Drag Reduction in Synthetic Seawater by Flexible and Rigid Polymer Addition Into a Rotating Cylindrical Double Gap Device

Abstract

Flexible and rigid long chain polymers in very dilute solutions can significantly reduce the drag in turbulent flows. The polymers successively stretch and coil by interacting with the turbulent structures, which changes the turbulent flow and further imposes a transient behavior on the drag reduction (DR) as well as a subsequent mechanical polymer degradation. This time-dependent phenomenon is strongly affected by a number of parameters, which are analyzed here, such as the Reynolds number, polymer concentration, polymer molecular weight, and salt concentration. This last parameter can dramatically modify the polymeric structure. The investigation of the salt concentration's impact on the DR is mostly motivated by some potential applications of this technique to ocean transport and saline fluid flows. In the present paper, a cylindrical double gap rheometer device is used to study the effects of salt concentration on DR over time. The reduction of drag is induced by three polymers: poly (ethylene oxide) (PEO), polyacrylamide (PAM), and xanthan gum (XG). These polymers are dissolved in deionized water both in the presence of salt and in its absence. The DR is displayed from the very start of the test to the time when the DR achieves its final level of efficiency, following the mechanical degradations. The presence of salt in PEO and XG solutions reduces the maximum DR, DRmax, as well as the time to achieve it. In contrast, the DR does not significantly change over the time for PAM solutions upon the addition of salt.