Computational and Experimental Study of the Unsteady Convection of Entropy Waves Within a High Pressure Turbine Stage

This paper describes the transport and the interaction of pulsating entropy
  waves generated by combustor burners within a high pressure turbine stage for
  aeronautical application. Experiments and Computational Fluid Dynamics (CFD)
  simulations were carried out in the context of the European Research Project
  RECORD.
Experimental campaigns considering burner-representative temperature
  fluctuations injected upstream of an un-cooled high-pressure gas turbine
  stage have been performed in the high-speed closed-loop test-rig of the Fluid
  Machine Laboratory (LFM) of Politecnico di Milano (Italy). The turbine
  geometry is representative of a transonic high pressure gas turbine stage.
  The pulsating entropy waves are injected at the stage inlet in streamwise
  direction featuring a 7% over-temperature with respect to the main flow in a
  frequency range 30-90 Hz. Four different azimuthal positions of the entropy
  wave generators with respect to the stator leading edges were tested, in all
  cases inject ed at 75% of the blade span.
Detailed time-resolved temperature measurements (in the range of 0-200 Hz)
  upstream and downstream of the stage, as well as in the stator-rotor axial
  gap were performed. Downstream of the rotor, phase resolved aerodynamic
  measurements (in the frequency range of 0-100 kHz) were performed as well.
  Time-accurate CFD simulations with and without entropy fluctuations imposed
  at the stage inlet were performed with the TRAF code, developed by the
  University of Florence. A numerical post-processing procedure, based on the
  DFT (Discrete Fourier Transform) of the conservative variables as been implemented to
  extract the low frequency content connected to the entropy fluctuations filtering
  the higher frequencies due, for instance, to the rotor/stator interaction. This
  approach numerically replicates the experimental temperature acquisition
  strategy, allowing a direct comparison between numerical and experimental
  temperature fluctuations.
Measurements highlighted a significant attenuation of the entropy wave spot
  throughout their transport within the stator channel and their interaction
  with the rotor blade rows, highly depending on their injection azimuthal
  position. Downstream of the rotor, the entropy waves are spread over the
  pitch and undergo a migration in the spanwise direction strongly dependent on
  the interaction with the secondary flows.
Simulations show an overall good agreement with the experiments on the
  measurement traverses, especially at the stage outlet and allow for explaining
  with a high level of detail the complex interaction phenomena occurring within
  the stage. By exploiting the combination of experiments and simulations, the
  aerodynamic and thermal implications of the temperature fluctuation injected
  upstream of the stage were properly assessed, thus allowing suggest useful
  information to the designer.  
The comparison with the experiments confirms the accuracy of the CFD method to
  solve the periodic, but characterized by a low frequency content event,
  associated to the entropy wave fluctuation.