(90d) Differential Kinetics of Polyox Degradation During Drag Reduction in a Smooth Pipe

The effectiveness of drag-reducing polymer additives invariably decreases with increasing downstream distance, on account of polymer degradation by scission in the turbulent flow. The specific objective of the present work was to differentiate between the degradation of polymer in the entrance region and the degradation that occurs in the downstream test pipeline. The importance of distinguishing between these two types of degradation is that only the latter, "intrinsic", degradation in the test pipeline section is meaningful for scaling up laboratory results to real pipelines. The present experiments employed 0.2 to 20 wppm aqueous solutions of a polyethyleneoxide (PEO) polymer, molecular weight ~5x106, flowing through a smooth 5 mm ID test pipe, with electro-polished bore, of length about 180 diameters, made up of six pressure-tapped segments. The test pipe was part of a single-pass progressive cavity pump-driven flow system fed from two 180 liter tanks that held premixed polymer solutions. Pressure differences between each of six tap pairs were measured by a transducer and logged on a data acquisition computer. Degradation was sensitively detected as a bifurcation between the pressure drops measured between upstream (j) and downstream (k) segments of the test pipe at fixed flowrate. In the polymeric regime of drag reduction, degradation was found to be insignificant from onset to a characteristic "falloff" wall shear stress ?äw^ and became significant thereafter with the ratio of falloff to onset wall shear stresses ~5 essentially independent of polymer concentration. The severity of degradation, measured by the estimated ratio of (still-active/original) polymer concentrations at a section, was found to be essentially independent of polymer concentration but increased significantly with increasing wall shear stress. At a fixed wall shear stress, for example Tw = 200 Pa, corresponding to solvent Re ~ 50000, the apparent first-order degradation rate constant kapp ~ 2 s-1. An attempt is made to physically interpret these degradation results in terms of the polymeric sublayer model of drag reduction.