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Experimental studies of large particles in Newtonian and non-Newtonian fluids
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, Centres, SeRC - Swedish e-Science Research Centre. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

In everyday human life, laminar flow is arguably an exception whereas turbulent flow is the norm. Yet, the former has been much better understood, naturally since laminar flow renders itself to treatment in a relatively easier fashion compared to turbulence with its chaotic dynamics across multiple scales in space and time. A parallel analogy in terms of sophistication of dynamics can be drawn between single phase and multiphase flows; the latter being the norm yet poorly understood due to numerous complexities arising on account of the huge parameter space involved. It is also remarkable that numerical studies are more prevalent in this field and there is a dearth of experimental results, which are important for both validation purposes and as a beacon to navigate research in practically relevant directions. This work has emerged to address the above issues. The attention has been largely directed towards understanding the flow of spherical particles in a square duct at moderately high concentrations using Particle Image Velocimetry (PIV) with refractive-index-matched (RIM) hydrogel particles. Fluids with Newtonian, viscoelastic and elastoviscoplastic rheology have been investigated due to their presence in natural and industrially relevant flows. Experiments and Direct Numerical Simulations (DNS) with spherical particles in a round pipe with turbulent flow of a Newtonian fluid are also conducted to extend and generalise the observations made in the square duct.

With the ability to optically interrogate the bulk of the flow at high particle concentrations (20\% in this work), many interesting measurements are made possible, focussing on the turbulent regime. For the Newtonian fluid, the pressure drop or, equivalently, the energy required to pump the fluid-particle mixture is a complex function of particle size and concentration in the duct. This phenomenon arises due to the particle concentration distribution, with a local maxima at the core and the walls, and its resulting effect on the dominant stresses in the system i.e.\ the Reynolds shear stress and particle-induced stress. Particles also migrate in a similar fashion in a turbulent flow of viscoelastic suspending fluid but, with a larger tendency to accumulate in the core compared to its Newtonian counterpart at the same Reynolds number leading to a faster rise in total stress with concentration. Finally, for the \textit{thick} elastoviscoplastic fluid, the single-phase flow is laminar but it exhibits turbulence-like fluctuations when particles are added, which are distributed in exotic configurations depending on the interplay between the viscoelastic forces and the ensuing secondary flows as well as inertial forces. On the other hand, a quantitative comparison between simulations and experiments for particles transported along the floor of the duct under turbulent conditions has helped in reinforcing confidence in both approaches.

We believe that these results will establish more confidence in the experimental usage of hydrogel particles for studying the flow of moderately dense suspensions. A natural extension would be the investigation of flow geometries more complex than a pipe or a square duct. Our results at higher Reynolds numbers is expected to motivate numerical simulations which are capable of investigating the detailed causes behind these observations, which are still unclear as of now. The information provided about the overall drag and the associated particle concentration and stress distribution will be helpful in painting a unified picture of turbulent suspension dynamics for a comprehensive range of flow rates and particle sizes. Future studies, either experimental or numerical, bearing similarities or deviations from our observations would also be constructive, for e.g.\ in assessing the sensitivity of the system to parameters that may be overlooked in the present study.

##### Abstract [sv]

Man kan med visst fog hävda att turbulens är normen och laminär strömning mer ovanlig i vårt dagliga liv. Förståelsen av laminär strömning är å andra sidan bättre, vilket är naturligt eftersom det ofta är enklare att studera laminärt flöde än turbulens med sin kaotiska dynamik som sträcker sig över ett stort intervall av tids- och längdskalor. En parallell analogi vad gäller komplexitet och sofistikerad dynamik kan göras mellan enfas- och flerfasströmning. Det senare fallet är vanligast, men det är inte lika väl förstått och beskrivet på grund av att komplexiteten ökar snabbt när nya parametrar såsom partikelstorlek och koncentration införs. Det bör också nämnas att numeriska simuleringar dominerar i flerfasforskningen, varför det råder brist på detaljerade experimentella resultat, trots att dessa är viktiga både för validering av simuleringar och för att identifiera relevanta forskningsproblem.

