| 1. |
- Farzaneh Kaloorazi, Meisam, 1982, et al.
(författare)
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Pore-Scale Transport and Two-Phase Fluid Structures in Fibrous Porous Layers: Application to Fuel Cells and Beyond
- 2021
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Ingår i: Transport in Porous Media. - : Springer Science and Business Media LLC. - 1573-1634 .- 0169-3913. ; 136:1, s. 245-270
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Tidskriftsartikel (refereegranskat)abstract
- We present pore-scale simulations of two-phase flows in a reconstructed fibrous porous layer. The three-dimensional microstructure of the material, a fuel cell gas diffusion layer, is acquired via X-ray computed tomography and used as input for lattice Boltzmann simulations. We perform a quantitative analysis of the multiphase pore-scale dynamics, and we identify the dominant fluid structures governing mass transport. The results show the existence of three different regimes of transport: a fast inertial dynamics at short times, characterised by a compact uniform front, a viscous-capillary regime at intermediate times, where liquid is transported along a gradually increasing number of preferential flow paths of the size of one–two pores, and a third regime at longer times, where liquid, after having reached the outlet, is exclusively flowing along such flow paths and the two-phase fluid structures are stabilised. We observe that the fibrous layer presents significant variations in its microscopic morphology, which have an important effect on the pore invasion dynamics, and counteract the stabilising viscous force. Liquid transport is indeed affected by the presence of microstructure-induced capillary pressures acting adversely to the flow, leading to capillary fingering transport mechanism and unstable front displacement, even in the absence of hydrophobic treatments of the porous material. We propose a macroscopic model based on an effective contact angle that mimics the effects of the such a dynamic capillary pressure. Finally, we underline the significance of the results for the optimal design of face masks in an effort to mitigate the current COVID-19 pandemic.
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| 2. |
- Jareteg, Adam, 1989, et al.
(författare)
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Investigation of steam regeneration strategies for industrial-scale temperature-swing adsorption of benzene on activated carbon
- 2021
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Ingår i: Chemical Engineering and Processing: Process Intensification. - : Elsevier BV. - 0255-2701. ; 167
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale separation of substances present at low concentrations is readily performed by adsorption in packed beds that requires recurring energy-intensive regeneration of the adsorbent. The present work uses numerical simulations previously developed for industrial-scale packed-bed benzene sorption on activated carbon with temperature-swing regeneration by steam to investigate the influence of steam properties and regeneration strategy on total energy performance and breakthrough behaviour. It is shown that using saturated steam lowers both the steam mass and energy consumption during regeneration of a fixed amount of benzene, whereas using superheated steam returns the bed to a more fresh-like state after each regeneration stage. The most promising variation tried implies a 19% reduction in the energy consumption. Furthermore, the importance of accounting for the real industrial cycling conditions in the optimization of packed-bed adsorbers is highlighted. It is shown that the participation of different sections of the bed during adsorption varies with the regeneration strategy, but is never as localized as predicted from a model for a fresh bed without cycling. Finally, the present results also show that the effluent purity attained during regeneration increases when high-temperature saturated steam is used, e.g. a 60-degree increase in steam temperature raises the purity by 11%.
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| 3. |
- Kannan, Ananda Subramani, 1989, et al.
(författare)
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A hydrodynamic basis for off-axis Brownian diffusion under intermediate confinements in micro-channels
- 2021
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 143
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Tidskriftsartikel (refereegranskat)abstract
- The mobility of a Brownian particle diffusing in a micro-channel is heterogeneous and spatially dependent on the surrounding hydrodynamic resistance fields. The positional asymmetry of such a diffusing particle leads to anisotropies in the observed diffusive behavior. In this paper, we probe such directionally varying diffusive behavior of a spherical nanoparticle diffusing at a location off-set from the centerline of a square micro-channel in a quiescent fluid. This investigation is carried out over varying degrees of intermediate hydrodynamic confinements. A coupled Langevin-immersed boundary method is used for these assessments. We observe that the co-axial diffusivity may be slightly enhanced during off-axis hindered diffusion when compared with a corresponding centerline diffusive behavior. We attribute this increased particle diffusivity to a reduced co-axial fluid resistance through a hydrodynamic basis derived using steady-state CFD solutions to the corresponding Stokes problem. For co-axial motion, the particle creates a recirculating flow pattern around itself when moving along the centerline, whereas it drags along the fluid in between itself and the wall when in close proximity to the latter. These contrasting flow behaviors are responsible for the unexpected enhancement of the co-axial diffusivity for some off-axis positions under intermediate hydrodynamic confinements.
