Quantitative description of polymer drag reduction: Effect of polyacrylamide molecular weight distributions

•Long-term experiments on polymer degradation in turbulent pipe flow.•Comparison of DR in rotational and pipe flow.•Relation of molecular weight distribution and drag reduction.•Mw sets minimum drag reduction.•High molecular weight tail drives overshoot. The effect of molecular weight distribution of polyacrylamide (PAAm) on drag reduction was studied in two flow geometries. Commercial PAAm with different weight averaged molecular weights (Mw = 5 × 105 to 1.8 × 107 g/mol) were investigated in turbulent pipe and rotational flows. Comparison of PAAm with different molecular weight distributions showed that drag reduction is not only a function of the averaged molecular weight. Broader polymer molecular weight distributions provided increased drag reduction over polymers of same average molecular weight but with a more narrow distribution. The role of distribution widths is of significance as polymer degradation in turbulent flows causes narrowing of the molecular weight distributions. Multiple linear regression was employed to connect weight fractions of polyacrylamide with drag reduction. Multiple linear regression was successfully applied to describe drag reduction in turbulent pipe and rotational flows indicating that drag reduction can be quantitatively derived from the molecular weight distribution of PAAm. [Display omitted]