PLIF Measurement of Turbulent Diffusion in Drag-Reducing Flow with Dosed Polymer Solution from a Wall

Experimental investigation of the relationship between mass transfer and turbulent drag reduction of the drag-reducing channel flow with dosed polymer solution from a wall was carried out. Planar laser induced fluorescence (PLIF) measurement was employed to investigate the mass transfer of dosed polymer solution. In addition, the polymer concentration distribution was measured directly by extracting samples from the channel flow (Sampling method). In the PLIF measurement, Reynolds number based on the channel height was set to 20000 and poly(ethylene oxide) was used as a polymer. The polymer solution with a concentration from 10 ppm to 200 ppm was dosed at 3 L/min from the whole surface of the wall. As a result, in the case of water flow, dosed dyed water was ejected from the wall and was well diffused by the strong turbulent eddy motion. In contrast, when the polymer solution was dosed from the wall, the diffusion was largely suppressed in the near-wall region and drag reduction occurred. This result indicates that turbulent diffusion was suppressed in the near-wall region and momentum transport in the wall-normal direction was also largely suppressed. Moreover, because the polymer solution could be provided continuously into the channel flow downstream of the leading edge of the blowing wall, the drag reduction rate was reduced downstream. Finally, we estimated the Sherwood number based on the mass transfer logic, and the relationship between the drag reduction and mass transfer was discussed.