15th European Conference on Turbomachinery Fluid dynamics & Thermodynamics

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Axial Turbines


Filippo Merli - Graz University of Technology, Austria
Nicolas Krajnc* - Graz University of Technology, Austria
Asim Hafizovic - Graz University of Technology, Austria
Emil Göttlich - Graz University of Technology, Austria


The accurate efficiency assessment of a high-pressure turbine (HPT) stage is complicated by the presence of upstream and downstream purge flows. In fact, the aerodynamic efficiency calculation is often based on the mass flow averaged values of total temperature at the stage inlet and outlet planes. This approach inevitably introduces uncertainties, due to the non-linearity of the main flow distribution and the presence of purge flows, whose distribution is usually unknown and therefore assumed as uniform. This paper presents and applies an alternative method to calculate the total-to-total efficiency of a fully purged HPT stage, without incurring in the abovementioned sources of error. Such method relies on seed gas concentration measurements at the HPT stage outlet plane, to track the purge flow scattering inside the annulus. After the results of the “distribution-based” method are compared to the standard “average-based” efficiency for nominal purge mass flows, the efficiency drop between the case without purge and the nominal purge case is investigated in detail. The experimental tests were carried out in the Transonic Test Turbine facility at Graz University of Technology. The dual-spool test setup includes a HPT stage, an intermediate turbine duct and a low-pressure turbine stage, running at engine-relevant operating conditions. All the HPT stage cavities are independently fed with cooling air, provided by a secondary air system (SAS). The SAS has been recently integrated to allow the injection of tracer gas in the desired purge line. By sampling the annulus flow with a sniffer probe, the purge flow distribution can be quantitatively obtained and utilized for determining the stage performance.

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