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CivilComp Proceedings
ISSN 17593433 CCP: 89
PROCEEDINGS OF THE SIXTH INTERNATIONAL CONFERENCE ON ENGINEERING COMPUTATIONAL TECHNOLOGY Edited by: M. Papadrakakis and B.H.V. Topping
Paper 98
Viscoelastic LowReynoldsNumber Flows in MixingSeparating Cells A. Afonso^{1}, M.A. Alves^{1}, R.J. Poole^{2}, P.J. Oliveira^{3} and F.T. Pinho^{1}^{,}^{4}
^{1}Chemical Engineering Department, CEFT, Faculty of Engineering, University of Porto, Portugal
A. Afonso, M.A. Alves, R.J. Poole, P.J. Oliveira, F.T. Pinho, "Viscoelastic LowReynoldsNumber Flows in MixingSeparating Cells", in M. Papadrakakis, B.H.V. Topping, (Editors), "Proceedings of the Sixth International Conference on Engineering Computational Technology", CivilComp Press, Stirlingshire, UK, Paper 98, 2008. doi:10.4203/ccp.89.98
Keywords: mixingseparating, low Reynolds number, viscoelastic fluids, upperconvected Maxwell model, elastic instabilities, flow bifurcation.
Summary
An investigation of Newtonian and viscoelastic flows is carried out on the
mixingseparating geometry of Cochrane et al. [1]. This consists of a low Reynolds
number flow in two opposed channels sharing a common wall, where a gap allows
the interaction between the two flows (Re<40).
For Newtonian fluids the flow is antisymmetric, due to the antisymmetry of the fullydeveloped inlet conditions and the symmetry of the flow geometry. Increasing the gap size increased the reversed flow rate ratio (R_{r}), here defined as the ratio between the reversed and total flow rates. The creeping flow of upperconvected Maxwell (UCM) fluids showed two distinct flow patterns and for a combination of critical flow geometries, it was possible to identify a new steady bistable bifurcation pattern at low inertia and high elasticity. For normalized gap sizes below a critical value the reversed flow is slightly enhanced by viscoelasticity, followed by a strong decrease in R_{r} towards zero as the Deborah number De further increases. Above a supercritical gap size, viscoelasticity is responsible for a continuous increase in R_{r}. For a nearcritical gap size it was possible to observe a sudden jump between the two flow conditions at slightly different Deborah numbers. Flow inertia was found to increase the critical Deborah number for the steady flow bifurcation at a particular value of the gap nondimensional width. Inertia naturally enhances the straight flow case and at Re=5, R_{r} always decreased with Deborah number for De<=0.6 and for the investigated gap sizes. These predictions suggest the need for experiments with very viscous elastic fluids in order to detect the supercritical behavior, which has so far not been reported in the literature. References
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