Downstream of a modern combined cycle gas turbine there is generally an abrupt end of the turbine inner hub wall leading to a backward facing step with separation and flow recirculation on the hub axis. This can lead to a strong hub wake with high losses in the center of the diffuser. The interaction between the end of the hub and the downstream axial diffuser can have a significant influence on diffuser flow field and hence on overall gas turbine performance. To minimize the hub wake the flow can be guided to the axis by reducing the diameter of the hub. But this can still lead to separation on the extended hub and to a strong hub wake. This paper describes experimental and numerical investigations of the use of flow injection to control the flow separation on the hub and to guide the main flow towards the axis of the diffuser to reduce the hub wake.

A new test rig has been built at the Institute of Thermal Turbomachinery and Machinery Laboratory of the University of Stuttgart, Germany (ITSM). This rig features an active flow control device based on hub flow injection with a Coanda-effect to influence the hub wake flow. In addition to experimental studies, steady CFD (computational fluid dynamics) simulations have been performed for this configuration and validated using the experimental data.

Test results and flow field computations confirm that the flow control device tested can have a beneficial influence on diffuser performance at specific conditions. If it is the casing flow which separates then hub injection has little effect. If the casing flow is attached, and a hub separation limits the performance, then hub jet flow injection can be highly beneficial. The results also indicate that the mass flow injected in the hub jet control device has to be carefully balanced.

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