Materials, Methods & Technologies, Volume 6, 2012

Rezumat: Microbubble saturation of the near-wall liquid flow is one of the most efficient and economic methods of turbulent friction reduction with a minimal environmental impact. It is shown in this work that the key parameter for friction reduction is void fraction of bubbles in a buffer zone of the boundary layer, whose value is determined by the gas flow rate, liquid velocity, downflow distance behind the gas generator, and gravitation orientation of the wall. In the initial region dependence between reduction of local wall friction and void gas fraction obeys the law close to the linear one. An increase in concentration near the wall above some critical values leads to intensive bubble coalescence; as a result the flow in the two-phase boundary layer turns to the film-bubble regime with typical amplification of gas phase buoyancy. At this, in the flow under the wall friction at relatively large distances from the gas generator can stay close to minimal, and in the flow above the wall it starts usually increasing. Understanding of physical foundations facilitates optimization of microbubble saturation as the method for reduction of turbulent friction on boats.