| 1. |
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| 2. |
- Battista, F., et al.
(författare)
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Fractal scaling of turbulent premixed flame fronts : Application to LES
- 2015
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Ingår i: International Journal of Heat and Fluid Flow. - : Elsevier BV. - 0142-727X .- 1879-2278. ; 51, s. 78-87
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Tidskriftsartikel (refereegranskat)abstract
- The fractal scaling properties of turbulent premixed flame fronts have been investigated and considered for modeling sub-grid scales in the Large-Eddy-Simulation framework. Since the width of such thin reaction fronts cannot be resolved into the coarse mesh of LES, the extent of wrinkled flame surface contained in a volume is taken into account. The amount of unresolved flame front is estimated via the "wrinkling factor" that depends on the definition of a suitable fractal dimension and the scale at which the fractal scaling is lost, the inner cut-off length e. In this context, the present study considers laboratory experiments and one-step reaction DNS of turbulent premixed jet flames in different regimes of turbulent premixed flames. Fractal dimension is found to be substantially constant and well below that typical of passive scalar fronts. The inner cut-off length shows a clear scaling with the dissipative scale of Kolmogorov for the regimes here considered. These features have been exploited performing Large Eddy Simulations. Good model performance has been found comparing the LES against a corresponding DNS at moderate Reynolds number and experimental data at higher Reynolds numbers.
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| 3. |
- Bäbler, Matthäus, et al.
(författare)
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Numerical simulations of aggregate breakup in bounded and unbounded turbulent flows
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 766
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Tidskriftsartikel (refereegranskat)abstract
- Breakup of small aggregates in fully developed turbulence is studied by means of direct numerical simulations in a series of typical bounded and unbounded flow configurations, such as a turbulent channel flow, a developing boundary layer and homogeneous isotropic turbulence. The simplest criterion for breakup is adopted, whereby aggregate breakup occurs when the local hydrodynamic stress sigma similar to epsilon(1/2), with epsilon being the energy dissipation at the position of the aggregate, overcomes a given threshold sigma(cr), which is characteristic for a given type of aggregate. Results show that the breakup rate decreases with increasing threshold. For small thresholds, it develops a scaling behaviour among the different flows. For high thresholds, the breakup rates show strong differences between the different flow configurations, highlighting the importance of non-universal mean-flow properties. To further assess the effects of flow inhomogeneity and turbulent fluctuations, the results are compared with those obtained in a smooth stochastic flow. Furthermore, we discuss the limitations and applicability of a set of independent proxies.
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| 4. |
- Costa, Pedro, et al.
(författare)
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Effects of the finite particle size in turbulent wall-bounded flows of dense suspensions
- 2018
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 843, s. 450-478
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Tidskriftsartikel (refereegranskat)abstract
- We use interface-resolved numerical simulations to study finite-size effects in turbulent channel flow of neutrally buoyant spheres. Two cases with particle sizes differing by a factor of two, at the same solid volume fraction of 20% and bulk Reynolds number are considered. These are complemented with two reference single-phase flows: the unladen case, and the flow of a Newtonian fluid with the effective suspension viscosity of the same mixture in the laminar regime. As recently highlighted in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501), a particle-wall layer is responsible for deviations of the mesoscale-averaged statistics from what is observed in the continuum limit where the suspension is modelled as a Newtonian fluid with (higher) effective viscosity. Here we investigate in detail the fluid and particle dynamics inside this layer and in the bulk. In the particle-wall layer, the near-wall inhomogeneity has an influence on the suspension microstructure over a distance proportional to the particle size. In this layer, particles have a significant (apparent) slip velocity that is reflected in the distribution of wall shear stresses. This is characterized by extreme events (both much higher and much lower than the mean). Based on these observations we provide a scaling for the particle-to-fluid apparent slip velocity as a function of the flow parameters. We also extend the scaling laws in Costa etal. (Phys. Rev. Lett., vol.117, 2016, 134501) to second-order Eulerian statistics in the homogeneous suspension region away from the wall. The results show that finite-size effects in the bulk of the channel become important for larger particles, while negligible for lower-order statistics and smaller particles. Finally, we study the particle dynamics along the wall-normal direction. Our results suggest that single-point dispersion is dominated by particle-turbulence (and not particle-particle) interactions, while differences in two-point dispersion and collisional dynamics are consistent with a picture of shear-driven interactions.
