Characterization of the Flow Generated in an Annular Test Facility
Annular cascades are particularly useful to study the three dimensional phenomena encountered in turbomachines. The Non-Rotating Annular Test Facility of the Swiss Federal Institute of Technology in Lausanne (EPFL) consists in a radial-axial inlet nozzle and allows the investigation of steady and unsteady flows in compressor and turbine geometries. This facility has been designed to generate subsonic or supersonic flow regimes in a test section without shock interferences and residual wakes from the pre-swirl guide vanes located in the radial entry. Two separate radial flows with different inlet flow angles are firstly generated before merging to counteract the pressure gradients induced by the tunnel curvature and obtain the desired pressure distribution in the test section. Actually, while pressure and velocity measurements are satisfactory, high turbulence levels are identified experimentally over the test channel height. In order to understand the mechanisms occuring in the radial-axial nozzle and quantify their resulting effects on the test section flow pattern, a numerical investigation was performed for different stagger angles of the pre-swirl guide vanes. The distribution of conservative quantities in the (r-z) plane along the nozzle geometry highlights the combined effect of the channel curvature, the swirl angle of the flow and the viscous effects generated along the tunnel endwall. It is shown that vortical structures formed along the nozzle walls induce a significant turbulence intensity and should be taken into account in the evaluation of the experimental results.