Correlations among surfactant drag reduction, additive chemical structures, rheological properties and microstructures in water and water/co-solvent systems

Under appropriate conditions, surfactants in water are known to self-assemble into threadlike micelles which reduce the drag of the solution in turbulent flow compared to that of the water solvent at the same flow rate. The phenomenon is called turbulent drag reduction (DR). Using surfactant DR additives (DRA) can save up to 70% pumping energy in turbulent pipe flow water circulating systems, such as district cooling/heating systems, in which a large amount of water is temperature controlled in a central station and recirculated within a district to heat/cool the buildings therein. A new approach to energy saving in district cooling systems is to replace water with 20% ethylene glycol (EG) in water as the cooling medium, which can be cooled down to -5°C (compared to 5°C for water). The coolant typically warms up to 15°C and is then returned to the central station for recooling. The temperature difference for the 20%EG/W medium is 20°C (-5°C to 15°C), twice as much as the 10°C for water (5°C to 15°C), increasing its cooling capacity and reducing the amount of recirculating coolant and pumping energy needed by about 50%. Pumping energy could be reduced by an additional 50% if effective surfactant DRAB can be used in such mixed solvents. However, co-solvents such as EG are known to inhibit micelle formation which may decrease the effectiveness of DRAs compared to pure water systems. This study investigated and developed effective surfactant DRAs in several water/co-solvent systems at low temperatures. DR, rheological, cryogenic transmission electron microscopy (cryo-TEM) and 1H NMR experiments are being carried out to develop correlations among DR, rheological properties and micelle microstructures. In addition to the practical application in district cooling systems using EG-water mixed solvent or other co-solvent systems, the results of this study provide more fundamental understanding of the effects of solvent properties on threadlike micelle microstructure, drag reduction and system rheology, which are poorly understood now.