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| 5. |
- Costa, Pedro, et al.
(författare)
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Universal Scaling Laws for Dense Particle Suspensions in Turbulent Wall-Bounded Flows
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 117:13
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Tidskriftsartikel (refereegranskat)abstract
- The macroscopic behavior of dense suspensions of neutrally buoyant spheres in turbulent plane channel flow is examined. We show that particles larger than the smallest turbulence scales cause the suspension to deviate from the continuum limit in which its dynamics is well described by an effective suspension viscosity. This deviation is caused by the formation of a particle layer close to the wall with significant slip velocity. By assuming two distinct transport mechanisms in the near-wall layer and the turbulence in the bulk, we define an effective wall location such that the flow in the bulk can still be accurately described by an effective suspension viscosity. We thus propose scaling laws for the mean velocity profile of the suspension flow, together with a master equation able to predict the increase in drag as a function of the particle size and volume fraction.
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| 6. |
- Fornari, Walter, et al.
(författare)
-
Reduced particle settling speed in turbulence
- 2016
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 808, s. 153-167
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Tidskriftsartikel (refereegranskat)abstract
- We study the settling of finite-size rigid spheres in sustained homogeneous isotropic turbulence (1111) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We study semi-dilute suspensions at different Galileo numbers, Ga. The Galileo number is the ratio between buoyancy and viscous forces, and is here varied via the solid-to-fluid density ratio rho(p)/rho(f), The focus is on particles that are slightly heavier than the fluid. We find that in HIT, the mean settling speed is less than that in quiescent fluid; in particular, it reduces by 6 %-60 % with respect to the terminal velocity of an isolated sphere in quiescent fluid as the ratio between the latter and the turbulent velocity fluctuations it is decreased. Analysing the fluid particle relative motion, we find that the mean settling speed is progressively reduced while reducing rho(p)/rho(f) due to the increase of the vertical drag induced by the particle cross-flow velocity. Unsteady effects contribute to the mean overall drag by about 6%-10%. The probability density functions of particle velocities and accelerations reveal that these are closely related to the features of the turbulent flow. The particle mean-square displacement in the settling direction is found to be similar for all Ga if time is scaled by (2a)/u' (where 2a is the particle diameter and a is the turbulence velocity root mean square).
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| 7. |
- Fornari, Walter, et al.
(författare)
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Rheology of Confined Non-Brownian Suspensions
- 2016
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 116:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couette flow using direct numerical simulations. We find that if the width of the channel is a (small) integer multiple of the sphere diameter, the spheres self-organize into two-dimensional layers that slide on each other and the effective viscosity of the suspension is significantly reduced. Each two-dimensional layer is found to be structurally liquidlike but its dynamics is frozen in time.
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| 8. |
- Fornari, Walter, 1989-, et al.
(författare)
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Rheology of extremely confined non-Brownian suspensions
- 2016
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 116:1
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We study the rheology of confined suspensions of neutrally buoyant rigid monodisperse spheres in plane-Couetteflow using Direct Numerical Simulations.We find that if the width of the channel is a (small) integer multiple of the spherediameter, the spheres self-organize into two-dimensional layersthat slide on each other and the effective viscosity of the suspension issignificantly reduced. Each two-dimensional layer is found to be structurallyliquid-like but its dynamics is frozen in time.
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| 9. |
- Fornari, Walter, 1989-, et al.
