(358e) Turbulent Flow of Diutan Biopolymer Solutions and Carbon Nanotube Suspensions in a 4.6 mm ID x 200 L/D Smooth Pipe

Turbulent flow of dilute aqueous solutions of a polysaccharide biopolymer, Diutan, and dilute aqueous suspensions of multi-walled carbon nanotubes 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, MW ~ 5.0E6, had end-to-end length Le ~ 6.0 um, formal contour length Lc ~ 11.7 um and chain diameter Dch ~ 1.5 nm. 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 = 5000, flow enhancements relative to solvent S' = [(1/√f)_p - (1/√f)_n]_Re√f increased almost linearly with increasing concentration, with intrinsic flow enhancements [S’] = Lim_c→0[S’/c] = 0.10±0.02.

Industrial grade multi-walled carbon nanotubes, abbr MWCNT, of average diameter and length (Dch, Le) = (12 nm, 30 um), were used. Suspensions of concentration C from 1 to 1000 wppm were made in deionized water containing 900 wppm of polyvinylpyrolidone surfactant. On account of their scarcity, nanotube suspensions were tested in brief “scanning” runs of 30 s duration at each of four Re = (8300, 16700, 33000, 67000). First, two scans at C = 0, that is, of the 900 wppm PVP surfactant solution alone, served to establish a basis, yielding P-K results of 1/√f (11.2±0.1, 12.3±0.1, 13.3±0.1, 14.5±0.1) at Re√f (790, 1440, 2670, 4890) respectively. Thereafter, scans of MWCNT suspensions at each of c = (1.0, 10, 100, 1000) wppm provided sets of 1/√f (11.2±0.2, 12.2±0.1, 13.2±0.1, 14.4±0.1) at Re√f (770, 1450, 2650, 4850) respectively that were virtually indistinguishable from the basis scans at c = 0 wppm. Suspensions of C = 1 to 1000 wppm MWCNTs in 900 wppm PVP surfactant thus exhibited Newtonian turbulent flow behavior.

Under similar flow conditions, solutions of Diutan biopolymer, with Le ~ 6 um and aspect ratio Le/Dch ~ 4000, exhibit Type B turbulent drag reduction with intrinsic slip [S’] = 0.10, whereas suspensions of multi-walled carbon nanotubes, with Le ~ 30 um and aspect ratio Le/Dch ~ 2500, both the same order of magnitude as Diutan, exhibit Newtonian turbulent flow, without any hint of drag reduction. It would be most interesting to divine what skeletal attribute(s) allow(s) drag reduction by Diutan biopolymer but forbid(s) drag reduction by carbon nanotubes.