| 1. |
- Jouybari, Nima Fallah, et al.
(författare)
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An investigation of forces on a representative surface in a pulp flow through rotating and non-rotating grooves
- 2023
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Ingår i: Journal of the Brazilian Society of Mechanical Sciences and Engineering. - : Springer. - 1678-5878 .- 1806-3691. ; 45:5
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Tidskriftsartikel (refereegranskat)abstract
- Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close to the stagnation point using Bernoulli’s principle is carried out to provide a better understanding of the pressure coefficient variation with angular velocity in the non-rotating groove. A comparison of pressure coefficients obtained numerically with those calculated by considering the compression effects revealed that the comparison effects are dominating in the pulp flow within the groove.
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| 2. |
- Jouybari, Nima Fallah, et al.
(författare)
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Investigation of Post-Darcy Flow in Thin Porous Media
- 2021
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Ingår i: Transport in Porous Media. - : Springer. - 0169-3913 .- 1573-1634. ; 138:1, s. 157-184
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Tidskriftsartikel (refereegranskat)abstract
- We present numerical simulations of post-Darcy flow in thin porous medium: one consisting of staggered arrangements of circular cylinders and one random distribution of cylinders bounded between walls. The simulations span a range of Reynolds numbers, 40 to 4000, where the pressure drop varies nonlinearly with the average velocity, covering nonlinear laminar flow to the fully turbulent regime. The results are compared to those obtained by replacing the bounding walls with symmetric boundaries with the aim to reveal the effect of bounding walls on microscopic characteristics and macroscopic measures, i.e., pressure drop, hydrodynamic dispersion and Reynolds stresses. We use large eddy simulation to directly calculate the Reynolds stresses and turbulent intensity. The simulations show that vortical structures emerge at the boundary between the cylinders and the bounding walls causing a difference between the microscopic flow in the confined and non-confined porous media. This affects the averaged values of pressure drop, the hydrodynamic dispersion and the Reynolds stresses. Finally, the distance between the bounding walls is altered with the particle Reynolds number kept constant. It is observed that the difference between results calculated in confined and non-confined cases increases when the bounding walls are narrower.
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| 3. |
- Fallahjoybari, Nima, et al.
(författare)
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A comparative study of different heat transfer enhancement mechanisms in a partially porous pipe
- 2021
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Ingår i: SN Applied Sciences. - : Springer Science and Business Media LLC. - 2523-3963 .- 2523-3971. ; 3:10
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Tidskriftsartikel (refereegranskat)abstract
- The effect of porous material position on the heat transfer inside a pipe working in a turbulent regime is studied here to obtain a detailed understanding of the heat transfer enchantment mechanisms in different porous substrate positions. To this end, an in-house Fortran code is developed to solve the governing equations using the finite volume method and SIMPLE algorithm. Turbulent flow in porous media is modeled using a modified version of k–ε model. The flow field and heat transfer inside the partially filled pipe are investigated for the two cases of central and boundary configurations. The porous and flow characteristics including Reynolds number, Darcy number, the conductivity ratios of solid to fluid and the thickness of inserted porous layer are varied and the heat transfer performance is studied in different cases. It is observed that two entirely different phenomena enhance the heat transfer in central and boundary configurations. While the channeling of fluid between the porous media and the pipe wall highly affects the heat transfer performance in the former, the thermal conductivity of porous media plays a highly critical role in the latter configuration. It is shown that, for the same filling ratio, inserting the porous layer at the core of the pipe is more effective than placing it at the wall. Investigating porous materials with different solid conductivities revealed that covering the pipe wall with a porous material is justified only for solid matrixes with high thermal conductivities.
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| 4. |
- Fallahjoybari, Nima, et al.
(författare)
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A Subgrid-Scale Model for Turbulent Flow in Porous Media
- 2019
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Ingår i: Transport in Porous Media. - : Springer. - 0169-3913 .- 1573-1634. ; 129:3, s. 619-632
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Fallahjoybari, Nima, et al.
