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

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied

Scaling relations for the length of coaxial oxy-flames with and without swirl

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N2, CO2 or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy- flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH∗ chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream

Tabulated chemistry for turbulent combustion modeling and simulation

Cette thèse se situe dans le cadre de la simulation numérique de la combustion turbulente à l’aide de méthodes de tabulation de la cinétique chimique. En approximant la structure fine des flammes turbulentes, ces méthodes prennent en compte des effets fins de cinétique chimique pour un faible coup dans les calculs numériques. Ceci permet de prédire les champs de température et d’espèces chimiques incluant les polluants. Le champ d’application de la chimie tabulée a d’abord été réservé à la simulation des écoulements moyens (RANS) dans une hypothèse de faible nombre de Mach pour une combustion dite "conventionnelle". Cependant, le développement actuel de nouvelles technologies de combustion ainsi que celui de modèles numériques plus avancés que les approches RANS nécessite d’étendre ce champ d’application. Les travaux de cette thèse ont mené au développement de nouveaux modèles de chimie tabulée afin de répondre à ces nouvelles exigences. L’émergence de nouvelles technologies comme la combustion sans flamme nécessite le développement de modèles dédiés. Ce mode de combustion présente en effet des structures de flamme mixtes. C’est pourquoi un modèle de tabulation de la cinétique chimique nommé UTaC (Unsteady flamelets Tabulated Chemistry) est proposé pour prédire la combustion diluée à haute température qui caractérise la combustion sans flamme. Le modèle est basé sur la tabulation de solutions instationnaires de flammelettes non-prémelangées qui s’auto-allument. Les pertes thermiques et la dilution variable des gaz brûlés sont négligés dans le cadre de cette thèse par soucis de simplification et de clarté de la validation du modèle. Le modèle est appliqué au cas d’un jet de combustible dilué dans un environnement de gaz vicié qui favorise l’auto-allumage comme moyen de stabilisation d’une flamme liftée. Plusieurs simulations RANS sont réalisées en faisant varier le combustible utilisé. Enfin, une simulation aux grandes échelles (LES) est aussi conduite pour le mélange méthane/air. Plusieurs codes numériques dédiés à la LES sont basés sur une formulation compressible des équations de Navier-Stokes. Cependant les méthodes de tabulation ne permettent pas directement de prendre en compte les effets acoustiques. Un modèle appelé TTC (Tabulated Thermo-chemistry for Compressible flows) a été créé afin d’introduire les méthodes de chimie tabulée dans les codes numériques compressibles. Pour cela, le calcul de la température est reformulé ainsi que le traitement des conditions aux limites à l’aide d’ondes caractéristiques. Enfin, l’application de modèle RANS de tabulation de la cinétique chimique à la LES est souvent faite sans tenir compte des spécificités de la simulation aux grandes échelles. Ainsi, les fonctions de densité de probabilités de type ß qui traduisent l’interaction de la combustion avec la turbulence en RANS sont utilisées telles quelles en LES. Nous montrerons que cette hypothèse est mauvaise car elle ne conserve pas l’intégrale du terme source dans une flamme prémélangée. Un nouveau modèle de chimie tabulée nommé F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation) est alors développé spécifiquement pour la simulation aux grandes échelles de la combustion parfaitement prémélangée. Le modèle est basé sur le filtrage de flammes laminaires de prémélange mono-dimensionelles.The thesis subject is located in the domain of numerical simulation of turbulent combustion through tabulated chemistry methods. These methods allow to include detailed chemistry effects at low cost in numerical simulation by approximating the fine scales structure of turbulent flames. Prediction of temperature and chemical species including pollutants becomes then possible. Tabulated chemistry models were first dedicated to low Mach-number RANS approaches for "conventional" combustion applications. However, the current uprising of new combustion configurations and of more precise numerical modeling than RANS approach requires to widen these range of applications. For that purpose, this thesis led to the development of new tabulated chemistry models. Flameless combustion is one of these new combustion technology that requires dedicated models. Indeed, complex flame structures are encountered in this combustion mode. That is why a tabulated chemistry model called UTaC (Unsteady flamelets Tabulated Chemistry) is derived to simulate high temperature diluted combustion which characterizes flameless combustion. The model lies on the tabulation of laminar unsteady non-premixed flamelets that auto-ignite. Heat losses and variation of dilution with burnt gases are neglected in the topic of this thesis for brevity and simplification of the model validation. The investigated configuration is a fuel jet diluted in a vitiated coflow. The hot coflow promotes auto-ignition in the lifted flame stabilization mechanism. Several RANS computations are performed by changing the fuel composition. Finally, a Large Eddy Simulation (LES) is also realized using a methane/air mixture as the impinging fuel stream. Several numerical codes for LES use a fully compressible formulation of Navier-Stokes equations. However, tabulated chemistry techniques do not take into account acoustic perturbations. A model called TTC (Tabulated Thermo-chemistry for Compressible flows) formalism is therefore developed in order to include tabulated chemistry in compressible CFD codes. TTC formalism consists in reformulating both temperature computation inside the numerical code and the characteristic boundary treatment. Finally, application of tabulated chemistry model to LES is usually done by a straightforward derivation from its RANS version without taking into account LES requirements. Indeed, ß-probability density functions which accounts for turbulence-chemistry interaction in RANS are used in LES although this technique does not conserve the source terms integral in premixed flames. A new model, F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation), is then derived specifically for LES of perfectly premixed combustion. This model is based on filtering of 1D laminar premixed flamelets

