Suppression of rotating stall in vaneless diffuser by wall roughness control

In order to suppress the rotating stall which occurs at small flow rates in the vaneless diffuser of centrifugal blowers with backswept blade impeller, the authors have proposed an effective technique in which the completely rough wall was locally distributed on the vaneless diffuser walls. On the other hand, the effect of wall friction on the 3-D boundary layer separation was analyzed by using the 3-D boundary layer calculation code. By the increase in wall roughness only on a part of the hub side diffuser wall, the flow rate of rotating stall inception was decreased markedly by about 30% almost without deteriorating the diffuser pressure recovery as predicted by the theoretical analysis. Consequently, the rotating stall margin could be significantly improved by the proper arrangement of wall roughness distribution.