Flow Control of Vaneless Diffuser Rotating Stall

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In order to elucidate the role of the diffuser wall boundary layers in diffuser rotating stalls, boundary layer suction was applied at various radial locations. The onset flow was increased by the suction owing to the decrease in the main flow rate, showing that the current explanation attributing to the reverse flow is not adequate.A 2-D inviscid, nonlinear diffuser flow anaysis was made. At smaller flow rates, flow patterns similar to that observed in experiments appeared when the vorticity from the inpeller constitutes a certain pattern. However, the onset condition is too severe compared with experiments and the instability did not occur when the effects of skin friction on the diffuser wall was taken into account. Thus, the simulated instability looks to be different from those observed experimentally.Discussions on the energy balance of rotating stall were made from the results of an active control of the rotating stall. The control was made by introducing a fluctuating flow at the diffuser inlet, which is in a certain phase relation with the rotating stall. Discrete frequency component due to rotating, stall completely disappeared in the whole diffuser indicating that the control is not due to a mere linear superposition of the disturbance but the rotation stall flow itself is changed by the addition of the disturbance. The energy transferred to the diffuser flow by the displacement effect of the fluctuating flow was related with the controlled rotating stall amplitude. It was shown that the amplitude decreased when the energy is negative. From this result, the quantitative discussion was made as to the relation between the amplitude and the energy balance of vaneless diffuser rotating stall.