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Energy loss due to the wall skin friction in a turbomachinery impeller is directly related to the mechanical efficiency. Generally, the flow is modeled as a flow around an enclosed rotating disk, and we can estimate the energy loss of the impeller by applying the analytical or experimental results of the friction moment for a rotating disk. Several analytical and experimental studies have been performed in order to examine the friction moment and reduce the skin friction drag. The flow behavior of the boundary layer on the rotating disk surface is strongly affected by the vortices that are formed the spiral streak for the disk rotation stationary. In order to reduce the drag caused by disk friction, the behavior of the vortices in the boundary layer must be controlled. In the present study, the drag reduction phenomena of an enclosed rotating disk were clarified experimentally using a disk with several fine spiral grooves made by a wet etching process. Experiments were carried to measure the frictional moment of a disk having 120∼160 fine spiral grooves, the depths of which ranged from 100∼200 μm, in Newtonian fluids. The shape of the grooves was obtained by referring to spiral streaks in the boundary layer. The Reynolds number range is 4×104 < Re < 6×105. The maximum drag reduction ratio is approximately 15%, and the phenomena depend on the number, the spiral angle, and the depth of the grooves. The experimental results for the velocity profile and the flow visualization clarified that the fine spiral grooves control the secondary flow of the boundary layer on the disk surface and delays the generation of local turbulence in the transition range.

Drag Reduction of an Enclosed Rotating Disk with Fine Spiral Grooves