Abstract
The paper deals with selected phenomena present in a blade cascade flow field. The presented research was performed on prismatic blade cascades composed mostly of the tip and root sections of the last stage rotor blade of a large output steam turbine. The unstarted supersonic flow on the cascade inlet, the separation of the flow and the swirl line behind the trailing edge have been identified as the possible sources of the unsteady force effects.
Highlights
The modern steam turbines of large power output utilize very long blades in the last stage of the rotor
During the start up or shut down of the turbine, off design conditions appear in the turbine, which can generate swirl lines and flow separation
One of important aerodynamic sources of unsteady flow fields, which usually are the dominant reason of unsteady loading of blades, is a swirl line arising at the trailing edge in the near wake [4,5,6]
Summary
The modern steam turbines of large power output utilize very long blades in the last stage of the rotor. The tip of the blade operates at supersonic inlet velocities, while the root of the blade operates at subsonic inlet velocities In this setup, it is certain that the transonic inlet flow conditions will occur on the blade at its nominal speeds, unstarted supersonic flow will always be present. During the start up or shut down of the turbine, off design conditions appear in the turbine, which can generate swirl lines and flow separation. These phenomena can only be present during off design conditions, they can prove fatal for the turbine. It is important to note the majority of phenomena leading to unsteady force effects occurs in the region of tip section which is not rigid
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