Viscoelasticity of a surfactant and its drag-reducing ability

Abstract

There is considerable interest in the use of viscoelastic cationic surfactant-counterion mixtures in district heating and cooling systems to reduce pressure losses. A recent field test in a secondary system near Prague showed a 30+% reduction in pumping energy requirements.

We have studied a number of commercial surfactants and we report here results of rheological, drag reduction and turbulence measurements on Arquad 18–50 (octadecyl trimethyl ammonium chloride (AR 18)) with an excess of sodium salicylate (NA). The concentration studied was 1.6 mM AR 18 and 4.0 mM NA which is about one third the concentration for excellent drag reduction in this surfactant's effective temperature range 30–90°C.

Viscosity, η, vs. shear rate,D, first normal stress difference,N ₁, vs. shear rate, drag reduction (as pressure drop,i=ΔP/1) vs. average velocity,U _ave, in a 39.4 mm tube for AR 18, and turbulence intensity data for three drag reducing surfactants are reported.

Of particular interest are the generally low turbulence intensities in all three directions which correspond to reduced heat, mass and momentum transfer rates compared to water, and the existence of large normal stress differences at 20°C for AR 18, a temperature at which no drag reduction occurs with this surfactant, indicating that normal stress effects do not correlate directly with drag reduction.

The effect of time of pumping on increasing drag reduction demonstrates that this factor overwhelms the expected increase in drag reduction as temperature is raised from 18–19°C to 40.5°C.