Reconstruction and Analysis of the Acoustic Transfer Matrix of a Reheat Flame From Large-Eddy Simulations

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
Mirko Bothien ◽  
Demian Lauper ◽  
Yang Yang ◽  
Alessandro Scarpato
Keyword(s):  
Transfer Matrix ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Temperature Fluctuations ◽  
Large Eddy Simulations ◽  
Lean Premixed Flames ◽  
Single Output ◽  
Auto Ignition ◽  
Large Eddy ◽  
Flame Response

Lean premix technology is widely spread in gas turbine combustion systems, allowing modern power plants to fulfill very stringent emission targets. These systems are, however, also prone to thermoacoustic instabilities, which can limit the engine operating window. The thermoacoustic analysis of a combustor is thus a key element in its development process. An important ingredient of this analysis is the characterization of the flame response to acoustic fluctuations, which is straightforward for lean-premixed flames that are propagation stabilized, since it can be measured atmospherically. Ansaldo Energia's GT26 and GT36 reheat combustion systems feature a unique technology where fuel is injected into a hot gas stream from a first combustor, which is propagation stabilized, and auto-ignites in a sequential combustion chamber. The present study deals with the flame response of mainly auto-ignition stabilized flames to acoustic and temperature fluctuations for which a computational fluid dynamics system identification (SI) approach is chosen. The current paper builds on recent works, which detail and validate a methodology to analyze the dynamic response of an auto-ignition flame to extract the flame transfer function (FTF) using unsteady large-Eddy simulations (LES). In these studies, the flame is assumed to behave as a single-input single-output (SISO) or a multi-input single-output (MISO) system. The analysis conducted in GT2015-42622 qualitatively highlights the important role of temperature and equivalence ratio fluctuations, but these effects are not separated from velocity fluctuations. Hence, this topic is addressed in GT2016-57699, where the flame is treated as a multiparameter system and compressible LES are conducted to extract the frequency-dependent FTF to describe the effects of axial velocity, temperature, equivalence ratio, and pressure fluctuations on the flame response. For lean-premixed flames, a common approach followed in the literature assumes that the acoustic pressure is constant across the flame and that the flame dynamics are governed by the response to velocity perturbations only, i.e., the FTF. However, this is not necessarily the case for reheat flames that are mainly auto-ignition stabilized. Therefore, in this paper, we present the full 2 × 2 transfer matrix of a predominantly auto-ignition stabilized flame, and hence, describe the flame as a multi-input multi-output (MIMO) system. In addition to this, it is highlighted that in the presence of temperature fluctuations, the 2 × 2 matrix can be extended to a 3 × 3 matrix relating the primitive acoustic variables as well as the temperature fluctuations across the flame. It is shown that only taking the FTF is insufficient to fully describe the dynamic behavior of reheat flames.

Reconstruction and Analysis of the Acoustic Transfer Matrix of a Reheat Flame From Large-Eddy Simulations

2017 ◽  
Author(s):  
Mirko Bothien ◽  
Demian Lauper ◽  
Yang Yang ◽  
Alessandro Scarpato
Keyword(s):  
Transfer Matrix ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Temperature Fluctuations ◽  
Large Eddy Simulations ◽  
Lean Premixed Flames ◽  
Single Output ◽  
Auto Ignition ◽  
Large Eddy ◽  
Flame Response

