Drag reduction by centrally-injected polymer “threads”

In the extensive literature on polymer drag reduction, it has occasionally been reported that a continuous thread of a high-concentration polymer solution injected into the axis of a pipe produces a drag-reduction effect on the water-flow in the pipe. The “thread” seems to persist through the length of the pipe and little if any diffusion of polymer to the walls of the pipe is apparent. All previous experiments have been at a relatively low Reynolds number, and the purpose of the present work was to evaluate the technique at pipe Reynolds numbers above 10⁵. Experiments were carried out in a 53 mm diameter stainless steel pipe 20 m in length, constructed to high tolerances. The polymer, Separan AP-203, was injected as a 0.5% solution from an axially placed nozzle at the bellmouth entrance. The experiments show that the central thread provided drag-reduction almost equivalent to pre-mixed solutions of the same total polymer concentration flowing in the pipe. Overall concentrations of 1, 2, 4, and 20 parts-per-million were used. Moreover, the effects were additive: 2 ppm thread overall concentration plus 2 ppm pre-mixed gave drag reductions equivalent to 4 ppm of either type. Reynolds numbers of up to 300 000 were investigated. Attempts to visualize the polymer thread were inconclusive. The dyed polymer thread was viewed through a window at the downstream end of the pipe. The predominate appearance was that of a dispersion of globules of high-concentration polymer.