Noise Reduction in the Blade Tip Region of an Axial Fan

2.2 Experimental Methods for Localizing / Characterizing Sources

A study of velocity and pressure fluctuations in the tip clearance flow of an axial fan for two different rotor blade tip designs is presented. Results determined with the standard blade tip design are compared to those from modified blade design with a tip winglet. Comparison of integral sound parameters indicates significant noise level reduction for the modified blade tip design. Noise reduction depends on the topology of flow structures in the area of gap-flow with the emphasis on the decay of vortex structures from larger to smaller local – time scales. In order to observe this phenomenon, a new experimental method was developed. This method is based on simultaneous measured values of local flow velocity and pressure. It enhances the identification and determination of the local flow structure and its turbulent flow properties. Besides, energy distribution within the phase space together with generation and dissipation of the turbulent kinetic energy within the gap-flow determines characteristic sources of acoustic emission of the fan. Locations of attractors within the phase space were also determined in order to support basic assumptions and conclusions:
Transition from integral operating points of the fan with minimum to those with maximum air-flows significantly affects the topologic structures of attractors; it is quite deterministic and linked with the expected conversion of the pressure into kinetic energy during the period that coincides with the position of the blade tip passage over the observed sensor. The following inter-blade areas exert larger velocity fluctuations typical for decay of larger eddy structures and are consequently closely linked with wideband noise generation. Emitted sound power depended significantly on the shape of tip presented here. The largest difference coincides with the best operating range of the fan where best efficiency and the lowest noise level were observed. The relation between turbulent kinetic energy production and emitted noise level was explained on a phenomenological level.