| 1. |
- Burström, Per, et al.
(författare)
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Discrete and continuous modelling of convective heat transport in a thin porous layer of mono sized spheres
- 2017
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Ingår i: Heat and Mass Transfer. - : Springer. - 0947-7411 .- 1432-1181. ; 53:1, s. 151-160
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Tidskriftsartikel (refereegranskat)abstract
- Convective heat transport in a relatively thin porous layer of monosized particles is here modeled. The size of the particles is only one order of magnitude smaller than the thickness of the layer. Both a discrete three-dimensional system of particles and a continuous one-dimensional model are considered. The methodology applied for the discrete system is Voronoi discretization with minimization of dissipation rate of energy. The discrete and continuous model compares well for low velocities for the studied uniform inlet boundary conditions. When increasing the speed or for a thin porous layer however, the continuous model diverge from the discrete approach if a constant dispersion is used in the continuous approach. The new result is thus that a special correlation must be used when using a continuous model for flow perpendicular to a thin porous media in order to predict the dispersion in proper manner, especially in combination with higher velocities.
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| 2. |
- Burström, Per, et al.
(författare)
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Modelling heat transfer during flow through a random packed bed of spheres
- 2018
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Ingår i: Heat and Mass Transfer. - : Springer. - 0947-7411 .- 1432-1181. ; 54:4, s. 1225-1245
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Tidskriftsartikel (refereegranskat)abstract
- Heat transfer in a random packed bed of monosized iron ore pellets is modelled with both a discrete three-dimensional system of spheres and a continuous Computational Fluid Dynamics (CFD) model. Results show a good agreement between the two models for average values over a cross section of the bed for an even temperature profiles at the inlet. The advantage with the discrete model is that it captures local effects such as decreased heat transfer in sections with low speed. The disadvantage is that it is computationally heavy for larger systems of pellets. If averaged values are sufficient, the CFD model is an attractive alternative that is easy to couple to the physics up- and downstream the packed bed. The good agreement between the discrete and continuous model furthermore indicates that the discrete model may be used also on non-Stokian flow in the transitional region between laminar and turbulent flow, as turbulent effects show little influence of the overall heat transfer rates in the continuous model.
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| 3. |
- Fukagata, K., et al.
(författare)
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Dynamics of Brownian particles in a turbulent channel flow
- 2004
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 0947-7411 .- 1432-1181. ; 40:9, s. 715-726
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent channel flows with suspended particles are investigated by means of numerical simulations. The fluid velocity is computed by large eddy simulation. Motion of small graphite particles with diameter of 0.01-10 mum, corresponding to the Schmidt number, Sc, of 2.87 x 10(2)-6.22 x 10(6) and the particle relaxation time in wall unit, tau(p)(+), of 9.79 x 10(-5)-4.51, is computed by Lagrangian particle tracking. Relation between the particle relaxation time and the computed deposition velocity is found to be in good agreement with an empirical relation. The statistics of the particle motion in the vicinity of the wall are studied. Clear differences are found in dynamical behavior of particles with different sizes. Medium size particles show a strong dependence on the structure of the fluid flow, while small and large particles are considerably less sensitive.
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| 4. |
- Ghanbarpour, Morteza, et al.
(författare)
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An investigation of thermal performance improvement of a cylindrical heat pipe using Al2O3 nanofluid
- 2016
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Ingår i: Heat and Mass Transfer. - : Springer. - 0947-7411 .- 1432-1181. ; , s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- In this study, effect of Al2O3 nanofluid on thermal performance of cylindrical heat pipe is investigated. An analytical model is employed to study the thermal performance of the heat pipe utilizing nanofluid and the predicted results are compared with the experimental results. A substantial change in the heat pipe thermal resistance, effective thermal conductivity and entropy generation of the heat pipe is observed when using Al2O3 nanofluid as a working fluid. It is found that entropy generation in the heat pipe system decreases when using a nanofluid due to the lower thermal resistance of the heat pipe which results in an improved thermal performance. It is shown that the proposed model is in reasonably good agreement with the experimental results and can be used as a fast technique to explore various features of thermal characteristics of the nanofluid based heat pipe.
