Detection of rotating stall precursor in vaneless diffuser of a centrifugal blower

On the transient process of rotating stall in a centrifugal blower with a backward-leaning blades impeller, fluctuations of wall static pressure and velocity were measured in the impeller and in the vaneless diffuser by means of the high-response pressure sensor and the semiconductor laser 2-focus velocimeter respectively. It was found that a small intermittent fluctuation occurs prior to the rotating stall due to the interaction between the impeller passage flow and the diffuser boundary layer flow immediately downstream of the impeller exit, which occurs under the condition with the radially-inward flow in the vaneless diffuser. The focus is placed on disturbances of wavelength larger than the blade spacing in the present auto-correlation analysis of the pressure fluctuation. The appropriate value of the auto-correlation coefficient is proposed for a quantitative precursor scale of rotating stall in the vaneless diffuser.