Abstract
The present authors are carrying out the research on the unstable flow occurred in a mixed flow vaneless diffuser system. In the experiment, a high-speed mode of rotating stall was measured. In order to clarify the cause of the high-speed mode, the instability of swirling flow in a pipe was examined using a test equipment composed of a swirling flow generator, a straight pipe and a hub. Main results are as follows. In the case without the hub, the high-speed mode was observed for relatively short pipes. In the case with the hub, the same mode was measured for the short hub with the small diameter. It was theoretically clarified that the high-speed mode measured was based on the rotation of a vortex core. A lower-speed mode can be explained by the small perturbation analysis which assumed a radial mode.
