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

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

Perfect preferential orientation of nitrogen-vacancy defects in a synthetic diamond sample

We show that the orientation of nitrogen-vacancy (NV) defects in diamond can be efficiently controlled through chemical vapor deposition (CVD) growth on a (111)-oriented diamond substrate. More precisely, we demonstrate that spontaneously generated NV defects are oriented with a ~ 97 % probability along the [111] axis, corresponding to the most appealing orientation among the four possible crystallographic axes. Such a nearly perfect preferential orientation is explained by analyzing the diamond growth mechanism on a (111)-oriented substrate and could be extended to other types of defects. This work is a significant step towards the design of optimized diamond samples for quantum information and sensing applications.Comment: 6 pages, 4 figure

Histoire du LSD. De l’ergot de seigle à l’utilisation thérapeutique

National audienceLe LSD, de l’allemand Lysergsäurediethylamid, est une substance hallucinogène utilisée à but récréatif. Également connue sous le nom d’« acide », cette molécule à propriétés psychotropes est classée en France comme un stupéfiant illicite selon l’arrêté du 22 février 1990..

An Acceleration Method for Numerical Studies of Conjugate Heat Transfer With a Self-Adaptive Coupling Time Step Method: Application to a Wall-Impinging Flame

International audienceThe application of large-eddy simulations to conjugate heat transfer problems can promisingly provide accurate results, including fluctuating heat loads which are critical for thermal fatigue. Such simulations rely on separate solvers and a coupling methodology which must be accurate and robust. In this context, the Hybrid-Cell Neumann-Dirichlet (HCND) coupling approach can adapt dynamically the coupling frequency given a desired accuracy. However, in order to determine statistics (mean, RMS,. . .) in a permanent regime, this approach must benefit from an acceleration technique which is here first derived and validated. Two configurations of a wall-impinging flame are then simulated: a quasi-steady case and a pulsated case. The former enables to validate the ability of the accelerated HCND method to predict a steady state wall temperature, while the latter highlights the retained acceleration which does not alter the fluctuations in wall temperature and wall heat flux. Both cases benefit from the self-adaptation of the coupling period provided by the method

Teaching Sodium Fast Reactors in CEA

International audienceAmong the Fast Neutron Reactor Systems, the SFR has the most comprehensive technological basis as result of the experience gained from worldwide operation of several experimental, prototype, and commercial size reactors since the 1940s. This experience corresponds to about 402 years of operation by end of 2010. Six reactors are in operation BOR60, BN600 and BN800 in Russia, Joyo in Japan, FBTR in India and CEFR in China. One reactor is being commissioned PFBR (500MWe) in India and several projects are currently developed FBR1 and 2 in India, BN1200 in Russia, JSFR in Japan, PGSFR in Korea, CFR-600 in China and ASTRID in France. In order to support operation of existing reactors, design activities for new projects and decommissioning of old reactors, it is mandatory to develop skills, more particularly among the young generation, who will operate these new reactors. In addition, education and training is also essential to share the knowledge among the teams involved in Research and Development. Several strategies are developed at the national level, or within multilateral framework, like EU or IAEA to support development of Fast Reactors.In France, to answer to this increasing demand of Education and Training, four sessions are proposed, within the frame of INSTN (French National Institute for Nuclear Science and Technology)-SFR History, main options, design and operational feedback-SFR Functional analysis and design-SFR Safety and operation-SFR Sodium structures interactionsThe French Na School (ESML) provides since 1975 also several sessions dedicated to Na facilities design, safe operation, handling, and also decommissioning and sodium treatment. Beside courses, practical exercises are organized during each session. 10 different modules are available, ranging in length from 1 to 5 days. CEA contributes also to the organization of European Sessions dedicated to Sodium Fast Reactors, organized within the frame of the European Commission (CP-ESFR, ESNII+ , ESFR-SMART) This Education and Training strategy is a key element for the future of the development of Sodium Fast Reactors, and more particularly ASTRID project. CEA is ready to share training experience and to collaborate with other foreign Education and Training Entities

LES of bifurcation and hysteresis in confined annular swirling flows

This paper presents a LES based study of two swirling confined jet configurations corresponding to an aeronautical injection system. The objectives are to demonstrate that LES codes become sensitive to numerical parameters (grid, SGS model) in such cases and that this is due to the fact that these flows are close to bifurcating conditions because of the presence of swirl and confinement walls. To demonstrate this, in the first configuration ("full swirler"), the swirler/plenum ensemble is computed while only the swirler without plenum is computed in the second ("adjustable swirler"): this simplification allows to vary swirl continuously and explore bifurcation diagrams where the control parameter is the mean swirl number. These numerical results are compared to a similar study performed experimentally by Vanierschot and Van den Bulck (2007) [1]. They confirm that certain confined swirling flows are intrinsically submitted to bifurcations. In the context of LES this leads to a large sensitivity of the simulation results to numerical parameters, a property which is not observed in most other non swirling or non confined situations