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Secondary vortex structures in turbine stage rotors

The flow through a turbine stage is extremely complex due to the presence of numerous secondary flows and vortex structures in the stator and rotor cascades. Firstly, horseshoe vortices are formed at blade leading edges of the stator and rotor blades, near hub and tip endwalls. Inside the rotor passage these vortices are believed to mix, partially or entirely, with passage vortices forming as a result of passage cross flows. At the same time, the trailing edges of the stator and rotor blades are the sources of wakes shed downstream and being convected with the main flow into the next cascades. Also flow separations, occasionally observed at rotor passages, can frequently lead to the creation of additional large-scale vortices of various orientations. Permanent interactions between all the abovementioned main flow structures, not mentioning those revealing smaller or varying  intensity, such as corner or leakage vortices for instance, make studying the turbine flow extremely difficult.

  

The interaction between secondary vortices (right) observed at a distance of  0.1 rotor chord length downstream of the rotor of a  high-pressure turbine stage (left). Recognised vortex structures: 1 – stator wake; 2 – stator tip passage vortex; 5 – rotor hub passage vortex ; 6 – stator tip trailing shed vortex; 7 – stator hub passage vortex; 8 - rotor wake; 9 - rotor hub trailing shed vortex.

The presence of secondary vortices dynamically interacting with each other is a source of numerous unfavourable effects both in a local and global scale. In the local scale the vortices are the source of fluctuations of flow parameters recorded in the vicinity of their instant locations. In the global scale they produce additional losses thus decreasing the efficiency of stage operation.

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