Transpiration cooling, obtained by injecting a coolant through a porous surface, is receiving increased consideration as an effective means of maintaining wails exposed to a hot gas stream at temperatures that can be tolerated by the material. Applications for which this cooling method is considered are the cooling of different structural elements such as turbine blades or the walls of combustion chambers in Jet engines and parts of the skin of missiles. In some of these appli¬cations the surface roughness inherently connected with a porous materi¬al may exceed the limit for which such a surface is considered as hydraulically smooth.nIn order to obtain information on friction and flow characteristics of rough surfaces under the condition of fluid injection, an investi¬gation with turbulent air flow vas made in a rectangular channel. Pro¬visions were made so that porous samples with varying degrees of rough¬ness could be installed in the lover wall.nVelocity profiles and friction factors for these surfaces are presented for ratios of air-injection velocity to mainstream velocity ranging from 0 to approximately 0.0170. For zero injection velocity the friction data compared favorably with available data on impermeable rough surfaces. It was found that the shape of the velocity profile is decidedly influenced by the fluid injection. The results also indicate in a qualitative manner the expected decrease in friction with increase in fluid injection.
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