In a stationary two-pass coolant channel system the fluid flow is investigated experimentally using the PIV (Particle Image Velocimetry). The cooling system consists of a trapezoidal leading edge channel, a 180 deg. bend and an almost rectangular second channel. The cross sections of the channels were adapted to the shape of a typical turbine blade. It has ribbed walls in both channels to enhance heat transfer performance. Ribs are placed at the top and bottom walls of the channels. The system was analyzed at the German Aerospace Center (DLR) with a small scaled model and at the Institute of Aerospace Thermodynamics (ITLR) of the Stuttgart University with a large scaled model to ease near rib flow analysis. Heat transfer results are not the objective of this paper, they are investigation topics of the ITLR which is in close cooperation [SCHUBERT (2003)]. At DLR presently rotation effects are studied. As a first step to understand the existing flow phenomena, the system is analyzed in non-rotating mode. During future work it will rotate about an axis orthogonal to the centreline of the straight passes. The results shown in this paper demonstrate the effect of the 180° bend with isothermal flow condition excluding any buoyancy. Turbulent channel flow with a REYNOLDS number of 40,000, derived with the hydraulic diameter of the second pass, was investigated. Two models either with smooth or ribbed walls were investigated. Some kinds of flow visualizations were conducted such as oil flow visualization technique for obtaining wall streamlines and laser light sheet visualization technique to detect vortex structures and separations. The results presented in this paper clarify the complex flow situation given by the two-pass system with the inherent turn. Especially in the bend area separation regions and vortices with high local turbulence are apparent. The presence of ribs changes the fluid motion by generating additional vortices impinging the leading and trailing wall. This very demanding measuring task represents a benchmark test case for the application of PIV and later of Stereo PIV, respectively.

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