(296f) Drag Reduction and Degradation of Diutan Biopolymer Solutions during Turbulent Flow in a 4.6 Mm ID x 200 L/D Smooth Pipe

Turbulent flow of dilute aqueous solutions of a polysaccharide biopolymer, Diutan, was explored experimentally in a 4.6 mm ID x 200 L/D smooth pipe. The test pipe was seamless stainless steel with electropolished bore, of internal diameter and bore roughness (D, k) = (4.60 mm, 0.18 microns), and comprized 7 segments, each of L/D = 30 with a pressure tap near its downstream end. It was installed in a single-pass, progressing cavity pump-driven flow system, fed from a 200 liter tank. Pipe Reynolds numbers varied from 8000 to 80000 and pipe wall shear stresses from 8 to 600 Pa. Friction factors for deionized water in all pipe segments adhered to the Prantdl-Karman law within ±0.2 units of 1/√f, indicative of fully-developed, hydrodynamically smooth turbulent flow.

The Diutan biopolymer, of MW ~ 4.0E6, is a double-helix, with contour length Lc ~ 4.5 um, chain diameter Dch ~ 1.8 nm and aspect ratio Lc/Dch ~ 2600. Aqueous Diutan solutions of concentrations C from 1 to 100 wppm exhibited Type B drag reduction, characteristic of extended macromolecules, yielding turbulent flow segments roughly parallel to, but displaced upwards from, the Prandtl-Karman law, the more so with increasing concentration. At fixed Re√f = 2500, flow enhancements relative to solvent S' = [(1/√f)_p - (1/√f)_n]_Re√f increased almost linearly with increasing concentration, providing intrinsic flow enhancement [S’] = Lim_c→0[S’/c] = 0.10±0.02.

The experiments were also able to detect turbulent flow-induced Diutan degradation, that has not previously been reported. Degradation was visible as a slight downward dip of the overall P-K friction factor data at the highest flowrates and also as a slight dip of downstream relative to upstream points in each data cluster. These yielded a quantitative Diutan "degradation modulus" (kdeg/Tw) = 0.0035 +/- 0.0005 (1/Pa s), which is ~ 1/3 of previously known moduli for conventional vinyl-backbone polymers, such as PEO W309 MW 11E6 (kdeg/Tw) = 0.011 +/- 0.002 and PAMH B1120 MW 18E6 (kdeg/Tw) = 0.012 +/- 0.001. The foregoing show the Diutan biopolymer to be ~ 3 times more resistant to degradation than the PEOs and PAMHs commonly used for turbulent drag reduction.