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| 5. |
- Gourdon, Mathias, 1980, et al.
(författare)
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Heat transfer for falling film evaporation of industrially relevant fluids up to very high Prandtl numbers
- 2016
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Ingår i: Waerme-Stoffuebertrag Thermo-Fluid Dyn. - : Springer Science and Business Media LLC. - 0042-9929 .- 1432-1181. ; 52:2, s. 379-391
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Tidskriftsartikel (refereegranskat)abstract
- In many industrial applications, falling film evaporation is an attractive technique for solvent removal due to high heat transfer and low residence times. Examples are the powder production in the dairy industry and in kraft pulp production process to remove water from so called black liquor. Common for both applications is that the fluids exhibit high viscosities in industrial practice. In this paper, results from experimental studies on both black liquor and a dairy product are reported for Prandtl numbers up to 800. The results are compared with several existing correlation in literature, and the need for a modified correlation is recognized especially to cover higher Prandtl-numbers. The following correlation for the turbulent flow region with 3
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| 6. |
- Haraldsson, H. O., et al.
(författare)
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Effect of solidification on drop fragmentation in liquid-liquid media
- 2001
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 0947-7411 .- 1432-1181. ; 37:4-5, s. 417-426
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents results of experimental and analytical investigation on molten alloy drop fragmentation in water pool. Emphasis is directed towards delineating the roles which melt to coolant heat transfer and melt solidification play in the fragmentation process. The strong impact of coolant temperature upon fragmentation process is addressed. A set of 23 drop fragmentation experiments were performed, in which 8 experiments employed a low melting point alloy, cerrobend-70 and 15 experiments using Pb-Bi eutectic alloy as drop fluid. The results show strong impact of coolant temperature on particle size distribution of the fragmented drops. A linear stability analysis of the interface between the two liquid fluids with thin crust growing between them, is performed. A modified dimensionless Aeroelastic number, for Kelvin-Helmholtz instability, is obtained and used as a criteria for fragmentation of molten drops penetrating into another liquid coolant media with lower temperature. The nondimensionalized mean diameter of the fragmented particles is correlated with the Aeroelastic number.
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| 7. |
- Hofgren, Henrik, et al.
(författare)
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Evaluation of Planck mean coefficients for particle radiative properties in combustion environments
- 2015
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 1432-1181 .- 0947-7411. ; 51:4, s. 507-519
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Tidskriftsartikel (refereegranskat)abstract
- Thermal radiation is the dominating form of heat transfer in several combustion technologies that combust solid fuels, such as pulverized coal combustion and fixed bed combustion. The thermal radiation originates from the hot combustion gases and particles. For accurate modelling of thermal radiation in these environments the selection of the radiative transport model and radiative property model is important. Radiative property models for gases have received huge attention and several well documented models exist. For particles, soot has received considerable attention whereas other particles have not to a similar extent. The Planck mean coefficients are most commonly used to describe the radiative properties of the particles. For gases the Planck mean absorption coefficient is known to give large deviations from recognised exact models in predicting the radiative heat transfer. In this study the use of Planck mean coefficients for particles are investigated and compared to spectral models. Two particle mass size distributions of fly ash are used, representing biomass and coal combustion. The evaluation is conducted in several combustion-like test cases with both gases and particles. The evaluation shows that using Planck mean coefficients for particles, in combustion-like situations, can give large errors in predicting the radiative heat flux and especially the source term. A new weighted sum of grey gas approach is tested and evaluated. It includes both the particles and gases to better account for the non-greyness of the fly ash absorption coefficient.
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| 8. |
- Hosseinalipour, Seyed Mostafa, et al.