(författare)
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CFD Simulation Of Pulp Flow In Rotating And Non-Rotating Grooves
- 2022
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Ingår i: Proceedings of the International Mechanical Pulping Conference. ; , s. 24-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The present study deals with the numerical simulation of softwood pulp flow in the rotating and non-rotating grooves in an aim to investigate the fluid flow and forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp in different consistencies is available from the experimental measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness and angular velocity from which the pressure coefficient and shear forces values are obtained. It is found that the pressure increases at the stagnation point located at the gap entrance in the non-rotating groove due to tangential motion of the upper wall which induces the helical motion of the pulp flow in the groove’s cavity. However, such an effect is not observed in the rotating cavity close to the groove inlet. Meanwhile, by moving further along the channel length toward the outlet the helical motion is enhanced and an increase in the pressure is observed at the stagnation point. The shear forces over the representative surface are found to be independent of representative surface’s location and it is in the same level in the rotating and non-rotating grooves. In addition to the numerical simulations, an analytical discussion is also presented to provide a deeper understanding of pressure coefficient and shear forces variations with different parameters in the rotating and non-rotating grooves.
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| 6. |
- Fallahjoybari, Nima, et al.
(författare)
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Heat Transfer Modification within the Porous Layer of a Partially Filled Pipe at High Reynolds Number including Dispersion Effects
- 2020
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Ingår i: Journal of Porous Media. - : BEGELL HOUSE INC. - 1091-028X .- 1934-0508. ; 23:11, s. 1101-1121
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Tidskriftsartikel (refereegranskat)abstract
- The present study deals with the fluid flow and heat transfer in a pipe partially filled with porous media at a high Reynolds number. In addition to turbulence effects in this situation, interstitial fluid mixing within the pores plays an important role, causing significant thermal dispersion. The aim of the present study is to evaluate and compare the effects of these phenomena on the heat transfer rate in a pipe partially filled with porous media at the core and over the wall. The parameters studied are Darcy number, solid/fluid conductivity ratio, Reynolds number, Prandtl number, and porous layer thickness. The results showed that dispersion effects increase with the increase of Darcy number, porous layer thickness, and Prandtl number. A similar trend is also observed for the variation of turbulence effects with these parameters except for Darcy number. It was also found that the heat transfer rate in a pipe partially filled with a porous layer over the wall is mainly affected by thermal dispersion and turbulence effects inside the porous media, while the Nusselt number is not affected by these phenomena when the porous media is inserted at the core.
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| 7. |
- Fallahjoybari, Nima, et al.
(författare)
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Investigation of pulp flow helicity in rotating and non-rotating grooves
- 2021
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Ingår i: Proceedings of the Institution of mechanical engineers. Part E, journal of process mechanical engineering. - : SAGE Publications. - 0954-4089 .- 2041-3009. ; 235:6, s. 2045-2058
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Tidskriftsartikel (refereegranskat)abstract
- Numerical simulation of pulp flow in rotating and non-rotating grooves is carried out to investigate the effect of pulp rheological properties and groove geometry on the rotational motion of the pulp flow. The eucalyptus pulp suspension is considered as a working fluid in the present study whose apparent viscosity correlation is available from the experimental measurements reported in the literature. The simulations are carried out with OpenFoam for different values of pulp material, fiber concentrations, and groove cross-section. Helicity is introduced to measure the turnover rate of pulp flow in the groove due to the importance of such motion on the final properties of the pulp flow. A measurement of helicity magnitude and its distribution along the groove revealed that a change in the pulp material would significantly affect the flow structures within the groove. Further investigation on the effects of fiber concentration, c, showed that this parameter does not have a significant effect on the averaged helicity magnitude for c = 2.0 and 2.5, whereas the helicity distribution over the groove cross-section changes clearly for c = 1.5. The results showed that the helicity level is negligible for almost half of the cavity cross-section in the non-rotating groove simulations, which can be considered as a shortcoming of the original geometry of the groove. Therefore, a smaller cross-section for the groove is considered through which an enhancement in the helicity magnitude is observed.
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| 8. |
- Fallahjoybari, Nima, et al.
