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The effect of particle density in turbulent channel flow laden with finite size particles in semi-dilute conditions
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.ORCID iD: 0000-0002-4346-4732
2016 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 3, 033301Article in journal (Refereed) Published
Resource type
Text
Abstract [en]

We study the effect of varying the mass and volume fraction of a suspension of rigid spheres dispersed in a turbulent channel flow. We performed several direct numerical simulations using an immersed boundary method for finite-size particles changing the solid to fluid density ratio R, the mass fraction χ, and the volume fraction φ. We find that varying the density ratio R between 1 and 10 at constant volume fraction does not alter the flow statistics as much as when varying the volume fraction φ at constant R and at constant mass fraction. Interestingly, the increase in overall drag found when varying the volume fraction is considerably higher than that obtained for increasing density ratios at same volume fraction. The main effect at density ratios R of the order of 10 is a strong shear-induced migration towards the centerline of the channel. When the density ratio R is further increased up to 1000, the particle dynamics decouple from that of the fluid. The solid phase behaves as a dense gas and the fluid and solid phase statistics drastically change. In this regime, the collision rate is high and dominated by the normal relative velocity among particles.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016. Vol. 28, no 3, 033301
Keyword [en]
Low-Reynolds-Number, Numerical-Simulation, Molecular Dimensions, Suspensions, Rheology, Spheres, Fluid, Microstructure, Statistics, Stress
National Category
Other Physics Topics
Identifiers
URN: urn:nbn:se:kth:diva-187293DOI: 10.1063/1.4942518ISI: 000373600600023Scopus ID: 2-s2.0-84959563104OAI: oai:DiVA.org:kth-187293DiVA: diva2:929853
Funder
EU, European Research Council, ERC-2013-CoG-616186Swedish Research Council
Note

QC 20160520

Available from: 2016-05-20 Created: 2016-05-19 Last updated: 2017-11-16Bibliographically approved
In thesis
1. Suspensions of finite-size rigid particles in laminar and turbulent flows
Open this publication in new window or tab >>Suspensions of finite-size rigid particles in laminar and turbulent flows
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Dispersed multiphase flows occur in many biological, engineering and geophysical applications such as fluidized beds, soot particle dispersion and pyroclastic flows. Understanding the behavior of suspensions is a very difficult task. Indeed particles may differ in size, shape, density and stiffness, their concentration varies from one case to another, and the carrier fluid may be quiescent or turbulent. When turbulent flows are considered, the problem is further complicated by the interactions between particles and eddies of different size, ranging from the smallest dissipative scales up to the largest integral scales. Most of theinvestigations on this topic have dealt with heavy small particles (typically smaller than the dissipative scale) and in the dilute regime. Less is known regarding the behavior of suspensions of finite-size particles (particles that are larger than the smallest length scales of the fluid phase).

In the present work, we numerically study the behavior of suspensions of finite-size rigid particles in different flows. In particular, we perform direct numerical simulations using an immersed boundary method to account for the solid phase. Firstly, the sedimentation of spherical particles slightly smaller than the Taylor microscale in sustained homogeneous isotropic turbulence and quiescent fluid is investigated. The results show that the mean settling velocity is lower in an already turbulent flow than in a quiescent fluid. By estimating the mean drag acting on the particles, we find that non stationary effects explain the increased reduction in mean settling velocity in turbulent environments. Moreover, when the turbulence root-mean-square velocity is larger than the terminal speed of a particle, the overall drag is further enhanced due to the large particles cross-flow velocities.

We also investigate the settling in quiescent fluid of oblate particles. We find that at low volume fractions the mean settling speed of the suspension is substantially larger than the terminal speed of an isolated oblate. This is due to the formation of clusters that appear as columnar-like structures.

Suspensions of finite-size spheres are also studied in turbulent channel flow. We change the solid volume and mass fractions, and the solid-to-fluid density ratio in an idealized scenario where gravity is neglected. The aim is to independently understand the effects of these parameters on both fluid and solid phases statistics. It is found that the statistics are substantially altered by changes in volume fraction, while the main effect of increasing the density ratio is a shear-induced migration toward the centerline. However, at very high density ratios (∼ 1000) the solid phase decouples from the fluid, and the particles behave as a dense gas.

In this flow case, we also study the effects of polydispersity by considering Gaussian distributions of particle radii (with increasing standard deviation), at constant volume fraction. We find that fluid and particle statistics are almost unaltered with respect to the reference monodisperse suspension. These results confirm the importance of the solid volume fraction in determing the behavior of a suspension of spheres.

We then consider suspensions of solid spheres in turbulent duct flows. We see that particles accumulate mostly at the corners. However, at large volume fractions the particles concentrate mostly at the duct core. Secondary motions are enhanced by increasing the volume fraction, until excluded volume effects are so strong that the turbulence activity is reduced. The same is found for the mean friction Reynolds number.

The inertial migration of spheres in laminar square duct flows is also investigated. We consider dilute and semi-dilute suspensions at different bulk Reynolds numbers and duct-to-particle size ratios. The highest particle concentration is found in regions around the focusing points, except at very large volume fractions since particles distribute uniformly in the cross-section. Particles also induce secondary fluid motions that become more intense with the volume fraction, until a critical value of the latter quantity is reached.

Finally we study the rheology of confined dense suspensions of spheres in simple shear flow. We focus on the weakly inertial regime and show that the suspension effective viscosity varies non-monotonically with increasing confinement. The minima of the effective viscosity occur when the channel width is approximately an integer number of particle diameters. At these confinements, the particles self-organize into two-dimensional frozen layers that slide onto each other.

Place, publisher, year, edition, pages
Kungliga Tekniska högskolan, 2017
Series
TRITA-MEK, ISSN 0348-467X
Keyword
Suspensions, complex fluids, sedimentation, rheology, turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217812 (URN)978-91-7729-607-2 (ISBN)
Public defence
2017-12-15, D3, Lindstedtsvägen 5, Stockholm, 16:57 (English)
Opponent
Supervisors
Funder
EU, European Research Council, ERC-2013-CoG-616186, TRITOS
Note

QC 20171117

Available from: 2017-11-17 Created: 2017-11-16 Last updated: 2017-11-17Bibliographically approved

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