Generation of broadband noise by turbulence in centrifugal fans

An experimental investigation has been conducted to improve the understanding of aerodynamic sources of noise in forward curved squirrel cage centrifugal fans. Experiments were performed on a thirty-two blade ventilating fan running in air and discharging through an anechoically terminated discharge duct.; Identification of noise sources at the inlet of the centrifugal fan was made by measuring the mean and fluctuating velocities at 140 locations inside the impeller, the radiated noise, and static pressure distribution around the scroll of the fan. Results from the mean and fluctuating axial and radial velocities inside the fan inlet indicated a strong separation region near the inlet. The total axial flow entering the impeller was found to pass only through the inner-most 60% of the impeller. Furthermore, the axial extent of this flow separation region was found to be a strong function of circumferential position around the inlet. The highest velocity fluctuations were recorded near the cutoff (45 degs from the top measured clockwise) and at a region 180 degs from the top. The velocity spectra, at locations near the inlet of the impeller, clearly show a highly flattened white noise type spectrum in the frequency range 0-2000 Hz, indicating a high level of broadband energy in the separation region nearest to the inlet of the impeller. Coherence functions between the two fluctuating velocity components obtained at different locations indicate that they are partially correlated with one another. Coherency between the velocity fluctuations and the measured far-field noise were found to be uniformly low over the entire frequency range (0-2000 Hz) at all the locations inside the impeller.; A novel Annular Inlet Guide (AIG) was designed, fabricated and successfully tested to reduce the size and strength of the separation region that has been observed near the inlet end of the impeller. The presence of the AIG made the flow more uniform in the exhaust duct, indicating that that flow in that scroll was redistributed more evenly across the width of the impeller. With the AIG installed, a 6 dB reduction in the first harmonic of the blade passing frequency and a 10 dB reduction in the second harmonic of the BPF was achieved. An overall reduction or 3 dB in the frequency range 0-2000 Hz was also measured. The reduction of noise by the redistribution of the flow occurs by two mechanisms: (a) the maximum velocities over the cutoff are lower, thereby lowering the sound pressure levels at the BPF and its harmonics, and (b) the size and strength of the separation region in the impeller is reduced which reduces the generation of broadband noise. The aerodynamic performance of the fan was not degraded by the presence of the AIG in the fan inlet. The benefits to fan performance because of redistribution of the flow was offset by losses induced by the presence of the AIG. (Abstract shortened by UMI.)...