(författare)
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Sedimentation of finite-size spheres in quiescent and turbulent environments
- 2016
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 788, s. 640-669
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Tidskriftsartikel (refereegranskat)abstract
- Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yetlittle is known about the behavior of finite-size particles inhomogeneous isotropic turbulence.To fill this gap, we perform Direct Numerical Simulations of sedimentation in quiescent and turbulent environments using anImmersed Boundary Method to accountfor the dispersed rigid spherical particles. The solid volume fractions considered are 0.5-1%,while the solid to fluid density ratio 1.02.The particle radius is chosen to be approximately 6 Komlogorov lengthscales.Sedimentation of a dispersed solid phase is widely encountered in applications and environmental flows, yet little is known about the behaviour of finite-size particles in homogeneous isotropic turbulence. To fill this gap, we perform direct numerical simulations of sedimentation in quiescent and turbulent environments using an immersed boundary method to account for the dispersed rigid spherical particles. The solid volume fractions considered are phi = 0.5-1%, while the solid to fluid density ratio rho(p)/rho(f) = 1.02. The particle radius is chosen to be approximately six Kolmogorov length scales. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. The reductions with respect to a single particle in quiescent fluid are approximately 12 % and 14% for the two volume fractions investigated. The probability density function of the particle velocity is almost Gaussian in a turbulent flow, whereas it displays large positive tails in quiescent fluid. These tails arc associated with the intermittent fast sedimentation of particle pairs in drafting kissing tumbling motions. The particle lateral dispersion is higher in a turbulent flow, whereas the vertical one is, surprisingly, of comparable magnitude as a consequence of the highly intermittent behaviour observed in the quiescent fluid. Using the concept of mean relative velocity we estimate the mean drag coefficient from empirical formulae and show that non-stationary effects, related to vortex shedding, explain the increased reduction in mean settling Velocity in a turbulent environment.
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| 10. |
- Fornari, Walter, 1989-, et al.
(författare)
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Settling of finite-size particles in turbulence at different volume fractions
- 2018
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Ingår i: Acta Mechanica. - : Springer Science and Business Media LLC. - 0001-5970 .- 1619-6937. ; 230:2, s. 413-430
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Tidskriftsartikel (refereegranskat)abstract
- We study the settling of finite-size rigid spheres in quiescent fluid and in sustained homogeneous isotropic turbulence (HIT) by direct numerical simulations using an immersed boundary method to account for the dispersed solid phase. We consider semi-dilute and dense suspensions of rigid spheres with solid volume fractions ϕ= 0.5 - 10 % , solid-to-fluid density ratio R= 1.02 , and Galileo number (i.e., the ratio between buoyancy and viscous forces) Ga= 145. In HIT, the nominal Reynolds number based on the Taylor microscale is Re λ ≃ 90 , and the ratio between the particle diameter and the nominal Kolmogorov scale is (2 a) / η≃ 12 (being a the particle radius). We find that in HIT the mean settling speed is less than that in quiescent fluid for all ϕ. For ϕ= 0.5 % , the mean settling speed in HIT is 8 % less than in quiescent fluid. However, by increasing the volume fraction the difference in the mean settling speed between quiescent fluid and HIT cases reduces, being only 1.7 % for ϕ= 10 %. Indeed, while at low ϕ the settling speed is strongly altered by the interaction with turbulence, at large ϕ this is mainly determined by the (strong) hindering effect. This is similar in quiescent fluid and in HIT, leading to similar mean settling speeds. On the contrary, particle angular velocities are always found to increase with ϕ. These are enhanced by the interaction with turbulence, especially at low ϕ. In HIT, the correlations of particle lateral velocity fluctuations oscillate around zero before decorrelating completely. The time period of the oscillation seems proportional to the ratio between the integral lengthscale of turbulence and the particle characteristic terminal velocity. Regarding the mean square particle displacement, we find that it is strongly enhanced by turbulence in the direction perpendicular to gravity, even at the largest ϕ. Finally, we investigate the collision statistics for all cases and find the interesting result that the collision frequency is larger in quiescent fluid than in HIT for ϕ= 0.5 - 1 %. This is due to frequent drafting–kissing–tumbling events in quiescent fluid. The collision frequency becomes instead larger in HIT than in still fluid for ϕ= 5 - 10 % , due to the larger relative approaching velocities in HIT, and to the less intense drafting–kissing–tumbling events in quiescent fluid. The collision frequency also appears to be almost proportional to the estimate for small inertial particles uniformly distributed in space, though much smaller. Concerning the turbulence modulation, we find that the mean energy dissipation increases almost linearly with ϕ, leading to a large reduction of Re λ .
