Polymer drag reduction regeneration

Polymers are largely used as drag reducers in turbulent flows. One of the main problems is the fall in efficiency over time. Such a fall in efficiency is commonly attributed to mechanical degradation. We show here that another mechanism is quite essential: polymer de-aggregation. We present that de-aggregation is not permanent; molecules re-aggregate over time, partially regenerating the drag reduction. We conduct a sequence of tests with three kinds of polymers: two flexible (polyacrylamide and polyethylene oxide) and one rigid (xanthan gum). The solutions were pre-sheared under a high Reynolds number to force the drag reduction to fall. Independent of the kind of polymer tested, the re-aggregation is slow, taking days to reach its maximum level. The drag reduction for 150 ppm of PAM recovered 83 % of its initial value 21 days before pre-shearing. Interestingly, the level of drag reduction for the xantham gum solution recovered 100 %. When the drag reducers are flexible polymers, the mechanical degradation and de-aggregation are equally crucial for drag reduction (DR) fall. Differently, the fall of DR is exclusively related to de-aggregation in the case of xantham gum, further proving that this kind of polymer is rigid.