Intrinsic thermoacoustic instabilities

It is accepted that the thermoacoustic behavior of a given combustion system can be analyzed by investigating how its natural acoustic modes are perturbed by the flame dynamics. As a result, the resonance frequency and structure of the resulting thermoacoustic mode – understood as a perturbed acoustic mode – are slightly modified with respect to the natural acoustic mode counterpart. However, experimental evidence shows that the frequency of unstable thermoacoustic modes sometimes lies far away from the natural acoustic frequencies of the system under study. In many cases, this frequency cannot be associated with hydrodynamic or entropy-related instabilities. In recent years, the intrinsic thermoacoustic (ITA) feedback loop has been formally recognized as the responsible mechanism in some of those situations. Theory and devoted experiments have been developed that have enormously contributed to the understanding of the particular behavior of intrinsic thermoacoustic instabilities. The present review encapsulates in a single theoretical framework the theory presented in the collection of today existing ITA papers, which spread through different cases of study regarding acoustic boundaries – anechoic, partially or fully reflecting – and geometries – duct flames, combustors composed by three coaxial ducts and annular configurations. Several examples are shown that summarize the most relevant results on ITA theory to this day. This review paper also gives special attention to the categorization of ITA modes, given the fact that there is no current agreement on the definition of an ITA mode: one example in this review paper explicitly shows that the proposed categorization methods can indeed be contradictory. Of high interest is also the review of papers illustrating the coexistence of thermoacoustic modes of acoustic and ITA nature, which in turn relate to the recently discovered exceptional points in the thermoacoustic spectrum. Additionally, this paper discusses the ‘counter-intuitive’ evidence that shows that ITA modes can be destabilized when acoustic dissipative elements are added into the system. Finally, it is shown how a single-mode Galerkin expansion may be able to model some ITA eigenfrequencies. This result is suggested in some recent works and is not obvious. The practical relevance of ITA modes in industrial combustion chambers of gas turbines is also discussed together with suggestions for future studies.