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| 11. |
- Fornari, Walter, 1989-, et al.
(författare)
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The effect of polydispersity in a turbulent channel flow laden with finite-size particles
- 2018
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier. - 0997-7546 .- 1873-7390. ; 67, s. 54-64
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Tidskriftsartikel (refereegranskat)abstract
- We study turbulent channel flows of monodisperse and polydisperse suspensions of finite-size spheres by means of Direct Numerical Simulations using an immersed boundary method to account for the dispersed phase. Suspensions with 3 different Gaussian distributions of particle radii are considered (i.e. 3 different standard deviations). The distributions are centered on the reference particle radius of the monodisperse suspension. In the most extreme case, the radius of the largest particles is 4 times that of the smaller particles. We consider two different solid volume fractions, 2% and 10%. We find that for all polydisperse cases, both fluid and particles statistics are not substantially altered with respect to those of the monodisperse case. Mean streamwise fluid and particle velocity profiles are almost perfectly overlapping. Slightly larger differences are found for particle velocity fluctuations. These increase close to the wall and decrease towards the centerline as the standard deviation of the distribution is increased. Hence, the behavior of the suspension is mostly governed by excluded volume effects regardless of particle size distribution (at least for the radii here studied). Due to turbulent mixing, particles are uniformly distributed across the channel. However, smaller particles can penetrate more into the viscous and buffer layer and velocity fluctuations are therein altered. Non trivial results are presented for particle-pair statistics.
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| 12. |
- Lashgari, Iman, et al.
(författare)
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Channel flow of rigid sphere suspensions : Particle dynamics in the inertial regime
- 2016
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Ingår i: International Journal of Multiphase Flow. - : Elsevier. - 0301-9322 .- 1879-3533. ; 78, s. 12-24
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Tidskriftsartikel (refereegranskat)abstract
- We consider suspensions of neutrally-buoyant finite-size rigid spherical particles in channel flow and investigate the relation between the particle dynamics and the mean bulk behavior of the mixture for Reynolds numbers 500 ≤ Re ≤ 5000 and particle volume fraction 0 ≤ Φ ≤ 0.3, via fully resolved numerical simulations. Analysis of the momentum balance reveals the existence of three different regimes: laminar, turbulent and inertial shear-thickening depending on which of the stress terms, viscous, Reynolds or particle stress, is the major responsible for the momentum transfer across the channel. We show that both Reynolds and particle stress dominated flows fall into the Bagnoldian inertial regime and that the Bagnold number can predict the bulk behavior although this is due to two distinct physical mechanisms. A turbulent flow is characterized by larger particle dispersion and a more uniform particle distribution, whereas the particulate-dominated flows is associated with a significant particle migration towards the channel center where the flow is smooth laminar-like and dispersion low. Interestingly, the collision kernel shows similar values in the different regimes, although the relative particle velocity and clustering clearly vary with inertia and particle concentration.
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| 13. |
- Lashgari, Iman, et al.
(författare)
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Transition and self-sustained turbulence in dilute suspensions of finite-size particles
- 2015
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Ingår i: Theoretical and Applied Mechanics Letters. - : Elsevier BV. - 2095-0349. ; 5, s. 121-125
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Tidskriftsartikel (refereegranskat)abstract
- We study the transition to turbulence of channel flow of finite-size particle suspensions at low volume fraction, i.e., Φ ≈ 0.001. The critical Reynolds number above which turbulence is sustained reduces to Re ≈ 1675, in the presence of few particles, independently of the initial condition, a value lower than that of the corresponding single-phase flow, i.e., Re ≈ 1775. In the dilute suspension, the initial arrangement of the particles is important to trigger the transition at a fixed Reynolds number and particle volume fraction. As in single phase flows, streamwise elongated disturbances are initially induced in the flow. If particles can induce oblique disturbances with high enough energy within a certain time, the streaks breakdown, flow experiences the transition to turbulence and the particle trajectories become chaotic. Otherwise, the streaks decay in time and the particles immigrate towards the channel core in a laminar flow.
