Análise experimental da redução de arrasto em soluções com polímeros rígidos e flexíveis em uma geometria rotativa desde os primeiros instantes do escoamento turbulento: efeitos de número de Reynolds, concentração, massa molecular, temperatura e salinidade marinha

Abstract

The drag reduction by the dilute addition of high–molecular weight polymers in a turbulent flow has been extensively studied since the phenomenon wasfirst observed over 60 years ago. Over the yearsreducers in flow systems have been successfully applied and represent a great potential benefit to many industrial processes. However, the phenomenon is not completely understoodand many aspects of the problem remain unclear. Some important issues are related to the development of turbulent structuresafter the additive injectionand to the breakingof the polymer molecules. These two phenomena impose a transient behavior on the polymer efficiency. Over time, at the very beginningof the test, drag reduction (DR)assumes a minimum value (sometimes negative) before reaching itsmaximum efficiency. When the degradation becomes important, DRstarts to decrease until itachievesits asymptotic value, a time in which the polymer scission stops and the molecular weight distribution reaches a steady state. In the present work isstudiedthe drag reduction development from the very beginning of a turbulent flow into a rotating cylindrical double gap device. The DRis induced by three different polymers: Polyethylene Oxide (PEO), Polyacrylamide (PAM) and Xanthan Gum (XG). The first two are known as flexible molecules while the last one is considered rigid.The tests are conducted for a range ofReynolds number, polymer concentration, molecular weight, temperatureand synthetic sea saltconcentration. The initial results discuss the very beginning of the turbulent flow in which DRpresents negative values,due to gain of extensional viscosity caused by polymer stretching. The remaining results discuss the use of synthetic seawater as a solvent.In this case the onset of drag reduction occurred at higher Reynolds number and had significant influence on the extent of drag reduction on PEO and XG solutions