Development and drag-reducing performance of a water-soluble polymer coating

Adding a small amount of polymers can achieve significant drag reduction effects. However, for external flows, the common homogeneous mixing and diffusing injection methods are not feasible. As an alternative, the present work developed a novel water-soluble polymer composite coating. The coating made use of the film-forming property of polyvinyl alcohol with polyethyleneoxide (PEO, a well-known drag reduction polymer) incorporated into it. When the coating dissolved, PEO continuously dispersed into the external flow. The surface characteristics of the water-soluble polymer coating were characterized. Drag reduction tests were conducted using a gravity circulation system. The coating exhibited a maximum drag reduction rate (DR) of 7% in the coating section and 27% in the downstream section. The larger percentage and the greater molecular weight of PEO not only promoted polymer drag reduction but also increased the surface roughness. Competition between effects of drag reduction and surface roughness led to complex effects in the coating section. It was also found that partial coating could induce significant drag reduction effects. The optimal length ratio of coated to total surface was related to the polymer characteristics and the speed of the main flow. The 1/4 and 1/2 coating resulted in a maximum DR of approximately 7% (Re = 27 523) in the test plate section for coatings with 10 000 wppm PEO, while the 1/4 coating had a maximum DR of approximately 9% (Re = 11 468) for coatings with 20 000 wppm PEO. These results indicated that such drag-reducing composite polymer coatings have great potential to be applied in underwater equipment.