Previous studies have indicated a potential for improving the performance of a Turbine Center Frame (TCF) duct by optimizing the clocking position between the high-pressure-turbine (HPT) vanes and TCF struts. To assess the impact of clocking on the performance, a new test vehicle with a clockable ratio of HPT vanes to TCF struts, consisting of an HPT stage (aero-dynamically representative of the second-stage HPT engine), a TCF duct with non-turning struts, and a first-stage low-pressure turbine vane, was designed and tested in the transonic test turbine facility (TTTF) at Graz University of Technology.

This paper quantifies the performance impact of clocking and describes the mechanisms causing TCF flow field changes, leveraging both experimental and numerical data. Other areas in the TCF duct impacted by the choice of the HPT vane circumferential position including the strength of unsteady HPT-TCF interaction modes, TCF strut incidence changes, and carry-over effects to the first LPT vane are additionally highlighted.

Five-hole-probe (5HP) area traverses and kielhead-rake traverses were used to asses the flow field at the TCF-exit and calculate the pressure loss. The flow field at the TCF exit shows significant differences depending on the circumferential position of the HPT vane. A relative performance benefit of 5% was achieved. A series of unsteady RANS simulations were performed to support the measured results, understand and characterize the relevant loss mechanisms. The observed performance improvement was related to interaction between the HPT secondary -flow structures and the TCF struts.

The impact of the HPT vane clocking on the unsteady flow field downstream of the TCF was investigated using Fast-Response Aerodynamic Pressure Probe (FRAPP) area traverses, analyzed by means of modal decomposition. In this way the individual azimuthal modes were ranked by their amplitude and a dependency of the clocking position was observed and quantified.

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