Turbulent drag reduction is a striking phenomenon in which the presence of small quantities of additive (in some cases a few ppm) in a carrier fluid can reduce turbulent pressure losses by up to 90%. Highpolymer drag-reducing additives have been successfully used in many crude oil and finished petroleum product pipelines all over the world.
While useful in once-through applications such as pipelines, polymer additives are not suitable for recirculating flows as they are susceptible to irreversible (permanent) mechanical degradation in regions of high stress. For recirculation flows, additives which are not sensitive to degradation by shear or extensional flows are needed or, if they do degrade, their structures must recover or repair quickly. Many surfactant additives can recover from mechanical degradation in seconds and so are effective in recirculating flows.
To utilize low-cost energy or waste heat, closed-loop district heating is used in many cities in northern Europe, Japan, and the U.S. to heat homes, businesses, and factories and to provide hot water. Alternatively, large chillers can provide low-temperature water for circulation through a district cooling system. District cooling systems are becoming increasingly important in the U.S. and Japan. The use of surfactant drag-reducing additives in these systems conserves fuel and thus reduces pollutants entering the environment and also reduces the size of pumps and piping. They can also increase throughput. Preliminary field tests have been encouraging.
My current drag-reduction research is focused on cationic, zwitterionic, mixed surfactant, and nonionic surfactant/water and ethylene glycol/water systems suitable for use at temperatures to at least 100 C for district heating systems and from -5 to 15 C for cooling systems. Understanding the influence of the chemical structure of the surfactant on its micellar structure, drag-reducing efficiency, and temperature range and the influence of micellar size and shape on drag-reducing ability will permit tailormaking useful surfactants for these and other applications. To this end, rheological techniques such as normal stress, extensional viscosity, dynamic viscosity, and flow birefringence measurements, and NMR, SANS, and cryogenic transmission electron microscopy (cryo-TEM) are utilized to characterize surfactants solutions. An international collaborative research project sponsored by NEDO (Japan) to develop surfactant additivies for water/ ethylene glycol systems effective to -5C was completed recently.
In studies of the effects of chemical structure of cationic surfactants and of their counterions, my students and I have discovered unusual rheological and microstructure phenomena. We showed that non-viscoelastic surfactant solutions which are water-like in their rheological behavior could be drag-reducing and that the belief that thread-like surfactant micelle network microstructures in the quiescent state are required for drag reduction is not generally true. Vesicle systems can be transformed into dragreducing, thread-like structures under stress. We have also demonstrated that the limiting Friction Factor-Reynolds number drag-reducing asymptote for high polymers proposed by Virk many years ago is exceeded by some surfactant systems, and we have offered a new asymptote for surfactants as well as a new turbulent mean velocity profile asymptote. Recent work has focused on developing techniques to enhance heat transfer in drag-reducing solutions by temporarily degrading their microstructures in heat exchangers while allowing them to recover downstream.