Vortex development induced by part gap geometry and endwall configurations for variable stator vanes in a compressor cascade

Marcel Gottschall, Konrad Vogeler

abstract:

Modern aircraft engines are equipped with adjustable compressor stator vanes to gain aerodynamic stability in a wider operation range. Unfortunately, the consequential variable radial gaps between vane and casing are responsible for increasing aerodynamic losses in the endwall regions. Hence, the understanding of the tip clearance vortex is a key issue for developing new compressors with enhanced efficiency and improved pressure ratios.
The article describes experimental investigations on the 3D flow field develop- ment near the endwall which is caused by specific part gap configurations and endwall concepts. As it was found in previous investigations, these different geometry configu- rations for variable stator vanes result in distinctive compressor performances. Their beneficial or harmful characteristics increase with the stagger angle at off design com- pressor operation. This work is aimed to explain the resulting outlet loss distributions and flow turning with a closer look inside the passage to the origin of the vortex system and a more detailed analysis of the interaction between clearance vortex and secondary flow field. In particular, the impact of different cascade and profile loadings due to stagger angle variation to the vortex development is adressed here.
All clearance and endwall configurations were implemented to a linear cascade of scaled modern stator profiles. The analysis was introduced at two stagger angles. Qualitative measurements were conducted with particle image velocimetry inside a blade passage in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Additional 3D numerical RANS calculations were carried out and compared with the measurements.
Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vor- tex development and its impact to the blade boundary layers downstream the passage is visualized and enhances the understanding of the geometry effect. The same applies to the endwall concepts without radial clearances. Their effect to the endwall bound- ary layer is shown and explains the beneficial characteristics compared to a reference geometry. Furthermore, the reasons for the deviant numerical predictions, particularly at high stagger angles, are analysed with the flow field visualisation inside the passage and allow a calibration of higher turbulence models for future investigations.

reference:

Marcel Gottschall, Konrad Vogeler
"Vortex development induced by part gap geometry and endwall configurations for variable stator vanes in a compressor cascade"
Paper No. 159, 10th European Turbomachinery Conference, Lappeenranta, Finland, 2013