Session: 43-01 Unsteady Flows in Turbines

Paper Number: 82256

82256 - The Role of Turbine Operating Conditions on Combustor-Turbine Interaction – Part 2: Loading Effects 

Modern aeroengine combustors burn a lean and premixed mixture featuring an unsteady heat release rate. Furthermore, the combustion air is swirled to stabilize the flame. Therefore, the harsh environment exiting the combustor leads to the production of vorticity and temperature perturbations. If the latter are unsteady, they are referred as entropy waves (EWs). Interacting with the first turbine stage, these unsteadiness  impact on the stage aerodynamics, make more challenging the blade cooling and are sources of indirect combustion noise production. The first of these issues is addressed in these combined papers Part I and Part 2 by means of experimental campaigns: a combination of EWs and swirl profile is injected upstream of an axial turbine stage to study the transport and effect on the stage aerodynamics due to these combustor non-uniformities. The novelty of these two papers lies in the role of the turbine operating condition on the combustor-turbine interaction. Considering that aero-engines work at different operating points (OPs) during a flight, the importance of investigating the combustor-turbine interaction at several OPs is consequential.

In order to carry out the experimental campaign, the engine-representative disturbances are generated by a novel combustor simulator that is able to combine the generation of EW and a swirl profile, as well. The EW superimposed on a swirl profile is injected at 50% of the blade height. The ratio between the number of combustor simulators and the stator blades is 1:2, i.e. there are 11 combustor simulators out of the 22 stator blades. Two different injection positions and four injection cases are considered: uniform inlet condition, injection of a swirl profile, injection of both swirl profile and EW, the latter at two different frequencies. Experimental measurements are carried out through the stage measuring the upstream injected disturbance, the stator and rotor downstream aerothermal flow field. The unsteady flow field is characterized through a fast response aerodynamic pressure probe and a micro S-type thermocouple.  A five-hole pressure probe is used to measure the steady flow field downstream of the combustor simulator and the stator.

Part 2 of the two papers analyses the effect of the turbine rotor loading obtained changing the rotational speed at the same expansion ratio. In addition to the design OP, two further are tested by loading or unloading the rotor. The expansion ratio chosen for this analysis is subsonic.

Results show weak changes in the EW transport across the stator, being the expansion ratio constant and the reaction degree not really modified. As already documented in previous research on the same turbine, the swirl profile is the main responsible for the changes in the aerodynamics stator downstream. The EW transport in the rotor modifies significantly depending on the OP, due to a very different rotor loading, and specifically incidence angle. In the unloaded and loaded OPs the EW magnitude attenuates more than in the loaded condition. Furthermore, the interaction with the rotor secondary flows is different leading to a different spreading and position of the EW downstream of the rotor. Unlike all the other OPs, in case of the loaded OP the temperature difference due to the EW has a main role on the stage aero-thermal flow field because a slight change in rotor incidence could dramatically modify how the rotor works.

Presenting Author: Andrea Notaristefano Politecnico di Milano

Presenting Author Biography: Andrea Notaristefano received his Master's degree in Mechanical Engineering at the Politecnico di Milano. Currently, he is a PhD student at Politecnico di Milano in Turbomachinery Fluid Dynamics. His PhD focuses on combustor-turbine interaction with the aim of assessing the impact of combustor non-uniformities on the aero-thermal flow field of a first turbine stage.

Authors:

Andrea Notaristefano Politecnico di Milano
Paolo Gaetani Politecnico di Milano