Stall characteristics in a radial diffuser

Detailed measurements of stall characteristics, at a constant impeller speed of 3000 RPM, were carried out in a radial vaneless diffuser model. The mass flow rate through the blower was gradually reduced until flow instability occurred in the diffuser. This enabled the onset and propagation of rotating stall to be fully described. The blower was operated without the vaneless diffuser in order to ascertain the cause of the flow instability. It was discovered that the impeller did not stall at the flow rates at which the blower was operated with the diffuser. When the critical flow angles measured at the diffuser inlet, and midway between the diffuser walls, were compared with earlier reported values in the open literature they were found in good agreement. The number of rotating stall cells found in this study was one with its second and third harmonics. The strength of the instability associated with the stall cell structure increased when the flow coefficient was reduced further below the critical flow coefficient.