A spectral deferred correction strategy for low Mach number reacting flows subject to electric fields

We propose an algorithm for low Mach number reacting flows subjected to electric field that includes the chemical production and transport of charged species. This work is an extension of a multi-implicit spectral deferred correction (MISDC) algorithm designed to advance the conservation equations in time at scales associated with advective transport. The fast and nontrivial interactions of electrons with the electric field are treated implicitly using a Jacobian-Free Newton Krylov approach for which a preconditioning strategy is developed. Within the MISDC framework, this enables a close and stable coupling of diffusion, reactions and dielectric relaxation terms with advective transport and is shown to exhibit second-order convergence in space and time. The algorithm is then applied to a series of steady and unsteady problems to demonstrate its capability and stability. Although developed in a one-dimensional case, the algorithmic ingredients are carefully designed to be amenable to multidimensional applications

Numerical study of ignition and inter-sector flame propagation in gas turbine

For safety reasons, in-flight relight of the engine must be guaranteed over a wide range of operating conditions but the increasing stringency of pollutant emission constraints requires the development of new aero-engine combustor whose design might be detrimental to the ignition capability. To improve the knowledge of the ignition process in aeronautical gas turbines and better combine conflicting technological solutions, current research relies on both complex experimental investigation and high fidelity numerical simulations. In this work, numerical study of the ignition process in gas turbines from the energy deposit to the light-around is performed with several objectives: increase the level of confidence of Large Eddy Simulations tool for the analysis of the ignition process, investigate the mechanisms controlling ignition in conditions representative of realistic aeronautical gas turbine flows and improve the low-order methodologies for the prediction of ignition performance. In a first part, LES of the single burner installed at CORIA (France) is carried out and allows to highlight the LES accuracy and to build a database on which the main mechanisms controlling the ignition success are identified. Based on these results, a methodology is developed to predict the ignition performance at a low computational cost using the non-reacting flow statistics only. In a second part, the light-around process is studied on two experimental set-ups and the very good agreement of the LES results with experiments is the starting point from an analysis of the mechanisms driving the flame propagation process

Multiphase Flow LES Study of the Fuel Split Effects on Combustion Instabilities in an Ultra Low-NOx Annular Combustor

International audienceThis paper describes the application of a coupled acoustic model/large-eddy simulation approach to assess the effect of fuel split on combustion instabilities in an industrial ultra-low-NOx annular combustor. Multiphase flow LES and an analytical model (analytical tool to analyze and control azimuthal modes in annular chambers (ATACAMAC)) to predict thermoacoustic modes are combined to reveal and compare two mechanisms leading to thermoacoustic instabilities: (1) a gaseous type in the multipoint zone (MPZ) where acoustics generates vortex shedding, which then wrinkle the flame front, and (2) a multiphase flow type in the pilot zone (PZ) where acoustics can modify the liquid fuel transport and the evaporation process leading to gaseous fuel oscillations. The aim of this paper is to investigate these mechanisms by changing the fuel split (from 5% to 20%, mainly affecting the PZ and mechanism 2) to assess which mechanism controls the flame dynamics. First, the eigenmodes of the annular chamber are investigated using an analytical model validated by 3D Helmholtz simulations. Then, multiphase flow LES are forced at the eigenfrequencies of the chamber for three different fuel split values. Key features of the flow and flame dynamics are investigated. Results show that acoustic forcing generates gaseous fuel oscillations in the PZ, which strongly depend on the fuel split parameter. However, the correlation between acoustics and the global (pilot + multipoint) heat release fluctuations highlights no dependency on the fuel split staging. It suggests that vortex shedding in the MPZ, almost not depending on the fuel split, is the main feature controlling the flame dynamics for this engin

Étude numérique de l'allumage et de la propagation inter-secteur dans les turbines à gaz

Pour des raisons de sécurité, les moteurs aéronautiques doivent pouvoir redémarrer en vol sur toute leur plage d'opération. Mais les contraintes sur les émissions polluantes nécessitent le développement de nouvelles chambres de combustion dont la conception peut détériorer les capacités d'allumage du moteur. Afin d'améliorer la compréhension du processus d'allumage et d'aider à l'optimisation de la conception, les recherches actuelles combinent les études expérimentales de plus en plus complexes et les simulation numériques hautes fidélités. Dans ce travail, l'étude numérique du processus d'allumage des chambres de combustion aéronautiques, de l'étincelle à la propagation azimutale de la flamme, est conduite avec plusieurs objectifs: améliorer la robustesse et la confiance de l'outil LES pour l'étude de l'allumage, étudier les mécanismes qui affectent l'allumage dans des conditions représentatives des conditions réelles et enfin améliorer les méthodes bas-ordre pour la prédiction des performances d'allumage. Dans une première partie, la SGE d'un monobruleur installé au CORIA permet de mettre en évidence les bons résultats de la LES et de construire une base de données pour l'analyses des mécanismes d'extinction. Ces données sont aussi utilisées pour développer une méthodologie permettant de prédire les performances d'allumage à bas coût en utilisant les résultats d'une SGE non-réactive. Dans une seconde partie, la propagation inter-secteur est investiguée par l'étude de deux cas expérimentaux et la SGE est capable de reproduire les modes de propagation mais aussi les temps d'allumage avec précision. Sur la bases de ces bons résultats, une analyse plus fine de la simulation permet d'identifier les mécanismes qui contrôlent la propagation de la flamme.For safety reasons, in-flight relight of the engine must be guaranteed over a wide range of operating conditions but the increasing stringency of pollutant emission constraints requires the development of new aero-engine combustor whose design might be detrimental to the ignition capability. To improve the knowledge of the ignition process in aeronautical gas turbines and better combine conflicting technological solutions, current research relies on both complex experimental investigation and high fidelity numerical simulations. In this work, numerical study of the ignition process in gas turbines from the energy deposit to the light-around is performed with several objectives: increase the level of confidence of Large Eddy Simulations tool for the analysis of the ignition process, investigate the mechanisms controlling ignition in conditions representative of realistic aeronautical gas turbine flows and improve the low-order methodologies for the prediction of ignition performance. In a first part, LES of the single burner installed at CORIA (France) is carried out and allows to highlight the LES accuracy and to build a database on which the main mechanisms controlling the ignition success are identified. Based on these results, a methodology is developed to predict the ignition performance at a low computational cost using the non-reacting flow statistics only. In a second part, the light-around process is studied on two experimental set-ups and the very good agreement of the LES results with experiments is the starting point from an analysis of the mechanisms driving the flame propagation process

