Noise and aerodynamic performance in a centrifugal blower with low solidity cascade diffuser

In the present study, the diffuser performance and the sound pressure level were measured in a low specific type centrifugal blower with/without a low solidity cascade diffuser (LSD), and the frequency spectra were compared between the LSD and the vaneless diffuser. The flow behavior was also simulated using a Navier- Stokes solver by ANSYS-CFX. The higher pressure rise in the diffuser was obtained by means of the LSD at flow rates smaller than the design flow rate, however, noise level increased significantly at off-design conditions, comparing with the case of vaneless diffuser. It is found that the overall sound pressure level does not always increase by means of the LSD in the low flow rate range where the reverse flow does not reach to the impeller exit, on the other hand, the noise increases significantly when the reverse flow comes to the impeller exit. This noise increase is based on the broadband noise due to interaction between the reverse flow and the impeller, and also the discrete frequency noise defined by the cross product of the rotational speed of the impeller and the number of LSD.