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| 14. |
- Lashgari, Iman, et al.
(författare)
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Transition to Turbulence in the Presence of Finite Size Particles
- 2015
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Ingår i: Procedia IUTAM. - : Elsevier. - 2210-9838. ; , s. 211-217
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Konferensbidrag (refereegranskat)abstract
- We study the transition from laminar to turbulent flow in a channel seeded with finite-size neutrally buoyant particles. A fixed ratio of 10 between the channel height and the particle diameter is considered. The flow is examined in the range of Reynolds numbers 500 ≤ Re ≤; 5000 and the particle volume fractions 0.001 ≤ Φ ≤; 0.3. We report a non-monotonic behavior of the threshold value of the Reynolds number above which the flow becomes turbulent, in agreement with previous experimental studies. The mean square velocity fluctuations and Reynolds shear stress of the fluid phase are reduced by increasing the particle volume fraction at a fixed Re=1500, while the mean square velocities of the solid phase are enhanced monotonically suggesting a transition from fluid to particle dominated dynamics at high volume fraction.
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| 15. |
- Lashgari, Iman, et al.
(författare)
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Turbulent channel flow of a dense binary mixture of rigid particles
- 2017
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Ingår i: Journal of Fluid Mechanics. - : CAMBRIDGE UNIV PRESS. - 0022-1120 .- 1469-7645. ; 818, s. 623-645
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Tidskriftsartikel (refereegranskat)abstract
- We study turbulent channel flow of a binary mixture of finite-sized neutrally buoyant rigid particles by means of interface-resolved direct numerical simulations. We fix the bulk Reynolds number and total solid volume fraction, Re-b = 5600 and Phi = 20 %, and vary the relative fraction of small and large particles. The binary mixture consists of particles of two different sizes, 2h/d(l) = 20 and 2h/d(s) = 30 where h is the half-channel height and d(l) and d(s) the diameters of the large and small particles. While the particulate flow statistics exhibit a significant alteration of the mean velocity profile and turbulent fluctuations with respect to the unladen flow, the differences between the mono-disperse and bi-disperse cases are small. However, we observe a clear segregation of small particles at the wall in binary mixtures, which affects the dynamics of the near-wall region and thus the overall drag. This results in a higher drag in suspensions with a larger number of large particles. As regards bi-disperse effects on the particle dynamics, a non-monotonic variation of the particle dispersion in the spanwise (homogeneous) direction is observed when increasing the percentage of small/large particles. Finally, we note that particles of the same size tend to cluster more at contact whereas the dynamics of the large particles gives the highest collision kernels due to a higher approaching speed.
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| 16. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Asymmetric Two-phase Flows Resistance in Homogeneous and Heterogeneous Anisotropic Porous Microstructure
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Two-phase flows in porous media exhibit anomalous behaviours at low capillary numbers due to the complex mechanism of interaction between flow spatial configuration and topology of the microstructure. In this study, we investigate the asymmetrical nature of the two-phase flow resistance induced by the anisotropic features of the porous microstructure. We perform pore-scale direct numerical simulations of two-phase flows in porous media composed of solid particles with different shapes and orientations, using the Lattice-Boltzmann method. The results indicate that the infiltration of a fluid into a single pore is regulated by the topological traits of the pore, including its anisotropy. These traits determine a geometrical characteristic length of the pore ℓp quantifying the flow resistance, which is directional-dependent: if the capillary length ℓγ=γ/pc (i.e. the ratio between surface tension and capillary pressure) falls below the characteristic pore length ℓγ<ℓp, pore infiltration occurs, otherwise the fluid remains trapped. We extend the analysis to heterogeneous anisotropic microstructure in order to investigate the effect of the spatial configuration of the pores on the global flow resistance. *This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 790744.
