Significance of polymer elasticity on drag reduction performance in dispersed oil-in-water pipe flow

Mechanisms proposed for the drag reduction by high molecular weight polymers range from stress anisotropy, elasticity and the formation of intermolecular associations. This experimental study tests the role of elasticity as indicated by the measurement of normal force, from which Weissenberg number can be calculated. Cationic drag reducing polyacrylamides were injected in a dispersed oil-in-water flow through 30.6-mm pipe. Four polymers were used to investigate the effect of polymer molecular weight and charge density. Three polymers with different molecular weights and 10% charge density were investigated along with a polymer with 45% charge density. The flow in the test section was set at 0.3 oil fraction and mixture velocities of 0.8, 1.0, 1.2 and 1.5 m/s (Reynolds number above 34,400) with polymer concentrations from 10 to 40 ppm. Generally, the maximum drag reduction was obtained for polymer concentrations in the test section higher than 20 ppm. It increased with flow velocity and polymer molecular weight. The rheological measurements indicated that the elasticity of the polymers increase with the increase in molecular weight and change density. Similarly, the maximum drag reduction increased with the increase in molecular weight and charge density, which indicates a correlation between the efficiency of the drag reducing polymer and its elasticity. Drag reduction increased also with the increase in mixture velocity (or Reynolds number). On the other hand, drag reduction was generally independent of the hydrodynamic volume of the polymer above intrinsic viscosity of 4700 cm³/g.