Evaluation and Analysis of the Stochastic Unsteadiness in the Last Stage of a Counter-Rotating Two-Spool Turbine Rig

This paper describes the results of an extensive measurement campaign carried out in the counter-rotating two-spool transonic turbine at Graz University of Technology. The test setup consists of a high pressure (HP) stage, a diffusing mid turbine frame with turning struts (TMTF) and a shrouded low pressure (LP) rotor. The two rotors are mounted on mechanically independent shafts in order to provide engine-representative operating conditions. The rig was designed in cooperation with MTU Aero Engines and Volvo Aero within the EU project DREAM (ValiDation of Radical Engine Architecture SysteMs). A fast response aerodynamic pressure probe has been employed to provide time-resolved aerodynamic area traverses inside the facility. The test rig exhibits very complex interactions. In fact, the presence of two rotors induces oscillations at frequencies that corresponds to the linear combinations of the two blade passing frequencies. For this reason, particular care should be taken in evaluating the stochastic unsteadiness. The paper presents a method based on Fourier filtering. The total pressure random fluctuations are obtained by selectively filtering, in the frequency domain, the deterministic unsteadiness due to the two rotors and their non-linear interactions and then applying the inverse Fourier transform. The stochastic fluctuating component of the total pressure is strictly correlated to the turbulence intensity. Hence, its determination allows the discussion of the interaction processes on the last stage of the turbine. The unsteadiness of the HP rotor influences considerably the aerodynamic of the intermediate duct. However, the losses upstream of the LP rotor are strictly correlated to the steady evolution of the flow that diffuses in the duct. Downstream of the LP rotor, the turbulent fluctuations are instead induced, mainly, by the unsteady interaction of the strut and the rotor blade.