A review of studies of heat transfer enhancement in turbulent drag reducing surfactant solutions

Abstract

Drag reduction in turbulent flow of hydrocarbons containing small amounts of high polymer was first reported by Toms about 70 years ago. Previously, Mysels and his coworkers had observed similar behavior in solutions of aluminum disoaps. A few years later, drag reduction behavior was observed in dilute aqueous-surfactant solutions in which long wormlike micelles were present. In the late 1970’s this phenomenon found its first commercial application when high molecular weight polymer was added to crude oil flowing through the 800-mile Alyeska pipeline. Crude flow was increased by about 25% with no additional pumps. However high molecular weight polymers are not suited for use in recirculation systems because the high shear encountered in pumps breaks the primary chemical bonds within the polymer chains. The resulting low molecular weight polymer chain fragments are not efficient drag reducers, and they do not reassemble. On the other hand, surfactant micelles are held together by secondary forces and they reform (self-associate) very quickly after break-up in high shear regions (pumps). Thus, they are effective in recirculation systems. District heating systems are widely used to heat buildings in urban locations in northern Europe and are also found in the US, Canada, Eastern Europe and other locales. These systems circulate hot water and exchange heat with each building thus relieving the buildings of the need for heat sources (furnaces) and the related investment, space, and maintenance required. They generally utilize cheap waste heat from nearby power plants to heat the circulating water. District cooling systems with the same advantages are utilized in some warm climate regions, particularly the United States and Japan. Adding a drag reducing surfactant additive to the recirculating water could decrease pumping energy requirements of these systems by 50% or more. There is, however, a serious problem with this scheme as the large reduction in friction loss is accompanied by an even larger reduction in heat transport. Thus, to utilize drag reducing surfactant additives in district heating or cooling systems, heat transport must be enhanced in order to transfer heat to or from each building’s internal recirculation system. Investigators have studied a number of heat transfer enhancement schemes to overcome this problem focusing on temporarily destroying the drag reducing micelle structure at the entrance to the heat exchanger using static mixers, honeycombs, other obstructions, ultrasonic radiation, UV radiation of photosensitive surfactant systems, etc. and others have studied altering the turbulent flow pattern in the heat exchanger. While some of these were effective, generally the required energy input was too great for them to be of practical value. These studies will be reviewed here.