Title: A study on the rotating stall in vaneless diffusers of centrifugal fans. 1st Report. Rotational speeds of stall cells, critical inlet flow angle.
Abstract: The rotational speeds of stall cells in vaneless diffusers and critical inlet flow angles for rotating stalls under the condition of no scroll were studied experimentally. Two experimental equations on the 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 on the critical inlet flow angle is derived from the experimental data. The equation is usefull to predict the onset of the rotating stall. In the experiment, a weak velocity fluctuation was found at the position of the reverse flow on the diffuser wall just before the rotating stall. It is concluded that the reverse flow is the cause for the rotating stall in the vaneless diffuser. The conditions of the reverse flow layers on the diffuser wall just before the rotating stall are made clear by a numerical analysis. The properties of the fully developed rotating stall are also presented.