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Fibre orientation and fibre streaks in turbulent wall bounded flow
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-9976-8316
Show others and affiliations
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-96616OAI: oai:DiVA.org:kth-96616DiVA: diva2:531416
Note

QS 2012

Available from: 2012-06-07 Created: 2012-06-07 Last updated: 2014-10-03Bibliographically approved
In thesis
1. Orientation of elongated particles in shear and extensional flow
Open this publication in new window or tab >>Orientation of elongated particles in shear and extensional flow
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Elongated particles in fluid flows are a big part of the world we are living in. Gaining knowledge on how particles behave in different fluid flows can potentially increase the efficiency of industrial processes and decrease the world's energy consumption as well as improve the properties of future materials.

In this thesis, the orientation of elongated particles in two different flows are studied. The first case is a dilute fibre suspension in a turbulent flow and the second case is a semi-dilute fibril dispersion in a laminar flow. The fibres (cellulose acetate) are at least three orders of magnitude larger than the fibrils (nano-fibrillated cellulose).

The turbulent flow case is half of a full channel flow, characterised by the friction Reynolds number, and is experimentally examined. This experiment is closely related to the papermaking process. Laser Doppler velocimetry measurements are preformed without fibres in order to make sure that the flow is turbulent and fully developed. Images of the fibres in the flow are acquired using a CCD-camera, from which it is possible to detect the fibres in an image processing step and extract both the positions and orientations of the fibres. A large parameter study is carried out, where the aspect ratio of the fibres, concentration and Reynolds number are changed. Short fibres are observed to align perpendicular to the flow, while the longer fibres are found to align in the flow direction. The fibres are also seen to accumulate in streamwise streaks, believed to be caused by velocity structures in the turbulent flow.

The second flow case studied focusses on a semi-dilute dispersion in a laminar flow. It includes both experiments and numerical calculations of the fibril orientation. The aim of this study is to demonstrate that it is possible to control the fibril orientation with a fluid. In a semi-dilute dispersion, fibrils are interacting. However, no flocs or networks are formed. A flow focusing apparatus is used in order to hydrodynamically accelerate the dispersion with an outer fluid (sheath) flow. The mean orientation in the flow direction is experimentally studied by detecting the birefringence of the flowing dispersion. The orientation distribution is calculated by solving the Smoluchowski equation. The fibrils are seen to align in the flow direction both in the experiments and the calculations. Moreover, the alignment is found to increase with increasing acceleration.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. viii, 25 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:11
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-95282 (URN)978-91-7501-404-3 (ISBN)
Presentation
2012-06-15, Seminarierummet, Brinellvägen 32, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Note
QC 20120607Available from: 2012-06-07 Created: 2012-05-21 Last updated: 2012-06-07Bibliographically approved
2. Hydrodynamic stability and turbulence in fibre suspension flows
Open this publication in new window or tab >>Hydrodynamic stability and turbulence in fibre suspension flows
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. viii, 34 p.
Series
Trita-MEK, ISSN 0348-467X ; 2012:10
Keyword
fluid mechanics, fibre suspension, turbulence, image analysis, hydrodynamic stability, nano-fibrillated cellulose
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-95279 (URN)978-91-7501-403-6 (ISBN)
Presentation
2012-06-12, Seminarierummet, Brinellvägen 32, Stockholm, 09:00
Opponent
Supervisors
Note
QC 20120613Available from: 2012-06-13 Created: 2012-05-21 Last updated: 2012-06-13Bibliographically approved
3. Orientation of elongated, macro and nano-sized particles in macroscopic flows
Open this publication in new window or tab >>Orientation of elongated, macro and nano-sized particles in macroscopic flows
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Non-spherical particles are present all around us, in biological, industrial and environmental processes. Making predictions of their impact on us and systems in our vicinity can make life better for everyone here on earth. For example, the ash particles from a volcano eruption are non-spherical and their spreading in the atmosphere can hugely impact the air traffic, as was also proven in 2010. Furthermore, the orientation of the wood fibres in a paper sheet influences the final properties of the paper, and the cause of a specific fibre orientation can be traced back to the fluid flows during the manufacturing process of the paper.

In this thesis, experimental and numerical work is presented with the goal to understand and utilize the behavior of elongated particles in fluid flows. Two different experimental setups are used. The first one, a turbulent half channel flow, aims at increasing the understanding of how particles with non-zero inertia behave in turbulence. The second setup is an attempt to design a flow field with the purpose to align nanofibrils and create high performance cellulose filaments.

Experiments were performed in a turbulent half channel flow at different flow set- tings with dilute suspensions of cellulose acetate fibres having three different aspect ratios (length to width ratio). The two main results were firstly that the fibres agglom- erated in streamwise streaks, believed to be due to the turbulent velocity structures in the flow. Secondly, the orientation of the fibres was observed to be determined by the aspect ratio and the mean shear, not the turbulence. Short fibres were oriented in the spanwise direction while long fibres were oriented in the streamwise direction.

