Session: 43-01 Unsteady Flows in Turbines

Paper Number: 81707

81707 - The Role of Turbine Operating Conditions on Combustor-Turbine Interaction – Part 1: Change in Expansion Ratio 

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 1 of the two papers analyses the effect of the expansion ratio describing the outcomes for two different stage expansion ratios, namely 1.4 and 1.76, that lead to subsonic and transonic flow conditions in the stage. The OPs are chosen keeping the stator outlet flow angles at midspan equal. From an aerodynamic point of view, the comparison of these OPs allows to study the Reynolds number effect on combustor-turbine interaction. Furthermore, the interest of increasing the expansion ratio, thus the flow acceleration, is of concern regarding the aforementioned acoustic issue of combustor-turbine interaction, that will be object of future studies. Results show that for both the operating conditions the swirl profile mainly driven the changes in the stage aerodynamics. The aerothermal flow field downstream of the stator is impacted significantly by the combustor simulator disturbances. Conversely, rotor downstream the differences in aerodynamics between the uniform and non-uniform injections become negligible. However, the EW still persists at the stage outlet and it is transported differently depending on whether the OP and the injection positions are.

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