Turbulent Pipe Flow Drag Reduction of Dilute Polymer Solutions As a Tool for Prediction of Weakly Viscoelastic Fluid Behavior in Extensional Flow Fields

Turbulent pipe flow drag reduction of high molecular weight, water soluble polymers in dilute solution has been studied for over 50 years with no definitive resolution as to the mechanism for the drag reduction. What has been shown by many researchers is that significant drag reduction (up to 85%) can be obtained with very small amounts (< 50 ppm) of high molecular weight, flexible polymers, at very low concetrations (below the entanglement concentration c*), and that this behavior is somehow related to the extension of the molecule in the turbulent flow field. This has lead to numerous theories related to length, time, and energy scales.

Polyethylene oxide (PEO) has been the most studied of these water soluble polymers becasue it can be obtained commercially in a variety of high molecular weights (> 1 million). However, these PEO materials also have a broad molecular weight distribution and often a high molecular weight tail (> 5 million). The goal of these studies was to attempt to determine if the effectiveness of PEO in turbulent pipe flow drag reduction could be related to it's molecular weight or molecular weight distribution. The ultimate goal would be to determine if the turbulent pipe flow drag reduction apparatus could be used to characterize or predict the behavior of weakly viscolelastic fluids in various extensional flow applicationssuch as wiping and flushing of surfaces, high speed water jets, inkjet printing, etc. To date, it has been very difficult to charatcterize the extesnional flow behavior of weakly viscoelastic fluids at very low polymer concentrations with shear viscosities near those of water.

Data on fanning friction factor (f) and %drag reduction (%DR) vs. Reynolds Number (Re) in turbulent pipr flow will be presented for several PEO materials in the dilute regime (50ppm to 1ppm). Multiple passes through the system have been used to study the decrease in %DR with the apparent molecular weight degradation. PEO materials of varying initial molecular weight were studied. An attempt will be made to correlate all these results with the effectiveness of these weakly viscolelastic fluids in commercial applications.