Lean premix technology is widely spread in gas turbine combustion systems, allowing modern power plants to fulfill very stringent emission targets. These systems are, however, also prone to thermoacoustic instabilities, which can limit the engine operating window. The thermoacoustic analysis of a combustor is thus a key element in its development process. An important ingredient of this analysis is the characterization of the flame response to acoustic fluctuations, which is straightforward for lean-premixed flames that are propagation stabilized, since it can be measured atmospherically. Ansaldo Energia’s GT26 and GT36 reheat combustion systems feature a unique technology where fuel is injected into a hot gas stream from a first combustor, which is propagation stabilized, and auto-ignites in a sequential combustion chamber. The present study deals with the flame response of mainly auto-ignition stabilized flames to acoustic and temperature fluctuations for which a CFD system identification approach is chosen. The current paper builds on recent works, which detail and validate a methodology to analyze the dynamic response of an auto-ignition flame to extract the Flame Transfer Function (FTF) using unsteady Large-Eddy Simulations (LES). In these studies, the flame is assumed to behave as a Single-Input Single-Output (SISO) or Multi-Input Single-Output (MISO) system. The analysis conducted in GT2015-42622 qualitatively highlights the important role of temperature and equivalence ratio fluctuations, but these effects are not separated from velocity fluctuations. Hence, this topic is addressed in GT2016-57699, where the flame is treated as a multi-parameter system and compressible LES are conducted to extract the frequency-dependent FTF to describe the effects of axial velocity, temperature, equivalence ratio and pressure fluctuations on the flame response. For lean-premixed flames, a common approach followed in the literature assumes that the acoustic pressure is constant across the flame and that the flame dynamics are governed by the response to velocity perturbations only, i.e., the FTF. However this is not necessarily the case for reheat flames that are mainly auto-ignition stabilized. Therefore, in this paper we present the full 2 × 2 transfer matrix of a predominantly auto-ignition stabilized flame and hence describe the flame as a Multi-Input Multi-Output (MIMO) system. In addition to this, it is highlighted that in presence of temperature fluctuations the 2 × 2 matrix can be extended to a 3 × 3 matrix relating the primitive acoustic variables as well as the temperature fluctuations across the flame. It is shown that only taking the FTF is insufficient to fully describe the dynamic behavior of reheat flames.

Numerical Analysis of Equivalence Ratio Fluctuations in a Partially Premixed Gas Turbine Combustor Using Large Eddy Simulations

2018 ◽  
Vol 141 (4) ◽  
Author(s):  
Ping Wang ◽  
Qian Yu ◽  
Prashant Shrotriya ◽  
Mingmin Chen
Keyword(s):  
Reaction Mechanism ◽  
Equivalence Ratio ◽  
Flame Temperature ◽  
Premixed Flames ◽  
Large Eddy Simulations ◽  
Combustion Model ◽  
Inlet Channel ◽  
Large Eddy ◽  
Partially Premixed Flames ◽  
Partially Premixed

In the present work, the fluctuations of equivalence ratio in the PRECCINSTA combustor are investigated via large eddy simulations (LES). Four isothermal flow cases with different combinations of global equivalence ratios (0.7 or 0.83) and grids (1.2 or 1.8 million cells) are simulated to study the mixing process of air with methane, which is injected into the inlet channel through small holes. It is shown that the fluctuations of equivalence ratio are very large, and their ranges are [0.4, 1.3] and [0.3, 1.2] for cases 0.83 and 0.7, respectively. For simulating turbulent partially premixed flames in this burner with the well-known dynamically thickened flame (DTF) combustion model, a suitable multistep reaction mechanism should be chosen aforehand. To do that, laminar premixed flames of 15 different equivalence ratios are calculated using three different methane/air reaction mechanisms: 2S_CH4_BFER, 2sCM2 reduced mechanisms and GRI-Mech 3.0 detailed reaction mechanism. The variations of flame temperature, flame speed and thickness of the laminar flames with the equivalence ratios are compared in detail. It is demonstrated that the applicative equivalence ratio range for the 2S_CH4_BFER mechanism is [0.5, 1.3], which is larger than that of the 2sCM2 mechanism [0.5, 1.2]. Therefore, it is recommended to use the 2S_CH4_BFER scheme to simulate the partially premixed flames in the PRECCINSTA combustion chamber.

scholarly journals Understanding the diesel-like spray characteristics applying a flamelet-based combustion model and detailed large eddy simulations

2019 ◽  
Vol 21 (1) ◽  
pp. 134-150 ◽  
Author(s):  
Eduardo J Pérez-Sánchez ◽  
Jose M Garcia-Oliver ◽  
Ricardo Novella ◽  
Jose M Pastor
Keyword(s):  
Flame Stabilization ◽  
Large Eddy Simulations ◽  
Combustion Model ◽  
Progress Variable ◽  
Auto Ignition ◽  
Large Eddy ◽  
Engine Combustion ◽  
Spatial Fields ◽  
Lift Off ◽  
Good Agreement