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| 17. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Characterization of fluid-mechanic efficiency of porous electrodes using X-ray computed tomography and Lattice-Boltzmann simulations
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Flow batteries are a promising solution for storing the energy produced by carbon-free intermittent renewable sources. They are constituted by liquid electrolytes flowing and reacting into carbon felt porous electrodes. In order to enhance their performances it is necessary to maximise electrolyte dispersion and reaction together and to minimize the flow resistance through the porous media [1]. It is well reported in the literature how peculiar and anomalous can be the dispersion behaviour in natural porous media [2] and in fibrous materials [3]; however, it is still badly known how much it affects the performances of real electrodes of flow batteries. To clarify this issue, real carbon felt electrodes have been reconstructed by means of X-ray computed tomography (CT). In particular, a metrological micro-CT system has been used to obtain accurate three-dimensional reconstructions, which have then been used as input for Lattice-Boltzmann flow simulations coupled with a Lagrangian Particle Tracking algorithm, in order to investigate the dispersion and reaction behaviour of tracers through the medium microstructures, see e.g. Fig.1. For each material, macroscopic dispersion, reaction efficiencies and flow resistance have been evaluated from the underlying microscopic statistics of flow and tracer trajectories. From these analyses, the overall fluid-mechanic efficiency of each material has been evaluated and compared allowing the identification of the optimal porous microstructure for flow battery applications, which shows the highest dispersion and reaction rate together with the lowest pressure drop. [1] Wang B., Kuo J., Bae S.C. and Granick S., 2012. Nature Materials, 11(6): 481-5. [2] Alotto P., Guarnieri M. and Moro F., 2014. Renewable and Sustainable Energy Reviews, 29, pp.325-335. [3] Maggiolo D., Picano F. and Guarnieri M., 2016. Physics of Fluids, 28(10), p.102001.
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| 18. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Flow and dispersion in anisotropic porous media: A lattice-Boltzmann study
- 2016
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1089-7666 .- 1070-6631. ; 28:10
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Tidskriftsartikel (refereegranskat)abstract
- Given their capability of spreading active chemical species and collecting electricity, porous media made of carbon fibers are extensively used as diffusion layers in energy storage systems, such as redox flow batteries. In spite of this, the dispersion dynamics of species inside porous media is still not well understood and often lends itself to different interpretations. Actually, the microscopic design of efficient porous media, which can potentially and effectively improve the performances of flow batteries, is still an open challenge. The present study aims to investigate the effect of fibrous media micro-structure on dispersion, in particular the effect of fiber orientation on drag and dispersion dynamics. Several lattice-Boltzmann simulations of flows through differently oriented fibrous media coupled with Lagrangian simulations of particle tracers have been performed. Results show that orienting fibers preferentially along the streamwise direction minimizes the drag and maximizes the dispersion, which is the most desirable condition for diffusion layers in flow batteries’ applications.
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| 19. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Particle based method and X-ray computed tomography for pore-scale flow characterization in VRFB electrodes
- 2019
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Ingår i: Energy Storage Materials. - : Elsevier BV. - 2405-8297. ; 16, s. 91-96
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Tidskriftsartikel (refereegranskat)abstract
- Porous electrodes are pivotal components of Vanadium Redox Flow Batteries, which influence the power density, pressure drop losses, activation overpotentials, limit current density, bulk and contact resistance, and ohmic losses. The quantification of the fluid-mechanic efficiency of porous electrodes based on their real geometry is a useful measure, as it primarily affects the mass transport losses and the overall battery performances. Although several studies, both numerical and experimental, have been devoted to the electrode enhancement, most analyses are carried out under the simplifying assumption of linear, macrohomogeneous and isotropic behavior of the fluid mechanics in the porous material. We present an original approach built on the Lattice-Boltzmann Method and Lagrange Particle Tracking that makes use of pore-scale accurate geometrical data provided by X-ray computed tomography with the aim of studying the dispersion and reaction rates of liquid electrolyte reactants in the flow battery porous electrode. Following this methodology, we compare the fluid-dynamic performances provided by a commonly used carbon felt and an unconventional material, that is, a carbon vitrified foam. Surprisingly, results unveil the possibility of achieving higher fluid-mechanic efficiencies with the foam electrode, whose intrinsic microstructure promotes higher reaction rate.
