Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

LES evaluation of the effects of equivalence ratio fluctuations on the dynamic flame response in a real gas turbine combustion chamber

Large Eddy Simulations (LES) of a lean swirl-stabilized gas turbine burner are used to analyze mechanisms triggering combustion instabilities. To separately study the effect of velocity and equivalence ratio fluctuations, two LES of the same geometry are performed: one where the burner operates in a “technically” premixed mode (methane is injected by holes in the vanes located in the diagonal passage upstream of the chamber) and the second one where the flow is fully premixed in the diagonal passage. The inlet is acoustically modulated and the mechanisms affecting the dynamic flame response are identified. LES reveals that both cases provide similar averaged (non-)pulsated flame shapes. However, even though the mean flames are only slightly modified, the delays change when mixing is not perfect. LES fields and a simple model for the methane jets trajectories show that mixing in the diagonal passage is not sufficient to damp heterogeneities induced by unsteady fuel flow rate and varying fuel jet trajectories. These mixing fluctuations are phased with velocity oscillations and modify the flame response to forcing. Local fields of delays and interaction indices are obtained, showing that the flame is not compact and is affected by fluctuations of mixing

Large eddy simulation and acoustic analysis of multi-burner industrial gas turbines

Des mesures de plus en plus restrictives sur les émissions polluantes poussent les constructeurs de turbines à gaz à développer de nouvelles technologies ainsi qu'à faire travailler les machines dans des conditions opérationnelles inédites. Il arrive que ces conditions opérationnelles produisent des effets indésirables qui déclenchent des instabilités de combustion. Seuls des tests poussés depuis la phase de conception à la mise au point des derniers réglages permettent de prévoir ces instabilités. Mais ces tests sont quasiment impossibles sur une vraie turbine et souvent une version simplifiée est utilisée pour les essais expérimentaux ainsi que dans les études numériques. Une simplification habituelle est alors de n'étudier que l'un des brûleurs de la turbine complète (qui peut en compter plusieurs dizaines) et d'extrapoler ces résultats. Cette démarche rend impossible l'étude de deux mécanismes: l'interaction entre brûleurs voisins et le développement de modes acoustiques azimutaux dans des chambres annulaires. Dans cette thèse nous utilisons la Simulation aux Grandes Echelles (SGE) pour étudier ces deux effets et de fac¸on plus générale pour comprendre la structure des flammes dans ces foyers et leur stabilisation. Premièrement la stabilisation de flamme à l'aide de flammes pilote est étudié dans une configuation de laboratoire. Après, l'influence de modes azimutaux sur des turbines à gaz annulaires est évaluée en utilisant une méthode numérique développée pendant cette thèse qui permet d'évaluer l'impact de ces modes sur une turbine en n'étudiant que un seul brûleur. Puis une SGE d'un brûleur triple est présentée. L'impact des brûleurs latéraux sur le brûleur central est évalué. En dernier, la faisabilité d'une simulation complète de chambre annulaire est démontrée. ABSTRACT : Pollutant emissions restrictions have driven gas turbine manufacturers to employ new technologies and to operate these systems in extreme operating conditions. These operating conditions produce in some cases combustion instabilities which can have dramatic effects for the turbine. Extended experimental and numerical studies are then required to analyze the possible behavior of the end design. Unfortunately tests on the real set-up are not possible and simplified cases are used. Although gas turbines can contain up to 30 burners blowing into the same annular chamber, tests in laboratories are often performed on one single burner. This simplification obviously suppresses two mechanisms: burner/burner interaction and the possibility of acoustic azimuthal modes of the full annular chamber. The objective of this thesis is to use Large Eddy Simulation (LES) to investigate whether these mechanisms are important or not. An additional issue considered is the effect of the pilot fuel injection on flame stability. First, the influence of a pilot flame on the flame stabilization of a laboratory scale burner is studied. Then, using a numerical method developed during this thesis, the impact of an azimuthal mode on a turbine is analyzed using single burner LES. A LES of a triple burner configuration is presented and the impact of the side burners on the central burner analyzed. To finish, the feasibility of full chamber LES is demonstrated

Acoustic and Large Eddy Simulation studies of azimuthal modes in annular combustion chambers

