This paper presents the experimental and numerical evaluation and comparison of the different flow fields downstream of a turbine center frame duct and a low-pressure turbine stage, generated by varying the inlet flow conditions to the turbine center frame duct. The measurements were carried out in an engine-representative two-stage two-spool test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. The rig consists of a high-pressure (HPT) and a low-pressure (LPT) turbine stage, connected via a turbine center frame (TCF) with non-turning struts. Four individual high-pressure turbine purge flow rates and two low-pressure turbine purge flow rates were varied to achieve different engine-relevant TCF and LPT inlet flow conditions. The experimental data was acquired by means of five-hole-probe area traverses upstream and downstream of the TCF, and downstream of the LPT. A steady RANS simulation taking all purge flows in account was used for comparison and additional insight are gained from a numerical variation of the HPT and LPT purge flow rates. The focus of this study is on the impact of the variations in TCF inlet conditions on the secondary flow generation through the TCF duct and the carry-over effects on the exit flow field and performance of the LPT stage. Existing work is limited by either investigating multi-stage LPT configurations with generally very few measurements behind the first stage or by not including relevant HPT secondary flow structures in setting up the LPT inflow conditions. This work addresses both of these shortcomings and presents new insight into the TCF and LPT aerodynamic behavior at varying the HPT and LPT purge flows. The results demonstrate the importance of the HPT flow structures and their evolution through the TCF duct for setting up the LPT inflow conditions, and ultimately for assessing the performance of the first LPT stage.

Previous studies have indicated a potential for improving the performance of a Turbine Center Frame (TCF) duct by op- timizing 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 tur- bine facility (TTTF) at Graz University of Technology. This paper quantifies the performance impact of clocking and describes the mechanisms causing TCF flow field changes, lever- aging both experimental and numerical data. Other areas in the TCF duct impacted by the choice of the HPT vane circumfer- ential position including the strength of unsteady HPT-TCF in- teraction 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 tra- verses were used to asses the flow field at the TCF-exit and calcu- late the pressure loss. The flow field at the TCF exit shows signif- icant differences depending on the circumferential position of the HPT vane. A relative performance benefit of 5% was achieved.

This paper presents the experimental and numerical evaluation and comparison of the different flow fields downstream of a turbine center frame duct and a low-pressure turbine stage, generated by varying the inlet flow conditions to the turbine center frame duct. The measurements were carried out in an engine-representative two-stage two-spool test turbine facility at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. The rig consists of a highpressure (HPT) and a low-pressure (LPT) turbine stage, connected via a turbine center frame (TCF) with non-turning struts. Four individual high-pressure turbine purge flow rates and two low-pressure turbine purge flow rates were varied to achieve different engine-relevant TCF and LPT inlet flow conditions. The experimental data was acquired by means of five-hole-probe area traverses upstream and downstream of the TCF, and downstream of the LPT. A steady RANS simulation taking all purge flows in account was used for comparison and additional insight are gained from a numerical variation of the HPT and LPT purge flow rates. The focus of this study is on the impact of the variations in TCF inlet conditions on the secondary flow generation through the TCF duct and the carry-over effects on the exit flow field and performance of the LPT stage.

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.

The paper discusses the impact of rotor purge flows on the unsteady flow field downstream of a two-stage, two-spool test turbine. The analyzed setup is representative of the second high-pressure turbine (HPT) and the first low-pressure turbine (LPT) stage in a modern turbofan aero-engine, with a turbine center frame (TCF) with non-turning struts in-between the two turbines. All measurements were carried out for an engine-representative test vehicle setup at the Transonic Test Turbine Facility at Graz University of Technology. The test rig features a secondary air system delivering five purge flows with independent temperature and mass flow control to the HPT and LPT cavities. This work extends the results shown in two recent publications analyzing the time-resolved flow through the same two-stage setup at fixed purge flow rates. The paper aims to provide additional input about the driving sources of unsteadiness in gas turbines for aeronautic applications, by isolating the HPT and LPT purge air contributions. The time-resolved flow field at the LPT exit was acquired with a Fast Response Aerodynamic Pressure Probe (FRAPP) for three different purge conditions (reference case, no HPT purge case, no LPT purge case), to separate and quantify the impact of HPT and LPT purge contributions on the main flow field. The so-called Rotor Synchronic Averaging (RSA) technique was used as phase-averaging approach, to account for the unsteadiness due to both rotors. Proper Orthogonal Decomposition (POD) was then applied to isolate the most important structures and identify their origins. The comparison of the three data-sets shows a significant influence of the HPT purge on the entire flow field at the LPT exit, even though the HPT is located far upstream, while the LPT purge impact appears to mostly affect the end-wall region.