In exhaust diffusers of steam turbines pressure oscillations may occur at certain operating conditions, due to shock–boundary layer interactions. These self-excited flow oscillations are caused by the high-speed clearance flow between blade tip and outer contour, leading to an excitation of the running blades. The paper describes the behavior of this unsteady flow phenomenon in the diffuser, identified with steady state and unsteady measurements at the wall, along with probe measurements behind the runner blading. A deeper physical understanding was gained by simulation of this flow pattern on the base of the water analogy.

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