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Hellström, J. Gunnar I.ORCID iD iconorcid.org/0000-0002-8360-9051
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Publications (10 of 68) Show all publications
Hadi Jafari, P., Wingren, A., Hellström, J. G. & Gebart, R. (2019). Effect of process parameters on the performance of an air-blown entrained flow cyclone gasifier. International Journal of Sustainable Energy
Open this publication in new window or tab >>Effect of process parameters on the performance of an air-blown entrained flow cyclone gasifier
2019 (English)In: International Journal of Sustainable Energy, ISSN 1478-6451, E-ISSN 1478-646XArticle in journal (Refereed) Epub ahead of print
Abstract [en]

Entrained flow gasification of biomass in a cyclone reactor combined by a gas engine has been applied in Nordic countries as one of the preferred methods for generating combined heat and power in small scales. The purpose of the current study was to optimise the gasification plant efficiency and understanding the influence of operating conditions. The experiments were carried out in a 2.4 MW(th) commercial gasification power plant. The gasifier was operated in optimum at a rather low lambda around 0.27 and a temperature of 950°C. The lower heating value of the clean product gas at this lambda was 5.95 MJ/Nm3. The experimental results also were compared with the predicted values from thermodynamic equilibrium calculations by Factsage 7.0. The performance of five different types of biofuels including torrefied spruce, peat, rice husk, bark and stemwood were assessed and compared with each other using thermodynamic equilibrium and available experimental data.

Place, publisher, year, edition, pages
Taylor & Francis, 2019
Keywords
Cyclone gasification, Airblown, Biomass, Cold gas efficiency, Process performance
National Category
Fluid Mechanics and Acoustics Energy Engineering
Research subject
Fluid Mechanics; Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-74459 (URN)10.1080/14786451.2019.1626858 (DOI)
Funder
Bio4Energy, 191503
Available from: 2019-06-13 Created: 2019-06-13 Last updated: 2019-06-17
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
Bin Asad, S. M., Lundström, S., Andersson, A. G., Hellström, J. G. & Leonardsson, K. (2019). Wall shear stress measurement on curve objects with PIV in connection to benthic fauna in regulated rivers. Water, 11(4), Article ID 650.
Open this publication in new window or tab >>Wall shear stress measurement on curve objects with PIV in connection to benthic fauna in regulated rivers
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2019 (English)In: Water, ISSN 2073-4441, E-ISSN 2073-4441, Vol. 11, no 4, article id 650Article in journal (Refereed) Published
Abstract [en]

The flow characteristics in the vicinity of a set of half-cylinders of different sizes simulating benthic objects were studied experimentally using particle image velocimetry (PIV). The cylinders were mounted on the bottom of an open channel, and the influence of the flow speed on the distribution of the shear stress along the bottom geometry was investigated. Of special interest was how the shear stress changes close to the wall as a function of the flow speed and cylinder arrangement. It was found that the shear stress varies significantly as a function of position. This implies habitat heterogeneity allowing benthic invertebrates with different shear stress tolerance exists when the bottom consists of differently sized stones. It was also shown that direct measurements of near wall velocity gradients are necessary to accurately calculate the wall shear stress for more complex geometries.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
shear stress, PIV, benthic fauna, river, stones
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-74913 (URN)10.3390/w11040650 (DOI)000473105700020 ()2-s2.0-85065032227 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-06-24 (johcin)

Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-08-16Bibliographically approved
Andersson, L. R., Larsson, S., Hellström, J. G., Andreasson, P., Andersson, A. G. & Lundström, S. (2018). Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations. Engineering, 10, 399-416
Open this publication in new window or tab >>Characterization of Flow Structures Induced by Highly Rough Surface Using Particle Image Velocimetry, Proper Orthogonal Decomposition and Velocity Correlations
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2018 (English)In: Engineering, ISSN 1947-3931, Vol. 10, p. 399-416Article in journal (Refereed) Published
Abstract [en]

