Turbulent duct flow of non-Newtonian liquids
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 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