Studies on Noise and Flow Fields in a Squirrel Cage Fan

There is a wide application of squirrel cage fan in heating and ventilating systems where a reduction of noise with fluid mechanics origin is of importance. This paper presents an aero-acoustic study on a squirrel-cage fan and the relation between unsteady fluid flow and the noise generated in the fan is investigated. Noise measurements in the exit channel of a fan are presented together with laser Doppler velocity measurements outside the rotor. Both measurements are Fourier transformed and important frequency contents are analyzed. Velocity measurements show that the largest fluctuations happen just after the inlet and close to the cut-off. The power spectra of velocity components near the rotor inlet and the cut off are high in comparison with other circumferential and axial positions. Blade passing frequency is dominant in noise and velocity data with a considerable volume of white band noise. It is not present at locations with no flow through the rotor. Fan performance and noise are available for different inlet geometries. An optimization of the inlet configuration changed the maximum flow rate but the sound power level was similar at the same volumetric flow regardless of the inlet shape.