Drag reduction in dilute fibre suspensions: Some mechanistic aspects

Sharma, Rahul; Seshadri, V.; Malhotra, Rashmi

doi:10.1016/0009-2509(79)85117-9

Cited by 13 publications

(10 citation statements)

References 13 publications

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“…This would reduce radial momentum transport rates by reducing the intensity of the Townsend-Bakewell roll waves, thereby explaining the nonlinear synergism of some of the studied additives. Sharma, Sheshadri and Malhotra [12] conducted experiments with polymer-fibre mixed systems. Methafos, a cellulose derivative, is mixed with asbestos fibres.…”

Section: Drag Reduction By Polymer-fibre Mixturesmentioning

confidence: 99%

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

Reddy

Singh

1985

Rheol Acta

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Section: Drag Reduction By Polymer-fibre Mixturesmentioning

confidence: 99%

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

Reddy

Singh

1985

Rheol Acta

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“…Lee et al first observed the synergistic effect where drag reduction using mixtures of polymers and fibers exceeds the sum of the drag reductions using either additives alone (Lee et al, 1974). The injection of fibers at the boundary and centerline of a turbulent pipe flow further shows that the synergy is a wall phenomenon (Sharma et al, 1979). Metzner et al speculate based on the observation of this synergy that fibers and polymers reduce turbulent drag by different mechanism.…”

Section: Introductionmentioning

confidence: 99%

Polymer induced drag reduction in exact coherent structures of plane Poiseuille flow

Graham

2007

Physics of Fluids

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Nonlinear traveling waves that are precursors to laminar-turbulent transition and capture the main structures of the turbulent buffer layer have recently been found to exist in all the canonical parallel flow geometries. The present work examines the effect of polymer additives on these "exact coherent states" (ECS) in the plane Poiseuille geometry, using the FENE-P constitutive model for polymer solutions. In experiments with a given fluid, Reynolds and Weissenberg numbers are linearly related (i.e. Wi/Re = const).In this situation, we study the effects of viscoelasticity on velocity field and polymer stress field along some experimental paths, which represent different flow behaviors as Re (and Wi) increases. The changes to the velocity field for the viscoelastic nonlinear traveling waves qualitatively capture many of those experimentally observed in fully turbulent flows of polymer solutions at low to moderate levels of drag reduction: drag is reduced, streamwise velocity fluctuations increase, wall-normal and spanwise velocity fluctuations decrease. The mechanism underlying these observations is the suppression of streamwise vortices by the polymer forces exerted on the fluid. Specifically, at sufficiently high wall shear rates, viscoelasticity completely suppress these streamwise vortices in the near-wall region, as is found in experiments in the maximum drag reduction (MDR) regime. The mean shear stress balance for the nonlinear traveling waves show that Reynolds shear stress decreases and polymer stress increases monotonically with the increase of viscoelasticity, as are found in full turbulence. The study of the influence of the viscoelasticity on the turbulent kinetic energy and Reynolds stress budgets shows that as Re (and Wi) increases, there is a consistent decrease in the production, diffusion and dissipation of turbulent kinetic energy. The decrease in the velocity pressure gradient term leads to a redistribution of the turbulent kinetic energy among the streamwise, iii wall-normal and spanwise directions. The influence of the rheological parameters on the viscoelastic ECS is analyzed. It is found that the degree of drag reduction is determined primarily by the extensional viscosity and Weissenberg number. The optimum wavelength conditions under which the viscoelastic ECS first come into existence are also investigated. The wavelengths in streamwise and spanwise directions and the wallnormal extent of the ECS all increase monotonically with the increase of viscoelasticity, as is found in experiments. Examination of the parameter space (Re, Wi) in which ECS exist indicates that at least at the Reynolds number considered here, the transition behaviors from laminar to turbulent flow, from no drag reduction to drag reduction, and from moderate drag reduction to MDR can be connected to the birth, evolution and death of these ECS, respectively.In order to test the scenario that we infer from these results, we next implement direct numerical simulation (DNS), studying the transient and statistical...

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“…It is generally believed that drag reduction is due to the suppression of turbulent eddies by fibers or other particles. Several studies have suggested that drag reduction in fiber suspensions is especially due to dampening of radial velocity fluctuations in the turbulent core, in contrast to drag reduction in polymer solutions, in which drag reduction is due to dampening near the wall . This implies less turbulent dispersion and reduced lateral mixing, which appears to contradict the experimental results of improved mixing for the hardwood fibers.…”

Section: Resultsmentioning

confidence: 91%

In‐line jet mixing of liquid‐pulp‐fiber suspensions: Effect of fiber properties, flow regime, and jet penetration

et al. 2012

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Mixing effectiveness was determined experimentally for side jet injection into pipe flow for water and pulp suspensions for a range of fiber mass concentrations (0-3.0%), mainstream velocities (0.5-5.0 m/s), and side-stream velocities (1.0-12.7 m/s). The mixing quality was measured in cross-sectional planes along the pipe using electrical resistance tomography and quantified by a modified mixing index, derived from the coefficient of variation of conductivity. Mixing depended strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in-line jet mixing in water are applicable to the mixing in suspensions, with small differences likely due to differences in fiber network strength and influences of fiber-turbulence interactions in modifying turbulent structures in the bulk. When a suspension flows as a plug, however, the mixing differs greatly from that in water, depending on the fiber network strength in the core of the pipe.

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Drag reduction in dilute fibre suspensions: Some mechanistic aspects

Cited by 13 publications

References 13 publications

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fibre mixtures in recirculatory turbulent flow of water

Polymer induced drag reduction in exact coherent structures of plane Poiseuille flow

In‐line jet mixing of liquid‐pulp‐fiber suspensions: Effect of fiber properties, flow regime, and jet penetration

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