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| 4. |
- Kannan, Ananda Subramani, 1989, et al.
(författare)
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Assessment of hindered diffusion in arbitrary geometries using a multiphase DNS framework
- 2021
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 230
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Tidskriftsartikel (refereegranskat)abstract
- The hydrodynamics around a Brownian particle has a noticeable impact on its hindered diffusion in arbitrary geometries (such as channels/pores) due to reduced mobility close to walls. These effects are difficult to describe at sub-pore scales, wherein a complete analytical solution of the underlying hydrodynamics is challenging to obtain. Here, we propose a coupled Langevin-multiphase direct numerical simulation (DNS) framework, that fully resolves the hydrodynamics in such systems and consequently provides an on-the-fly capability to probe local instantaneous particle diffusivities. We validate and establish the capabilities of this framework in square micro-channels (under varying degrees of hydrodynamic confinement) and in an arbitrary pore. Our results show that directional variations in mean-squared displacements, velocity auto-correlation functions and diffusivities of the Brownian particle, due to inherent asymmetries in the geometry are adequately captured. Further, a local anisotropy in the hydrodynamic resistances along the co-axial direction of the channel is also noted.
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| 5. |
- Kannan, Ananda Subramani, 1989, et al.
(författare)
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The Knudsen Paradox in Micro-Channel Poiseuille Flows with a Symmetric Particle
- 2021
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Ingår i: Applied Sciences. - : MDPI AG. - 2076-3417. ; 11:1, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- The Knudsen paradox—the non-monotonous variation of mass-flow rate with the Knudsen number—is a unique and well-established signature of micro-channel rarefied flows. A particle which is not of insignificant size in relation to the duct geometry can significantly alter the flow behavior when introduced in such a system. In this work, we investigate the effects of a stationary particle on a micro-channel Poiseuille flow, from continuum to free-molecular conditions, using the direct simulation Monte-Carlo (DSMC) method. We establish a hydrodynamic basis for such an investigation by evaluating the flow around the particle and study the blockage effect on the Knudsen paradox. Our results show that with the presence of a particle this paradoxical behavior is altered. The effect is more significant as the particle becomes large and results from a shift towards relatively more ballistic molecular motion at shorter geometrical distances. The need to account for combinations of local and non-local transport effects in modeling reactive gas–solid flows in confined geometries at the nano-scale and in nanofabrication of model pore systems is discussed in relation to these results.
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| 6. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Asymmetric invasion in anisotropic porous media
- 2021
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 104:4
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Tidskriftsartikel (refereegranskat)abstract
- We report and discuss, by means of pore-scale numerical simulations, the possibility of achieving a directional-dependent two-phase flow behavior during the process of invasion of a viscous fluid into anisotropic porous media with controlled design. By customising the pore-scale morphology and heterogeneities with the adoption of anisotropic triangular pillars distributed with quenched disorder, we observe a substantially different invasion dynamics according to the direction of fluid injection relative to the medium orientation, that is depending if the triangular pillars have their apex oriented (flow aligned) or opposed (flow opposing) to the main flow direction. Three flow regimes can be observed: (i) for low values of the ratio between the macroscopic pressure drop and the characteristic pore-scale capillary threshold, i.e., for Δp0/pc≤1, the fluid invasion dynamics is strongly impeded and the viscous fluid is unable to reach the outlet of the medium, irrespective of the direction of injection; (ii) for intermediate values, 1<Δp0/pc≤2, the viscous fluid reaches the outlet only when the triangular pillars are flow-opposing oriented; (iii) for larger values, i.e., for Δp0/pc>2, the outlet is again reached irrespective of the direction of injection. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering. We thus argue that the invasion process occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness, which in the case of flow-opposing pillars is halved. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening, to predict the asymmetric invasion dynamics. This peculiar behavior opens up the possibility of fabrication of porous capillary valves to control the flow along certain specific directions.
