Drag Reduction Does Not Always Increase with Increased Flow Velocity

Abstract

In view of industry's current thinking about drag reduction, an article by Avnapov et al., "The Effect of Adding Polyisobutylene on the Capacity of Pipelines," is of considerable interest. Their Fig. Pipelines," is of considerable interest. Their Fig. 2, when replotted (see Fig. 1) reveals that drag reduction is not a continuously increasing function of the flow velocity as has been postulated by others working in this field. This was revealed by the calculation of drag reduction for 50 ppm polyisobutylene (PIB) in gas condensate based on Curves 1 and 2 of Fig. 2 of the Russian article (see Table 1). These data, plotted in Fig. 1, show that the drag reduction increases as velocity increases up to a "critical" velocity of 9.8 ft/sec. As the velocity is increased beyond 9.8 ft/sec, the drag reduction gradually decreases. To my knowledge, this has not been shown elsewhere, perhaps because no one had tested for drag reduction at perhaps because no one had tested for drag reduction at such high velocities. The Reynolds number for a 0.8-cp, 0.8-gm/cc fluid flowing at 16 ft/sec in a 6-in pipe about 700,000 which is rather high. We have obtained experimental results similar to the above in a drag reduction test in which diesel fuel and gasoline were flowed through a 6-in.-diameter conduit at 8.7 and 11.4 ft/sec, respectively. TABLE 1 - DATA FROM AVNAPOV ET AL.* /1 (units unknown) Velocity Drag No PIB 50 ppm PIB Reduction cm/sec ft/sec (Curve 1) (Curve 2) (percent) 200 6.55 3.5 2.5 28.5 250 8.2 6.5 4.0 38.4 300 9.8 11.1 6.0 45.9 400 13.1 19.5 11.0 43.6 500 16.4 28.5 16.5 42.1 P. 673