VISCOELASTICITY OF A SURFACTANT AND ITS DRAG-REDUCING ABILITY

There is considerable interest in the use of viscoelastic cationic sur factant-counterion mixtures in district heating and cooling systems to reduce pressure losses. A recent field test in a secondary system nea r Prague showed a 30+% reduction in pumping energy requirements. We ha ve studied a number of commercial surfactants and we report here resul ts of rheological, drag reduction and turbulence measurements on Arqua d 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 excellen t drag reduction in this surfactant's effective temperature range 30-9 0 degrees C. Viscosity, eta, vs. shear rate, D, first normal stress di fference, N-1, vs. shear rate, drag reduction (as pressure drop, i = D elta P/1) vs, average velocity, U-ave, in a 39.4 mm tube for AR 18, an d turbulence intensity data for three drag reducing surfactants are re ported. Of particular interest are the generally low turbulence intens ities in all three directions which correspond to reduced heat, mass a nd momentum transfer rates compared to water, and the existence of lar ge normal stress differences at 20 degrees C for AR 18, a temperature at which no drag reduction occurs with this surfactant, indicating tha t normal stress effects do not correlate directly with drag reduction. The effect of time of pumping on increasing drag reduction demonstrat es that this factor overwhelms the expected increase in drag reduction as temperature is raised from 18-19 degrees C to 40.5 degrees C.