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Finite-size spherical particles in a square duct flow of an elastoviscoplastic fluid: an experimental study
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-6694-7406
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0003-3637-8956
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0002-2504-3969
KTH, Superseded Departments (pre-2005), Mechanics.ORCID iD: 0000-0002-4346-4732
(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: urn:nbn:se:kth:diva-257675OAI: oai:DiVA.org:kth-257675DiVA, id: diva2:1347689
Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2019-09-03Bibliographically approved
In thesis
1. Experimental studies of large particles in Newtonian and non-Newtonian fluids
Open this publication in new window or tab >>Experimental studies of large particles in Newtonian and non-Newtonian fluids
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
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
urn:nbn:se:kth:diva-257681 (URN)978-91-7873-278-4 (ISBN)
Public defence
2019-09-26, Kollegiesalen, Brinellvägen 8, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20190903

Available from: 2019-09-03 Created: 2019-09-02 Last updated: 2019-09-10Bibliographically approved

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