Incipient rotating stall characteristics measured by semiconductor L2F and pressure transducer in vaneless diffusers of centrifugal blowers

In order to clarify the transient behavior from the non-stall condition to the developed rotating stall in vaneless diffuser of a centrifugal blower, blade-to-blade variations of velocity and wall pressure were measured immediately upstream and downstream of the impeller exit as well as in the vaneless diffuser by means of the semiconductor laser 2-focus velocimeter and the pressure sensor respectively. The specified auto-correlation coefficient of large scale disturbances with the wavelength larger than twice of the blade spacing show that the intermittent stall occurs prior to the developed rotating stall. By analyzing the flow-fluctuationellipse and the correlation coefficient with respect to the radial and tangential components of velocity fluctuation, it is found that the intermittent stall as well as the developed rotating stall are triggered by the interaction between the reverse flow in the diffuser entry region and the jet-wake like flow discharged from the impeller.