(577f) Macroscale Insensitivity of Type B Drag Reduction By Two Biopolymers | AIChE

(577f) Macroscale Insensitivity of Type B Drag Reduction By Two Biopolymers

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Turbulent drag reduction by distilled water solutions of two polysaccharide biopolymers, Diutan and Xanthan, was studied in three electropolished pipes of 4.57, 7.82 and 15.8 mm ID. The experiments encompassed solvent-based Reynolds numbers from 2000 to 200000, pipe wall shear stresses from 1 to 900 Pa, and biopolymer concentrations from 2 to 2000 weight parts per million. Solutions of both biopolymers exhibited Type B drag reduction, characteristic of extended macromolecules, in all three pipes. Low concentrations, c < 200 wppm, yielded turbulent flow segments roughly parallel to, but displaced upwards from, the Prandtl-Karman law, resembling ladder rungs that ascended with increasing concentration. High concentrations, c > 500 wppm, exhibited a continuous transition from shear-thinning laminar flow at low Re to turbulent flow along the maximum drag reduction asymptote at high Re. At fixed high Re√f = 5000, low concentration flow enhancements relative to solvent S' = [(1/√f)p - (1/√f)n]Re√f increased almost linearly with increasing concentration, while the highest concentration solutions all exhibited flow enhancements S' ~ 20, approaching the asymptotic maximum drag reduction. At the same fixed Re√f =5000, intrinsic flow enhancements [S’] = Limc→0[S’/c] (1/√f units/wppm) in each of the 4.57, 7.82, 15.85 mm ID pipes were, respectively, for Diutan [0.12±0.01, 0.12±0.03, 0.12] and for Xanthan [0.12±0.02, 0.13±0.02, 0.11±0.03]. The invariance of intrinsic slip [S’] with respect to pipe diameter for each biopolymer implies, more generally, that Type B drag reduction is insensitive to turbulence macroscale. This newly-found macroscale insensitivity combined with long-known microscale insensitivity suggest that Type B drag reduction is insensitive to turbulence parameters, being mainly controlled by parameters of the extended macromolecular additive. It is also curious that both biopolymers induced virtually identical intrinsic slips [S’] = 0.12±0.03, despite Diutan having the higher molecular weight M = 5.0E6 vs 2.5E6 and longer formal contour length Lc ~ 11.7 mm vs 3.2 mm relative to Xanthan, suggesting an influence of other skeletal attributes, perhaps stiffness and aspect ratio.

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