Scaleout of Pipeline Drag Reduction by Wall-Injected Polyox to a Flat Plate Boundary Layer

Turbulent drag reduction by wall-injected Polyox solutions was studied by simultaneous axial pressure profile and flow visualization measurements at Reynolds numbers from 50000 to 100000 in a transparent test pipe of ID 16 mm and L/D 430. Friction factors for deionized water solvent adhered to the Prandtl-Karman law for fully-developed turbulent flow in smooth pipes. The P309 Polyox, of MW ~ 8x106 with Nbb ~ 0.5x106 backbone chain links, was injected as a concentrated solution Cinj =1000 wppm through 6 equi-spaced circumferential ports at friction-normalized injection velocities Vinj+ ~ 1 << pipeline mean velocity V+ ~ 25. Downstream polymer concentrations Cline varied from 0.5 to 20 wppm, exhibiting eventual fractional drag reductions DRline from 0.15 to the asymptotic maximum drag reduction MDR ~ 0.80 possible at the present Re. Polymer dissolution into the flow was assessed from the initial increase in fractional drag reduction towards the eventual downstream DRline, which provided a characteristic development distance (L/D)dr and also from flash photography which provided a characteristic distance (L/D)pv where red-dyed strands of the injected solution disappeared. Finally, an attempt was made to “scaleout” the pipeline drag reduction observations to a flat plate boundary layer by treating the latter as an equivalent pipe flow with UCL = Uo, the free-stream velocity, R+ = del.uô/í , with del the thickness and uô friction velocity at the boundary layer trailing edge L, and Type B slip S' taken from pipe flow results at concentration Cbl corresponding to the polymer expenditure rate per unit of wetted area.