Polymer degradation of dilute solutions in turbulent drag reducing flows in a cylindrical double gap rheometer device
Résumé
The drag reduction by high molecular weight polymer additives in a turbulent flow is an important phenomenon that has received the attention of a number of researchers. However, the efficiency of those additives is not constant. Turbulence is also responsible for breaking the polymer molecules, decreasing their ability to reduce drag. This degradation phenomenon has recently received its deserved attention in the literature and investigations that take into account the effect of concentration, molecular weight, Reynolds number, and temperature can be found, although these parameters have not yet been explored in very wide ranges. In the present work we investigate this degradation phenomenon for aqueous solutions of two different polymers: Polyacrylamide (PAM) and Polyethylene oxide (PEO), in a cylindrical double gap rheometer device. The dependence of degradation on molecular weight, concentration, temperature, and Reynolds number is analysed for a wide range of these parameters. Our main results are displayed in terms of drag reduction (DR). All tests are performed to compute DR for a long period of time including the values obtained from the very beginning of the process. It is shown that DR increases with time until achieving a maximum value before starting to decrease as a consequence of degradation. We also display the results using a relative drag reduction quantity, DR′, defined as the ratio of the current drag reduction to the maximum one obtained for a non-degraded solution. We propose an alternative decay function that relates DR′ as a function of the Reynolds number, concentration, molecular weight, and temperature.