A study on the rotating stall in vaneless diffusers of centrifugal fans. 1st Repor. Rotational speeds of stall cells, critical inlet flow angle.

The rotational speeds of stall cells in vaneleess diffusers and critical inlet flow angles for rotating stalls under the condition of no scroll were studied experimentally. Two experimental equations for rotational speeds are derived and a prediction method is presented. The predicted rotational speeds agree well with measured values in the literature. A simple equation for the critical inlet flow angle is derived from the experimental data. This equation is useful for predicting the onset of a rotating stall. In the experiment, a weak velocity fluctuation was found at the position of reverse flow on the diffuser wall just before the rotating stall. It is concluded that reverse flow in the cause of rotating stall in a vaneless diffuser. The conditions of reverse flow layers on the diffuser wall just before a rotating stall are made clear by a numerical analysis. The properties of a fully developed rotating stall are also presented.