Certain surfactant solutions are drag reducing in turbulent shear flows. Because they "self-repair" after mechanical degradation, they are very attractive for reducing pumping energy losses in recirculating water applications such as district heating and cooling systems. Some surfactant systems reduce drag below the Virk limiting drag reduction asymptote for high polymers. The surfactant asymptote is 40% below that for polymers and the limiting velocity profile slope for highly drag reducing surfactant systems is about twice that for polymers. Like polymers, surfactants show low turbulence intensities normal to the wall and may exhibit zero Reynolds stresses requiring the postulation of an "elastic" stress to satisfy the total stress balance. The influences of counterion chemical structure and shear on microstructures, rheology and therefore drag reduction of cationic surfactant solutions are also addressed. Viscoelasticity, high extensional/shear viscosity ratios and threadlike microstructures have been proposed as necessary physical criteria for surfactant drag reduction. Recently, however, several non-viscoelastic drag reducing surfactant solution systems (zero first normal stress differences, no recoil and no stress overshoot) have been reported. Most drag reducing surfactant solutions have extensional to shear viscosity ratios of 100 or more. However, two solutions with a low ratio in the shear/extensional rate range of 20～1000 s~(-1) have been observed. The ratio tends to increase at higher extensional rates, however, so the second criterion may be valid. Finally, cryo-TEM images of some drag reducing surfactant micelle microstructures which lacked threadlike structures in the quiescent state have been observed. However, Zheng et al. showed that vesicle microstructures in the quiescent state can change to threadlike micelles under shear, supporting the third criterion.
展开▼
