On the response of a generic swirl flame to transverse planar acoustics

Heat release rate fluctuations are invariably generated at flames under the influence of acoustic and/or hydrodynamics fluctuations. At the same time, heat release rate fluctuations support acoustic fluctuations, such that, when flames are placed in a confined environment (as is usually the case in practical applications driven by combustion), a feedback loop may establish between heat release rate fluctuations and acoustic fluctuations corresponding to the acoustic resonance mode of the confinement. The associated large amplitude fluctuations (in heat release rate as well as pressure) are detrimental to the system and must, therefore, be avoided. Consequently, the stability limits of the combustor govern the operating envelope of the system. This envelope must, at the same time, be tuned to achieve low pollutant emissions. As an example, low NOx emissions can be achieved when running combustors in the lean premixed mode, where fuel-lean (or air-excess) reactant mixture is premixed before combustion. For this reason, lean premixed combustion was introduced in gas turbine combustors. Unfortunately, combustors are more susceptible to thermoacoustic instability when lean premixed flames are employed. For this reason, understanding, prediction, and mitigation of thermoacoustic instabilities has been a research involvement for engineers and researchers dealing with combustion, particularly in the last few decades, since the introduction of lean premixed combustion in gas turbine combustors. As the instability critically depends on the response of flames to perturbations, understanding flame response is key to understanding thermoacoustic coupling. This thesis falls under the category of investigations that focus on the study of the response of gaseous burner stabilized flames to acoustic forcing. In particular, the focus is the problem of thermoacoustic instability associated with a relatively recent development in gas turbine combustor configuration: annular combustion chambers. Existing information on flame response that is applicable to low frequency thermoacoustic instability is based on studies—experiments and numerical computations—dealing with acoustically-forced flames where the axis of propagation of planar acoustic waves is aligned with the burner axis. During thermoacoustic instability in annular combustors, individual flames experience a two-dimensional acoustic field, such that the flame is affected by acoustic fluctuations in