Thermoacoustic oscillations in can-annular combustors

• Applicant: Alessandro Orchini, Ph.D.
• Subject Area: Fluid Mechanics
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 422037803

Thermoacoustic oscillations have been a plaguing issue in the development of high-performance, low-emission combustion systems for more than half a century - and continue to do so, in particular for stationary gas turbines. This dynamic phenomenon results from an interaction of unsteady combustion and the acoustic resonances of the chamber and is strongly undesirable because it severely restricts the operating range of the engine. While much of the research over the last decade in this field has been devoted to annular combustors, present high-efficiency H-class gas turbines exclusively feature can-annular combustor architectures. Very little literature exists on the subject. Nonetheless, recent work performed at Ansaldo Energia Switzerland shows that industry itself has started investigating the physics of can-annular combustors.In this type of system, combustion takes place in a number of cans (typically 12 or 16), without any coupling of aerodynamics and thermodynamics between the cans. The annular turbine inlet, common to all cans, however, provides for acoustic coupling between adjacent cans. On an abstract level, the thermoacoustic dynamics of a can-annular combustor can then be characterized as a system of weakly coupled self-excited oscillators. One oscillator represents an acoustic mode of an isolated can, driven by the flame, and weak coupling is provided by the gap between two cans at the turbine inlet. This type of system features ubiquitous appearance in nature and technology and is known to exhibit peculiar linear and nonlinear phenomena, such as localization and synchronization. Although can-annular systems have been used for power generation for quite some time, the role of these phenomena in thermoacoustics is presently neither appreciated nor understood. This project therefore has the following objectives: 1) reveal the generic thermoacoustic properties of can-annular systems, 2) characterize and model the aeroacoustic coupling between two cans, 3) establish a modeling and prediction framework for thermoacoustic oscillations in can-annular combustors, and 4) develop and validate mitigation measures for unstable modes in can-annular systems. The generic thermoacoustic properties are exposed based on linear and nonlinear analyses of can-annular model systems, with particular focus on localization and synchronization phenomena and on the effect of noise. The acoustic coupling between two cans is investigated in a dedicated experimental set-up. Acoustic transmission coefficients between two cans are measured in the linear and nonlinear regime, and detailed flow investigations of the aerodynamic phenomena at the gap allow for physics-based modeling of the coupling mechanism. A generic can-annular experiment with electro-acoustic feedback serves as a modular test-bench to verify the theoretical findings and the modeling framework, and to explore the effect of coupling, noise and asymmetry parameters on the system dynamics.

• DFG Programme: Research Grants
• International Connection: Norway, Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Cooperation Partners: Professor Dr.-Ing. Jonas Moeck; Professor Dr. Nicolas Noiray