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Investigation of the steady and unsteady performance of a transonic HP turbine

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VKI PHDT 2002-04, Investigation of the steady and unsteady performance of a transonic HP turbine, ISBN 978-2-930389-45-1

Investigation of the steady and unsteady performance of a transo

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Investigation of the steady and unsteady performance of a transonic HP turbine
By G. Paniagua,  published in 2002, ISBN 978-2-930389-45-1
PhD Thesis from the von Karman Institute/ Université Libre de Bruxelles, Belgium, September 2002

The high-pressure (HP) turbine, which powers the core of the whole gas turbine engine, contributes significantly to the engine cost. Conventional HP turbines are composed of two stages, however, the request of further decreases of specific fuel consumption and size has led to the development of highly loaded single-stage HP turbines. The fluid dynamics involved in this new generation of turbines is dominated by shock interactions. Blade row interference effects (potential, viscous, and specially shock interactions) are major factors penalising the aero-thermal performance of the turbine.

This research focuses on the steady and unsteady flow field downstream of the stator and rotor. The influences of Reynolds number and pressure ratio are addressed resulting in 4 distinct test conditions. Additionally, at nominal conditions different mass flow rates of cold flow have been ejected between the stator and the rotor platform. The experiments are carried out in the large VKI short duration turbine facility. 3-D Navier-Stokes simulations aid in the interpretation of this very complex flow interaction. The research is of great interest to all turbine designers, as it would provide guidelines for the design of cheaper-simpler and more reliable engines. The European Union and most of the European manufacturers funded the research.

Special attention was paid to the development of accurate instrumentation both for the measurement of low and high frequency phenomena. As the experiments are carried out in a blowdown facility, with typical running time » 0.5 s, detailed studies have been developed to ensure that the very accurate micro-thermocouples and pneumatic pressure probes are fast enough to resolve the transient conditions. In the course of this work original tools to evaluate probe response have been developed together with numerical routines to identify the transfer function of the measurement chain and compensate the signal in case of slow response. The innovative numerical post-processing routine is based on the discretisation of n-th order linear systems. Concerning the high frequency phenomena piezoelectric pressure sensors and cold wire thermo-resitors are used to measure the unsteady pressure and temperature. A large quantity of measurements was gathered:

  • steady stage inlet total pressure and temperature span-wise profiles
  • steady and unsteady static pressure downstream of the vane at hub and tip endwalls
  • steady and unsteady static pressure around the rotor blade at several blade heights
  • steady and unsteady total pressure and temperature downstream of the stage
  • steady stage exit flow angle

The time averaged pitch-wise static pressure field downstream of the vane shows large variations due to the vane trailing edge shock system. The time resolved static pressure fluctuations downstream of the vane indicate the change of origin, incidence and strength of the stator trailing edge shock system as the rotor turns and changes the local blockage. At the stage outlet, the effects of passage vortices could be identified, however, the rotor wake could not be recognised. The total temperature field exhibits large fluctuations at the tip due to the periodic passage of the hot tip leakage flow. Downstream of the stage in the pitch-wise direction, there is a clear total pressure variation associated with the stator, unclear whether due to the stator wake or the potential field.

Because the flow ingested/ejected between stator and rotor platform is a very small amount (1.5% of the core flow) it is often neglected during the aerodynamic design. An important conclusion of this work has been the significant influence of the hub-cavity flow on the rotor loading, due to large increase of the vane downstream static pressure at hub (7%) and consequent lower incidence. Heat transfer measurements show that the time-averaged pressure side is little affected by wake and shock passages. On the suction side it seems that the boundary layer does not even reach a turbulent state. The rotor unsteady loading at 15% is drastically affected by the vane shock, the ejection of the cavity flow diminishes significantly the amplitude of the fluctuations, especially near the leading edge.

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Manufacturer von Karman Institute for Fluid Dynamics

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