DRAG CHARACTERISTICS OF A DRAG-REDUCING SURFACTANT SOLUTION FLOWING OVER A SUDDEN-EXPANSION PIPE

The minor loss characteristics of a drag-reducing surfactant solution flowing over a circular sudden-expanded pipe have been investigated experimentally with an expansion ratio of 1:1.52. The surfactant used is cetyltrimethyl ammonium bromide (CTAB) with concentrations of 1×10 ^{- 4} and 2×10^-4 by weight. The maximum drag reduction rate for both solutions is achieved 70% in the fully developed flow in straight pipes. But at lower inlet Reynolds numbers than the critical one, the expansion loss coefficient is only 10%~20% below that for water, while at inlet Reynolds numbers much higher than the critical one, it is found to be much greater than that for water and to approach 1.5 times one for water at the Reynolds number at which the friction factor reaches that for water. Furthermore, a much longer distance is required for the micelle solution flowing across the sudden-expanded step, than 7.8 times the diameter (45 times the step height) of expansion-downstream pipe for water in order to reform a fully developed flow in the downstream. And as inlet flow for the solution of concentration 2 ×10^-4 loses its drag-reducing efficiency, approximately 158 times diameter (920 times the step height) of the expansion downstream pipe is necessary for reforming the fully developed drag-reducing flow in the downstream. From the present rheological measuring results for the surfactant solutions, the drag and its development behaviour of the sudden expansion pipe can be considered to be closely related to the time characteristics in forming and relaxing of the netlike micelle structure induced by shearing.