Improvements in aerodynamic performance and noise due to low solidity cascade diffuser in a centrifugal blower

A high efficiency and a wide operating range are required in recent centrifugal compressors and blowers. Low solidity circular cascade diffuser (LSD) was proposed in place of vaned diffuser and channel diffuser for achieving both higher pressure ratio and wider operating range. In the present study, aerodynamic performance and noise characteristics of LSD were investigated changing a radial location of LSD leading-edge. It is known that an interaction occurs between jet-wake flow discharged from the rotating impeller and the LSD leading edge, however jet-wake flow pattern is almost uniformalized until the radial position of R=1.20 in the vaneless diffuser. Aiming at reducing the noise generated by the interaction mentioned above, the LSD leading edge was moved from R_LSD=1.10 to 1.15 and 1.20 in a centrifugal blower with low specific speed. The frequency spectra of noise were analyzed and the noise due to LSD were discussed from the view point of overall noise, discrete frequency noise and broadband noise. In order to clarify the relationship between the flow field and the noise, flow behavior in the impeller as well as in the diffuser was simulated using a Navier-Stokes solver ANSYS-CFX. According to the present experimental results, about 40% improvement in diffuser performance and about 11% improvement in unstable flow range were achieved in small flow rates by the LSD compared with the vaneless diffuser except for a small deterioration in diffuser performance at the large flow rate. On the other hand, the noise increased clearly in the case of R_LSD=1.10 at the large flow rate and the small flow rates as well. By locating the LSD blade leading edge downstream farther from the impeller exit, a remarkable reduction in overall noise was achieved without deterioration of diffuser performance. It is found that the noise increase due to LSD is mainly dependent on the broadband noise based on the jet-wake flow, the interaction between the reverse flow and the impeller blades results in the discrete frequency noise defined by the frequency of cross product of the rotational speed of the impeller and the number of LSD blade, and a significant reduction in noise is achieved by uniformalization of the jetwake flow.