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| 20. |
- Maggiolo, Dario, 1985, et al.
(författare)
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Pore trapping mechanisms in two-phase flows through fuel cells porous media
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The peak performances in fuel cells are strongly limited by flooding of the electrodes. Flooding occurs as a consequence of the intense electrochemical reaction in the cell, when, at high current density, a high amount of water is produced and trapped in the gas diffusion layers (GDLs). The trapped water blocks the pores of the fibrous GDLs and impedes the uniform diffusion of fuel gas along the cell area. Therefore, a fast removal of water from the porous layer is demanded. How to enhance such a mechanism in order to improve the water management remains still an open question. In the present study we address this problem with an innovative bottom-up approach: by means of Lattice-Boltzmann simulations, we investigate the effects of the microstructure at the pore-scale on the two-phase flows dynamics, in order to optimise the GDLs design and obtain an efficient water management at the macroscale. Results show that, during imbibition, the wetting phase can be trapped at the pore throat, impeding liquid removal. The trapping mechanism is primarily governed by the thermodynamic energy barriers induced by the pore expansions, in a complementary way to Haines jump during drainage. This undesired phenomenon is exacerbated in hydrophobic media during imbibition. These findings suggest a possible new route for innovative design of gas diffusion layers in fuel cells applications.
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| 21. |
- Niazi Ardekani, Mehdi, et al.
(författare)
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Drag reduction in turbulent channel flow laden with finite-size oblate spheroids
- 2017
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 816, s. 43-70
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Tidskriftsartikel (refereegranskat)abstract
- We study suspensions of oblate rigid particles in a viscous fluid for different values of the particle volume fractions.Direct numerical simulations have been performed using a direct-forcing immersed boundary method to account for the dispersed phase, combined with a soft-sphere collision model and lubrication corrections for short-range particle-particle and particle-wall interactions. With respect to the single phase flow, we show that in flows laden with oblate spheroids the drag is reduced and the turbulent fluctuations attenuated.In particular, the turbulence activity decreases to lower values than those obtained by only accounting for the effective suspension viscosity.To explain the observed drag reduction we consider the particle dynamics and the interactions of the particles with the turbulent velocity field and show that the particle wall layer, previously observed and found to be responsible for the increased dissipation in suspensions of spheres, disappears in the case of oblate particles.These rotate significantly slower than spheres near the wall and tend to stay with their major axes parallel to the wall, which leads to a decrease of the Reynolds stresses and turbulence production and so to the overall drag reduction.
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| 22. |
- Niazi Ardekani, Mehdi, et al.
(författare)
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Heat transfer in laminar Couette flow laden with rigid spherical particles
- 2018
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 834, s. 308-334
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Tidskriftsartikel (refereegranskat)abstract
- We study heat transfer in plane Couette flow laden with rigid spherical particles by means of direct numerical simulations. In the simulations we use a direct-forcing immersed boundary method to account for the dispersed phase together with a volume-of-fluid approach to solve the temperature field inside and outside the particles. We focus on the variation of the heat transfer with the particle Reynolds number, total volume fraction (number of particles) and the ratio between the particle and fluid thermal diffusivity, quantified in terms of an effective suspension diffusivity. We show that, when inertia at the particle scale is negligible, the heat transfer increases with respect to the unladen case following an empirical correlation recently proposed in the literature. In addition, an average composite diffusivity can be used to approximate the effective diffusivity of the suspension in the inertialess regime when varying the molecular diffusion in the two phases. At finite particle inertia, however, the heat transfer increase is significantly larger, smoothly saturating at higher volume fractions. By phase-ensemble-averaging we identify the different mechanisms contributing to the total heat transfer and show that the increase of the effective conductivity observed at finite inertia is due to the increase of the transport associated with fluid and particle velocity. We also show that the contribution of the heat conduction in the solid phase to the total wall-normal heat flux reduces when increasing the particle Reynolds number, so that particles of low thermal diffusivity weakly alter the total heat flux in the suspension at finite particle Reynolds numbers. On the other hand, a higher particle thermal diffusivity significantly increases the total heat transfer.
