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Fallahjoybari, N., Lundström, S. & Hellström, J. G. (2020). Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media. International Journal of Heat and Fluid Flow, 81, Article ID 108523.
Open this publication in new window or tab >>Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media
2020 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 81, article id 108523Article in journal (Refereed) Published
Abstract [en]

In the present study, the importance of the thermal dispersion and the turbulent heat flux in porous media and their effects on the macroscopic distribution of thermal energy are investigated. To this end, turbulent flow and heat transfer within five unit-cells mimicking porous media are solved using large eddy simulation. It is shown that the thermal dispersion and the turbulent heat flux are negligible as compared to the convection term in the macroscopic energy equation. When further scrutinizing this equation, it is revealed that except for the longitudinal components of the thermal dispersion, the other components of thermal dispersion and turbulent heat flux may be neglected away from the boundaries as compared to the interfacial heat transfer. Visualizations of vortices show that the size of the turbulence structures within the cells is of the same order as the size of the pores; therefore, the turbulent heat flux is limited to the intra-pore level. Finally, a discussion is provided on the accuracy of the gradient type diffusion model commonly used for turbulent heat flux in porous media in the absence of macroscopic turbulence. It is shown that the intra-pore turbulence does not affect the macroscopic transport of thermal energy within the porous media studied.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Turbulence, Porous media, LES, Thermal dispersion, Turbulent heat flux
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-77217 (URN)10.1016/j.ijheatfluidflow.2019.108523 (DOI)
Note

Validerad;2020;Nivå 2;2019-12-18 (johcin)

Available from: 2019-12-18 Created: 2019-12-18 Last updated: 2019-12-18Bibliographically approved
Fallahjoybari, N. & Lundström, S. (2020). Performance improvement of a solar air heater by covering the absorber plate with a thin porous material. Energy, 190, Article ID 116437.
Open this publication in new window or tab >>Performance improvement of a solar air heater by covering the absorber plate with a thin porous material
2020 (English)In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 190, article id 116437Article in journal (Refereed) Published
Abstract [en]

The effect of covering the absorber plate of a solar air heater with a thin porous media is investigated in the present study. Simulations are carried out for turbulent flow and heat transfer in the solar heater and within the porous layer. The effects of different parameters such as Reynolds number, Darcy number and solid to fluid thermal conductivity ratio on the thermal and thermo-hydraulic performances of a solar air heater are studied. It is observed that the implementation of a thin porous layer over the absorber plate significantly increases the thermal and thermo-hydraulic performances of the solar air heater. The maximum increase in the thermal and thermo-hydraulic performances is more than 5 times of those obtained in a solar heater without porous medium. Meanwhile, the maximum increase in the frictions factor of the porous solar heater is 2 times of that in a solar heater without porous media at the maximum Reynolds number studied. The proposed configuration also reduces the risk of hot spots since no entrapped eddies are formed over the absorber plate. It is shown that the turbulence produced at the porous-fluid interface penetrates into the thin porous layer and enhances the heat transfer from the absorber plate. The results also reveal that the conduction heat transfer within the porous layer highly affects the thermal and thermo-hydraulic performances of the solar heater.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Solar air heater, Porous media, Turbulence, Nusselt number, Thermo-hydraulic performance
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-76958 (URN)10.1016/j.energy.2019.116437 (DOI)2-s2.0-85075359673 (Scopus ID)
Note

Validerad;2020;Nivå 2;2020-01-10 (johcin)

Available from: 2019-11-29 Created: 2019-11-29 Last updated: 2020-01-10Bibliographically approved
Storli, P.-T. & Lundström, S. (2019). A New Technical Concept for Water Management and Possible Uses in Future Water Systems. Water, 11(12), Article ID 2528.
Open this publication in new window or tab >>A New Technical Concept for Water Management and Possible Uses in Future Water Systems
2019 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 12, article id 2528Article in journal (Refereed) Published
Abstract [en]

A new degree of freedom in water management is presented here. This is obtained by displacing water, and in this paper is conceptually explained by two methods: using an excavated cavern as a container for compressed air to displace water, and using inflatable balloons. The concepts might have a large impact on a variety of water management applications, ranging from mitigating discharge fluctuation in rivers to flood control, energy storage applications and disease-reduction measures. Currently at a low technological readiness level, the concepts require further research and development, but the authors see no technical challenges related to these concepts. The reader is encouraged to use the ideas within this paper to find new applications and to continue the out-of-the-box thinking initiated by the ideas presented in this paper. 

