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

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..

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

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

Large Eddy Simulation of flows in industrial compressors: a path from 2015 to 2035

A better understanding of turbulent unsteady flows is a necessary step towards a breakthrough in the design of modern compressors. Due to high Reynolds numbers and very complex geometry, the flow that develops in such industrial machines is extremely hard to predict. At this time, the most popular method to simulate these flows is still based on a Reynolds Averaged Navier-Stokes (RANS) approach. However there is some evidence that this formalism is not accurate for these components, especially when a description of time-dependent turbulent flows is desired. With the increase in computing power, Large Eddy Simulation (LES) emerges as a promising technique to improve both knowledge of complex physics and reliability of flow solver predictions. The objective of the paper is thus to give an overview of the current status of LES for industrial compressor flows as well as to propose future research axes regarding the use of LES for compressor design. While the use of wall-resolved LES for industrial multistage compressors at realistic Reynolds number should not be ready before 2035, some possibilities exist to reduce the cost of LES, such as wall-modelling and the adaptation of the phase lag condition. This paper also points out the necessity to combine LES to techniques able to tackle complex geometries. Indeed LES alone, i.e. without prior knowledge of such flows for grid construction or the prohibitive yet ideal use of fully homogeneous meshes to predict compressor flows, is quite limited today

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

Model of Dust Thermal Emission of Comet 67p-Churyumov-Gerasimenko for the Rosetta-MIRO Instrument

The ESA's Rosetta spacecraft will arrive at comet 67P/Churyumov-Gerasimenko in 2014. The study of gas and dust emission is primary objective of several instruments on the Rosetta spacecraft, including the Microwave Instrument for the Rosetta Orbiter (MIRO). We developed a model of dust thermal emission to estimate the detectability of dust in the vicinity of the nucleus with MIRO. Our model computes the power received by the MIRO antenna in limb viewing as a function of the geometry of the observations and the physical properties of the grains. We show that detection in the millimeter and submillimeter channels can be achieved near perihelion