Measurement of Rotating Stall in a Centrifugal Blower by Semiconductor Laser 2-Focus Velocimeter (1st Report, Measurement of Blade-to-Blade Distribution of Velocity Fluctuation)

For detecting the rotating stall observed at a low flow rate in a centrifugal blower with a backward-leaning blade impeller, blade-to-blade variation of velocity fluctuation was measured in the impeller, immediately downstream of the impeller and in the vaneless diffuser. Measurement was conducted by means of the spot-focus-type semiconductor laser 2-focus velocimeter developed by the authors. This L2F velocimeter has a narrow-band spatial-filter effect within ±1.5 degrees of flow angle fluctuation. The fluctuations of velocity and flow angle were analyzed on the basis of the probability density distribution. The change of the blade-to-blade relative velocity distribution due to flow rate was clearly shown, and it was determined that the origin of rotating stall was in the vaneless diffuser because the blade-to-blade velocity fluctuation distribution in the impeller was hardly affected by the large velocity fluctuation due to the rotating stall which occurred in the vaneless diffuser.