High Reynolds number flow inside a channel of rectangular cross section is examined using Particle Image Velocimetry. One wall of the channel has been replaced with a surface of a roughness representative to that of real hydropower tunnels, i.e. a random terrain with roughness dimensions typically in the range of ≈10% - 20% of the channels hydraulic radius. The rest of the channel walls can be considered smooth. The rough surface was captured from an existing blasted rock tunnel using high resolution laser scanning and scaled to 1:10. For quantification of the size of the largest flow structures, integral length scales are derived from the auto-correlation functions of the temporally averaged velocity. Additionally, Proper Orthogonal Decomposition (POD) and higher-order statistics are applied to the instantaneous snapshots of the velocity fluctuations. The results show a high spatial heterogeneity of the velocity and other flow characteristics in vicinity of the rough surface, putting outer similarity treatment into jeopardy. Roughness effects are not confined to the vicinity of the rough surface but can be seen in the outer flow throughout the channel, indicating a different behavior than postulated by Townsend’s similarity hypothesis. The effects on the flow structures vary depending on the shape and size of the roughness elements leading to a high spatial dependence of the flow above the rough surface. Hence, any spatial averaging, e.g. assuming a characteristic sand grain roughness factor, for determining local flow parameters becomes less applicable in this case.

Place, publisher, year, edition, pages
Scientific Research Publishing, 2018
Keywords
CFD, Validation, Hydraulic Roughness, PIV, Hydropower
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71097 (URN)10.4236/eng.2018.107028 (DOI)
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2019-08-12Bibliographically approved
Andersson, R., Burman, A., Hellström, J. G. & Andreasson, P. (2018). Inlet Blockage Effects in a Free Surface Channel With Artificially Generated Rough Walls. In: Daniel Bung ; Blake Tullis (Ed.), Proceedings of the 7th IAHR International Symposium on Hydraulic Structures: . Paper presented at 7th International Symposium on Hydraulic Structures, Aachen, Germany, 15-18 May 2018 (pp. 723-732).
Open this publication in new window or tab >>Inlet Blockage Effects in a Free Surface Channel With Artificially Generated Rough Walls
2018 (English)In: Proceedings of the 7th IAHR International Symposium on Hydraulic Structures / [ed] Daniel Bung ; Blake Tullis, 2018, p. 723-732Conference paper, Published paper (Refereed)
Abstract [en]

When considering free surface flow in channels, it is essential to have in-depth knowledge about the inlet flow conditions and the effect of surface roughness on the overall flow field. Hence, we hereby investigate flow inside an 18m long channel by using Particle Tracking Velocimetry (PTV) and Acoustic Doppler Velocimetry (ADV). The roughness of the channel walls is generated using a diamond-square fractal algorithm and is designed to resemble the actual geometry of hydropower tunnels. Four different water levels ranging from 20 to 50cm are investigated. For each depth, the inlet is blocked by 25 and 50% at three positions each, at the centre, to the right and to the left in the flow-direction. The flow is altered for each depth to keep the flow velocity even throughout the measurements. PTV is applied to measure the velocity of the free water surface; four cameras are placed above the setup to capture the entirety of the channel. The results show a clear correlation between roughness-height and velocity distribution at depths 20-30 cm. The surface roughness proved effective in dispersing the subsequent perturbations following the inlet blockage. At 50cm, perturbations from the 50% blockage could be observed throughout the channel. However, at 20cm, most perturbations had subsided by a third of the channel length. The ADV was used to capture the velocity in a total of 375 points throughout the channel, at a depth of 50 cm with no inlet perturbations.