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
water management, reservoirs, hydropower plants, pumped storage power plants, hydropeaking, environmental flows
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-77286 (URN)10.3390/w11122528 (DOI)
Note

Validerad;2020;Nivå 2;2020-01-07 (johcin)

Available from: 2020-01-03 Created: 2020-01-03 Last updated: 2020-01-07Bibliographically approved
Fallahjoybari, N. & Lundström, S. T. (2019). A Subgrid-Scale Model for Turbulent Flow in Porous Media. Transport in Porous Media, 129(3), 619-632
Open this publication in new window or tab >>A Subgrid-Scale Model for Turbulent Flow in Porous Media
2019 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 129, no 3, p. 619-632Article in journal (Refereed) Published
Abstract [en]

Given the analogy between the filtered equations of large eddy simulation and volume-averaged Navier–Stokes equations in porous media, a subgrid-scale model is presented to account for the residual stresses within the porous medium. The proposed model is based on the kinetic energy balance of the filtered velocity field within a pore; hence, when using the model, numerical simulations of the turbulent flow in the pores are not required. The accuracy of the model is validated with available data in the literature on turbulent flow through packed beds and staggered arrangement of square cylinders. The validation yields that the model successfully captures the effect of the pore-scale turbulent motion. The model is then used to study turbulent flow in a wall-bounded porous media to assess its accuracy.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Turbulence, Porous media, Volume average, LES, Subgrid-scale model
National Category
Engineering and Technology Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-76122 (URN)10.1007/s11242-019-01296-w (DOI)000480750200001 ()2-s2.0-85070677723 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-10-01 (johcin)

Available from: 2019-09-26 Created: 2019-09-26 Last updated: 2019-12-04Bibliographically approved
Wibron, E., Ljung, A.-L. & Lundström, S. (2019). Comparing Performance Metrics of Partial Aisle Containments in Hard Floor and Raised Floor Data Centers Using CFD. Energies, 12(8), Article ID 1473.
Open this publication in new window or tab >>Comparing Performance Metrics of Partial Aisle Containments in Hard Floor and Raised Floor Data Centers Using CFD
2019 (English)In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 12, no 8, article id 1473Article in journal (Refereed) Published
Abstract [en]

In data centers, efficient cooling systems are required to both keep the energy consumption as low as possible and to fulfill the temperature requirements. The aim of this work is to numerically investigate the effects of using partial aisle containment between the server racks for hard and raised floor configurations. The computational fluid dynamics (CFD) software ANSYS CFX was used together with the Reynolds stress turbulence model to perform the simulations. Velocity measurements in a server room were used for validation. Boundary conditions and the load of each rack were also retrieved from the experimental facility, implying an uneven load between the racks. A combination of the performance metrics Rack Cooling Index (RCI), Return Temperature Index (RTI) and Capture Index (CI) were used to evaluate the performance of the cooling systems for two supply flow rates at a 100% and 50% of operating condition. Based on the combination of performance metrics, the airflow management was improved in the raised floor configurations. With the supply flow rate set to operating conditions, the RCI was 100% for both raised floor and hard floor setups. The top- or side-cover fully prevented recirculation for the raised floor configuration, while it reduced the recirculation for the hard floor configuration. However, the RTI was low, close to 40% in the hard floor case, indicating poor energy efficiency. With the supply flow rate decreasing with 50%, the RTI increased to above 80%. Recirculation of hot air was indicated for all the containments when the supply rate was 50%, but the values of RCI still indicated an acceptable performance of the cooling system

