Polymer thread injection, drag reduction

A polymer of the polyacrylamide type was injected as a 0.5% solution from an axially-placed nozzle at the bellmouth entrance. The experiments showed that the central thread provided drag reduction almost equivalent to premixed solutions of the same total polymer concentration flowing in the pipe. Overall concentrations of 1, 2, 4, and 20 ppm were used. Moreover, the effects were additive 2 ppm thread overall concentration plus 2 ppm premixed gave drag reductions equivalent to 4 ppm of either type. Reynolds numbers of up to 300,000 were investigated. In other experiments, a number of different polymer fluids were injected on the centerline of a water pipe-flow facility [857]. Two distinct flow regions were identified ... 

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 [856]. 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. 

Experiments are reported in which.a concentrated polymer solution is injected into the centre of a turbulent pipe flow. Drag reduction is obtained even if the polymer forms a liquid thread which is conveyed in the core region of the flow, i.e. no significant part of the injected polymer is present in the near-wall region. This type of drag reduction differs from that found in homogeneous solutions and seems to be due to an interaction between the polymer thread and the large-scale structure. 

If the concentration of the injected polymer solution is decreased, drag reduction increases much more quickly from the injection point. This may be explained by the fact that no polymer threads or strings are observed. Therefore the polymer is transported very quickly to the wall region. 

Fig. 4. Experimental setup for measurement of (a) drag reduction and (b) tracer particle trajectories (c) profiles of turbulent intensity in the longitudinal direction A, Water O, polymer injection (water phase) , poisoner injection (polymer thread) , premixed...

Hoyt, J. W. and Sellin, R. H. J., Drag reduction by centrally-injected polymer threads, Rheol. Acta 27 518 (1988). 

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