Keywords
Hydraulic roughness, PTV, diamond-square algorithm, free-surface flows
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-71096 (URN)10.15142/T3P644 (DOI)2-s2.0-85054178430 (Scopus ID)9780692132777 (ISBN)
Conference
7th International Symposium on Hydraulic Structures, Aachen, Germany, 15-18 May 2018
Available from: 2018-10-04 Created: 2018-10-04 Last updated: 2018-10-12Bibliographically approved
Pantea, H. J., Misiulia, D., Hellström, J. G. & Gebart, R. (2018). Modeling of Particle-Laden Cold Flow in a Cyclone Gasifier. Journal of Fluids Engineering - Trancactions of The ASME, 141(2), Article ID 021302.
Open this publication in new window or tab >>Modeling of Particle-Laden Cold Flow in a Cyclone Gasifier
2018 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 141, no 2, article id 021302Article in journal (Refereed) Published
Abstract [en]

Isothermal transient Eulerian–Lagrangian simulation of the turbulent gas–solid flow in a cyclone gasifier with two inlet tubes at 890 °C has been performed. The single-phase gas flow is modeled using SSG Reynolds stress turbulence model. Ten thousand representative solid particles of different sizes are injected from each inlet continuously at every second of simulation time. Particles are finally stopped as soon as they arrive at the outlet or reach the bottom plate of the gasifier. The effect of particle-to-gas coupling on the pressure and velocity of the flow and particles motion inside the gasifier is studied. The numerical approach can reasonably predict the impact of particle load on the gas flow as presented in the experimental results. Single particles are traveled throughout the transient gas flow field by using Lagrangian approach. High temperature of the gas flow inside the gasifier has significant effects on the swirl intensity reduction, damping the turbulence in the core region, pressure, and particle behaviors. However, the presence of solid particles does not have a notable influence on the swirl intensity and turbulence.

Place, publisher, year, edition, pages
The American Society of Mechanical Engineers (ASME), 2018
National Category
Engineering and Technology Energy Engineering Fluid Mechanics and Acoustics
Research subject
Energy Engineering; Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-70480 (URN)10.1115/1.4040929 (DOI)000452773200012 ()2-s2.0-85052003087 (Scopus ID)
Note

Validerad;2018;Nivå 2;2018-08-31 (svasva)

Available from: 2018-08-17 Created: 2018-08-17 Last updated: 2019-01-30Bibliographically approved
Risberg, D., Westerlund, L. & Hellström, J. G. (2017). Computational fluid dynamics simulation of indoor climate in low energy buildings computational set up. Thermal Science, 21(5), 1985-1998
Open this publication in new window or tab >>Computational fluid dynamics simulation of indoor climate in low energy buildings computational set up
2017 (English)In: Thermal Science, ISSN 0354-9836, E-ISSN 2334-7163, Vol. 21, no 5, p. 1985-1998Article in journal (Refereed) Published
Abstract [en]

In this paper CFD was used for simulation of the indoor climate in a part of a low energy building. The focus of the work was on investigating the computational set up, such as grid size and boundary conditions in order to solve the indoor climate problems in an accurate way. Future work is to model a complete building, with reasonable calculation time and accuracy. A limited number of grid elements and knowledge of boundary settings are therefore essential. An accurate grid edge size of around 0.1 m was enough to predict the climate according to a grid independency study. Different turbulence models were compared with only small differences in the indoor air velocities and temperatures. The models show that radiation between building surfaces has a large impact on the temperature field inside the building, with the largest differences at the floor level. Simpling the simulations by modelling the radiator as a surface in the outer wall of the room is appropriate for the calculations. The overall indoor climate is finally compared between three different cases for the outdoor air temperature. The results show a good indoor climate for a low energy building all around the year.

