Experimental research of noise reduction possibilities of an axial fan with an additional contra-rotor

This work refers to investigations of the noise propagation of a fan with an inlet contra-rotating rotor. The main goal was to find if there were regions of rotational frequency where the noise levels emitted by the system may be lower than expected. The measurements were performed at Lodz University of Technology in the aeroacoustic anechoic chamber. Constant rotational speed of the first rod-rotor was set-up as constant with varying rotational speed of the second rotor (the fan). The system was similar to one of the NASA earlier experiments. The analysis showed correlations between the emitted noise levels and the rotational speeds of the rotors. Next, the noise emitted only by the second rotor (the fan) was compared with the noise of two co-operating rotors. Some regions, where the noise of the fan was reduced due to the presence of the second contra-rotating rotor at the inlet, gave a base for future investigations. The issue is important especially in cases where quiet electric drives can be used and the aerodynamic noise is crucial. The presented results can be used in similar systems in modern flow machinery, aeroengines configurations, double-rotor drones, as well as, due to the simplicity of the system and its geometry, for verification of numerical methods.