Drag reduction scale-up criteria.

Abstract

Available drag reduction correlations are presented and their utility for engineering scale-up to a scale-up from a correlation based on friction velocity and the availability of some turbulent flow data with the process fluid of interest is compared. A further purpose is to investigate the consequences of polymer modification of the mean flow profile in the outer layer. Emphasis is placed on accuracy and ability to portray the drag reducing activity of complex systems. In the case of very dilute polymeric systems in large conduits, very accurate predictions may be needed to assess the performance of a particular additive. Similarly, equations capable of detailed predictions of observed behaviour can provide needed insight into the mechanism of drag reduction. Inasmuch as the semi-logarithmic resistance law, e.g., the Seyer and Metzner model of turbulent viscoelastic flow, provides a scale-up criterion significantly more complicated to use than a number of other models, an engineering approximation to this model is described. It is shown that for accurate predictions to be made at modest levels of drag reduction activity, the model must be modified to compensate for thickening of the sublayer. (A)