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  • 1.
    Fällman, Monika
    et al.
    KTH, Superseded Departments, Mechanics.
    Lundell, Fredrik
    KTH, Superseded Departments, Mechanics.
    Holm, Richard
    KTH, Superseded Departments, Mechanics.
    Söderberg, L. Daniel
    STFI-Packforsk.
    A critical evaluation of ultrasound velocity profiling aiming towards measurements in fibre suspensionsManuscript (preprint) (Other academic)
  • 2.
    Holm, Richard
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Fluid mechanics of fibre suspensions related to papermaking2005Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    This thesis deals with fluid dynamic mechanisms related to papermaking, specif- ically: the initial dewatering mechanisms during roll-forming and fibre motion in sedimentation and in shear flow.

    Pressure and wire position measurements have been conducted in a model resembling the forming zone and the measured pressure distributions are shown to have more complex patterns than the simple model p / T/R (where T is the wire tension and R is the roll radius). It is shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. In addition, it is shown that the drainage has a stabilizing effect on the dewatering pressure.

    The flow around the forming roll has also been modelled with the assump- tion that the wire is impermeable. A non-linear equation for the position of the wire is derived that clearly shows that the Weber number, We, is an im- portant parameter. The equation is linearized around the trivial solution and has a standing wave solution with a specific wavelength that scales with the We-number.

    Motion of non-Brownian fibre settling in a Newtonian fluid at a small but finite Reynolds number has been studied experimentally. Two different regimes of sedimentation were identified. For dilute suspensions, fibres gener- ally fall without flipping and may travel at velocities larger than that of an isolated particle. In the semi-dilute regime we found the settling process to be dominated by large-scale fluctuations. The velocity fluctuations scale with the suspension volume concentrationφ according toφ1/3, which is similar to the findings for settling spheres.

    The influence of shear on fibre orientation in the near wall region was studied in cellulose acetate fibre suspensions. At low concentration and low aspect ratio fibres were observed to orient perpendicular to the streamwise direction (named rollers) in the near wall region whereas the orientation further into the suspension was unchanged. As the concentration and aspect ratio increased the fraction of rollers decreased.

    Finally, an evaluation of a commercial Ultra Velocity Profiler unit in fibre suspensions are presented. The idea was to determine the velocity and characterise the turbulence from ultra sound echoes from particles in the fluid. However, the spatial and/or temporal resolution of the measurements did not permit turbulence characterisation. These limitations might be possible to overcome and some procedures are proposed and evaluated.

  • 3.
    Holm, Richard
    et al.
    KTH, Superseded Departments, Mechanics.
    Storey, S.
    Martinez, Mark
    Söderberg, L. Daniel
    KTH, Superseded Departments, Mechanics.
    Visualization of streaming-like structures during settling of dilute and semi-dilute rigid fibre suspensions2004In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal (Other academic)
  • 4.
    Holm, Richard
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory. KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    A theoretical analysis of the flow stability in roll forming of paper2005In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 20, no 2, p. 212-216Article in journal (Refereed)
    Abstract [en]

    This paper deals with the fundamental mechanisms that control partial roll dewatering in papermaking. The flow around the forming cylinder is modelled in a cylindrical coordinate system and the wire is assumed to be impermeable. The governing equations are reduced based on a discussion where the magnitude of the different terms is estimated. Given this reduced set of equations a non-linear equation for the position of the wire is deduced. This clearly shows that one of the most important parameters is the Weber number, We, which is the non-dimensional number that can be obtained by comparing the effect of wire tension in relation to the momentum of the incoming headbox jet. The characteristics of the non-linear equation are discussed and the equation is linearized around the trivial solution to the equations, which gives that the wire is displaced a constant distance from the roll along the whole wrap. The linear equation has a standing wave solution with a specific wavelength that scales with We. This solution is compared to previously measured profiles regarding the wavelength of the waves.

  • 5.
    Holm, Richard
    et al.
    KTH, Superseded Departments, Mechanics.
    Söderberg, L. Daniel
    STFI-Packforsk.
    Influence of shear on fibre orientation in the near wall region2004Article in journal (Other academic)
  • 6.
    Holm, Richard
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, L. Daniel
    STFI-Packforsk.
    Shear influence on fibre orientation: Dilute suspension in the near wall region2007In: Rheologica Acta, ISSN 0035-4511, E-ISSN 1435-1528, Vol. 46, no 5, p. 721-729Article in journal (Refereed)
    Abstract [en]

