Interactions between Multiple Unstable Transversal Modes in High-Frequency Thermoacoustic Systems

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In the past, linear and nonlinear dynamics related to non-compact, multi-dimensional, high-frequency thermoacoustic oscillations in air-breathing, premixed combustion systems have been investigated with a single mode assumption. This implies the existence of only one linearly unstable acoustic mode that governs the limit-cycle oscillations, while further modes are presumed as linearly stable and of negligible relevance. However, this single mode notion is questionable especially in the high-frequency regime beyond the concerned combustion chamber's cut-on frequency, where multiple linearly unstable modes exist and interact. A non-consideration of all possibly unstable modes would compromise a stable and reliable combustor operation. For the first time, the proposed project seeks to establish a fundamental understanding on physical mechanisms that govern high-frequency thermoacoustic oscillations constituted by multiple unstable modes. For this purpose, a lab-scale, swirl-stabilized, premixed combustor in which multiple transversal modes in the high-frequency regime are linearly unstable serves as a benchmark system. Specifically, the proposed project is divided into three main research objectives: The first one aims to unravel linear flame driving mechanisms and nonlinear combustor dynamics of the second transversal mode. This includes, besides understanding of principal physics, the development of appropriate mathematical quantification and simulation tools. Specific knowledge on interaction mechanisms between multiple unstable transversal modes in terms of physical explanations and modeling approaches shall be generated through the second objective. Finally, the third objective seeks to develop and validate reliable system identification methodologies to extract relevant thermoacoustic parameters (e.g. linear growth rates) that are particularly applicable to multi-modal systems.