindrical channels. Important findings from this study are presented by the membrane structures, water diffusivity at different water saturations, and calculations of the tortuosity within the membrane. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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| 4. |
- Johansson, Erik, et al.
(författare)
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Modeling mesoscopic solidification using dissipative particle dynamics
- 2016
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 101, s. 207-216
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Tidskriftsartikel (refereegranskat)abstract
- Dissipative particle dynamics with energy conversion (DPDe) is a simulation technique that has been used to model thermal transport characteristics and heat transfer at mesoscale. This study shows further development of the DPDe method capturing solid/liquid phase-change phenomena and its application to water freezing in a parallel-plate straight channel. In this work, the weighting functions of the random and dissipative forces are modeled as functions of temperature in order to correctly predict the temperature dependent properties of the fluid in a two-dimensional domain. An equation of state is incorporated in the model in order to model the solidification of water. Careful consideration is taken to couple the latent heat of the system to real world units, and the solidification predicted using this model is compared to a well known analytical solution. The developed model is employed to simulate the thermally developing flow in a parallel-plate channel with constant wall temperatures below the freezing point. A liquid pump is introduced along with a region initiating the liquid temperature in order to create the thermally developing flow. The investigations show that the fluid velocity has a small effect on the time it takes for the channel to freeze completely. The dominating factor will be the temperature of the solid walls in the domain. The simulations also show that when a higher wall temperature is applied, the solid/liquid interface will be rougher due to mesoscopic fluctuations of heat and momentum. (C) 2015 Elsevier Masson SAS. All rights reserved.
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| 5. |
- Johansson, Erik O., et al.
(författare)
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Modeling of a non-periodic boundary condition with entrance and exit in dissipative particle dynamics
- 2015
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Ingår i: ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM 2015, collocated with the ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems. - 9780791856871
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Konferensbidrag (refereegranskat)abstract
- Dissipative particle dynamics (DPD) have been widely used for the simulations of dynamics of both simple and complex fluids at nano/micro scales. In these simulations, periodic boundaries are usually employed in the main flow direction and the characterization of the flow and heat transfer is based on fully developed conditions. In the real nano/micro-fluidic devices, however, there are entrances and exits and the flow and temperature fields are not the same at different positions, making the periodic boundary conditions ill-suited due to problems with conservation of energy and momentum. This is the motivation of the present study to generate the non-periodic boundary condition having an entrance and an exit in the the DPD system and study the heat transfer characteristics in the entrance region. In this study, the entrance and exit regions are modelled for simulations of the flow in a parallel-plate channel based on the available methodology originally introduced for molecular dynamics. In this methodol-ogy, a body force acts on the DPD particles at the entrance region of the solution domain to generate the entrance region. This is region is so-called pump region. Also, a region to initiate the DPDe temperature was located followed by the pump region. Forced convection heat transfer of water flowing through a parallel-plate channel with constant wall temperature was simulated using this method. The simulations were implemented for different body forces in the pump region. The results were evaluated in terms of velocity, temperature and number density distributions in the channel and showed the effects of the compressibility of the DPD fluid and random movement (or Brownian motion). In addition, the Reynolds and Nusselt numbers were calculated to investigate their effects on the heat transfer characteristics at the entrance region.
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| 6. |
- Yamada, Toru, et al.
(författare)
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Application of many-body dissipative particle dynamics to determine liquid characteristics
- 2015
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Ingår i: International Journal of Numerical Methods for Heat & Fluid Flow. - 1758-6585. ; 25:7, s. 1619-1637
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Tidskriftsartikel (refereegranskat)abstract
- Purpose - The purpose of this paper is to find out the applicability of the many-body dissipative particle dynamics (MDPD) method for various real fluids by specifically focusing on the effects of the MDPD parameters on the MDPD fluid properties. Design/methodology/approach - In this study, the MDPD method based on van der Waals (vdw) equation of state is employed. The simulations are conducted by using LAMMPS with some modifications of the original package to include the many-body features in the simulation. The simulations are investigated in a three-dimensional Cartesian box solution domain in which MDPD particles are distributed. In order to evaluate the MDPD liquid characteristics for a stationary liquid film, self-diffusivity, viscosity, Schmidt number (Sc) and surface tension, are estimated for different MDPD parameters. The parameters are carefully selected based on previous studies. A set of single-droplet simulations is also performed to analyze the droplet characteristics and its behavior on a solid-wall. Besides, the relationship between the characteristic length in the DPD simulations and scaling parameters for the stationary liquid-film case is discussed by employing the Ohnesorge number. Findings - The results show that the liquid properties in the MDPD simulations can be widely ranged by varying the MDPD parameters. The values are highly influenced by the many-body feature in the conservative force which is not included in the original DPD method. It is also found that the wetting ability of the MDPD fluid on solid walls can be easily controlled by changing a many-body parameter. The characteristic length between the MDPD reduced unit and real unit is related for the stationary liquid-film case by employing the Ohnesorge number. Originality/value - The present parametric study shows that the liquid properties in the MDPD method can vary by carefully controlling the MDPD parameters, which demonstrates the high-potential applicability of the method for various real fluids. This will contribute to research areas in multi-phase transport phenomena at nano and sub-micron scales in, for example, fuel cells, batteries and other engineering devices involving porous media.
