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Modeling and identification of Limit Cycle Oscillations in thermoacoustic instabilities
Thermoacoustic instabilities are nonlinear phenomena that arise when acoustic waves adversely couple with unsteady heat release, determining the onset of Limit Cycle Oscillations. The project aims at deriving models and system identification approaches for their investigations.
Keywords: System identification; limit cycles; thermoacoustics; periodic systems; nonlinearities
In thermoacoustic instabilities, a detrimental interaction between acoustic waves and heat transfer determines the onset of self-sustained oscillations, or Limit Cycle Oscillations (LCO), in the system. This problem is particularly relevant in jet engines, where a complex combustion dynamics takes place, and therefore research in this domain has largely focused on suppressing LCO to improve efficiency. Conversely, from an energy harvesting perspective, one could think of exploiting the LCO to extract mechanical power from the heat-excited waves and convert it into electricity. Accurate models are thus paramount in order to enable reliable predictions and feedback control solutions to the active suppression (or harvesting) problem.
Most of the literature for control-oriented models has considered simplified representations. While this has proven sufficient to justify the presence of LCO, research has suggested that it might not be able to capture other behaviours, like for example non-harmonic oscillators and the coexistence of multiple attractors.
Modeling, and the closely related task of system identification, of thermoacoustic instabilities is thus still an open problem and an active area of research.
In thermoacoustic instabilities, a detrimental interaction between acoustic waves and heat transfer determines the onset of self-sustained oscillations, or Limit Cycle Oscillations (LCO), in the system. This problem is particularly relevant in jet engines, where a complex combustion dynamics takes place, and therefore research in this domain has largely focused on suppressing LCO to improve efficiency. Conversely, from an energy harvesting perspective, one could think of exploiting the LCO to extract mechanical power from the heat-excited waves and convert it into electricity. Accurate models are thus paramount in order to enable reliable predictions and feedback control solutions to the active suppression (or harvesting) problem. Most of the literature for control-oriented models has considered simplified representations. While this has proven sufficient to justify the presence of LCO, research has suggested that it might not be able to capture other behaviours, like for example non-harmonic oscillators and the coexistence of multiple attractors. Modeling, and the closely related task of system identification, of thermoacoustic instabilities is thus still an open problem and an active area of research.
The project articulates around two steps.
In the first, a numerical model to quantitatively describe thermoacoustic instabilities will be derived. This model will overcome limitations and/or simplifications of common control-oriented approaches and will provide a thorough understanding of the most important physical mechanisms featuring the problem.
In the second step, system identification strategies tailored for nonlinear systems experiencing LCOs will be developed. Nonlinear system identification is a far less developed topic than its linear counterpart, and approaches able to encode the existence of an LCO as qualitative constraint of the identification process are of great interest in many different engineering domains. A starting point for the research could be represented by classic nonlinear system identification schemes (e.g. Hammerstein–Wiener models; basis functions-based black box, semi-physical modelling grey box), moving then to approaches which directly exploit the information of periodicity of the response (in the form of LCO) and the physical knowledge gained in the first step. Aspects of experiment design will also be considered owing to the known problem of identifiability of oscillating systems.
The tasks of the project include:
1. literature review on the topic;
2. development of a model which captures the most common thermoacoustic instabilities;
3. proposal of system identification algorithms tailored for nonlinear systems exhibiting LCOs
The project articulates around two steps. In the first, a numerical model to quantitatively describe thermoacoustic instabilities will be derived. This model will overcome limitations and/or simplifications of common control-oriented approaches and will provide a thorough understanding of the most important physical mechanisms featuring the problem. In the second step, system identification strategies tailored for nonlinear systems experiencing LCOs will be developed. Nonlinear system identification is a far less developed topic than its linear counterpart, and approaches able to encode the existence of an LCO as qualitative constraint of the identification process are of great interest in many different engineering domains. A starting point for the research could be represented by classic nonlinear system identification schemes (e.g. Hammerstein–Wiener models; basis functions-based black box, semi-physical modelling grey box), moving then to approaches which directly exploit the information of periodicity of the response (in the form of LCO) and the physical knowledge gained in the first step. Aspects of experiment design will also be considered owing to the known problem of identifiability of oscillating systems. The tasks of the project include:
1. literature review on the topic;
2. development of a model which captures the most common thermoacoustic instabilities;
3. proposal of system identification algorithms tailored for nonlinear systems exhibiting LCOs
- Dr. Andrea Iannelli, iannelli@control.ee.ethz.ch
- Prof. Roy Smith, rsmith@control.ee.ethz.ch
- Dr. Andrea Iannelli, iannelli@control.ee.ethz.ch - Prof. Roy Smith, rsmith@control.ee.ethz.ch