Symmetry breaking in a 3D bluff-body wake

The dynamics of a three-dimensional axisymmetric blu -body wake are examined at low Reynolds regimes where transitions take place through spatio-temporal symmetry breaking. A linear stability analysis is employed to identify the critical Reynolds num- ber associated with symmetry breaking, and the associated eigenmodes, known as global modes. The analysis shows that the axisymmetric stable base ow breaks the rotational symmetry through a pitchfork m = 1 bifurcation, in agreement with previously reported results for axisymmetric wakes. Above this threshold, the stable base ow is steady and three-dimensional with planar symmetry. A three-dimensional global stability analysis around the steady re ectionally symmetric base ow, assuming no homogeneous direc- tions, predicts accurately the Hopf bifurcation threshold, which leads to asymmetric vortex shedding. DNS simulations validate the stability results and characterize the ow topology during the early chaotic regime

Investigation of initial droplet distribution and importance of secondary breakup model on lean blowout predictions of a model gas turbine combustor

In the present work, the importance of secondary droplet breakup and the impact of the initial droplet distribution on lean blowout (LBO) predictions is investigated to determine the sensitivity and robustness of the prediction to spray boundary conditions and secondary breakup models, respectively. This is accomplished using large-eddy simulations of a model gas turbine combustor, operating near the LBO limit. The flow field and gas phase chemistry is solved using an unstructured Navier-Stokes solver and a flamelet/progress variable formulation, respectively. Liquid spray is modeled using a polydisperse droplet injection and a Lagrangian spray evaporation model. Simulations with and without stochastic modeling of droplet breakup are performed. An analysis of the dropletWeber number indicates that large diameter droplets are above the criticalWeber number and should undergo secondary breakup. The present study demonstrates that secondary droplet breakup has a significant impact on the predicted droplet distribution and temporal evolution of the flame. Due to the large computational requirements of the current simulations, an ad-hoc approach to determining the initial droplet SMD and diameter is not feasible and a secondary droplet breakup model is required to reproduce the experimental trends

Fuel effects on lean blow-out in a realistic gas turbine combustor

Towards the implementation of alternative jet fuels in aviation gas turbines, testing in combustor rigs and engines is required to evaluate the fuel performance on combustion stability, relight, and lean blow-out (LBO) characteristics. The objective of this work is to evaluate the effect of different fuel candidates on the operability of gas turbines by comparing a conventional petroleum-based fuel with two other alternative fuel candidates. A comparative study of fuel properties is first conducted to identify physico-chemical processes that are affected by these fuels. Subsequently, large-eddy simulations (LES) are performed to examine the performance of these fuels on the stable condition close to blow-out in a referee gas turbine combustor. LES results are compared to available experimental data to assess their capabilities in reproducing observed fuel effects. It is shown that the simulations correctly predict the spray main characteristics as well as the flame position. The change in OH*-emissions for different fuel candidates is also qualitatively captured. An analysis of the flame anchoring mechanisms highlights the fuel effects on the flame position. Finally, the LBO-behavior is examined in order to evaluate the LBO-limit in terms of equivalence ratio and identify fuel effects on the blow-out behavior

Flame propagation in aeronautical swirled multi-burners: Experimental and numerical investigation

WOS:000340443400016International audienceDriven by pollutant emissions stringent regulations, engines manufacturers tend to reduce the number of injectors and rely on lean combustion which impacts the light-around phase of ignition. To improve knowledge of the ignition process occurring in real engines, current research combines fundamental and increasingly complex experiments with high fidelity numerical simulations. This work investigates the flame propagation, using a multi-injector experiment located at CORIA (France) in combination with Large Eddy Simulation (LES) obtained by CERFACS (France). The comparison of numerical fully transient ignition sequences with experimental data shows that LES recovers features found in the experiment. Global events such as the propagation of the flame front to neighboring swirlers are well captured by LES, with the correct propagation mode (spanwise or axial) and the correct overall ignition time delay. The detailed analysis of LES data allows to identify the driving mechanisms leading to each propagation mode. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved