Drag Reduction in Surfactant Solutions
Drag reducing surfactant solutions are characterized by the presence of rod-like micelles which are formed by single surfactant molecules above a characteristic concentration. This critical micelle concentration strongly depends on the temperature and the electrolyte concentration. Shear viscosity measurements of drag reducing surfactant solutions show that at shear rates above a critical value the viscosity suddenly increases due to the formation of a shear-induced state (SIS) in which the micelles coalescence to larger structures and are completely aligned in flow direction. The turbulent friction behaviour of these surfactant solutions is characterized by a critical wall-shear stress. The observed loss of drag reduction beyond this critical wall-shear stress is reversible.
Small-angle neutron scattering (SANS) experiments demonstrate that the alignment of the rod-like micelles in flow direction correlates with drag reduction. At low Reynolds numbers in the turbulent flow regime the dimensionless velocity profiles are very similar to those observed in dilute polymer solutions, whereas at maximum drag reduction conditions the shape of the profiles is similar to a laminar profile. The axial turbulence intensity is increased whereas the transverse turbulence intensity and the Reynolds shear stresses are strongly damped. An attempt is made to explain theses changes by an increased effective viscosity.
KeywordsWall Shear Stress Friction Factor Surfactant Solution Drag Reduction Reynolds Shear Stress
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