(författare)
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Large eddy simulation of turbulent pulp flow in a channel
- 2020
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Ingår i: Journal of Non-Newtonian Fluid Mechanics. - : Elsevier BV. - 0377-0257 .- 1873-2631. ; 285
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Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulation (LES) of turbulent flow of concentrated fiber suspension or pulp is carried out to investigate the flow and turbulence structures in a channel. The simulations are carried out for the turbulent flow of Eucalyptus pulp suspension using OpenFOAM for three fiber concentrations (c = 1.5, 2.0 and 2.5) and six different Reynolds numbers (6 <= Re-s <= 16,600). It is observed that the variation in flow regime is similar in the two lower fiber concentrations while the flow regime is highly affected by fiber concentration for c = 2.5. Visualizations of vortical structures for different Reynolds numbers and fiber concentrations are used to investigate different flow regimes. Variation of apparent viscosity with Reynolds number and fiber concentration is also presented to show its effect on the turbulent properties of fiber suspension flow. It is shown that the deviation of turbulent velocity profile from that of a Newtonian flow increases with an increase in Reynolds number and fiber concentration. Also, the extend of buffer layer increases at higher Re. Using the calculated turbulent velocity profile, the values of constant in logarithmic velocity profile is proposed for fiber suspension. Finally, a discussion is presented on the variation of turbulent intensities and Reynolds stress with Reynolds number and fiber concentration.
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| 9. |
- Fallahjoybari, Nima, et al.
(författare)
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Numerical Computation of Macroscopic Turbulent Quantities in a Porous Medium : an Extemsion to a macroscopic Turbulent model
- 2016
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Ingår i: Journal of Porous Media. - 1091-028X .- 1934-0508. ; 19:6, s. 497-513
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study is conducted using a standard numerical model for a porous medium consisting of a staggered arrangement of square cylinders. Fully developed macroscopic turbulent kinetic energy and dissipation rate are derived and analyzed for different porosities of the medium at different Reynolds numbers. The results obtained are used to extend the applicability range of an existing macroscopic turbulence model in porous media to low-Reynolds-number turbulent flows. It is shown that the levels of normalized macroscopic turbulent kinetic energy and dissipation rate are not constant over the entire range of Reynolds number. These quantities increase from lower levels at low Reynolds numbers up to an asymptotic value being independent of Reynolds number. The constants in the closure expression of the macroscopic turbulence equations are modified using the present results. Finally, in order to highlight the importance of the present modifications, the results of the macroscopic turbulence model before and after the modifications are compared for two cases.
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| 10. |
- Fatahian, Hossein, et al.
(författare)
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Improving the flow uniformity in compact parallel-flow heat exchangers manifold using porous distributors
- 2022
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Ingår i: Journal of thermal analysis and calorimetry (Print). - : Springer Science and Business Media LLC. - 1388-6150 .- 1588-2926. ; 147:22, s. 12919-12931
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Tidskriftsartikel (refereegranskat)abstract
- The present study deals with the numerical simulation of turbulent flow in a Z-type manifold in which the fluid is distributed via nine distribution tubes. One of the major drawbacks of such devices is the mal-distribution of flow within these tubes. The flow rates are usually low in the first tubes close to the header entrance and increase in the other tubes. To address this problem and achieve a more uniform flow distribution inside the manifold, a novel solution is introduced in the present study, which includes the insertion of thin layers of porous media at the inlet of distribution tubes. In addition, a parametric study is conducted to evaluate the effect of porous media geometrical parameters such as pore diameter, porosity, and porous layer thickness on flow distributions among the tubes. The results demonstrate that the proposed approach increases the uniformity of flow distribution specifically when a porous media with higher resistance against the fluid flow is inserted uniformly within the manifold. In this case, a standard deviation as small as Phi=0.0067 could be reached showing the high level of flow uniformity within the manifold. Also, a maximum pressure drop of 12.557 kPa is observed which is approximately 38% larger than that calculated in the manifold without porous insertions. Moreover, several non-uniform distributions of porous media are also investigated to further improve the flow uniformity and decrease the pressure drop. An improvement in the standard deviation of Phi=0.0043 is obtained in the case of #E3 with a non-uniform porous distribution with a 5% reduction in pressure drop compared to the pressure drop calculated in the corresponding uniform case #E1. The results reveal the effectiveness of the approach presented here to reach a more uniform flow distribution within the manifold without the need for re-designing and altering the manifold geometry which is usually proposed in the literature.