Det här föreliggande avhandlingsarbetet har ambitionen att bidra med experimentella resultat på flerfasströmning. Fokus har huvudsakligen varit att förstå flöden av suspensioner (blandningar) med sfäriska partiklar i kanaler med kvadratiska tvärsnitt och måttligt hög koncentration av partiklar. Partiklarna som använts är brytningsindexmatchade (refraction-index-matching, RIM) och har studerats med PIV (particle image velocimetry). Fluider med newtonsk, viskoelastisk och elastoviskoplastisk reologi har använts. Sådana fluider förekommer i flöden både i naturen och i industriella tillämpningar. Experiment och direkta numeriska simuleringar (DNS) av turbulent strömning med sfäriska partiklar i ett rör med cirkulärt tvärsnitt har också genomförts för att utöka och generalisera observationerna i den kvadratiska kanalen.

Tack vare möjligheten att optiskt studera stora delar av strömningen vid relativt höga partikelkoncentrationer (maximalt 20 procent) har den turbulenta strömningen karakterisats i detalj. Med newtonsk fluid är tryckfallet, eller energin som krävs för att pumpa suspensionen, en komplicerad funktion av partikelstorlek och koncentration. Detta beror på att partikelfördelningen kan ha maxima antingen i mitten av röret/kanalen eller längs väggarna, vilket i sin tur påverkar skjuvspänningarna i systemet (turbulenta Reynolds-spänningar och partikelinducerade spänningar). Suspensioner med partiklar i viskoelastiska fluider uppvisar liknande tendens till migrering, men i detta fall samlas partiklarna i större utsträckning i kanalens mitt jämfört med det newtonska fallet (vid samma värde på det så kallade Reynoldstalet, en parameter som beskriver strömningen). Detta leder till en snabbare ökning av den totala spänningen, och därmed även tryckfallet, med  ökande koncentration. I det tredje fallet med den elastoviskoplastiska fluiden så strömmar den laminärt utan partiklar. När partiklar tillsätts uppstår turbulensliknande fluktuationer. Partiklarna fördelas i olika exotiska konfigurationer beroende på samspelet mellan de viskoelastiska krafterna och de sekundära strömningar som uppstår i kvadratiska kanaler. Slutligen har en kvantitativ jämförelse mellan simuleringar och experiment för partiklar som transporteras längs rörväggarna bidragit till att stärka tilliten till de båda angreppssätten.

Resultaten som presenteras i denna avhandling styrker tilltron till experimentellt utnyttjande av hydrogelpartiklar för att studera partikelsuspensioner vid måttligt höga koncentrationer. En naturlig fortsättning skulle kunna vara att studera mer komplexa geometrier än raka rör och kanaler med cirkulärt respektive kvadratiskt tvärsnittt. De experimentella resultaten vid högre Reynoldstal motiverar också numeriska simuleringar som kan undersöka den detaljerade fysiken bakom observationerna. Mätningarna av det totala strömningsmotståndet med tillhörande partikel- och spänningsfördelning bidrar till en komplett bild av turbulent suspensionsdynamik för ett stort intervall av strömningshastigheter och partikelstorlekar. Framtida studier, experimentella såväl som numeriska, kan inriktas mot att studera hur systemets känslighet är för viktiga parametrar som inte inkluderats i den här studien.

##### Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019.
##### Series
TRITA-SCI-FOU ; 2019;39
##### Keywords [en]
turbulence, finite-size, particle-laden flows, duct flow, pipe flow, viscoelastic fluid, viscoplastic fluid
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
ISBN: 978-91-7873-278-4 (print)OAI: oai:DiVA.org:kth-257681DiVA, id: diva2:1347711
##### Public defence
2019-09-26, Kollegiesalen, Brinellvägen 8, Stockholm, 10:15 (English)
##### Note

