No abstract

The present paper is an attempt in understanding the stall inception mechanism in a low speed, contra rotating axial flow fan stage, using wavelet transforms. The rotors used in this study have relatively large tip gap (about 3% of the blade span) and aspect ratio of 3. The study was carried out near stall and at stall mass flow conditions for different speed ratios of rotor-2 to rotor-1. Unsteady pressure data from the casing wall mounted sensors are used to understand the stall inception mechanism. The wavelet transform clearly indicates that stall inception occurs mainly through long length scale disturbances for both rotors. It also reveals that short length disturbances occur simultaneously or intermittently in the case of rotor-1. The analysis shows the presence of a strong modal disturbance with 25–80% of the rotor frequency in the case of rotor-1 at the stall mass flow for all the speed combinations studied. The most interesting thing observed in the present study is that the frequency amplitude of the disturbance level is very small for both rotors.

Radial inflow distortion is encountered by any turbomachine in some form or the other. Radial inflow distortion alters the design blade loading distribution by redistributing the velocity along the blade span. The present experimental study is aimed to understand the influence of radially distorted inflow on the stall inception and propagation in a low speed contra rotating axial flow fan (CRAF). The study is carried out near-stall and at stall mass flow conditions for the design speed and off-design rotational speed combinations of the rotors. Total pressure rakes and Kiel probe rakes are used for flow characterization at the inlet of rotor-1 and rotor-2. Data from seven-hole probe measurement is also used to calculate the incidence angle at rotors inlet and the total press loss at relative frame of reference. Unsteady pressure data from the casing wall mounted sensors is used to understand the stall inception mechanism. The unsteady pressure data is analyzed using wavelet transforms and Fast Fourier Transforms (FFT). Wavelet transform is used to identify the stall inception mechanism, whereas FFT is used for quantification of the stall parameters. The study reveals that the performance and the stability of the CRAF stage is highly affected in case of hub covered radial distorted inflow condition and the stall incepts at a higher mass flow compared with the undistorted flow. However, the effect of tip covered radial distorted inflow on the performance and stability is negligible, and the stall incepts at nearly the same mass flow rate as that of the undistorted flow. Moreover, the CRAF stage gives better performance at higher mass flow for tip radially distorted inflow compared with the undistorted case. The wavelet transform clearly indicates that stall inception occurs mainly through long length-scale disturbances (LLSDs) for both the rotors. It is also observed that in some cases, spikes are embedded within LLSDs. The combined effect of stall initiated due to inflow distortion and direct interaction of the wake and the tip leakage vortex from rotor-1 with rotor-2 made the LLSDs and spikes type disturbances much stronger for rotor-2 compared with rotor-1.