Transient Process of Rotating Stall in Radial Vaneless Diffuser

Abstract

Detailed measurements of flow velocity and pressure field were undertaken in a radial vaneless diffuser. The unsteady flow pattern at critical flow rates for rotating stall was investigated, using a transient data recorder which can store the velocity and/or pressure signals prior to the stall onset. The results show the existence of prestall symptoms that can be characterized by velocity/pressure oscillation with a frequency of 1.45 times that detected during the succeeding rotating stall. The measurements of velocity field using an X-wire probe confirm that this velocity fluctuation is associated with the reversed flow region, and transfers in tangential direction at an angular velocity of about 15% of the impeller revolution. Although the duration of this prestall phenomena diminishes with the decreasing flow rate at which the stall takes place, it is confirmed that its occurence is commonly observed for the conditions covered by the present study.