Analysis of noise generated by low solidity cascade diffuser in a centrifugal blower

This paper deals with the effect of the blade leading edge location (RLSD) of a low solidity cascade diffuser (LSD) on noise and diffuser performance in a centrifugal blower. The noise of the LSD was measured and analyzed comparing with that of vaneless diffuser (VLD) in view points of overall noise, discrete frequency noise and broadband noise. The numerical flow analysis was conducted in the impeller and the diffuser by using a Navier-Stokes solver. The noise of the VLD varied little in a wide flow rate range, on the other hand, that of the LSD increased remarkably in the small flow rate by about 7 dB. The noise of the LSD did not increase near the design flow and was almost equal to that of the VLD. It was found that the increase in noise due to LSD is dependent mainly on the broadband noise between 600∼1000Hz, which was closely correlated to the lift force of the LSD blade. The two kinds of discrete frequency noise appeared due to an interaction between the rotating impeller and the LSD blade and another interaction between the rotating impeller blades and the reverse flow toward the impeller exit, but their influence on the overall noise were relatively small. By shifting the LSD blade leading edge location downstream from R_LSD=1.1 to 1.2, the noise was reduced by about 3 dB at the maximum without deterioration of the diffuser performance. The maximum lift coefficient of the LSD blade was achieved as high as 1.5 at the high attack angle of 17 degrees even in the case of R_LSD=1.2, resulting in improvement of the diffuser performance by about 40% and in reduction of the unstable flow range by about 11%.