Polymer-induced drag reducing flow has been investigated for over 60 years. One reason for this is that the drag reducers in flow systems have been successfully applied and represent a great potential benefit to many industrial processes. However, the phenomenon is not completely understood and many aspects of the problem remain unclear. Some important issues are related to the development of turbulent structures and to the breaking of the polymer molecules. These two phenomena impose a transient behavior on the polymer efficiency and the drag reduction, DR, can be clearly divided into three periods of time. Over time, at the very beginning of the test, DR assumes a minimum value (sometimes negative) before reaching its maximum efficiency. When degradation becomes important, DR starts to decrease until it achieves its asymptotic value, a time in which the polymer scission stops and the molecular weight distribution reaches a steady state. In the present paper, we study the drag reduction development from the very beginning of a turbulent flow into a rotating cylindrical double gap device. DR is induced by three different polymers: Poly (ethylene oxide) (PEO), Polyacrylamide (PAM) and Xanthan Gum (XG). The first two are known as flexible molecules while the last one is considered rigid. The goal here is to compare the effect of the different polymers on DR over time, paying particular attention to the difference between the rigid and the flexible molecules. The tests are conducted for a range of Reynolds numbers, concentrations and temperatures, from the very start to the time when the drag reduction achieves its final level of efficiency. The time to achieve the maximum efficiency is an increasing function of concentration and decreases with Reynolds and temperature in PEO solutions. Such time seems to be very short for the other polymers, less than 3s. It is worth noting that no loss of DR was observed for high concentrations of PAM, which suggests that PAM is more resistant than PEO. It is also shown that DR induced by XG is qualitatively different from that of the other agents. XG's solution is highly influenced by a pre-shearing, which suggests the existence of polymer aggregates. In addition, it seems that degradation do not occurs for solutions of XG. The observed loss of efficiency in high concentrations is, possibly, caused by de-aggregation during the test. (C) 2013 Elsevier B.V. All rights reserved.