In order to utilize the impressive properties (stiffness comparable to Kevlar) of the cellulose nanofibril in a macroscopic material, the alignment of the fibrils must be controlled. Here, a flow focusing device (resulting in an extensional flow), designed to align the fibrils, is used to create a cellulose filament with aligned fibrils. The principle is based on a separation of the alignment and the assembly of the fibrils, i.e. first align the fibrils and then lock the aligned structure. With this process, continuous filaments were created, with properties similar to that of the wood fibre at the same fibril alignment. However, the highest alignment (lowest angle) of the fibrils in a filament created was only 31o from the filament axis, and the next step is to increase the alignment. This thesis includes modeling of the alignment process with the Smoluchowski equation and a rotary diffusion. Finding a model that correctly describes the alignment process should in the end make it possible to create a filament with fully aligned fibrils.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. xiii, 81 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014.19
Keyword
Orientation, fibre, fibril, turbulent channel flow, particle streaks, flow focusing, cellulose nanofibrils, extensional flow, Smoluchowski, rotary diffusion
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-150493 (URN)978-91-7595-254-3 (ISBN)
Public defence
2014-09-25, Kollegiesalen, Brinellvägen 8, KTKH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20140908

Available from: 2014-09-08 Created: 2014-09-04 Last updated: 2014-09-08Bibliographically approved
4. Transitional and turbulent fibre suspension flows
Open this publication in new window or tab >>Transitional and turbulent fibre suspension flows
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis the orientation of macro-sized fibres in turbulent flows is studied, as well as the effect of nano-sized fibrils on hydrodynamic stability. The focus lies on enabling processes for new materials where cellulose is the main constituent. When fibres (or any elongated particles) are added to a fluid, the complexity of the flow-problem increases. The fluid flow will influence the rotation of the fibres, and therefore also effect the overall fibre orientation. Exactly how the fibres rotate depends to a large extent on the mean velocity gradient in the flow.

In addition, when fibres are added to a suspending fluid, the total stress in the suspension will increase, resulting in an increased apparent viscosity. The increase in stress is related to the direction of deformation in relation to the orientation of the particle, i.e. whether the deformation happens along the long or short axis of the fibre. The increase in stress, which in most cases is not constant neither in time nor space, will in turn influence the flow.

This thesis starts off with the orientation and spatial distribution of fibres in the turbulent flow down an inclined plate. By varying fibre and flow parameters it is discovered that the main parameter controlling the orientation distribution is the aspect ratio of the fibres, with only minor influences from the other parameters. Moreover, the fibres are found to agglomerate into streamwise streaks. A new method to quantify this agglomeration is developed, taking care of the problems that arise due to the low concentration in the experiments. It is found that streakiness, i.e. the tendency to agglomerate in streaks, varies with Reynolds number.

Going from fibre orientation to flow dynamics of fibre suspensions, the influence of cellulose nanofibrils (CNF) on laminar/turbulent transition is investigated in three different setups, namely plane channel flow, curved-rotating channel flow, and the flow in a flow focusing device. This last flow case is selected since it is can be used for assembly of CNF based materials. In the plane channel flow, the addition of CNF delays the transition more than predicted from measured viscosities while in the curved-rotating channel the opposite effect is discovered. This is qualitatively confirmed by linear stability analyses. Moreover, a transient growth analysis in the plane channel reveals an increase in streamwise wavenumber with increasing concentration of CNF. In the flow focusing device, i.e. at the intersection of three inlets and one outlet, the transition is found to mainly depend on the Reynolds number of the side flow. Recirculation zones forming downstream of two sharp corners are hypothesised to be the cause of the transition. With that in mind, the two corners are given a larger radius in an attempt to stabilise the flow. However, if anything, the flow seems to become unstable at a smaller Reynolds number, indicating that the separation bubble is not the sole cause of the transition. The choice of fluid in the core flow is found to have no effect on the stability, neither when using fluids with different viscosities nor when a non-Newtonian CNF dispersion was used. Thus, Newtonian model fluids can be used when studying the flow dynamics in this type of device.

As a proof of concept, a flow focusing device is used to produce a continuous film from CNF. The fibrils are believed to be aligned due to the extensional flow created in the setup, resulting in a transparent film, with an estimated thickness of 1 um.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. x, 56 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2014.22
Keyword
Fluid mechanics, fibre suspension, turbulence, laminar-turbulent transition, image analysis, hydrodynamic stability, cellulose nanofibrils.
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-153018 (URN)978-91-7595-287-1 (ISBN)
Public defence
2014-10-24, L1, Drottning Kristinas Väg 30, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Knut and Alice Wallenberg Foundation
Note

QC 20141003

Available from: 2014-10-03 Created: 2014-10-02 Last updated: 2015-03-06Bibliographically approved

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Prahl Wittberg, LisaSöderberg, Daniel

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