Loss of efficiency of polymeric drag reducers induced by high Reynolds number flows in tubes with imposed pressure

Abstract

This paper studies the loss of efficiency of polymeric drag reducers induced by high Reynolds number flows in tubes. The overall pressure was fixed and the apparatus was built so as to minimize the polymer degradation. We used three kinds of polymers: two flexible and one rigid. We conducted our tests to take into account the drag reduction (DR) for a wide range of concentrations of each polymer. The main results are displayed for the DR as a function of the number of passes through the apparatus. The mechanism of the loss of efficiency for the Xanthan Gum (XG) solutions (the rigid one) seems to be completely different from that observed for Poly (ethylene oxide) (PEO) and Polyacrylamide (PAM) (the flexible materials). While the PEO and PAM mechanically degrade by the action of the turbulent flow, the XG seems to remain intact, even after many passes through the pipe flow apparatus. From the practical point of view, it is worth noting that the PAM solutions are clearly more efficient than the PEO and XG. Another practical point that deserves attention is concerned with the asymptotic drag reduction found for XG. Although its maximum DR was significantly smaller than that found for PEO, the final value for both polymers were quite the same, which is obviously related to the intensified mechanical molecule scission in the PEO solutions. Our results for the relative drag reduction (the current value of DR divided by its maximum obtained at the first pass) was quite well fitted by the decay function proposed in our previous paper [A. S. Pereira and E. J. Soares, “Polymer degradation of dilute solutions in turbulent drag reducing flows in a cylindrical double gap rheometer device,” J. Non-Newtonian Fluid Mech. 179, 9–22 (2012)], in which a rotating apparatus was used. This strongly suggests that the physical mechanism that governs the degradation phenomenon is independent of the geometry. We also used a degradation model for PEO proposed by Vonlanthen and Monkewitz [“Grid turbulence in dilute polymer solution: Peo in water,” J. Fluid Mech. 730, 76–98 (2013)] to fit our data of relative drag reduction for PEO and PAM.

The Role Played by the Aging of Aloe Vera on Its Drag Reduction Properties in Turbulent Flows

“…The pipe system was the same used in [60,55,16] (see Fig. 1 in [60]) When the total amount of solution, initially in the pressure vessel, had passed through the pipeline, one round was finished.…”

“…The pipe system was the same used in [60,55,16] (see Fig. 1 in [60]) When the total amount of solution, initially in the pressure vessel, had passed through the pipeline, one round was finished. After that, the solution, now partially de-aggregated, returns to the pressure vessel from the storage tank by gravity, in a very slow motion, to avoid any uncontrolled polymer degradation or de-aggregation (here mainly de-aggregation), and, finally, one cycle has been completed and the same solution is then used for the next cycle.…”

“…Regarding polymeric drag reducers, a huge obstacle to their practical use is the mechanical molecular degradation. There are many experimental evidences showing that, under extensional flows, the molecules are stretched and eventually break down, reducing the molecular weight and their efficiency as drag reducers [62,60]. A pioneering work focused on the effects of polymer degradation on drag reduction is [43].…”

“…The main characteristics of XG, a very known rigid polymer, as a drag reducing agent (DRA) were studied by several researchers such as [36], [38], [20], [72], [11], [29], [62], [45], [6], [60], [55], [7] and [45], using a rotating apparatus. Ref.…”

“…Ref. [60], using a pipeline system, conducted many experiments with PEO (polyethylene oxide) and PAM (polyacrylamide), which are both flexible polymers, and with XG (a rigid polymer), to show that the latter does not really degrade mechanically. The authors argue that the observed drop of efficiency is related to another phenomenon, called de-aggregation, as stated by [57].…”