This investigation analyses the structure of spray A from engine combustion network (ECN), which is representative of diesel-like sprays, by means of large eddy simulations and an unsteady flamelet progress variable combustion model. A very good agreement between modelled and experimental measurements is obtained for the inert spray that supports further analysis. A parametric variation in oxygen concentration is carried out in order to describe the structure of the flame and how it is modified when mixture reactivity is changed. The most relevant trends for the flame metrics, ignition delay and lift-off length are well-captured by the simulations corroborating the suitability of the model for this type of configuration. Results show that the morphology of the flame is strongly affected by the boundary conditions in terms of the reactive scalar spatial fields and Z–T maps. The filtered instantaneous fields provided by the simulations allow investigation of the structure of the flame at the lift-off length, whose positioning shows low fluctuations, and how it is affected by turbulence. It is evidenced that small ignition kernels appear upstream and detached from the flame that eventually merge with its base in agreement with experimental observations, leading to state that auto-ignition plays a key role as one of the flame stabilization mechanisms of the flame.

scholarly journals An assessment of large eddy simulations of premixed flames propagating past repeated obstacles

2009 ◽  
Vol 13 (3) ◽  
pp. 513-540 ◽  
Author(s):  
S. R. Gubba ◽  
S. S. Ibrahim ◽  
W. Malalasekera ◽  
A. R. Masri
Keyword(s):  
Premixed Flames ◽  
Large Eddy Simulations ◽  
Large Eddy

Assessment of the validity of RANS knock prediction using the resonance theory

2019 ◽  
Vol 21 (4) ◽  
pp. 610-621 ◽  
Author(s):  
Corinna Netzer ◽  
Lars Seidel ◽  
Frédéric Ravet ◽  
Fabian Mauss
Keyword(s):  
Numerical Simulations ◽  
Direct Numerical Simulations ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Post Processing ◽  
Resonance Theory ◽  
Auto Ignition ◽  
Engine Knock ◽  
Large Eddy ◽  
Reynolds Averaged Navier Stokes

Following the resonance theory by Bradley and co-workers, engine knock is a consequence of an auto-ignition in the developing detonation regime. Their detonation diagram was developed using direct numerical simulations and was applied in the literature to engine knock assessment using large eddy simulations. In this work, it is analyzed if the detonation diagram can be applied for post-processing and evaluation of predicted auto-ignitions in Reynolds-averaged Navier–Stokes simulations even though the Reynolds-averaged Navier–Stokes approach cannot resolve the fine structures resolved in direct numerical simulations and large eddy simulations that lead to the prediction of a developing detonation. For this purpose, an engine operating point at the knock limit spark advance is simulated using Reynolds-averaged Navier–Stokes and large eddy simulations. The combustion is predicted using the G-equation and the well-stirred reactor model in the unburnt gases based on a detailed gasoline surrogate reaction scheme. All the predicted ignition kernels are evaluated using the resonance theory in a post-processing step. According to the different turbulence models, the predicted pressure rise rates and gradients differ. However, the predicted ignition kernel sizes and imposed gas velocities by the auto-ignition event are similar, which suggests that the auto-ignitions predicted by Reynolds-averaged Navier–Stokes simulations can be given a meaningful interpretation within the detonation diagram.

LES Evaluation of Non-Linear Effects on the Dynamic Flame Response in a Real Gas Turbine Combustion Chamber

2013 ◽  
Author(s):  
Sebastian Hermeth ◽  
Gabriel Staffelbach ◽  
Laurent Gicquel ◽  
Thierry Poinsot
Keyword(s):  
Gas Turbine ◽  
Linear Response ◽  
Equivalence Ratio ◽  
High Amplitude ◽  
Linear Case ◽  
Turbine Engine ◽  
Real Gas ◽  
Large Eddy ◽  
Non Linear ◽  
Flame Response

Large Eddy Simulations (LES) of a lean swirl-stabilized gas turbine burner are used to analyze the non-linear response of the flame in a real gas turbine engine. For this study, two LES of the same geometry are performed: one where the inlet is acoustically modulated with a low perturbation amplitude in order to obtain a linear response, and one where a high amplitude forcing is applied. The high amplitude forced LES reveals that the averaged mean flame shape changes significantly compared to the linearly forced LES. During the oscillation cycle the flame undergoes strong variations in shape, with a typical mushroom shape roll-up. Based on these predictions, the measured global flame response shows that a high forcing amplitude does not only lead to a change in the amplitude response but also changes its delay quite significantly. Although different, the global gain remains quite similar between the linear and non-linear case. However, local variations of this index clearly point to the non compactness of these flames. This also means that high local gain values have opposite effects in regions where the local time delay changes rapidly. One mechanism by which the two configurations differ is revealed by studying PDF of equivalence ratio. In the non-linear case significantly stronger equivalence ratio perturbations act on the flame.

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