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| 11. |
- Fatahian, H., et al.
(författare)
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Performance enhancement of Savonius wind turbine using a nanofiber-based deflector
- 2022
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Ingår i: Journal of the Brazilian Society of Mechanical Sciences and Engineering. - : Springer Science and Business Media Deutschland GmbH. - 1678-5878 .- 1806-3691. ; 44:3
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Tidskriftsartikel (refereegranskat)abstract
- This research presented a unique deflector called a nanofiber-based deflector for Savonius wind turbine to achieve the best performance with a high self-starting capability. The objective of this novel deflector produced from nanofibers was to diminish the detrimental effect of the complicated wake zone made behind the standard solid deflector, which had not yet been analyzed. Different PVA and PLA nanofibers concentration ratios were evaluated experimentally before being included in the numerical simulation to find the most appropriate porosity value and average pore size. These nanofibers with concentration ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 (v/v) were fabricated by electrospinning method. Then, the performance of conventional Savonius turbine and deflector arrangements was thoroughly studied using Computational Fluid Dynamics. Variations of power, torque, and flow structure were compared between configurations of the deflector and the conventional Savonius rotor to prove how the nanofiber-based deflector improved the performance of the turbine. It was revealed that a suitable porosity value existed for attaining the maximum performance of the Savonius turbine. Furthermore, utilizing a PVA75/PLA25 deflector with φ = 0.89, greater average torque and power coefficients (Cm and CP) values were obtained. It is worth mentioning that using PVA75/PLA25 and PVA deflectors with greater porosity values completely eliminated the severe vortices behind the deflector. In rotation angles ranging from 100°–200° to 290°–360°, PVA75/PLA25 deflector substantially increased the CP. The average CP was raised by 7 and 9% using solid and PVA75/PLA25 deflectors, respectively. Accordingly, the solid and PVA75/PLA25 deflectors enhance not only the power coefficient but also the ability to self-start. At a rotation angle of 0°, the PVA75/PLA25 deflector outperformed the solid deflector by 87%, improving the static torque coefficient (Cms).
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| 12. |
- Persson, Johan, et al.
(författare)
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Feeding of double disc refiners – rotor design evaluation
- 2022
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Ingår i: Proceedings of the International Mechanical Pulping Conference. ; , s. 69-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This work presents a hypothesis of how steam flow effect the chip flow in the Double disc (DD) refiner and test it with a numerical simulation. DD refiners are often considered one of the most energy efficient refiner models. However, feeding chips into these machines is not as easy as feeding single disc refiners due to the rotating geometries. It is our belief that to increase energy efficiency in refining we need to increase also the production rates. The authors have previously noticed that in a standard DD rotor, steam flowed both in the same direction as the flow of woodchips and in the opposite direction. It is our hypothesis that backwards flowing steam in and in close proximity to the critical transition from the non-rotating geometry to the rotating geometry is negative for the material flow. To evaluate the hypothesis a new rotor was designed to eliminate the backwards flow. The authors have previously presented a two way coupled multiphase model with steam flow modeled with Computational Fluid Dynamics and wood chips modeled as groups of connected spherical particles with Discrete Element Method with a momentum exchange model. This model was utilized to model the flow of steam and woodchips in a DD under normal operational parameters, with the conventional rotor and with the new rotor. The throughput of wood chips was evaluated and normalized with regards to the chip flow to the refiner. The flow was considerable more stable in the new rotor, the throughput was close to 100 % for the observed time window, and the steam flow was more uniform. The results of the simulation supports the hypothesis. The next step in the research would be to test the new rotor in full scale operation.
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