Experimental study of stall behavior in radial vaneless diffuser of centrifugal pump

A rotating stall phenomenon in radial vaneless diffuser of centrifugal pump has been investigated experimentally as the first stage of developing a method to suppress the stall. The test impeller is two-dimensional having six blades with outlet angles of 18.1 degrees and the diffuser is parallel walled with the outer diameter ratio of 1.90 to the impeller outlet and the height of 30 mm, which is the same of the impeller. Measured results are shown and discussed in the present paper as follows. The onset of rotating stall is triggered by a reverse flow occurring on the hub surface near the diffuser outlet. At that time the average angle of absolute flow velocity becomes less than 11.9 degrees at the section just downstream of the impeller. As the observed behaviors of rotating stall, change of stall cell numbers, amplitude and frequency of pressure fluctuation are clarified.