“… Figure 2. Comparisonbetween locations of the flame front deduced using LSA ( a ) and Schlieren techniques [ 48 , 49 ] ( b ) of a laminar conical premixed flame when the flame is modulated at f p =100 Hz for a perturbation level . , where .…”
Section: Determining the Flame Describing Functionmentioning
Combustion instabilities arise owing to a two-way coupling between acoustic waves and unsteady heat release. Oscillation amplitudes successively grow, until nonlinear effects cause saturation into limit cycle oscillations. Feedback control, in which an actuator modifies some combustor input in response to a sensor measurement, can suppress combustion instabilities. Linear feedback controllers are typically designed, using linear combustor models. However, when activated from within limit cycle, the linear model is invalid, and such controllers are not guaranteed to stabilize. This work develops a feedback control strategy guaranteed to stabilize from within limit cycle oscillations. A low-order model of a simple combustor, exhibiting the essential features of more complex systems, is presented. Linear plane acoustic wave modelling is combined with a weakly nonlinear describing function for the flame. The latter is determined numerically using a level set approach. Its implication is that the open-loop transfer function (OLTF) needed for controller design varies with oscillation level. The difference between the mean and the rest of the OLTFs is characterized using the ν-gap metric, providing the minimum required ‘robustness margin’ for an scriptH∞ loop-shaping controller. Such controllers are designed and achieve stability both for linear fluctuations and from within limit cycle oscillations.
“… Figure 2. Comparisonbetween locations of the flame front deduced using LSA ( a ) and Schlieren techniques [ 48 , 49 ] ( b ) of a laminar conical premixed flame when the flame is modulated at f p =100 Hz for a perturbation level . , where .…”
Section: Determining the Flame Describing Functionmentioning
Combustion instabilities arise owing to a two-way coupling between acoustic waves and unsteady heat release. Oscillation amplitudes successively grow, until nonlinear effects cause saturation into limit cycle oscillations. Feedback control, in which an actuator modifies some combustor input in response to a sensor measurement, can suppress combustion instabilities. Linear feedback controllers are typically designed, using linear combustor models. However, when activated from within limit cycle, the linear model is invalid, and such controllers are not guaranteed to stabilize. This work develops a feedback control strategy guaranteed to stabilize from within limit cycle oscillations. A low-order model of a simple combustor, exhibiting the essential features of more complex systems, is presented. Linear plane acoustic wave modelling is combined with a weakly nonlinear describing function for the flame. The latter is determined numerically using a level set approach. Its implication is that the open-loop transfer function (OLTF) needed for controller design varies with oscillation level. The difference between the mean and the rest of the OLTFs is characterized using the ν-gap metric, providing the minimum required ‘robustness margin’ for an scriptH∞ loop-shaping controller. Such controllers are designed and achieve stability both for linear fluctuations and from within limit cycle oscillations.
“…Examples of careful use of OH* and CH* for inferring overall features of the combustion system are given in the thermoacoustics literature (e.g. [55][56][57][58]).…”
The development of better laser-based experimental methods and the fast rise in computer power has created an unprecedented shift in turbulent combustion research. The range of species and quantities measured and the advent of kHz-level planar diagnostics are now providing great insights in important phenomena and applications such as local and global extinction, pollutants, and spray combustion that were hitherto unavailable. In simulations, the shift to LES allows better representation of the turbulent flow in complex geometries, but despite the fact that the grid size is smaller than in RANS, the push towards realistic conditions and the need to include more detailed chemistry that includes very fast species and thin reaction zones emphasize the necessity of a sub-grid turbulent combustion model. The paper discusses examples from current research with experiments and modelling that focus on flame transients (self-excited oscillations, local extinction), sprays, soot emissions, and on practical applications. These demonstrate how current models are being validated by experimental data and the concerted efforts the community is taking to promote the modelling tools to industry. In addition, the various coordinated International Workshops on non-premixed, premixed, and spray flames, and on soot are discussed and some of their target flames are explored. These comprise flames that are relatively simple to describe from a fluid mechanics perspective but contain difficult-to-model combustion problems such as extinction, pollutants and multi-mode reaction zones. Recently, swirl spray flames, which are more representative of industrial devices, have been added to the target flames. Typically, good agreement is found with LES and some combustion models such as the progress variable-mixture fraction flamelet model, the Conditional Moment Closure, and the Transported PDF method, but predicting soot emissions and the condition of complete extinction in complex geometries is still elusive.
“…Palies et al [2010] studied the flame dynamics under external-force excitation by using ICCD, which states unsteady heat release is due to the velocity pulsation. Li et al [2015] investigated the effect of acoustic perturbation on the hear release pulsation. Kim et al[2009] studied the dynamics of flame by OH*、CH* and CO2* chemiluminescence for different synthesis gas.…”
An experimental study on combustion instability is presented with focus on propane-air premixed swirling flames. Swirling flames under self-excited oscillation are studied by imaging of visible light and OH* chemiluminescence filter under several typical conditions. The dynamical characteristics of swirling flames were analysed by Dynamic Mode Decomposition (DMD) method. Three types of unstable modes in the combustor system were observed, which correspond to typical acoustic resonant modes (LF mode, C 1/4 mode and P 1/2 mode) of the combustor system. The combustion instability is in the longitudinal mode. Furthermore, the structure of downstream hot burnt gas under stable combustion and unstable combustion is studied by imaging of visible light and near-infrared light. Results show that there is a significant difference in the downstream flow under stable combustion and unstable combustion. The DMD spectrum of the flame and the downstream hot burnt gas obtained is the same, which is close to the characteristic frequency of acoustic pressure captured by the microphone signal. The visible light and near-infrared light imaging observation method adopted in this paper provides a new imaging method for the investigation of thermo-acoustic instability.
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