Flow control of rotating stall in a radial vaneless diffuser

The instabilities of a backflow layer on a diffuse wall and the main flow with vorticity have already been shown theoretically to cause the occurrence of rotating stall in a vaneless diffuser. These instabilities, however, have not yet been proven to exist experimentally. This study was carried out to examine the factors contributing to the occurrence of rotating stall using a jet installed in a diffuser. Rotating stall was completely suppressed by a jet that was set in the direction opposite to the vector of the impeller peripheral velocity, and amplified by the jet set in the same direction as that vector. The effects of the jets were confirmed by the experiment using the jets installed at positions other than the neighborhood of the diffuser wall. The results suggest that the instability of the main flow contributes to the onset of rotating stall. The factors contributing to the onset of rotating stall and the effect of the jet on the performance of the impeller-diffuser combination are discussed.