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| 23. |
- Picano, Francesco, et al.
(författare)
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Turbulent channel flow of dense suspensions of neutrally buoyant spheres
- 2015
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 764, s. 463-487
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Tidskriftsartikel (refereegranskat)abstract
- Dense particle suspensions are widely encountered in many applications and in environmental flows. While many previous studies investigate their rheological properties in laminar flows, little is known on the behaviour of these suspensions in the turbulent/inertial regime. The present study aims to fill this gap by investigating the turbulent flow of a Newtonian fluid laden with solid neutrally-buoyant spheres at relatively high volume fractions in a plane channel. Direct numerical simulation (DNS) are performed in the range of volume fractions Phi=0-0.2 with an immersed boundary method (IBM) used to account for the dispersed phase. The results show that the mean velocity profiles are significantly altered by the presence of a solid phase with a decrease of the von Karman constant in the log-law. The overall drag is found to increase with the volume fraction, more than one would expect if just considering the increase of the system viscosity due to the presence of the particles. At the highest volume fraction investigated here, Phi = 0.2, the velocity fluctuation intensities and the Reynolds shear stress are found to decrease. The analysis of the mean momentum balance shows that the particle-induced stresses govern the dynamics at high Phi and are the main responsible of the overall drag increase. In the dense limit, we therefore find a decrease of the turbulence activity and a growth of the particle induced stress, where the latter dominates for the Reynolds numbers considered here.
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| 24. |
- Rudski, Lawrence G., et al.
(författare)
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Stressing the Cardiopulmonary Vascular System: The Role of Echocardiography
- 2018
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Ingår i: Journal of the American Society of Echocardiography. - : MOSBY-ELSEVIER. - 0894-7317 .- 1097-6795. ; 31:5, s. 527-
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Forskningsöversikt (refereegranskat)abstract
- The cardiopulmonary vascular system represents a key determinant of prognosis in several cardiorespiratory diseases. Although right heart catheterization is considered the gold standard for assessing pulmonary hemodynamics, a comprehensive noninvasive evaluation including left and right ventricular reserve and function and cardiopulmonary interactions remains highly attractive. Stress echocardiography is crucial in the evaluation of many cardiac conditions, typically coronary artery disease but also heart failure and valvular heart disease. In stress echocardiographic applications beyond coronary artery disease, the assessment of the cardiopulmonary vascular system is a cornerstone. The possibility of coupling the left and right ventricles with the pulmonary circuit during stress can provide significant insight into cardiopulmonary physiology in healthy and diseased subjects, can support the diagnosis of the etiology of pulmonary hypertension and other conditions, and can offer valuable prognostic information. In this state-of-the-art document, the topic of stress echocardiography applied to the cardiopulmonary vascular system is thoroughly addressed, from pathophysiology to different stress modalities and echocardiographic parameters, from clinical applications to limitations and future directions.
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| 25. |
- Sardina, Gaetano, et al.
(författare)
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Continuous Growth of Droplet Size Variance due to Condensation in Turbulent Clouds
- 2015
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 115:18
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Tidskriftsartikel (refereegranskat)abstract
- We use a stochastic model and direct numerical simulation to study the impact of turbulence on cloud droplet growth by condensation. We show that the variance of the droplet size distribution increases in time as t(1/2), with growth rate proportional to the large-to-small turbulent scale separation and to the turbulence integral scales but independent of the mean turbulent dissipation. Direct numerical simulations confirm this result and produce realistically broad droplet size spectra over time intervals of 20 min, comparable with the time of rain formation.
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