Analysis of the thermo-acoustic response of an aero-engine injector using compressible LES

Authors

Cetin Ozan Alanyalioglu - Darmstadt Technical University, Germany
Hanna Reinhardt - Darmstadt Technical University, Germany
André Fischer - Rolls-Royce Deutschland, Germany
Claus Lahiri - Rolls-Royce Deutschland, Germany
Hendrik Nicolai - Darmstadt Technical University, Germany
Christian Hasse - Darmstadt Technical University, Germany

Abstract

The stringent emission requirements for the next generation of aero-engines have increased interest in combustion chamber designs that employ lean-burn combustion technologies. Although lean combustion offers promising reduction in NOx and particulate emissions, the flame becomes more susceptible to suffering from thermoacoustic instabilities. Such instabilities can become a major challenge in engine operation and development, which requires understanding the two-way interactions between the acoustics and the flame. Therefore, it becomes critical to characterize the flame acoustic response during the design cycle to assess overall combustion system stability. For this purpose, in addition to experimental studies, unsteady 3D simulations of the turbulent reactive flow are increasingly employed.  In particular, when using scale-resolving methods such as large-eddy simulations (LES), the high-resolution spatial and temporal information about the flow, mixing and combustion can provide unique insights into the underlying physical mechanisms. Such information is usually challenging to obtain through experimental measurements only under practically relevant operating conditions. Thus, the close integration of experiment and simulation is of particular importance, and the measurements provide essential reference and validation data for the LES. Considering that the thermoacoustic response of an aero-engine injector is determined by the strong interaction between turbulent flow, combustion and acoustics, the SCARLET (SCaled Acoustic Rig for Low Emission Technologies) single sector test rig provides acoustically controlled upstream and downstream conditions to characterize the acoustic response of full-scale aero-engine under realistic operating conditions. Optical flame transfer function measurements using OH* chemiluminescence are obtained as well as flame transfer matrix measurements using an entirely acoustical approach based on microphone data. In this work, we focus on reactive compressible LES of the SCARLET rig to predict the flame acoustic response. Acoustic forcing is carried out using a broadband excitation signal implemented through characteristics-based boundary conditions. The combustion process is modeled using a flamelet-based tabulated chemistry approach coupled with presumed probability density functions for the mixture fraction and the progress variable for turbulence chemistry interaction. Flame transfer functions, obtained using the global heat release rate fluctuations, are compared against experimentally measured optical flame transfer functions based on OH* chemiluminescence, as well as the flame transfer functions derived from acoustically obtained flame transfer matrices in conjunction with Rankine-Hugoniot relations. The results are compared with experimentally measured flame transfer functions and analyzed comprehensively considering underlying assumptions of each method.