    The purpose of this experimental work was to study the influence of shear close to a solid boundary on the fibre orientation in suspensions with different fibre aspect ratios and concentrations. We have studied a laminar suspension flow down an inclined plate. The fibre orientation in different wall parallel planes were measured. We applied an index-of-refraction (IR) matching method together with particle tracking techniques to obtain the fibre motion. The fibre orientation was extracted using a two-dimensional wavelet transform. The shear flow resulted in fibres perpendicularly oriented to the streamwise direction (rollers) in the near wall region. These rollers were observed in the experiment to perform a rolling-sliding motion down the inclined plate around a stable perpendicular orientation. As the distance to the wall increased the number of rollers decreased and the fibre orientation was unaffected from its initial streamwise orientation. As the aspect ratio increased the influence of shear on the fibre orientation decreased for all measured wall parallel planes. This was also the case for higher fibre concentrations. The purpose of this study was to contribute to the development of the capacity to control the sheet network structure in papermaking.

  • 7.
    Holm, Richard
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Faxén Laboratory. KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, L. Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Norman, Bo
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Paper Technology.
    Experimental studies on dewatering during roll forming of paper2005In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 20, no 2, p. 205-211Article in journal (Refereed)
    Abstract [en]

    Pressure and wire position measurements have been performed in an experimental facility, the KTH-Former, which intends to model the roll-forming zone of a paper machine. The measured pressure distributions in the forming zone are shown to have more complex patterns than the simple model p=T/R, which normally is referred to as the nominal pressure. It is also shown that an increase in wire tension has a similar effect as a decrease in flow-rate on the shape of the pressure distribution. This is a consequence of that the flow to a large extent is governed by the relation between the dynamic pressure and the nominal pressure. For the case of partial dewatering the suction peak that appears at the roll-wire separation point has a strong influence on the pressure distribution upstream. Finally, it is shown that the drainage has a stabilizing effect on the dewatering pressure.

  • 8.
    Lundell, Fredrik
    et al.
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Söderberg, Daniel
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Storey, Stefan
    Holm, Richard
    KTH, School of Engineering Sciences (SCI), Mechanics.
    THE EFFECT OF FIBRES ON LAMINAR-TURBULENT TRANSITION AND SCALES IN TURBULENT DECAY2005In: ADVANCES IN PAPER SCIENCE AND TECHNOLOGY: TRANSACTIONS OF THE 13TH FUNDAMENTAL RESEARCH SYMPOSIUM, VOLS 1-3 / [ed] IAnson SJ, BURY: PULP & PAPER FUNDAMENTAL RESEARCH SOCIETY , 2005, p. 19-34Conference paper (Refereed)
    Abstract [en]

    Two physical phenomena which determine fundamental possibilities of paper forming are studied. The two phenomena are (i) laminar/turbulent transition and (ii) decay of turbulence. At first, the relevance of the processes to paper making is reviewed and discussed. The state of the boundary layers (laminar or turbulent) on split vanes and the decay of turbulence in the free stream are found to be of uttermost importance for the control of layer purity, formation and other properties of the final paper. Experiments in which these two processes are studied by visualisations are presented. The experiments emphasize the impact of fibres on these processes, as compared to what is found with pure water. All experiments are performed in model experiments were the structures in the flow are visualised by the addition of small, flake-like particles. It is shown that the addition of fibres radically change the physics of the flow. In a water table experiment, the addition of fibres is seen to promote the production of turbulent spots. At high enough fibre concentrations, the flow of water and fibres is fully turbulent even if a flow of pure water is laminar. In decay of turbulence, the fibres are seen to radically change the energy transfer between different scales so that intermediate and small scales remain active for longer times. It is concluded that fibres have large effects on laminar-turbulent transition and turbulence decay and that improved knowledge of these effects are a corner stone in the understanding of head box flow and its relations to the resulting paper quality.

  • 9. Parkin, P.
    et al.
    Holm, Richard
    KTH, Superseded Departments, Mechanics.
    Medici, Davide
    KTH, Superseded Departments, Mechanics.
    The application of PIV to the wake of a wind turbine in yaw2001In: DLR-Mitteilung / [ed] Kompenhans, J., 2001, p. 155-162Conference paper (Refereed)
    Abstract [en]

    PIV has been used in a wind tunnel study of the wake of a 0.25m diameter two bladed model horizontal axis wind turbine (HAWT). Velocity fields of the wake from one to five rotor diameters downstream of the wind turbine model are shown, both with the turbine aligned in the flow and yawed. Data analysis is mainly based on time averaged velocity profiles of the wake for a range of yaw angles [0 ±10, ±20, ±30] in a plane parallel to the flow. Results show the size and persistence of the velocity deficit and tip vortices in the wake, and the wake deflection in yaw. It is shown that active control of turbine yaw angles could be an advantage for overall maximization of power output from wind farms.

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