Flow Properties and Heat Transfer of Drag-Reducing Surfactant Solutions

Abstract

The frictional resistance of fluids can be reduced by adding small amounts of certain polymers, a phenomenon first reported (Toms, 1948) known as drag reduction or Toms phenomenon. The added polymers might form thread-like structures in fluids, which interact with turbulent eddies due to their viscoelasticity. Since the polymer synthesis technology and cost effectiveness have been highly improved, polymer drag reduction has been adopted widely in large pipeline systems for crude oils and refined petroleum products. Presently, more than 40% of all the gasoline consumed in the United States has polymer drag reducer in it (Motier, 2002). However, mechanical degradation of polymer chains in high shear rate regions, such as pumps, is frequently observed, which lowers the molecular weight and causes a loss of drag reduction. For that reason, polymer drag reduction cannot be adopted for circulating flow systems. Drag reduction caused by surfactant solutions was first reported by Gadd (Gadd, 1966). Combinations of certain cationic surfactants with a suitable counter ion are often chosen as the drag-reducing agents. Some nonionic surfactants also show the drag-reducing effects, rendering the use of counter ions dispensable. A number of authors have pointed out that the surfactant molecules come together to form rod-like micelles, which are necessary for drag reduction. Figure 1 shows surfactant molecule and micelle structures. Drag-reducing surfactants form rod-like micelles, and their aggregates might be present in a solution. Figure 2 shows a transmission electron microscope (TEM) image of surfactant micelles (Shikata et al., 1988). Again, aggregates of rod-like micelles might interact with turbulent eddies and cause drag reduction. These aggregates suffer mechanical degradation in high shear rate regions, which is then repaired in lower shear stress regions, such as in flow through pipes.