Identification of noise generation and flow kinematics in the air gap for two different blade tip designs of an axial fan

Importance of the role of axial fans in our everyday life as well as in industrial sphere is crucial. Their presence dictates indispensable reduction of the emitted noise level. The aerodynamically born noise due to the kinematics of the air-flow in the gap between rotor and stator is one predominant source of emitted noise by most fans and is the most intensive within the human audible spectrum. Two different rotor blade tip shapes A and B were analysed in details and the results presented in this paper. Their influence on the emitted noise was determined by measurements of the integral aerodynamic characteristic and by the level of the sound pressure level of the fan. It was confirmed that the kinematics of the air-flow within the blade tip gap is significantly different for both observed blade tip designs and plays a crucial role when generation and size of the total emitted noise power is concerned. Frequency spectra of the measured noise pressure level for both observed blade designs was also essentially different and served to confirm the well-known fact that intensity of the sound pressure level decreases with the frequency, which helps to assess and identify mechanisms of the noise generation. A typical measured amplitude decrease of the sound pressure level was observed and explained in details for both blade tip versions for the frequency range f>1000 Hz: appropriate scaling for the blade tip A and for the blade tip B was determined as \(\omega^{-2.8}\) and \(\omega^{-4/3}\) respectively. Distinctive differences between the two observed frequency spectra were established. Identification of locations, sources and specificity of the generated noise together with their detailed explanation was made possible by measurements of local pressure, velocity and their variances and is supported by simultaneous experimental flow visualization.