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Mohammadi, R., Ghaderi, M. R. & Hajiyan, E. (2023). A Molecular Dynamics Simulation Study of In- and Cross-Plane Thermal Conductivity of Bilayer Graphene. Materials, 16(20), Article ID 6714.
Open this publication in new window or tab >>A Molecular Dynamics Simulation Study of In- and Cross-Plane Thermal Conductivity of Bilayer Graphene
2023 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 16, no 20, article id 6714Article in journal (Refereed) Published
Abstract [en]

Efficient thermal management of modern electronics requires the use of thin films with highly anisotropic thermal conductivity. Such films enable the effective dissipation of excess heat along one direction while simultaneously providing thermal insulation along the perpendicular direction. This study employs non-equilibrium molecular dynamics to investigate the thermal conductivity of bilayer graphene (BLG) sheets, examining both in-plane and cross-plane thermal conductivities. The in-plane thermal conductivity of 10 nm × 10 nm BLG with zigzag and armchair edges at room temperature is found to be around 204 W/m·K and 124 W/m·K, respectively. The in-plane thermal conductivity of BLG increases with sheet length. BLG with zigzag edges consistently exhibits 30–40% higher thermal conductivity than BLG with armchair edges. In addition, increasing temperature from 300 K to 600 K decreases the in-plane thermal conductivity of a 10 nm × 10 nm zigzag BLG by about 34%. Similarly, the application of a 12.5% tensile strain induces a 51% reduction in its thermal conductivity compared to the strain-free values. Armchair configurations exhibit similar responses to variations in temperature and strain, but with less sensitivity. Furthermore, the cross-plane thermal conductivity of BLG at 300 K is estimated to be 0.05 W/m·K, significantly lower than the in-plane results. The cross-plane thermal conductance of BLG decreases with increasing temperatures, specifically, at 600 K, its value is almost 16% of that observed at 300 K.

Place, publisher, year, edition, pages
MDPI, 2023
Keywords
bilayer graphene, in-plane thermal conductivity, cross-plane thermal conductivity, non-equilibrium molecular dynamics, anisotropic thermal transport
National Category
Energy Engineering Wood Science
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-101760 (URN)10.3390/ma16206714 (DOI)
Note

Validerad;2023;Nivå 2;2023-10-31 (joosat);

CC BY 4.0 License

Available from: 2023-10-23 Created: 2023-10-23 Last updated: 2023-10-31Bibliographically approved
Hajiyan, E., Couceiro, J., Hansson, L. & Sandberg, D. (2023). Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine. Paper presented at 13th International Conference Wood Science and Technology in the Third Millennium, ICWSE 2023, November 2 – 4, 2023, Brasov, Romania. Applied Sciences, 13(19), Article ID 11083.
Open this publication in new window or tab >>Drying Behaviour of Western Hemlock with Schedules Developed for Norway Spruce and Scots Pine
2023 (English)In: Applied Sciences, E-ISSN 2076-3417, Vol. 13, no 19, article id 11083Article in journal (Refereed) Published
Abstract [en]

Determining moisture content (MC) distribution during the drying of porous materials such as wood is crucial for developing drying schedules and assessing their suitability to achieve optimised processes. This study aimed to determine the causes of the unique drying behaviour and the well-known unusual longer drying time of western hemlock compared to other similar softwoods. In situ X-ray computed tomography (CT) was used to study the evolution of MC in timber during the drying process. The drying behaviour of western hemlock (Tsuga heterophylla (Raf.) Sarg.) was compared with Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) from green to oven-dried condition with industry-proposed drying schedules used for steering a custom-made experimental kiln combined with a CT scanner. CT scanning was performed at 30 min intervals during the complete drying period of 30 h, and the CT images were processed to calculate the MC evolution within the specimen. Western hemlock showed a considerably slower capillary-phase drying and did not go into the transition and diffusion phases when a schedule adapted to pine and spruce drying was applied for its drying. CT images and MC gradient calculations showed a lower drying rate and severe non-uniformity in MC distribution, which could be due to the effect of higher green MC and the presence of wet pockets. Furthermore, the evaporation front at the first 5 h of drying receded faster into the hemlock specimen, and as drying proceeded, it slowed down compared to other specimens.

