Apparent wall slip velocity measurements in free surface flow of concentrated suspensions

Medhi, Bhaskar Jyoti; Kumar, Ashish; Singh, Anugrah

doi:10.1016/j.ijmultiphaseflow.2011.03.006

Cited by 30 publications

(13 citation statements)

References 32 publications

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“…A few authors such as Koh, Hookham & Leal (1994) and more recently Medhi, Kumar & Singh (2011) observed a significant slip between the particles and the carrier fluid near a solid boundary. As Koh et al (1994) themselves admitted, the relative motion of the particle and fluid phases is difficult to understand if the density mismatches are vanishingly small.…”

Section: Discussionmentioning

confidence: 99%

The dam-break problem for concentrated suspensions of neutrally buoyant particles

2013

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This paper addresses the dam-break problem for particle suspensions, that is, the flow of a finite volume of suspension released suddenly down an inclined flume. We were concerned with concentrated suspensions made up of neutrally buoyant non-colloidal particles within a Newtonian fluid. Experiments were conducted over wide ranges of slope, concentration and mass. The major contributions of our experimental study are the simultaneous measurement of local flow properties far from the sidewalls (velocity profile and, with lower accuracy, particle concentration) and macroscopic features (front position, flow depth profile). To that end, the refractive index of the fluid was adapted to closely match that of the particles, enabling data acquisition up to particle volume fractions of 60 %. Particle migration resulted in the blunting of the velocity profile, in contrast to the parabolic profile observed in homogeneous Newtonian fluids. The experimental results were compared with predictions from lubrication theory and particle migration theory. For solids fractions as large as 45 %, the flow behaviour did not differ much from that of a homogeneous Newtonian fluid. More specifically, we observed that the velocity profiles were closely approximated by a parabolic form and there was little evidence of particle migration throughout the depth. For particle concentrations in the 52-56 % range, the flow depth and front position were fairly well predicted by lubrication theory, but taking a closer look at the velocity profiles revealed that particle migration had noticeable effects on the shape of the velocity profile (blunting), but had little impact on its strength, which explained why lubrication theory performed well. Particle migration theories (such as the shear-induced diffusion model) successfully captured the slow evolution of the velocity profiles. For particle concentrations in excess of 56 %, the macroscopic flow features were grossly predicted by lubrication theory (to within 20 % for the flow depth, 50 % for the front position). The flows seemed to reach a steady state, i.e. the shape of the velocity profile showed little time dependence.

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Section: Discussionmentioning

confidence: 99%

The dam-break problem for concentrated suspensions of neutrally buoyant particles

2013

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“…A quantitative description of the motion of a sediment at a wall is quite difficult [71][72][73][74][75][76], and there are still many open questions. The following problems may be of relevance, but are beyond the scope of the present investigation: resuspension of particles [77,78] and related processes [79]; shear-induced migration of particles in concentrated suspensions [73][74][75]; not a sharp interface [80] and shear-induced corrugation of interfaces [81]; wall slip (depending on surface properties and particle material) [82][83][84][85][86][87][88][89]; stick-slip events [90]. For the present investigation, it is only of importance how the motion of the sediment affects the bulk flow, as this is what controls the settling time and the concentration in the suspension during the settling process.…”

Section: Iib: Sediment Moves Along the Wallmentioning

confidence: 99%

On basic equations and kinematic-wave theory of separation processes in suspensions with gravity, centrifugal and Coriolis forces

Schneider

2017

Acta Mech

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Neglecting inertial and viscosity effects in the bulk flow is a common assumption in the analysis of separation processes in suspensions under the action of gravity or centrifugal and Coriolis forces. While there is a number of examples of particular solutions, the general form of the basic equations for three space dimensions, together with the appropriate boundary and initial conditions, is still uncertain and, with regard to certain aspects, even controversial. An essential point is a proper choice of the variables. Here it is proposed to introduce the mass density of the mixture, the mean mass velocity of the mixture and the total volume flux as a set of dependent variables. After some manipulations, a complete set of basic equations is obtained. It consists of two continuity equations, a generalized drift-flux relation, and two linearly independent components of a vector equation describing the total body force as irrotational. Then, by eliminating the mean mass velocity of the mixture from the set of unknowns, a generalized kinematic-wave equation is derived. It describes kinematic waves that are embedded in a bulk flow that may be one-, two-or three-dimensional. Concerning boundary conditions at solid walls, one has to ascertain whether the total body force at the wall points into the suspension or out of it. In the former case, a thin boundary layer of clear liquid is formed at the wall, whereas in the latter case a thin sediment layer may either stick at the wall or slide along it. Each of those three possibilities leads to a particular boundary condition for the bulk flow in terms of the dependent variables. In addition, initial conditions and kinematic shock relations are briefly discussed. Finally, the application of the kinematic-wave theory to the settling process in rotating tubes is outlined.

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“…The plateau visible in the shear stress behavior of especially 1.6% MFC (see also Fig 2(a)) is most likely related to existence of concentration gradient and, at higher values of flow rate, even of a thin particle depletion layer near the wall. The well-known explanation for such phenomenon is given by the combined effect of steric hindrance and repulsive hydrodynamic interaction between the tube wall and suspended particles (Barnes 1995;Medhi et al 2011;Leighton, Acrivos 1987;Jäsberg et al 2000). This interpretation is supported by Fig 7 (a) and (b) showing the measured dependence of viscosity at wall and of apparent slip velocity on wall shear stress, respectively.…”

Section: Boundary Layer Behaviormentioning

confidence: 99%

Characterization of micro-fibrillated cellulose fiber suspension flow using multi scale velocity profile measurements

Kataja

Haavisto²,

Salmela³

et al. 2017

Nordic Pulp &Amp; Paper Research Journal

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Rheological properties and boundary layer flow behavior of Micro Fibrillated Cellulose (MFC) suspended in water was studied using a novel velocity profiling rheometric technique. The method is based on measuring stationary velocity profiles of fluid flow in a straight tube simultaneously by Doppler Optical Coherence Tomography (DOCT) and by Ultrasound Velocity Profiling (UVP). The high resolution DOCT provides velocity profiles near the transparent tube wall, while UVP yields corresponding information in the interior parts of the flow. The data from the two instruments is combined into a comprehensive velocity profile including both the thin boundary layer near the wall and the interior parts of the flow. Within the boundary layer, concentration and thereby the viscosity of MFC is found to decrease towards the wall. At high flow rate, sublayer of virtually pure water is observed next to the wall, giving rise to apparent wall slip. The results from interior part of the flow show shear thinning behavior in qualitative agreement with results from conventional rheological methods. The results indicate that the new method can provide detailed experimental information on the rheology of MFC suspensions and their intricate boundary layer flow behavior, avoiding uncertainties inherent in many conventional rheological techniques.

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Apparent wall slip velocity measurements in free surface flow of concentrated suspensions

Cited by 30 publications

References 32 publications

The dam-break problem for concentrated suspensions of neutrally buoyant particles

The dam-break problem for concentrated suspensions of neutrally buoyant particles

On basic equations and kinematic-wave theory of separation processes in suspensions with gravity, centrifugal and Coriolis forces

Characterization of micro-fibrillated cellulose fiber suspension flow using multi scale velocity profile measurements

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