Abstract
Turbulent drag reduction in Couette flow was investigated for aqueous solutions of a nonionic polymer, poly(ethylene oxide) (PEO), a cationic surfactant, hexadecyltrimethylammonium choride (HTAC), and their mixtures. Consistent with literature data, drag reduction was observed for PEO solutions above a critical molecular weight, 0.91x105 < Mc < 3.04x105. Maximum drag reduction occurs at an optimum concentration, c*PEO, which scales inversely with molecular weight, and the %maximum drag reduction increased with molecular weight. For aqueous HTAC solutions, wall shear stress decreases with increasing HTAC concentration and levels off at an optimum concentration, c*HTAC, comparable to the critical micelle concentration. For HTAC/PEO mixtures, the critical PEO molecular weight for drag reduction decreases, interpreted as due to an increase in hydrodynamic volume because of binding of HTAC micelles to PEO. Consistent with this interpretation, at fixed PEO concentration, maximum drag reduction is observed at an optimum HTAC concentration, c*HTAC/PEO, comparable to the maximum binding concentration, MBC. The effect of ionic strength on surfactant solutions and polymer-surfactant complex solutions was also investigated. For HTAC in salt solutions, addition of salt decreases the optimum HTAC concentration, c*HTAC, and the % maximum drag reduction decreases with increasing ionic strength. On the other hand, for the aqueous PEO-HTAC solution, the wall shear stresses upon addition of HTAC beyond the respective CMC values are strongly reduced as ionic strength increases.
