PRE-STALL WAVES: PRECURSORS TO STALL INCEPTION IN A CONTRA-ROTATING AXIAL FAN

Stall in a compressor or a fan is often associated with pre-stall waves, that could act as precursors. The present study aims to understand in detail the pre-stall waves leading to instabilities in a low aspect ratio, low hub-tip ratio contra-rotating axial fan. Apart from a clean inflow condition, experiments on the contra-rotating fan are also carried out for two radial distortion conditions, namely, hub-radial and tip-radial distortions, and three circumferential distortion conditions, namely, simple-circumferential, hub-complex-circumferential and tip-complexcircumferential distortions. The results primarily concluded that operating rotor-2 at a speed higher than the design speed could possibly suppress the pre-stall disturbances. Towards the fully developed stall, the waves that are associated with low frequencies speed up and thus these waves become intermediate frequency waves. The fluid phenomena that trigger the stall are associated with high frequencies and these subsequently stretch to low frequencies at the onset of fully developed stall. The low-frequency waves and high frequency waves compromise to reach an intermediate frequency range during the fully developed stall. Further, it is observed that disturbances associated with low frequencies as well as high frequencies co-exist during the fully developed stall regime. There is also a region in the frequency spectra where no disturbances are excited and this region appears to be a "no excitation zone". This paper thus concludes that there possibly exists a mechanism through which the energy is transferred between different frequencies during the pre-stall and fully developed stall regimes.