Place, publisher, year, edition, pages
VINČA Institute of Nuclear Sciences, 2017
National Category
Energy Engineering Fluid Mechanics and Acoustics
Research subject
Energy Engineering; Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-66606 (URN)10.2298/TSCI150604167R (DOI)000414237000010 ()2-s2.0-85032913950 (Scopus ID)
Note

Validerad;2017;Nivå 2;2017-11-17(inah)

Available from: 2017-11-17 Created: 2017-11-17 Last updated: 2018-11-19Bibliographically approved
Khayamyan, S., Lundström, S., Hellström, G., Gren, P. & Lycksam, H. (2017). Measurements of Transitional and Turbulent Flow in a Randomly Packed Bed of Spheres with Particle Image Velocimetry. Transport in Porous Media, 116(1), 413-431
Open this publication in new window or tab >>Measurements of Transitional and Turbulent Flow in a Randomly Packed Bed of Spheres with Particle Image Velocimetry
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2017 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 116, no 1, p. 413-431Article in journal (Refereed) Published
Abstract [en]

Particle image velocimetry (PIV) has been used to investigate transitional and turbulent flow in a randomly packed bed of mono-sized transparent spheres at particle Reynolds number, (Formula presented.). The refractive index of the liquid is matched with the spheres to provide optical access to the flow within the bed without distortions. Integrated pressure drop data yield that Darcy law is valid at (Formula presented.). The PIV measurements show that the velocity fluctuations increase and that the time-averaged velocity distribution start to change at lower (Formula presented.). The probability for relatively low and high velocities decreases with (Formula presented.) and recirculation zones that appear in inertia dominated flows are suppressed by the turbulent flow at higher (Formula presented.). Hence there is a maximum of recirculation at about (Formula presented.). Finally, statistical analysis of the spatial distribution of time-averaged velocities shows that the velocity distribution is clearly and weakly self-similar with respect to (Formula presented.) for turbulent and laminar flow, respectively

Place, publisher, year, edition, pages
Springer, 2017
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-60561 (URN)10.1007/s11242-016-0781-0 (DOI)000394167300019 ()2-s2.0-84994430906 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-02-01 (andbra)

 

Available from: 2016-11-20 Created: 2016-11-20 Last updated: 2018-09-27Bibliographically approved
Khayamyan, S., Lundström, S., Gren, P., Lycksam, H. & Hellström, G. (2017). Transitional and Turbulent Flow in a Bed of Spheres as Measured with Stereoscopic Particle Image Velocimetry. Transport in Porous Media, 117(1), 45-67
Open this publication in new window or tab >>Transitional and Turbulent Flow in a Bed of Spheres as Measured with Stereoscopic Particle Image Velocimetry
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2017 (English)In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 117, no 1, p. 45-67Article in journal (Refereed) Published
Abstract [en]

Stereoscopic particle image velocimetry has been used to investigate inertia dominated, transitional and turbulent flow in a randomly packed bed of monosized PMMA spheres. By using an index-matched fluid, the bed is optically transparent and measurements can be performed in an arbitrary position within the porous bed. The velocity field observations are carried out for particle Reynolds numbers, (Formula presented.), between 20 and 3220, and the sampling is done at a frequency of 75 Hz. Results show that, in porous media, the dynamics of the flow can vary significantly from pore to pore. At (Formula presented.) around 400 the spatially averaged time fluctuations of total velocity reach a maximum and the spatial variation of the time-averaged total velocity, (Formula presented.) increases up to about the same (Formula presented.) and then it decreases. Also in the studied planes, a considerable amount of the fluid moves in the perpendicular directions to the main flow direction and the time-averaged magnitude of the velocity in the main direction, (Formula presented.), has an averaged minimum of 40% of the magnitude of (Formula presented.) at (Formula presented.) about 400. For (Formula presented.), this ratio is nearly constant and (Formula presented.) is on average a little bit less than 50% of (Formula presented.). The importance of the results for longitudinal and transverse dispersion is discussed.

Place, publisher, year, edition, pages
Springer, 2017
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-61843 (URN)10.1007/s11242-017-0819-y (DOI)000395621700003 ()2-s2.0-85009872759 (Scopus ID)
Note

Validerad; 2017; Nivå 2; 2017-03-06 (andbra)

Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2018-10-04Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-8360-9051

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