Place, publisher, year, edition, pages
MDPI, 2023
Keywords
CT scan, experimental research, image processing, moisture content, porous material, timber drying, wet pockets
National Category
Wood Science
Research subject
Wood Science and Engineering
Identifiers
urn:nbn:se:ltu:diva-101641 (URN)10.3390/app131911083 (DOI)
Conference
13th International Conference Wood Science and Technology in the Third Millennium, ICWSE 2023, November 2 – 4, 2023, Brasov, Romania
Note

Godkänd;2023;Nivå 0;2023-10-12 (joosat);Konferensartikel i tidskrift;

Part of special Issue: International Conference Wood Science and Engineering in the Third Millennium - ICWSE 2023

CC BY 4.0 License

Available from: 2023-10-12 Created: 2023-10-12 Last updated: 2023-10-12Bibliographically approved
Farahani, S. D., Farahani, A. D., Hajian, E. & Öztop, H. F. (2022). Control of PCM melting process in an annular space via continuous or discontinuous fin and non-uniform magnetic field. Journal of Energy Storage, 55(A), Article ID 105410.
Open this publication in new window or tab >>Control of PCM melting process in an annular space via continuous or discontinuous fin and non-uniform magnetic field
2022 (English)In: Journal of Energy Storage, ISSN 2352-152X, Vol. 55, no A, article id 105410Article in journal, Editorial material (Refereed) Published
Place, publisher, year, edition, pages
Elsevier, 2022
National Category
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-94386 (URN)10.1016/j.est.2022.105410 (DOI)000880313400004 ()2-s2.0-85135383146 (Scopus ID)
Available from: 2022-11-30 Created: 2022-11-30 Last updated: 2023-01-25Bibliographically approved
Farahani, S. D. & Hajiyan, E. (2021). Achieving uniform heat transfer coefficient in coaxial pulsating jet. Journal of thermal analysis and calorimetry (Print), 147(3), 2833-2846
Open this publication in new window or tab >>Achieving uniform heat transfer coefficient in coaxial pulsating jet
2021 (English)In: Journal of thermal analysis and calorimetry (Print), ISSN 1388-6150, E-ISSN 1588-2926, Vol. 147, no 3, p. 2833-2846Article in journal, Editorial material (Refereed) Published
Abstract [en]

The main aim of the present paper is to investigate the inverse design of uniform distribution of heat transfer coefficient on the target plate by employing coaxial jet in the steady and pulsating state. The objective function is defined as the root-mean-square of the deviation of the local Nusselt number on the target plate in computational fluid dynamics simulation from the desired Nusselt number. The heat transfer search method minimizes the objective function. The geometric and fluid parameters are taken into account as design variables. Firstly, optimization in the steady state for the target Nusselt numbers 7, 10 and 13 is carried out and the values of the objective function in the optimal state are found to be 0.51, 0.18 and 0.314, respectively. The range of design variables in the steady and pulsating state is similar, while the heat transfer rate of the pulsating state is higher than that of the steady state. The variations of velocity in the inner and outer nozzles with time are considered to have a sine–sine, cosine–sine and constant–sine behavior. Optimization is carried out in the pulsating state for the Nusselt numbers 33, 44, 55 and 57. The objective function for these numbers in the optimal state is less than 0.02. By employing the conjugate gradient method with adjoint equation and the optimal value of design variables, the distribution of Nusselt number on the target plate in the steady and pulsating state for the constant–sine velocity case is experimentally estimated. The difference between experimental results and the value of the target Nusselt number is about 4–14%, indicating a good agreement between the two approaches.

Place, publisher, year, edition, pages
Springer, 2021
Keywords
Coaxial jet, Uniform Nusselt number, Pulsating state, Inverse design
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:ltu:diva-94366 (URN)10.1007/s10973-021-10582-z (DOI)000621761900014 ()2-s2.0-85101752725 (Scopus ID)
Available from: 2022-11-30 Created: 2022-11-30 Last updated: 2023-05-08Bibliographically approved
Davoodabadi Farahani, S., Davoodabadi Farahani, A. & Hajiyan, E. (2021). Effect of PCM and porous media/nanofluid on the thermal efficiency of microchannel heat sinks. International Communications in Heat and Mass Transfer, 127, Article ID 105546.
Open this publication in new window or tab >>Effect of PCM and porous media/nanofluid on the thermal efficiency of microchannel heat sinks
2021 (English)In: International Communications in Heat and Mass Transfer, ISSN 0735-1933, E-ISSN 1879-0178, Vol. 127, article id 105546Article in journal (Refereed) Published
Abstract [en]

The present study is a genuine attempt to improve the thermal behavior and hydraulic performance of a conventional microchannel heat sink (MCHS) using porous material, phase change material (PCM) and nanofluid. Three-dimensional microchannels with square, circular and flattened circular geometries are examined numerically by considering the thermal conduction in solid parts. Various factors are examined, including the effect of the number of channels, fluid velocity passing through channels, porous substrate thickness, PCM layer thickness, Darcy number, porosity coefficient, thermal conductivity ratio in the porous material, volume fraction and nanoparticle diameter. According to the findings, for a critical thickness of the porous substrate, the thermal performance is at its minimum. With increasing values of Darcy number, porosity coefficient, nanoparticle diameter and PCM thickness, the thermal performance of MCHS decreases. An MCHS with square geometry displays a better thermal performance compared to the other two geometries. Furthermore, the use of porous material, nanofluid and PCM improves the thermal behavior compared with a conventional microchannel. The thermal performance of MCHS with PCM is approximately 47% and 17% higher than that of a microchannel with the porous substrate and a microchannel incorporating a combination of the porous layer and nanofluid.

