Erroneous Sources of Combustion noise in Acoustic Perturbation Equations (ESCAPE)

• Applicant: Professor Wolfgang Polifke, Ph.D.
• Subject Area: Technical Thermodynamics ; Energy Process Engineering ; Hydraulic and Turbo Engines and Piston Engines
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558721881

Thermoacoustic combustion instability and combustion noise are a hindrance to the development of flexible, reliable, low-emission, and carbon-neutral combustion technology for power generation and propulsion. These phenomena are a result of flow/flame/acoustic interactions in feedback across a wide range of scales. Their modelling, analysis and prediction with high-fidelity simulation is extremely demanding and compute intensive. Hybrid approaches, which separate the modelling of large and small scales, can reduce the computational demand by orders of magnitude. One such hybrid approach are the acousic perturbation equations (APE), developed originally for the prediction of aeroacoustic noise, and subsequently extended to reactive flows. Like other formulations of acoustic analogies, the APE separate the governing equations in a generalized wave operator and source terms, respectively. Analysis of thermo-acoustic sources in reactive flows with the APE framework has found source terms of significant strength in addition to the unsteady rate of heat release. In particular, acceleration of entropy gradients across the flame was found to be a significant, even dominant contributor to the overall sound emission. This finding is in contradiction to established understanding, which identifies unsteady heat release as the sole -- or at least the most important -- acoustic source in reactive flows. A satisfactory resolution of this controversy has not yet been presented. The goal of the ESCAPE project is to develop an explanation of these findings and elaborate their consequences. For clarification of this fundamental question, we will employ recently developed, original concepts and tools, in particular the linearized reactive flow equations and an arbitrary Lagrangian-Eulerian framework for (thermo-)acoustic sources. The research hypothesis is that the additional sources in the APE of reactive flows are spurious in the sense that they result from a misrepresention of flame movement in the framework of a perturbation equation with non-uniform base flow. If the results confirm this hypothesis, it will be necessary to either revise the interpretation of the source terms, or the formulation of the APE and related forms of perturbation equations. Conversely, if the hypothesis is refuted it will be necessary to introduce additional sources of perturbation energy in established hybrid approaches of flow/flame/acoustic interactions. ESCAPE addresses important unresolved questions in regard to the consistent and comprehensive modelling of thermoacoustic instability and combustion noise. The results of the project will increase our understanding of the fundamental physics of thermo-acoustic interactions and support the further development of efficient, accurate and comprehensive reduced-order modelling tools for the design of sustainable combustion technology.

• DFG Programme: Research Grants