The drag reduction performance by slowly releasing polymer solution through porous media

Polymer in the near-wall region was proven to be effective for reducing drag in external flows. This investigation proposed to slowly release polymer into main flow—rather than the slot injection—to weaken polymer diffusion, and improve drag reduction efficiency. Drag reduction performance by slowly releasing polymer solution into the channel turbulent flow through the porous media was investigated experimentally and systematically. Velocity statistics and polymer diffusion characteristics were quantitatively analyzed using particle image velocimetry and planar laser-induced fluorescence measurements. Comparison between the release of pure water and the polymer solution confirmed that the slow-release method restrained more polymer in the near-wall region than did the slot injection method, exhibiting greater drag reduction rate (DR). DR variations vs increasing the downstream distance were coupled with the slow-release rate, but oscillation characteristics appeared consistently in all cases, including the test with slot injection. This oscillation was expected to be related to the coil–stretch transition of the polymer molecules, which requires further exploration. Increasing the slow-release rate, concentration, and molecular weight was equivalent to increasing the effective drag-reducing polymer in the main flow, and the effects on DR were similar. Furthermore, the modified dimensionless polymer flux Km (including the molecular weight) was defined based on the dimensionless polymer flux K, which only considered the slow-release rate and the concentration. The DR vs log10Km shared similar characteristics with the previously reported DR vs log10K relationships. The characterization of Km could provide guidance to optimize the polymer drag reduction in practical application.