QC 20190903

Available from: 2019-09-03 Created: 2019-09-02 Last updated: 2019-09-10Bibliographically approved
##### List of papers
1. Experimental investigation of turbulent suspensions of spherical particles in a squareduct
Open this publication in new window or tab >>Experimental investigation of turbulent suspensions of spherical particles in a squareduct
2018 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 857, p. 748-783Article in journal (Refereed) Published
##### Abstract [en]

We report experimental observations of turbulent flow with spherical particles in a square duct. Three particle sizes, namely 2H/d(p) = 40, 16 and 9 (2H being the duct full height and d(p) being the particle diameter), are investigated. The particles are nearly neutrally buoyant with a density ratio of 1.0035 and 1.01 with respect to the suspending fluid. Refractive index matched-particle image velocimetry (RIM-PIV) is used for fluid velocity measurement even at the highest particle volume fraction (20 %) and particle tracking velocimetry (PTV) for the particle velocity statistics for the flows seeded with particles of the two largest sizes, whereas only pressure measurements are reported for the smallest particles. Settling effects are seen at the lowest bulk Reynolds number R-e2H approximate to 10 000, whereas, at the highest R-e2H approximate to 27 000, particles are in almost full suspension. The friction factor of the suspensions is found to be significantly larger than that of single-phase duct flow at the lower R-e2H investigated; however, the difference decreases when increasing the flow rate and the total drag approaches the values of the single-phase flow at the higher Reynolds number considered, R-e2H = 27 000. The pressure drop is found to decrease with the particle diameter for volume fractions lower than (sic) = 10% for nearly all R-e2H investigated. However, at the highest volume fraction (sic) = 20 %, we report a peculiar non-monotonic behaviour: the pressure drop first decreases and then increases with increasing particle size. The decrease of the turbulent drag with particle size at the lowest volume fractions is related to an attenuation of the turbulence. The drag increase for the two largest particle sizes at (sic) = 20 %, however, occurs despite this large reduction of the turbulent stresses, and it is therefore due to significant particle-induced stresses. At the lowest Reynolds number, the particles reside mostly in the bottom half of the duct, where the mean velocity significantly decreases; the flow is similar to that in a moving porous bed near the bottom wall and to turbulent duct flow with low particle concentration near the top wall.

##### Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2018
##### Keywords
multiphase flow, particle/fluid flow, suspensions
##### National Category
Mechanical Engineering
##### Identifiers
urn:nbn:se:kth:diva-239092 (URN)10.1017/jfm.2018.783 (DOI)000448523200001 ()
##### Funder
EU, European Research Council, ERC-2013-CoG-616186Swedish Research Council
##### Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2019-09-03Bibliographically approved
2. Buoyant finite-size particles in turbulent duct flow
Open this publication in new window or tab >>Buoyant finite-size particles in turbulent duct flow
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, no 4, article id 024303Article in journal (Refereed) Published
##### Abstract [en]

Particle image velocimetry and particle tracking velocimetry have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved direct numerical simulations (DNSs) have also been performed with the immersed boundary method at the same experimental conditions, bulk Reynolds number Re2H=5600, duct height to particle-size ratio 2H/dp=14.5, particle volume fraction Φ=1%, and particle to fluid density ratio ρp/ρf=1.0035. Good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution, and turbulent statistics of the two phases. Additional experimental results considering two particle sizes 2H/dp=14.5 and 9 and multiple Φ=1%, 2%, 3%, 4%, and 5% are reported at the same Re2H. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture in terms of fluid velocity statistics differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable detail, even in regions where the local volume fraction is relatively high.

