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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. Validerad;2023;Nivå 2;2023-05-29 (joosat);Licens fulltext: CC BY License

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.

Large eddy simulations of turbulent flow and passive scalar transport in a channel partially filled with an array of spherical particles, mimicking a layer of deposited material over a surface, are carried out to investigate the effect of turbulence mixing on the passive scalar transfer rate from the channel wall. It is observed that large turbulence structures generated at the permeable layer-fluid interface penetrate into the porous media and enhance the scalar transport from the channel wall. The small gap between the layer of particles and the wall also induces a channeling effect that implies turbulence mixing and high velocity near the wall resulting in an enhancement in scalar transport from the channel wall. The influence from the distance between the particles in one layer, particle diameter and the gap thickness on the passive scalar transport is also investigated. It is found that the highest Stanton number is derived for the largest distance between particles since this facilitates the motion of the large turbulent structures into the porous layer. Such motions will affect the scalar transport from the channel wall into the bulk. The channeling effect is observed to be the most important parameter when the particle diameter and the gap thickness are varied, and the highest Stanton number is achieved for the cases with the most effective channeling effect.

Water management diagnosis in polymer electrolyte membrane fuel cell is of great importance. The water produced in the fuel cell affects its performance and lifetime through the current distribution and the two phase flow pattern in the channel. The aim of the present study is to modify a segmented model in order to investigate a model-based water management diagnosis at different operating conditions. Simulations are conducted in three current densities: low (0.2 A / c m 2 ), medium (0.6 A / c m 2 ) and high (1 A / c m 2 ), four temperatures ranged from 40 to 70, two stoichiometries (2 and 3) and four inlet humidities (25%, 50%,75% and 100%). The results show that at fully saturation inlet condition, there is a uniform local current density for all three considered current densities. Also, two-phase pressure drop and output voltage have similar trends. Hence two phase pressure drop can be considered as a suitable criterion for water management diagnosis. At inlet humidities less than 50%, non-uniformity of local current density increases that leads to reduction of output voltage, especially at high current density. Generally, for non-saturated inlet condition, two phase pressure drop and output voltage may show different trends. Therefore, two-phase pressure drop can be used only as a criterion for the formation of water and not for water management diagnosis.

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.

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.

The present study aims to present an approach to mitigate the maximum solid temperature and its gradient inside the porous material in volumetric solar receivers. To this end, a porous receiver with non-uniform air velocity at the inlet is considered in the present study. Comparison of the results with those obtained for a porous receiver with uniform air velocity at the inlet reveals the ability of the new velocity distribution in reducing the maximum solid temperature and its gradient within the solid phase. The temperature distribution is obtained for different porosities and pore diameters in the porous receivers with uniform and non-uniform air velocity distributions at the inlet. It is observed that the proposed distribution of air velocity at the inlet decreases the maximum solid temperature within the porous receiver even for small porosities and pore diameters of porous media.

Turbulent flow in a composite porous/fluid domain is usually addressed in the literature assuming laminar flow inside the porous region. However, some of the recent studies revealed the high level of turbulent quantities inside the porous region of composite porous/fluid domains. Therefore, a comparison is made between the results of turbulent and laminar simulations in the present study in order to get more insight on the effects of turbulence inside the porous region on fluid flow and heat transfer in a pipe partially filled with a porous media. The effects of turbulence inside the porous layer on velocity, turbulence parameters, temperature distribution and Nusselt number are analysed for different Darcy numbers and several porous layer thicknesses. It is shown that the turbulence effects inside the porous layer are important even for pore base Reynolds numbers lower than the critical Reynolds number in porous media.

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

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 a ...

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 ≤Res≤ 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.

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.

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.

This paper presents pore scale simulation of turbulent combustion of air/methane mixture in porous media to investigate the effects of multidimensionality and turbulence on the flame within the pores of porous media. In order to investigate combustion in the pores of porous medium, a simple but often used porous medium consisting of a staggered arrangement of square cylinders is considered in the present study. Results of turbulent kinetic energy, turbulent viscosity ratio, temperature, flame speed, convective heat transfer and thermal conductivity are presented and compared for laminar and turbulent simulations. It is shown that the turbulent kinetic energy increases from the inlet of burner, because of turbulence created by the solid matrix with a sudden jump or reduction at the flame front due to increase in temperature and velocity. Also, the pore scale simulation revealed that the laminarization of flow occurs after flame front in the combustion zone and turbulence effects are important mainly in the preheat zone. It is shown that turbulence enhances the diffusion processes in the preheat zone, but it is not enough to affect the maximum flame speed, temperature distribution and convective heat transfer in the porous burner. The dimensionless parameters associated with the Borghi–Peters diagram of turbulent combustion have been analyzed for the case of combustion in porous media and it is found that the combustion in the porous burner considered in the present study concerns the range of well stirred reactor very close to the laminar flame region.

The present study deals with the generalization of a macroscopic turbulence model in porous media using a capillary model. The additional source terms associated with the production and dissipation of turbulent kinetic energy due to the presence of solid matrix are calculated using the capillary model. The present model does not require any prior pore scale simulation of turbulent flow in a specific porous geometry in order to close the macroscopic turbulence equations. Validation of the results in packed beds, periodic arrangement of square cylinders, synthetic foams, and longitudinal flows such as pipes, channels, and rod bundles against available data in the literature reveals the ability of the present model in predicting turbulent flow characteristics in different types of porous media. Transition to the fully turbulent regime in porous media and different approaches to treat this phenomenon are also discussed in the present study. Finally, the general model is modified so that it can be applied to lower Reynolds numbers below the range of fully turbulent regime in porous media.

Two macroscopic turbulence models, Pedras and de Lemos (P–dL) and Nakayama and Kuwahara (N–K), have been used previously in the literature for analyzing turbulent flows through porous media. The accuracy of these models is discussed in the present study, and a model is proposed for reacting flows through porous media. The additional source terms of macroscopic $$k-\varepsilon $$ equations representing production and dissipation of turbulent kinetic energy are modeled introducing two unknown model constants based on physical considerations. These unknown constants are determined from the results of microscopic simulations available in the literature and scale analysis in the present study. To substantiate the validity of the present model and its superiority over the previous models, eddy viscosity predicted by the present model is compared against the results of previous models and microscopic simulation of reacting flow through a simple but often used numerical model for porous media, a staggered arrangement of square cylinders. Unlike the previous models, results of the present model are in good agreement with the microscopic results in the preheat zone, and the present model accurately captures the laminarization of flow in the combustion zone.

This paper presents two dimensional simulations of air/methane combustion in cylindrical porous burner using models that explicitly consider the intra-pore levels of turbulent kinetic energy. Two most used turbulence models in porous media, proposed by Pedras and de Lemos (P–dL) and Nakayama and Kuwahara (N–K), have been used in the present study. Results of two macroscopic turbulence models are presented and compared for different Reynolds numbers and several flame locations. The values of turbulent kinetic energy and eddy viscosity calculated by two models are quite different due to the difference between the terms proposed for internal generation in turbulent kinetic energy and dissipation rate equations. It is found that P–dL model more accurately predicts the characteristics of reacting flow in porous media such as turbulent kinetic energy and eddy viscosity. Also, it is shown that the effects of turbulence in porous media are more significant when the flame stabilizes at the downstream of burner. Due to enhancement of heat and species diffusion, the preheating of reactants increases and the flame moves closer to the burner entrance when turbulence models are applied.