The effect of two different endwall-penny concepts for variable stator vanes in a compressor cascade

Marcel Gottschall, Ronald Mailach, Konrad Vogeler

abstract:

The article describes investigations on the influence of two possible endwall-penny concepts for variable stator vanes to secondary flow field development and the perfor- mance of high pressure compressors. The work is aimed to avoid or reduce the harmful radial gap at variable compressor stators which varies with the operating point in clear- ance width and geometry. Whilst concept 1 applies a piecewise flattened polygonal hub or casing to achieve constant radial gaps, the second idea uses a penny covering the whole vane chord with no radial clearances.
In a first step both approaches were transformed and implemented to a linear cascade of modern stator profiles. Measurements were conducted with a 5-hole-probe in several planes up- and downstream the cascade as well as pressure tappings on the airfoil and the endwall. Additional 3D numerical calculations were carried out to gain information about the flow field inside the cascade. These analysis were performed at three stagger angles and two characteristic Reynolds numbers with a constant cascade turning to be representative for the adjusting range of aircraft engines.
Compared to a reference case without endwall contour and no clearance the results indicate decreased efficiency due to higher total pressure losses for both concepts. The blockage at the penny edges near the endwall and the extended corner seperation in case of the polygonal endwall are found to be responsible for this behaviour. Considering the cascade loading in terms of blade turning, a small overall benefit was achieved with the large penny configuration caused by much higher overturning near the wall. These tendencies increase with the stagger angle at part-load conditions of the compressor.
Furthermore, these results were compared with a part gap configuration of a vari- able vane that was found to provide a benefit concerning pressure losses and blade turning in a previous work. For lower cascade loadings the large penny configuration still reaches the highest overall cascade turning. This benefit compared to the part gap configuration disappears at the high stagger angle. The higher magnitude of turning is achieved with a much higher flow angle deviation along the blade height and significant increased losses.

reference:

Marcel Gottschall, Ronald Mailach, Konrad Vogeler
"The effect of two different endwall-penny concepts for variable stator vanes in a compressor cascade"
Paper No. GT2012-68404, ASME Turbo Expo, Copenhagen, Denmark, 2012