##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-243895 (URN)10.1103/PhysRevFluids.4.024303 (DOI)000458160100003 ()2-s2.0-85062418601 (Scopus ID)
##### Note

QC 20190215

Available from: 2019-02-09 Created: 2019-02-09 Last updated: 2019-09-03Bibliographically approved
3. Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids
Open this publication in new window or tab >>Turbulence modulation by finite-size spherical particles in Newtonian and viscoelastic fluids
2019 (English)In: International Journal of Multiphase Flow, ISSN 0301-9322, E-ISSN 1879-3533, Vol. 112, p. 116-129Article in journal (Refereed) Published
##### Abstract [en]

We experimentally investigate the influence of finite-size spherical particles in turbulent flows of a Newtonian and a drag reducing viscoelastic fluid at varying particle volume fractions and fixed Reynolds number. Experiments are performed in a square duct at a Reynolds number Re2H of nearly 1.1 × 104, Weissenberg number Wi for single phase flow is between 1 and 2 and results in a drag-reduction of 43% compared to a Newtonian flow (at the same Re2H). Particles are almost neutrally-buoyant hydrogel spheres having a density ratio of 1.0035 ± 0.0003 and a duct height 2H to particle diameter dp ratio of around 10. We measure flow statistics for four different volume fractions ϕ namely 5, 10, 15 and 20% by using refractive-index-matched Particle Image Velocimetry (PIV). For both Newtonian Fluid (NF) and Visceolastic Fluid (VEF), the drag monotonically increases with ϕ. For NF, the magnitude of drag increase due to particle addition can be reasonably estimated using a concentration dependent effective viscosity for volume fractions below 10%. The drag increase is, however, underestimated at higher ϕ. For VEF, the absolute value of drag is lower than NF but, its rate of increase with ϕ is higher. Similar to particles in a NF, particles in VEF tend to migrate towards the center of the duct and form a layer of high concentration at the wall. Interestingly, relatively higher migration towards the center and lower migration towards the walls is observed for VEF. The primary Reynolds shear stress reduces with increasing ϕ throughout the duct height for both types of fluid.

Elsevier, 2019
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-243840 (URN)10.1016/j.ijmultiphaseflow.2018.12.015 (DOI)2-s2.0-85058816573 (Scopus ID)
##### Note

QC 20190215

Available from: 2019-02-07 Created: 2019-02-07 Last updated: 2019-09-03Bibliographically approved
4. Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study
Open this publication in new window or tab >>Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study
(English)Manuscript (preprint) (Other academic)
##### Abstract [en]

The present experimental study addresses the flow of a Yield Stress Fluid (YSF) with some elasticity (Carbopol gel) in a square duct. The behaviour of two fluids with lower and higher yield stress is investigated in terms of the friction factor and flow velocities at multiple Reynolds numbers $Re^* \in$ (1, 200) and, hence, Bingham numbers $Bi \in$ (0.01, 0.35). Taking advantage of the symmetry planes in a square duct, we reconstruct the entire 3-component velocity field from 2-dimensional Particle Image Velocimetry (PIV). A secondary flow consisting of eight vortices is observed to recirculate the fluid from the core towards the wall-center and from the corners back to the core. The extent and intensity of these vortices grows with increasing $Re^*$ or, alternately, as the plug-size decreases. The second objective of this study is to explore the change in flow in the presence of particles. To this end, almost neutrally-buoyant finite-size spherical particles with duct height, $2H$, to particle diameter, $d_p$, ratio of 12 are used at two volume fractions $\phi$ = 5 and 10\%. Particle Tracking Velocimetry (PTV) is used to measure the velocity of these refractive-index-matched spheres in the clear Carbopol gel, and PIV to extract the fluid velocity. Additionally, simple shadowgraphy is also used for qualitatively visualising the development of the particle distribution along the streamwise direction. The particle distribution pattern changes from being concentrated at the four corners, at low flow rates, to being focussed along a diffused ring between the center and the corners, at high flow rates. The presence of particles induces streamwise and wall-normal velocity fluctuations in the fluid phase; however, the primary Reynolds shear stress is still very small compared to turbulent flows. The size of the plug in the particle-laden cases appears to be smaller than the corresponding single phase cases. Similar to Newtonian fluids, the friction factor increases due to the presence of particles, almost independently of the suspending fluid matrix. Interestingly, predictions based on an increased effective suspension viscosity agrees quite well with the experimental friction factor for the concentrations used in this study.

##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-257675 (URN)
##### Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2019-09-03Bibliographically approved

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7891011121310 of 26
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