The objectives of this paper are the description of azimuthal instability modes found in annular combus- tion chambers using two numerical tools: (1) Large Eddy Simulation (LES) methods and (2) acoustic solv- ers. These strong combustion instabilities are difficult to study experimentally and the present study is based on a LES of a full aeronautical combustion chamber. The LES exhibits a self-excited oscillation at the frequency of the first azimuthal eigenmode. The mesh independence of the LES is verified before ana- lysing the nature of this mode using various indicators over more than 100 cycles: the mode is mostly a pure standing mode but it transitions from time to time to a turning mode because of turbulent fluctu- ations, confirming experimental observations and theoretical results. The correlation between pressure and heat release fluctuations (Rayleigh criterion) is not verified locally but it is satisfied when pressure and heat release are averaged over sectors. LES is also used to check modes predicted by an acoustic Helmholtz solver where the flow is frozen and flames are modelled using a Flame Transfer Function (FTF) as done in most present tools. The results in terms of mode structure compare well confirming that the mode appearing in the LES is the first azimuthal mode of the chamber. Moreover, the acoustic solver provides stability maps suggesting that a reduction of the time delay of the FTF would be enough to sta- bilise the mode. This is confirmed with LES by increasing the flame speed and verifying that this modi- fication leads to a damped mode in a few cycles

qprof: a gprof-inspired quantum profiler

We introduce qprof, a new and extensible quantum program profiler able to generate profiling reports of various quantum circuits. We describe the internal structure and working of qprof and provide three practical examples on practical quantum circuits with increasing complexity. This tool will allow researchers to visualise their quantum implementation in a different way and reliably localise the bottlenecks for efficient code optimisation.Comment: 10 pages, 11 figure

Preparing a Fortran legacy code for the upcoming exascale architectures

Preparing legacy codes for the upcoming exascale systems is a timely topic since the unveiling of the Frontier system in June 2022. In this work we describe the steps taken to prepare the AVBP code for this new step in computing ressources. AVBP [6] is a parallel CFD code that solves the three-dimensional compressible Navier-Stokes equations on unstructured and hybrid grids. AVBP is a cutting-edge software when it comes to distributed memory CPUs, scaling efficiently up to 200.000's of cores on Bluegene or AMD Epyc2 systems. However, other types of architectures such as ARM processors and accelerators are gaining popularity and play a significant role in the exascale era. We first explore the usage of ARM processors, then GPU accelerators through OpenACC[2] directives. This work highlights the difficulties of porting a legacy code to those architectures and solutions implememented so far for performance

LES of longitudinal and transverse self-excited combustion instabilities in a bluff-body stabilized turbulent premixed flame

Combustion dynamics of a V-flame in an afterburner-type configuration are investigated using high-order compressible large eddy simulations (LES) and compared to experimental results. Both self-excited longitudinal (100 Hz) and transverse (1400 Hz) modes observed in the experiments are captured by LES and instability mechanisms are discussed. LES results for all modes are compared to a Helmholtz solver output, showing that the transverse mode appearing in the LES is the 1Lx-2Ty-0Tz eigenmode of the chamber, affecting the velocity field symmetrically. The 1Lx fluctuation causes a symmetric flame roll-up which increases heat release rate fluctuations, closing the feedback loop. The 2Ty component of the mode is active along the flame holder axis and causes not only transverse fluctuations but also a reorganization of the mean flame along two main zones located on both sides of the zero acoustic velocity plane, a feature that has not been reported before. Dynamic mode decomposition (DMD) is used to extract the structure of the transverse mode from LES snapshots which is found to match the Helmholtz solver prediction. This study confirms the capacity of high-order LES to capture not only low-frequency oscillations but also high-order frequency transverse modes in combustion chambers

LES and acoustic analysis of thermo-acoustic instabilities in a partially premixed model combustor

Numerical simulations were performed using Large Eddy Simulation (LES) and acoustic analysis tools to study thermo-acoustic instabilities in an academic burner. The configuration studied corresponds to a methane/air burner installed at the University of Twente (The Netherlands). It operates under fuel-lean partially premixed conditions at atmospheric pressure, and was built to study thermo-acoustic instabilities in conditions representative of gas turbine Lean Premixed systems: gaseous fuel is injected upstream of the combustor and has a limited time to mix with air. Even though the objective is to burn in a premixed mode, the actual regime corresponds to a partially premixed flame where strong equivalence ratio variations are created especially during combustion instabilities. Capturing these modes with LES is a challenge: here, simulations for both stable and unstable regimes are performed. In the unstable case, the limit cycle oscillations (LCO) are characterized and compared to experimental results. Reasonable agreement is found between simulations and experiments