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
data center, aisle containment, computational fluid dynamics (CFD), measurements, energy efficient
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-74676 (URN)10.3390/en12081473 (DOI)000467762600066 ()2-s2.0-85065507938 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-18 (johcin)

Available from: 2019-06-18 Created: 2019-06-18 Last updated: 2019-06-18Bibliographically approved
Karlsson, L., Lycksam, H., Ljung, A.-L., Gren, P. & Lundström, S. (2019). Experimental study of the internal flow in freezing water droplets on a cold surface. Experiments in Fluids, 60(12), Article ID 182.
Open this publication in new window or tab >>Experimental study of the internal flow in freezing water droplets on a cold surface
Show others...
2019 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 60, no 12, article id 182Article in journal (Refereed) Published
Abstract [en]

The study of a freezing droplet is interesting in areas, where the understanding of build up of ice is important, for example, on wind turbines, airplane wings and roads. In this work, the main focus is to study the internal motion inside freezing water droplets using particle image velocimetry and to reveal if mechanisms such as natural convection and Marangoni convection have a noticeable influence on the flow within the droplet. The flow has successfully been visualized and measured for the first 25% of the total freezing time of the droplet when the velocity in the water is the highest and when the characteristic vortices can be seen. After this initial time period, the high amount of ice in the droplet scatters the PIV light sheet too much and the images retrieved are not suitable for analysis. Initially, it can be seen that the Marangoni effects have a large impact on the internal flow, but after about 15% of the total freezing time, the flow turns indicating increased effects of natural convection on the flow. Shortly after this time, almost no internal flow can be seen.

Place, publisher, year, edition, pages
Springer, 2019
National Category
Fluid Mechanics and Acoustics Applied Mechanics
Research subject
Fluid Mechanics; Experimental Mechanics
Identifiers
urn:nbn:se:ltu:diva-76844 (URN)10.1007/s00348-019-2823-1 (DOI)000497768000004 ()2-s2.0-85075101925 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-11-25 (johcin)

Available from: 2019-11-25 Created: 2019-11-25 Last updated: 2019-12-10Bibliographically approved
Xie, Q., Yang, J. & Lundström, S. (2019). Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load. Fluids, 4(1), Article ID 15.
Open this publication in new window or tab >>Field Studies and 3D Modelling of Morphodynamics in a Meandering River Reach Dominated by Tides and Suspended Load
2019 (English)In: Fluids, ISSN 2311-5521, Vol. 4, no 1, article id 15Article in journal (Refereed) Published
Abstract [en]

Meandering is a common feature in natural alluvial streams. This study deals with alluvial behaviors of a meander reach subjected to both fresh-water flow and strong tides from the coast. Field measurements are carried out to obtain flow and sediment data. Approximately 95% of the sediment in the river is suspended load of silt and clay. The results indicate that, due to the tidal currents, the flow velocity and sediment concentration are always out of phase with each other. The cross-sectional asymmetry and bi-directional flow result in higher sediment concentration along inner banks than along outer banks of the main stream. For a given location, the near-bed concentration is 2−5 times the surface value. Based on Froude number, a sediment carrying capacity formula is derived for the flood and ebb tides. The tidal flow stirs the sediment and modifies its concentration and transport. A 3D hydrodynamic model of flow and suspended sediment transport is established to compute the flow patterns and morphology changes. Cross-sectional currents, bed shear stress and erosion-deposition patterns are discussed. The flow in cross-section exhibits significant stratification and even an opposite flow direction during the tidal rise and fall; the vertical velocity profile deviates from the logarithmic distribution. During the flow reversal between flood and ebb tides, sediment deposits, which is affected by slack-water durations. The bed deformation is dependent on the meander asymmetry and the interaction between the fresh water flow and tides. The flood tides are attributable to the deposition, while the ebb tides, together with run-offs, lead to slight erosion. The flood tides play a key role in the morphodynamic changes of the meander reach.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
tidal meandering river, field measurements, 3D numerical model, flow features, sediment transport, erosion-deposition patterns
National Category
Water Engineering Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-72664 (URN)10.3390/fluids4010015 (DOI)000464467200002 ()
Note