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| 7. |
- Yamada, Toru, et al.
(författare)
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Diffusive-Ballistic Heat Transport in a Two-Dimensional Square Plate Using Energy Conserving Dissipative Particle Dynamics
- 2014
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Ingår i: ASME 2013 Heat Transfer Summer Conference collocated with the ASME 2013 7th International Conference on Energy Sustainability and the ASME 2013 11th International Conference on Fuel Cell Science, Engineering and Technology Volume 4: Heat and Mass Transfer Under Extreme Conditions. - 9780791855508 ; 4, s. 004-016
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Konferensbidrag (refereegranskat)abstract
- Diffusive-ballistic heat transport in a two-dimensional square plate was simulated using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be a two-dimensional square plate surrounded by walls at constant temperatures, where DPD particles are uniformly distributed. The effects of phonon mean free path was incorporated by its relation to the cutoff radius of energy interaction. This cutoff radius was obtained based on Knudsen number (Kn) using the existing phonon-boundary scattering models. The simulations for 0.1 < Kn < 10 were conducted with different modifications of the cutoff radius. The results are presented in form of temperature distributions in the solution domain and the effect of Knudsen number is discussed.
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| 8. |
- Yamada, Toru, et al.
(författare)
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Diffusive-ballistic heat transport in thin films using energy conserving dissipative particle dynamics
- 2013
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310. ; 61, s. 287-292
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Tidskriftsartikel (refereegranskat)abstract
- Diffusive-ballistic heat transport in thin films was simulated using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be two-dimensional with DPD particles distributed uniformly under constant temperature boundary conditions at the top and bottom walls and periodic boundaries at the side walls. The effects of phonon mean free path were incorporated by its relation to the cutoff radius of energy interaction. This cutoff radius was based on the Knudsen number using the existing phonon-boundary scattering models. The simulations for 0.1 < Kn < 10 were obtained with the different modifications of the cutoff radius. The results were presented in form of a nondimensional temperature profile across the thin film and were compared with the semi-analytical solution of the equation of phonon radiative transport (EPRT). When the phonon-boundary scattering is not considered, the DPDe simulation results have more discrepancies compared with the EPRT solution as Kn increases, indicating that the phonon-boundary scattering plays an important role when the heat transport across the film becomes more ballistic. The results demonstrate that the DPDe can simulate the diffusive-ballistic heat transport for a broad range of Kn, but phonon-boundary scattering should be considered for the accurate simulation of the ballistic heat transport. (C) 2013 Elsevier Ltd. All rights reserved.
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| 9. |
- Yamada, Toru, et al.
(författare)
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Dissipative particle dynamics simulations of water droplet flows in a submicron parallel-plate channel for different temperature and surface-wetting conditions
- 2016
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Ingår i: Numerical Heat Transfer; Part A: Applications. - : Informa UK Limited. - 1040-7782 .- 1521-0634. ; 70:6, s. 595-612
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Tidskriftsartikel (refereegranskat)abstract
- The effects of temperature-dependent thermophysical properties on droplet flow characteristics in a parallel-plate channel at submicron scale are investigated. The dissipative particle dynamics method with many-body (MDPD) and energy conservation (DPDe) configurations (MDPDe) was used. Droplet flows were simulated to study the effects of the temperature difference between top and bottom walls, body force on MDPDe particles, and wall-wetting conditions. The effects on the droplet flow were discussed. Droplet flows with a subzero wall temperature were simulated. An ice layer was formed on the wall. Its thickness and shape changed depending on surface wetting, temperature gradient, and body force.
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| 10. |
- Yamada, Toru, et al.
(författare)
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Numerical Study on the Diffusive-Ballistic Heat Transport in a Two-Dimensional Square Plate
- 2014
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Ingår i: ASME 2013 International Mechanical Engineering Congress and Exposition Volume 8C: Heat Transfer and Thermal Engineering. - 9780791856369 ; 8C, s. 08-015
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Konferensbidrag (refereegranskat)abstract
- The heat conduction in a two-dimensional square plate at nano/micro scales is investigated using numerical and semi-analytical methodologies which are energy conserving dissipative particle dynamics (DPDe) and Boltzmann transport equation (BTE) with the gray relaxation time approximation. The solution domain was considered to be a two-dimensional square plate surrounded by walls having constant temperatures. The numerical and semi-analytical results for different Knudsen numbers ranging from 0.1 to 10 are presented in form of temperature distributions in the solution domain. These results are compared with each other and the applicability of the DPDe model to two-dimensional structures is discussed.
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