(författare)
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Coriolis and buoyancy effects on heat transfer in viewpoint of field synergy principle and secondary flow intensity for maximization of internal cooling
- 2021
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Ingår i: Heat and Mass Transfer/Waerme- und Stoffuebertragung. - : Springer Science and Business Media LLC. - 0947-7411. ; 57:9, s. 1467-1483
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Tidskriftsartikel (refereegranskat)abstract
- The present investigation emphases on rotation effects on internal cooling of gas turbine blades both numerically and experimentally. The primary motivation behind this work is to investigate the possibility of heat transfer enhancement by dean vortices generated by Coriolis force and U-bend with developing turbulent in the view point of the field synergy principle and secondary flow intensity analysis. A two-passage internal cooling channel model with a 180° U-turn at the hub section is used in the analysis. The flow is radially outward at the first passage of the square channel and then it will be inward at the second passage. The study covers a Reynolds number (Re) of 10,000, Rotation number (Ro) in the range of 0–0.25, and Density Ratios (DR) at the inlet between 0.1–1.5. The numerical results are compared to experimental data from a rotating facility. Results obtained with the basic RANS SST k-ω model are assessed completely as well. A field synergy principle analysis is consistent with the numerical results too. The results state that the secondary flows due to rotation can considerably improve the synergy between the velocity and temperature gradients up to 20%, which is the most fundamental reason why the rotation can enhance the heat transfer. In addition, the Reynolds number and centrifugal buoyancy variations are found to have no remarkable impact on increasing the synergy angle. Moreover, vortices induced by Rotation number and amplified by Reynolds number increase considerable secondary flow intensity which is exactly in compliance with Nusselt number enhancement.
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| 9. |
- Kalisz, Sylwester, et al.
(författare)
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Influence of non-uniform flow distribution on overall heat transfer in convective bundle of circulating fluidized bed boiler
- 2005
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 0947-7411 .- 1432-1181. ; 41:11, s. 981-990
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Tidskriftsartikel (refereegranskat)abstract
- In the paper the results of comparative investigations on heat transfer performance of boiler convective bundle and its additional surface, i.e. membrane water wall are presented. For this purpose the non-uniform flow field was modelled in an isothermal test stand operated in self-modeling mode. Then the heat transfer characteristics of such arrangement were estimated by means of naphthalene heat/mass transfer analogy technique. The bundle samples in the shape of round bars (rods) were cast with naphthalene and placed in 27 locations in the bundle while water-wall-modeling samples were coated with naphthalene by painting. Both types of samples were exposed to cold air flow. The results were then compared to the mean heat transfer performance of the same bundle exposed to uniform flow field. The differences of approximately 10% were noticed. Moreover, the heat transfer coefficients for additional surface were even three times lower than those of the bundle. In view of results obtained in the work, the commonly made assumption of equality of heat transfer coefficients for both the bundle and its additional surface may lead to certain errors in heat transfer calculations and discrepancies between the calculated values of heating surfaces area and later operational needs of steam generator.
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| 10. |
- Karlsson, Linn, et al.
(författare)
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Modelling the dynamics of the flow within freezing water droplets
- 2018
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Ingår i: Heat and Mass Transfer. - : Springer. - 0947-7411 .- 1432-1181. ; 54:12, s. 3761-3769
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Tidskriftsartikel (refereegranskat)abstract
- The flow within freezing water droplets is here numerically modelled assuming fixed shape throughout freezing. Three droplets are studied with equal volume but different contact angles and two cases are considered, one including internal natural convection and one where it is excluded, i.e. a case where the effects of density differences is not considered. The shape of the freezing front is similar to experimental observations in the literature and the freezing time is well predicted for colder substrate temperatures. The latter is found to be clearly dependent on the plate temperature and contact angle. Including density differences has only a minor influence on the freezing time, but it has a considerable effect on the dynamics of the internal flow. To exemplify, in the vicinity of the density maximum for water (4 ∘C) the velocities are about 100 times higher when internal natural convection is considered for as compared to when it is not.
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