Validerad;2019;Nivå 2;2019-01-31 (svasva)

Available from: 2019-01-24 Created: 2019-01-24 Last updated: 2019-04-30Bibliographically approved
Misiulia, D., Antonyuk, S., Andersson, A. G. & Lundström, S. (2019). High-efficiency industrial cyclone separator: A CFD study. Powder Technology
Open this publication in new window or tab >>High-efficiency industrial cyclone separator: A CFD study
2019 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

The flow within an industrial scroll-inlet high-efficiency cyclone separator has been studied using RSM and LES simulations. Of particular interest is the effect of the gas outlet configuration, i.e. outlet scroll and radial bend, on the flow pattern, pressure drop and cyclone efficiency. A surprising phenomenon is that the inner vortex splits into two vortices for the cyclone with a conventional outlet pipe while if the cyclone is equipped with an outlet scroll or radial bend there is no split. The outlet scroll and radial bend increase the pressure losses by 5.1% and 6.4%, respectively. These installations, moreover, significantly destabilize the pressure losses and the amplitude of instantaneous pressure drop oscillations increases from 0.65% to 16.2% and 33.96%, respectively. The investigated outlet scroll and radial bend have practically no effects on the cyclone efficiency since the flow in the main separation zone is not affected by the gas outlet configuration.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Cyclone separator, Computational fluid dynamics, Vortex breakdown, Pressure drop, Collection efficiency
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-76944 (URN)10.1016/j.powtec.2019.10.064 (DOI)2-s2.0-85075881002 (Scopus ID)
Available from: 2019-11-28 Created: 2019-11-28 Last updated: 2019-12-16
Fallahjoybari, N., Lundström, S. & Hellström, J. G. (2019). Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media. Transport in Porous Media
Open this publication in new window or tab >>Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media
2019 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634Article in journal (Refereed) Epub ahead of print
Abstract [en]

The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Turbulence, Porous media, LES, Hydrodynamic dispersion, Reynolds stress
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-77111 (URN)10.1007/s11242-019-01365-0 (DOI)2-s2.0-85075452647 (Scopus ID)
Available from: 2019-12-09 Created: 2019-12-09 Last updated: 2019-12-09
Andersson, R., Hellström, J. G., Andreasson, P. & Lundström, S. (2019). Numerical investigation of a hydropower tunnel: Estimating localised head-loss using the manning equation. Water, 11(8), Article ID 1562.
Open this publication in new window or tab >>Numerical investigation of a hydropower tunnel: Estimating localised head-loss using the manning equation
2019 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 8, article id 1562Article in journal (Refereed) Published
Abstract [en]

The fluid dynamics within a water tunnel is investigated numerically using a RANS approach with the k-ε turbulence model. The computational model is based on a laser scan of a hydropower tunnel located in Gävunda, Sweden. The tunnel has a typical height of 6.9 m and a width of 7.2 m. While the average cross-sectional shape of the tunnel is smooth the local deviations are significant, where some roughness elements may be in the size of 5 m implying a large variation of the hydraulic radius. The results indicate that the Manning equation can successfully be used to study the localised pressure variations by taking into account the varying hydraulic radius and cross-sectional area of the tunnel. This indicates a dominant effect of the tunnel roughness in connection with the flow, which has the potential to be used in the future evaluation of tunnel durability. ANSYS-CFX was used for the simulations along with ICEM-CFD for building the mesh. 

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
ANSYS-CFX, Case-study, Head-loss, Hydropower, Rock tunnel, Surface roughness
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-75622 (URN)10.3390/w11081562 (DOI)000484561500036 ()2-s2.0-85070288117 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-08-21 (svasva)

Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-10-08Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-1033-0244

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