Keywords
Microchannel heat sink, Porous medium, Phase change material, Two-phase flow, Thermal performance
National Category
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-94364 (URN)10.1016/j.icheatmasstransfer.2021.105546 (DOI)000702863200001 ()2-s2.0-85113330518 (Scopus ID)
Available from: 2022-11-30 Created: 2022-11-30 Last updated: 2023-05-08Bibliographically approved
Farahani, S. D., Farahani, A. D. & Hajian, E. (2021). Efficacy of Magnetic Field on Melting Behavior of Phase Change Materials in an Enclosure with new Fins. Journal of Enhanced Heat Transfer, 28(7), 19-38
Open this publication in new window or tab >>Efficacy of Magnetic Field on Melting Behavior of Phase Change Materials in an Enclosure with new Fins
2021 (English)In: Journal of Enhanced Heat Transfer, ISSN 1065-5131, E-ISSN 1563-5074, Vol. 28, no 7, p. 19-38Article in journal (Refereed) Published
Abstract [en]

Enhancing the efficiency of thermal storage systems is important and necessary. Numerical research on investigating the effects of new fins, magnetic fields, and nanoparticles on the melting of phase change materials (PCM) in a finned cavity is presented in this paper. Effects of fin and its shape, porous fin, adding nanoparticles, the non-uniform magnetic field, and the oscillating magnetic field on melting of PCM are evaluated to determine the state that has the most thermal performance. The enthalpy-porosity method is employed for simulating the melting process. The results specify that the melting time in finned mode with circular, triangular, rectangular, sin-downward, and sin-upward shapes decreased by 44%, 46%, 51%, 53.9%, and 54%, respectively, compared to the no-fin mode. The liquid fraction is increased to 98% by adding nanoparticles. By employing the porous fins, the liquid fraction is decreased by increasing the porosity coefficient and Darcy number. For the sin-upward fin, the liquid fraction is increased by 54%, 48.29%, 22.99%, and 9.50% using a hybrid nanoparticle/non-uniform magnetic field, nanoparticles, porous fin, and non-uniform magnetic field, respectively, compared to the base case. The highest melting performance belongs to using the sin-upward fins with nanoparticles and a non-uniform magnetic field.

Place, publisher, year, edition, pages
Begell House, 2021
Keywords
melting performance of PCM, sinusoidal fins, porous media, nanoparticle, non-uniform magnetic field, oscillating magnetic field
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:ltu:diva-94388 (URN)10.1615/jenhheattransf.2021039381 (DOI)2-s2.0-85117787215 (Scopus ID)
Available from: 2022-11-30 Created: 2022-11-30 Last updated: 2023-05-08Bibliographically approved
Farahani, A. D., Davoodabadi Farahani, S. & Hajian, E. (2021). Efficacy of Magnetic Field on Nanoparticle-Enhanced Phase Change Material Melting in a Triple Tube with Porous Fin. Heat Transfer Research, 52(12), 43-65
Open this publication in new window or tab >>Efficacy of Magnetic Field on Nanoparticle-Enhanced Phase Change Material Melting in a Triple Tube with Porous Fin
2021 (English)In: Heat Transfer Research, ISSN 1064-2285, E-ISSN 2162-6561, Vol. 52, no 12, p. 43-65Article in journal (Refereed) Published
Abstract [en]

In the present paper, a numerical study is conducted to examine the effect of a nonuniform magnetic field on the melting process of phase change material (PCM) in a finned triple tube. Lauric acid is considered as the PCM. The effect of fin length, the radius of the middle tube, porous fin, nanoparticles, and nonuniform magnetic field on PCM melting performance has been explored. The enthalpy-porosity method is employed for simulating the melting process. The results show that the melting rate in a finned triple tube is higher than in a finned double tube. The melting rate of PCM decreases by increasing the radius of the middle tube and decreasing the length of the fins on this tube. The use of nanoparticles improves the melting performance. When using a porous fin, increasing the Darcy number and porosity coefficient has a negative effect on melting performance and reduces the liquid fraction. Utilizing a nonuniform magnetic field improves the melting performance. The liquid fraction is increased significantly as the intensity of the magnetic field is increased. Compared to the base case, the liquid fraction is increased by 19.29%, 81.57%, 3.26%, and 89.12%, respectively, utilizing nanoparticles, nonuniform magnetic field, porous fin, and nonuniform magnetic field/nanoparticle.

Place, publisher, year, edition, pages
Begell House, 2021
Keywords
triple tube, melting, porous fin, nonuniform magnetic field, NEPCM
National Category
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-94387 (URN)10.1615/heattransres.2021039023 (DOI)2-s2.0-85117799109 (Scopus ID)
Available from: 2022-11-30 Created: 2022-11-30 Last updated: 2023-10-11Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2839-9055

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