Orifice Flows of Solutions of Drag-reducing Polymers

Abstract

ALTHOUGH it is widely believed that the drag reduction effected by polymers of high molecular weight in solution must be associated with the elasticity of such solutions, it has not proved easy to demonstrate an elastic property common to all polymers at the low concentrations producing drag reduction. Thus although the resistance of elastic solutions to acceleration (that is, extension) has been proposed as a mechanism for drag reduction¹, this effect has been reported as small for polyethylene oxide, and undetectable for guar gum, at concentrations giving considerable drag reduction^{2, 3}. The two materials used by Giles were nonionic in character. In our experience, very low concentrations of ionic polymers, such as many of the commercial “Polyacrylamides”, do noticeably increase the extensional viscosity of water, as shown by the reduction of the flow rate through orifices, but this effect cannot be related to the drag-reducing ability of the polymers as it is greatly reduced (like the shear viscosity) by salt additions (0.5 molar sodium chloride, for example) which barely affect the drag reduction.