Study of Sound Source Distribution in Centrifugal Fan Using Transparent Wall

Experimental Methods for Localizing / Characterizing Sources

Sound source investigation is very important for reducing the fan noise. The sound photograph and beam forming method are recently used for this purpose. The axial fan is able to use this method because the impeller is directly observed from the microphone array. But at the case of centrifugal fan, the impeller is hidden in the casing or shroud and it is difficult to use those method directly. For example, it is used for the case at free from casing, but it is restricted only the open operation not to pressurized condition.
This research is focused to make clear the sound source distribution in the centrifugal fan with pressurized operation. For this purpose, It is used the transparent wall. The transparent wall is rolled the wall for the flow, but not wall for the sound. Transparent wall is researched by some researchers for the reduction of the noise and know the simple sound source map. But the application of the complex case for the fan is not known. In this research, it is cleared the characteristics of the transparent wall for the pressurized conditions. It is cleared that the transparent characteristics is varied so much at the pressurized condition.
Next the application for the pressurized condition of the tubular centrifugal fan is researched. The fan is consisted of the centrifugal impeller and axisymmetric casing. The air flows onto the impeller from the center of impeller and turns to radial direction in the impeller. The radial directional flow makes the large pressure rise in the impeller. After the impeller, the air turns to axial direction again. The flow pattern in the tubular fan is so complex that the large loss and noise are generated in the fan. We analyze the flow characteristics of the tubular fan with numerical simulation.It is attempted the sound transparent wall for the casing to know the sound source. The low frequency sound of about 500 Hz that is estimated the source being the impeller is clearly emerged from the outlet of the impeller and the high frequency noise that is estimated the source location of the inlet leakage flow is mainly radiated from the inlet of the fan.