Polymer structures and turbulent shear stability of drag reducing solutions

Abstract

THE phenomenon of frictional drag reduction in the turbulent flow of Newtonian fluids, by the addition of high molecular weight polymers, has been known for some time. Because of the great potential it offers in various technological applications this phenomenon has generated considerable engineering interest. Unfortunately, when a limited volume of polymer solution is continuously exposed to mechanical shearing action, either by repeated use¹ or by passing through long pipelines², drag reduction rapidly declines, indicating a rapid breakdown of the polymer molecules³. This polymer degradation greatly limits the application of drag reducing polymers. When considering the use of high molecular weight polymers in operating systems their shear stability must be considered as equally important as their effectiveness in drag reduction. Polymer degradation effect is likely to be caused by the scission of molecular entanglements or the breaking of individual molecules induced by the shear stresses associated with very high local shear rates⁴. But previous studies were made mainly with linear polymers such as polyethylene oxide (PEO) and polyacrylamide (PAM), because a linear structure was believed to be most effective⁵. We have studied the effect of polymer structure on turbulent drag reduction by synthesising drag-reducing agents of different structures⁶. Here we present the results of an investigation of the degradation behaviour of a highly branched PAM, and compare the results with those observed with ordinary linear polymers.