Comparative investigation of unsteady flow interactions in endwall regions of shrouded and unshrouded turbines

• Flow unsteadiness reduces the radial vorticity of endwall secondary vortices.
• The endwall flow interaction can be effectively utilized to control leakage losses.
• The shrouded turbine is of better unsteady performance than the unshrouded turbine.

The flow in turbomachinery is inherently unsteady, and the endwall losses are major sources of lost efficiency in turbine cascades. Therefore, the investigation of unsteady endwall flow interactions and the consideration of the effects into turbine design are valuable to improve the turbine performance. Comparative investigation into the physical mechanisms of unsteady endwall flow interactions of 1.5-stage shrouded and unshrouded turbines are performed by using a three-dimensional Navier–Stokes viscous solver. Emphasis is placed on how unsteady stator–rotor interactions affect turbine endwall secondary flows, and the feasibility of incorporating the unsteady endwall flow effects in turbine design is also discussed. The results show that unsteady interactions between upstream wake, tip leakage vortex/mixing zone and downstream passage vortex are the main factor affecting turbine endwall secondary flows. Unsteady interactions can reduce the radial vorticity of turbine endwall secondary flows, and the effects of these interactions on the streamwise vorticity of endwall secondary flows depend on upstream wake characteristics. The properly controlled unsteady interactions can reduce the size and intensity of endwall secondary flows, and thus improve the turbine performance. Because of the difference of turbine tip architectures, the periodic fluctuations of the flow in the shrouded turbine have smaller amplitude than those in the unshrouded turbine, and the shrouded turbine is of better unsteady performance than the unshrouded turbine.