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.
Highlights
Contra rotating turbomachines have several advantages like improved flow capacity, compact size, and higher aerodynamic performance in both ducted and unducted configurations
The present study on stall inception mainly focuses on the stall behavior at near stall mass flow and stall mass flow conditions for different speed combinations of rotor-1 and rotor-2
The overall performance map for the three speed combinations of the contra rotating axial flow fan (CRAF) stage is shown in Figure 2 and the corresponding design mass flow, near stall mass flow, and stall mass flow points are indicted in the figure
Summary
Contra rotating turbomachines have several advantages like improved flow capacity, compact size, and higher aerodynamic performance in both ducted and unducted configurations. This type of turbomachines differs from the conventional turbomachines in that there is no stator between two rotating rows of blades. Achieving wider and stable operating range is a very important design parameter in the design exercise To achieve such a compressor/fan design, a thorough understanding of the stall inception mechanism is necessary. Numerous investigations have been performed by various researchers on the stall inception mechanism of axial flow compressors or fans and a promising progress is seen in understanding the unsteady flow phenomena. Pismenny and Levy [11] carried out a theoretical investigation
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