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The effect of extensional flow on shear viscosity

Hodgkinson, Richard (2017) The effect of extensional flow on shear viscosity. PhD thesis, University of Sheffield.

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Shear rheology is conventionally studied under pure shearing flows, rather than more realistic mixed flows. Moving parallel surfaces and capillary rheometery are examples of the former, whilst the latter occurs whenever a flow accelerates or decelerates creating an additional component of extension, e.g. on passing through an industrial extrusion die. We postulate and gather supporting evidence that shear rheology is a function of not only shear, but both shear and extension rate, a factor with important consequences for fibre spinning and extrusion operations. The direction, as well as rate, of extensional deformation is important. A novel two-phase flow, planar extension experiment is developed and the surface coatings necessary to control the interface structure identified. Shear viscosity evolution is monitored, in-situ, under extensional flow, by optically measuring shear rates either side of a test fluid – reference fluid interface; issues due to optical refraction are critically addressed. Preliminary evidence is shown for a 1.2wt% 4x10^6 MW PEO solution that parallel (+ve) extensional flow, on the order of 11.5s-1 , causes a reduction in shear viscosity, and perpendicular (-ve) causes an increase in shear viscosity, supporting the hypothesis. A framework for a comparison experiment, with the same shear history but without extension, is presented. As part of this work, design criteria for planar hyperbolic extensional channels are critically assessed. In particular, expanding a hyperbola entrance region would maximise total Hencky strain, yet this region is almost never given rationalised consideration in literature. In this region the basis for the hyperbolic profile breaks down, and a new profiling strategy and channel form are presented, which is found to only differ significantly in this inlet region. A useful design limit of 130 degrees on channel inlet angle is identified. The new profile is compared to a hyperbolic profile through the use of CFD for wall slip flow, and a slight improvement in extension rate uniformity along the centreline found. Deviations are contrasted against assumptions made in the profiling strategy: comments are made with regards the possibility for “internal” shear to occur, and non-uniform extension rates are accordingly found to exist between streamlines in these channels despite the use of full wall slip in the simulations.

Item Type: Thesis (PhD)
Keywords: rheology, extensional flow, shear viscosity, mixed flow, shear thinning, polyethylene(oxide), two phase flow, particle image velocimetry, PIV, surface wetting
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Chemical and Biological Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.724403
Depositing User: Mr Richard Hodgkinson
Date Deposited: 02 Oct 2017 09:16
Last Modified: 12 Oct 2018 09:46
URI: http://etheses.whiterose.ac.uk/id/eprint/18327

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