Turbulent duct flow of non-Newtonian liquids

• Main Author: Smith, Sarah Elizabeth
• Published: University of Liverpool 2000
• Subjects: 532.54
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399184

The turbulent flow of non-Newtonian fluids in straight ducts has been investigated. Specifically, the fully developed circular pipe flow, axisymmetric sudden expansion flow and fully developed square duct flow were studied. The pipe flow study analysed previous measurements of the mean velocity profiles and friction factor-Reynolds number characteristics of different non-Newtonian fluids in pipe fully developed pipe flows. An investigation of different nondimensional parameters permitted initial progress on developing a correlation between drag reduction and fluid rheology to be made. Comparison of the ranking orders of drag reduction, fluid extensional viscosity and fluid elasticity revealed that these fluid properties are most strongly correlated with drag reduction at low shear/strain rates (that is, in the buffer and outer regions of the boundary layer). The sudden expansion geometry was investigated for flows of aqueous Xanthan gum solution and two reference Newtonian fluids. A smooth contraction was placed at the inlet to the sudden expansion. Few significant differences were observed between the mean flow behaviours of the test fluids for the turbulent Reynolds numbers tested (26,000 and 80,000). These results may reflect the manner in which the rigid, rod-like molecules found in Xanthan gum influence the flow behaviour. Turbulence measurements indicated that all three turbulence components were suppressed for the polymer solution flow within the free shear layer downstream of the expansion. The turbulent flow of two non-Newtonian fluids (a blend ofXanthan gum and Carboxymethylcellulose in water and an aqueous solution of polyacrylamide) in a square duct were compared with a turbulent Newtonian square duct flow. Although suppression of the transverse turbulence components was noted, the polymer solutions also strongly affected the behaviour of the secondary flows found in turbulent non-circular duct flows of Newtonian fluids. Specifically, the secondary flows appeared to be weakened in the polymer blend flow and completely suppressed in the polyacrylamide solution flow. It is anticipated that fluid elasticity is influential in this suppression