Assessment of Flame Transfer Function Formulations for the Thermoacoustic Analysis of Lean Burn Aero-engine Combustors

The numerical analysis of thermoacoustic instability in lean burn aero-engines requires proper Flame Transfer Functions (FTF) able to describe the complex physical phenomena characterizing the coupling between heat release rate fluctuations and the acoustic field which is further complicated by the use of liquid fuel together with advanced injection systems. In this work simple FTF formulations have been applied to the thermoacoustic analysis of a tubular combustor equipped with a PERM (Partially Evaporating and Rapid Mixing) injection system with the main aim of assessing their capabilities in the prediction of thermoacoustic instabilities in lean burn aero-engine combustors. The thermoacoustic behaviour of lean burn aero-engine combustors is strongly dependent on operating conditions and the simple FTF formulations assessed in this work seem to be inadequate to the study of the thermoacoustic stability in all conditions. This is mainly due to the presence of liquid fuel since the physical processes related to droplet evolution, in particular droplet breakup and evaporation, are highly influenced by the mean properties of the flow field determining different flame structures and thus a different dynamic response of the flame. In order to fulfill the objective of developing simple FTFs to be used in lean burn aero-engines, further investigations are required to understand the basic driving mechanisms that regulate the coupling between heat release rate fluctuations and the acoustic field with great attention to the impact of liquid fuel evolution and droplet dynamics.