Drag reduction in flow: Review of applications, mechanism and prediction

Drag-reduction effectiveness of xanthan gum in a rotating disk apparatus

A singular class of polymer reactions can be caused by mechanical stress. Sufficient storage of mechanical energy to break chemical bonds in the main chain is generally possible only on deformation of polymers of high molecular weight. The corresponding appropriate conditions of high stress may occur in both polymer processing and use. This review summarizes reports of such polymer stress reactions published principally after 1980. The survey is organized by polymer type and by analysis technique.

Recent studies of polymer reactions caused by stress

Mechanical degradation of dilute polymer solutions under turbulent flow

Turbulent drag reduction induced by dilute solutions of both water-soluble poly(ethylene oxide) (PEO) and oil-soluble polyisobutylene (PIB) under turbulent flow in a rotating disk apparatus was investigated. The concentration dependence of drag reduction for these systems was shown to obey an empirical drag reduction equation which has been previously applied to describe flow in circular pipes. Linear correlation between polymer concentration and drag reduction for different molecular weights of PEO and PIB was also obtained. The intrinsic concentration [C] was found to be very useful in normalizing the drag reduction data for different types of polymers, polymer molecular weights, concentrations, and rotational disk velocities. Explanation of the drag reduction phenomenon on a molecular level can be obtained from intrinsic drag reduction and [C].

Polymer-Induced Turbulent Drag Reduction

The flow-assisted degradation behavior of polystyrene was studied as a function of solvent, polymer concentration, molecular weight, and molecular weight distribution. To obtain data at concentrations as low as 100 parts per million by weight, turbulent drag reduction measurements were used to augment the usual analytical techniques of viscosity and gel permeation chromatography. Turbulent flow measurements were found to be a valuable technique for evaluating the effects of degradation: the drag reduction onset point provides information about the largest molecules in the sample while the flow rate dependence is related to the shape of the top part of the molecular weight distribution. For the polymers and flow conditions studied, the degradation causes a shift in the distribution to lower molecular weights with little change in the shape. This suggests a complex mechanism where the probability of bond scission is not random but varies along the polymer backbone.

D. L. Hunston

Papers

Flow-assisted degradation in dilute polystyrene solutions