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| 7. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Directional-dependent invasion dynamics in anisotropic porous media with customised disorder
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We show possibility of achieving a directional-dependent two-phase flow behaviour during the process of invasion of a viscous fluid into anisotropic porous media with customised pore-scale morphology and heterogeneity. Via pore-scale numerical simulations, we observe a substantially different invasion dynamics according to the medium orientation relative to the direction of fluid injection, i.e. with flow-aligned or flow-opposing oriented pillars. The porous medium anisotropy induces a lower effective resistance when the pillars are flow-opposing oriented, suppressing front roughening and capillary fingering, while promoting transverse invasion with respect to the direction of fluid injection. We argue that fluid infiltration occurs as long as the pressure drop is larger then the macroscopic capillary pressure determined by the front roughness. We present a simple approximated model, based on Darcy's assumptions, that links the macroscopic effective permeability with the directional-dependent front roughening. The model correctly predicts an intermediate flow regime, defined by a specific range of values of the ratio between the macroscopic pressure drop and the medium characteristic pore-scale capillary threshold, within which the injected viscous fluid reaches the outlet only whith flow-opposing oriented pillars. The prediction of the observed directional-dependent fluid conductance is important for e.g. the fabrication of porous materials that act as capillary valves to control the flow along certain specific directions. This work is supported by the Horizon 2020 research and innovation programme, Grant agreement No 790744, and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), Grant Numbers 2019-01261. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) at C3SE and HPC2N partially funded by the Swedish Research Council through Grant agreement no. 2018-05973.
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| 8. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Respiratory droplets interception in fibrous porous media
- 2021
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:8
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Tidskriftsartikel (refereegranskat)abstract
- We investigate, by means of pore-scale lattice Boltzmann simulations, the mechanisms of interception of respiratory droplets within fibrous porous media composing face masks. We simulate the dynamics, coalescence, and collection of droplets of the size comparable with the fiber and pore size in typical fluid-dynamic conditions that represent common expiratory events. We discern the fibrous microstructure into three categories of pores: small, large, and medium-sized pores, where we find that within the latter, the incoming droplets tend to be more likely intercepted. The size of the medium-sized pores relative to the fiber size is placed between the droplet-to-fiber size ratio and a porosity-dependent microstructural parameter L ϵ∗ = ϵ / (1 - ϵ), with ϵ being the porosity. In larger pores, droplets collection is instead inhibited by the small pore-throat-to-fiber size ratio that characterizes the pore perimeter, limiting their access. The efficiency of the fibrous media in intercepting droplets without compromising breathability, for a given droplet-to-fiber size ratio, can be estimated by knowing the parameter L ϵ*. We propose a simple model that predicts the average penetration of droplets into the fibrous media, showing a sublinear growth with L ϵ*. Permeability is shown also to scale well with L ϵ∗ but following a superlinear growth, which indicates the possibility of increasing the medium permeability at a little cost in terms of interception efficiency for high values of porosity. As a general design guideline, the results also suggest that a fibrous layer thickness relative to the fiber size should exceed the value L ϵ∗ in order to ensure effective droplets filtration.
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| 9. |
- Pettersson, Kaj, 1990, et al.
(författare)
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Contribution of dynamic capillary pressure to rainfall infiltration in thin homogeneous growth substrates
- 2021
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Ingår i: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 603
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Tidskriftsartikel (refereegranskat)abstract
- The use of green roofs to help mitigate storm water contributions to urban flooding has been gaining popularity but is hindered by the limited data on the performance of such roofs with regard to storm water runoff mitigation. The underlying issue stems from the inherent complexity of modeling subsurface multiphase flow. Modeling of this phenomena requires calculating the contributions of substrate microstructure characteristics, the influence of the wetting and non-wetting phases upon each other, and the effect of the microstructure on the wetting phase. Previously we have observed how the microstructure can affect detention, however the quantification of this relationship is still missing. In the present paper we present numerical simulations of wetting phase infiltration of a thin monodisperse packed bed in order to understand and quantify the impact of microstructure geometry on storm water infiltration of a green roof substrate. For a slightly hydrophilic case, (θ=82°), we find that a dominant mechanism underlying this relationship is the microstructure-induced dynamic behavior of the capillary pressure. We determine that at larger packing ratios (ratio of packed bed depth to particle size), the influence of hydraulic head diminishes and behaves conversely for thinner layers, particularly when larger pores are present. Indeed, thin beds composed of large particles can exhibit high flow velocities that in turn affect the capillary pressure within the substrate. We observe that the capillary pressure can shift from negative values denoting capillary suction to positive ones which cause valve-like blocking effects on the flow; dependent upon the flow velocity as determined by the microstructure. In particular, we find that the capillary pressure depends on the value of the pore-scale gravity-induced flow velocity, quantified through a characteristic Capillary number. The provided quantification of this relationship can be invaluable from a design perspective to understand the behavior of capillary pressure of different substrates under a variety of flow rates prior to testing substrate candidates. In addition, a comparison of the behavior of the dynamic component of capillary pressure to other works is undertaken. Flow homogeneity is also found to be linked to the flow velocity, and consequently to the microstructure.
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