Injection of Drag Reducing Additives Into Turbulent Water Flows: Results from Factorial Design

The basic objective of this experimental study is to optimize the process of injecting drag-reducing additives into a water flow. The initial phase consisted of a factorial design where the concentration and flowrate of the injected additive as well as the angle and width of the injection slot were varied independently. The experiments were conducted in a rectangular cross section channel that has an aspect ratio of ten to one. Slots were located in both of the larger walls at a streamwise location where the channel flow of water was fully developed. Drag reduction was deduced from wall pressure measurements and wall-layer concentration of the additive was deduced from colorimeter measurements. Additive concentration varied from 100 to 400 ppm. Injection flowrates ranged from 200 to 400 mlmin. The slot angle was either 15 or 25 degrees while the slot width was either 1.27 mm or 2.54 mm. The quantity which was optimized the merit function was the integral of the drag reduction with respect to streamwise distance beginning at the initial location of positive drag reduction and ending at the location where the additive was uniformly mixed with the water. The results showed that additive concentrations less than 1 ppm yield drag reduction on the order of 20. These results were achieved well downstream of the injector where it is hypothesized tha the additive molecules were in an excellent conformation for reducing drag.