Influence of Blade Aspiration on the Flow Quality of an Annular Axial Compressor Cascade

Experimental and numerical investigations were performed in order to develop a technique of active flow control by aspiration applied to an annular axial compressor cascade. Continuous aspiration is applied through shaped slots located on the suction side of the blades and close to the suction side on the hub. Test cases with aspiration are compared to test cases with less or without aspiration to identify the influence of the aspiration. Additional measurements were done on a second annular cascade with reduced blade number, hence higher loading. This allowed investigating if the aspiration devices could compensate the increased losses that occur at higher loading. A simple model is introduced to predict the static pressure rise due to the aspiration depending on the extracted massflow and the achieved deflection and stagnation pressure level at outlet compared to a reference case. The tests were carried out in the Non- Rotating Annular Test Facility of EPFL at an average inlet flow Mach number of 0.8. Results are presented for two incidence levels: for 3° incidence the aspiration on the blade complements the aspiration on the hub, improving the flow quality at outlet in terms of uniformity and average stagnation pressure level. This is advantageous for the compression in following stages. Secondary flow features and especially the hub corner separation are reduced but the tip clearance vortex is enlarged. Improved diffusion, loss levels and deflection as well as the flow extraction itself yield increased static pressure rise. For higher incidences the tested aspiration devices are less effective but the static pressure rise is still increased by the flow extraction. For the case of increased blade loading at 3° incidence performance similar to the case of medium loading is achieved, however deflection is reduced due to the wider blade passage. The aspiration rate was the same in both cases, the increased and medium loading.