Reduction and turbulence characteristics in sub-zero temperature range of cationic and zwitterionic surfactants in EG/water solvent

The addition of small amounts of surfactant to a solvent in turbulent flow can reduce friction drag greatly. A new approach to energy savings in district cooling systems is to use surfactant solutions at subzero temperatures as the cooling fluids. In this paper, experiments were conducted to study the drag-reduction performances of a commercial cationic surfactant solution (oleylmethylbiss-hydroxyethyl ammonium chloride) and a newly synthesized zwitterionic surfactant solution (oleyl trimethylaminimide) in a two-dimensional channel. For testing the drag reduction at subzero temperatures, for which little data have been reported and none in channel flow, a 20% ethylene glycol aqueous solution (EG/W) was used as a solvent. The surfactant concentration ranged from 50 to 1000 ppm and the temperatures were -5 and 25°C. The turbulence features of the surfactant drag-reducing flow were measured by using the PIV (particle image velocimetry) technique. It was found that both surfactant solutions showed drag-reducing characteristics, which were affected by concentration and temperature. The maximum drag reduction was 83% at 25°C for the 200 ppm zwitterionic surfactant solution. Compared with EG/W turbulent flow, the addition of surfactant suppressed the turbulent intensities and vortex fluctuations and the Reynolds stresses were zero. The effects of addition of NaNO₂ to the surfactant solutions were also investigated.