(75c) Polymer Drag Reduction in Taylor-Couette Flow: Routes for Improved Shear Stability

Adding high molecular weight
polymers to fluids can significantly reduce frictional drag when in turbulent
flow. However, the good drag reduction performance is often limited by the
time- and shear-dependent stability of the polymer, with polymer degradation
leading to reduced drag reduction. Reduced performance can be partially
mitigated by frequent dosing of the transport fluid, although routes to resist
degradation are desired. Currently, the mechanisms of polymer degradation are poorly
understood, and therefore routes to overcome such issues have resulted in
little clarity.

In
the current study, the drag reduction performance of a high molecular weight (>
1 MDa) polymer, Poly(2-dimethylamino)ethyl methacrylate) methyl chloride
quaternary salt (Poly DMAEMAq), was measured using a Taylor-Couette flow
geometry (double-walled gap, TA Instruments DHR-II) up to Reynolds number 5,500.
From a stress ramp test, the onset of drag reduction (relative to the solvent) occurred
at a critical shear rate of ~840 s^-1. The polymer concentration to attain
the highest drag reduction of 19% was 40 ppm,
as shown in Fig. 1A. Further increases in polymer concentration led to no
further enhancement. While demonstrating good drag reduction (maximum limited
by Re,), the highly charged (cationic) polymer exhibited poor shear stability
(Fig. 1B), with the effect of drag reduction diminishing within the first 100 s
of the test at a range of Reynolds numbers (1,500 – 5,000). Varying the polymer
concentration from 1 ppm to 40 ppm had negligible influence on the shear
stability, and increasing the shear rate up to ~5,300 s^-1 (Re
= ~5,000 ) resulted in a rapid drop off in performance.

To
overcome the poor shear stability, the current study has considered the
influence of polymer-surfactant (CTAB) co-addition and solution salinity. The
surfactant CMC in the presence of an equimolar concentration of sodium
salicylate (NaSal) was measured to be ~50 ppm. Compared to a polymer solution
only, the co-addition led to an initial slight increase in the drag reduction at
shear rates < 1,500 s^-1, which can be attributed to the drag
reduction by surfactant worm-like micelles. At higher shear rates, the
surfactant no longer provides a contribution to the overall drag reduction with
values comparable to the polymer only. However, when measuring the shear
stability of the drag reducing agent, slight improvements in the time-dependent
performance were detected. This effect is thought to relate to the increased salinity
of the solution and a change in polymer conformation (i.e. coiling through
charge screening). The research discusses the potential synergistic effects between
polymer and surfactants/solution salinity to optimize the potential of the polymer
for drag reduction applications.