Dynamics of flow-induced structure formation in drag reducing polymer solutions.

摘要

High molecular weight, flexible chain polymers are particularly effective as drag reducing agents in aqueous liquids at concentrations as low as a few parts per million by weight (ppm). Current research focuses on evaluating the roles of polymer concentration, solution composition, and shear flow history on the rheological and rheo-optical responses of several drag reducing systems, partially hydrolyzed polyacrylamide (HPAM), polyacrylamide (PAM), and poly(ethylene oxide) in water and salt solutions.; The steady state shear viscosity of the HPAM solutions in deionized water at 25°C for concentrations from 10 ppm to 4000 ppm demonstrate classic shear thinning for short shearing times. However, at or above 500 ppm, torque continuously rises over longer shearing times (∼5 minutes) suggesting aggregate formation.; Using a Couette flow cell, simultaneous measurement of turbidity, viscosity, and bire-fringence or dichroism and small-angle scattering patterns found two concentration regimes for HPAM in DI water. In the concentration range from 100 ppm to 1500 ppm, above a critical shear rate, solutions exhibit flow-induced turbidity, followed by near complete relaxation of the turbidity upon flow cessation. At higher concentrations, the solutions exhibit irreversible turbidity, reflecting the formation of liquid droplets during flow. All solutions exhibit irreversible precipitation upon storage after being sheared.; Characteristic length scales are determined from Mie theory for the transmittance data while the light scattering patterns, via the structure factor, provide a length scale, shape and orientation of the heterogeneous centers observed. Both interpretations demonstrate micron scale scattering structures being formed.; Work on the effects of strong shear on the molecular weight stability finds small degradation in the polymers' average molecular weight and molecular weight distribution. Experiments with HPAM and PAM solutions in a large channel flow loop at Re ≥ 20,000 provide evidence of the importance of chain degradation to changes in drag reduction with time. Losses in molecular weight are small in the case of HPAM and do not correspond to losses in drag reduction. The PAM solutions demonstrate no change in the molecular weight distribution over the first 13–16% loss in DR followed by a loss in the high molecular weight tail at greater losses in DR.