Performance improvement of shrouded turbines with the management of casing endwall interaction flows

The flow in turbomachinery is inherently unsteady, and the understanding of the flow interaction in the casing endwall region of shrouded turbine stages is a key requirement in achieving further improvements in performances. Thus, comparative numerical investigations of casing endwall flow interactions of 1.5-stage shrouded turbines with straight and bowed vanes are performed, in an attempt to incorporate flow unsteadiness in the control of casing endwall secondary flows and losses. The investigation focuses on a comparison of the flow interaction mechanisms of casing secondary vortices with upstream straight and bowed wakes, and the influences on the performance of casing endwall region are also discussed and quantified. The results show that the bowed wake at different radial sections passes different distances to reach rotor leading edge, which affects their consequent interactions with following blades. And, this is quite similar to clocking effects but with a limited extent. The interaction with upstream bowed wakes has a beneficial effect on casing endwall flow behavior resulting in weakened secondary flows. The combined unsteady effects on turbine performance depend on the balance of unsteady endwall mixing and endwall secondary flows, and the negative impact of these effects should be alleviated to achieve enhanced performance.