433241 Pipe Flow Development and Polymer Degradation during Turbulent Drag Reduction

Wednesday, November 11, 2015: 2:15 PM
150A/B (Salt Palace Convention Center)
Preetinder Virk, Mass. Institute of Technology, Cambridge, MA

Turbulent drag reduction, flow development, and degradation of aqueous polyethyleneoxide solutions was studied by axial pressure profile measurements in a smooth, segmented, pipe with electro-polished bore of ID 4.58 mm and overall L/D 180.  The test pipe assembly, of 6 identical segments, each of L/D 30 with a 0.38 mm ID pressure tap 2.4 L/D from its downstream end, was placed in a single-pass progressive cavity pump-driven flow system fed from two 180 liter tanks that held premixed polymer solutions.  A polyethyleneoxide polymer, Polyox N60K of MW 3.2x106, was used at concentrations C from 1 to 500 wppm in deionized water DW solvent.  

Individual friction factors for DW solvent at any fixed flowrate were constant to within 1.2% for all tap pairs, and all DW friction factors adhered to the Prandtl-Karman law 1/√f = 4.0 log Re√f - 0.4 within ±0.2 1/√f units for 300 < Re√f < 6000.  Thus the present solvent flows were all fully-developed by the time they reached the first tap pair 1&2 at mean L/D = 42, that is, solvent “entrance lengths” were Le,n/D < 42, in accord with established results of Le,n/D = 25 to 40 for turbulent Newtonian pipe flow.

Drag reduction by polymer solutions, quantitatively described by the flow enhancement  S’ = (1/√fp – 1/√fn)Re√f, varied over the entire possible range 0 < S’ < S’mdr, from S’ ~ 0, near onset on the Prandtl-Karman law, to S’ ~ 17, close to the maximum drag reduction asymptote 1/√f = 19.0 log Re√f - 32.4.  Three kinds of S’ vs L/D behavior, described by examples, could be discerned.  (i) At low C and low Re√f, example C = 1 wppm and Re√f < 2500, S’ was essentially independent of L/D, with flow development akin to solvent, Le,p/D ~ Le,n/D. (ii) At low C and high Re√f, example C = 1 wppm and Re√f > 2500, S’ was highest initially (L/D = 42) and decreased monotonically with increasing L/D, reflecting polymer degradation beyond a “falloff point” at Re√f^ ~2500.  (iii)  At high C and all Re√f, example C = 200 and 500 wppm and 600 < Re√f < 2500, S’ was lowest initially, increased with increasing L/D, and asymptotically attained a constant S’ ~ S’mdr.  A normalized summary of the preceding, (S’/S’mdr) = (0.70, 0.93, 0.97, 1.00, 1.00) at L/D = (42, 72, 102, 132, 162) suggest an entrance length of Le,mdr /D ~ 102 at maximum drag reduction, (S’/S’mdr) → 1.

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See more of this Session: Turbulent and Reactive Flows
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