Service Communautaire d'Information sur la Recherche et le Développement - CORDIS

Final Activity Report Summary - AETHER (Aero-acoustical and thermo-acoustical coupling in energy processes)

AETHER has constituted a unique and highly qualified training and research environment for early stage researchers and experienced researchers developing their careers into the field of energy engineering. The network, composed of 6 universities, 3 research centres and 5 industries, has been targeting three main sectors of activity: - gas turbines for electricity production, - industrial heaters for manufacturing (e.g. steel) industry, - domestic and district heating systems, e.g. based on distributed co-generation plants. The scientific objectives of AETHER addressed technological issues shared by these three sectors: gas transport and combustion systems are very sensitive to flow and acoustic perturbations. This sensitivity manifests itself as flow-induced pulsations and combustion instabilities that are not only detrimental to the performance and lifetime of the equipment, but also lead to increased emissions of pollutant species.

The scientific program, combining theoretical, numerical and experimental approaches, has been articulated around a number of Work Packages, aiming at:
- a better understanding of the elementary processes through which combustion and unsteady flow produce noise and are receptive to it;
- the modelling of these physical mechanisms through aero-acoustic and thermo-acoustic transfer functions and describing functions, both in the linear and nonlinear regimes;
- the incorporation of the modelled key elements of a given technological device into one-dimensional and three-dimensional simulations, capable to predict the stability limits, and eventually limit-cycle amplitude of the oscillations.

On the thermo-acoustic side, the behaviour of free premixed and diffusion flames in both laminar and turbulent regimes, submitted to flow rate modulations and mixture composition oscillations, has been thoroughly studied. The objective was to show how flame transfer functions can be obtained experimentally, from first principles or through high-accuracy numerical approaches. Another very important aspect considered in AETHER was the effect of effusion cooling. Viscous dampers, or liners, have an important role in providing acoustic damping for engineering systems, particularly as cooling liners can in principle be optimised for the purpose. Extensive experiments in liner systems have demonstrated that in some cases simple hole and liner compliance models are adequate in capturing the essential features of liner absorption within the linear range.

On the aero-acoustic side, an important effort has been devoted to the accurate simulation of the transient features of confined flows responsible for the generation of aerodynamic noise. Some of this work consisted in characterising with success the aero-acoustic behaviour of elements of pipelines (in particular, T-joints) using surprisingly crude two-dimensional flow models. More sophisticated and CPU-intensive Computational Fluid Dynamics (CFD) techniques such as Large Eddy Simulation or Detached Eddy Simulation have also been combined with numerical acoustic techniques as the Finite Element Method (FEM) or the Boundary Element Method (BEM). Another important aspect that was tackled during the project, though not anticipated at its beginning, was the role played by non-linear and non-normal effects on the flame and/or flow response. It has appeared that further progress in the analysis and control of aero- and thermo-acoustic instabilities requires a deeper understanding of nonlinear and non-normal interactions between flow field and acoustics.

To conclude, the most important scientific achievements of the AETHER project have substantially contributed to improving available techniques for the analysis of energy transport and conversion systems using both high-fidelity CPU-intensive and low-order fast methods. These tools have been developed for the analysis of complex aero- and thermo-acoustic systems; in particular, for geometries such as found in the three sectors of activity targeted.

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