Tabulation de la cinétique chimique pour la modélisation et la simulation de la combustion turbulente

The thesis subject is located in the domain of numerical simulation of turbulent combustion through tabulated chemistry methods. These methods allow to include detailed chemistry effects at low cost in numerical simulation by approximating the fine scales structure of turbulent flames. Prediction of temperature and chemical species including pollutants becomes then possible. Tabulated chemistry models were first dedicated to low Mach-number RANS approaches for "conventional" combustion applications. However, the current uprising of new combustion configurations and of more precise numerical modeling than RANS approach requires to widen these range of applications. For that purpose, this thesis led to the development of new tabulated chemistry models. Flameless combustion is one of these new combustion technology that requires dedicated models. Indeed, complex flame structures are encountered in this combustion mode. That is why a tabulated chemistry model called UTaC (Unsteady flamelets Tabulated Chemistry) is derived to simulate high temperature diluted combustion which characterizes flameless combustion. The model lies on the tabulation of laminar unsteady non-premixed flamelets that auto-ignite. Heat losses and variation of dilution with burnt gases are neglected in the topic of this thesis for brevity and simplification of the model validation. The investigated configuration is a fuel jet diluted in a vitiated coflow. The hot coflow promotes auto-ignition in the lifted flame stabilization mechanism. Several RANS computations are performed by changing the fuel composition. Finally, a Large Eddy Simulation (LES) is also realized using a methane/air mixture as the impinging fuel stream. Several numerical codes for LES use a fully compressible formulation of Navier-Stokes equations. However, tabulated chemistry techniques do not take into account acoustic perturbations. A model called TTC (Tabulated Thermo-chemistry for Compressible flows) formalism is therefore developed in order to include tabulated chemistry in compressible CFD codes. TTC formalism consists in reformulating both temperature computation inside the numerical code and the characteristic boundary treatment. Finally, application of tabulated chemistry model to LES is usually done by a straightforward derivation from its RANS version without taking into account LES requirements. Indeed, ß-probability density functions which accounts for turbulence-chemistry interaction in RANS are used in LES although this technique does not conserve the source terms integral in premixed flames. A new model, F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation), is then derived specifically for LES of perfectly premixed combustion. This model is based on filtering of 1D laminar premixed flamelets.Cette thèse se situe dans le cadre de la simulation numérique de la combustion turbulente à l’aide de méthodes de tabulation de la cinétique chimique. En approximant la structure fine des flammes turbulentes, ces méthodes prennent en compte des effets fins de cinétique chimique pour un faible coup dans les calculs numériques. Ceci permet de prédire les champs de température et d’espèces chimiques incluant les polluants. Le champ d’application de la chimie tabulée a d’abord été réservé à la simulation des écoulements moyens (RANS) dans une hypothèse de faible nombre de Mach pour une combustion dite "conventionnelle". Cependant, le développement actuel de nouvelles technologies de combustion ainsi que celui de modèles numériques plus avancés que les approches RANS nécessite d’étendre ce champ d’application. Les travaux de cette thèse ont mené au développement de nouveaux modèles de chimie tabulée afin de répondre à ces nouvelles exigences. L’émergence de nouvelles technologies comme la combustion sans flamme nécessite le développement de modèles dédiés. Ce mode de combustion présente en effet des structures de flamme mixtes. C’est pourquoi un modèle de tabulation de la cinétique chimique nommé UTaC (Unsteady flamelets Tabulated Chemistry) est proposé pour prédire la combustion diluée à haute température qui caractérise la combustion sans flamme. Le modèle est basé sur la tabulation de solutions instationnaires de flammelettes non-prémelangées qui s’auto-allument. Les pertes thermiques et la dilution variable des gaz brûlés sont négligés dans le cadre de cette thèse par soucis de simplification et de clarté de la validation du modèle. Le modèle est appliqué au cas d’un jet de combustible dilué dans un environnement de gaz vicié qui favorise l’auto-allumage comme moyen de stabilisation d’une flamme liftée. Plusieurs simulations RANS sont réalisées en faisant varier le combustible utilisé. Enfin, une simulation aux grandes échelles (LES) est aussi conduite pour le mélange méthane/air. Plusieurs codes numériques dédiés à la LES sont basés sur une formulation compressible des équations de Navier-Stokes. Cependant les méthodes de tabulation ne permettent pas directement de prendre en compte les effets acoustiques. Un modèle appelé TTC (Tabulated Thermo-chemistry for Compressible flows) a été créé afin d’introduire les méthodes de chimie tabulée dans les codes numériques compressibles. Pour cela, le calcul de la température est reformulé ainsi que le traitement des conditions aux limites à l’aide d’ondes caractéristiques. Enfin, l’application de modèle RANS de tabulation de la cinétique chimique à la LES est souvent faite sans tenir compte des spécificités de la simulation aux grandes échelles. Ainsi, les fonctions de densité de probabilités de type ß qui traduisent l’interaction de la combustion avec la turbulence en RANS sont utilisées telles quelles en LES. Nous montrerons que cette hypothèse est mauvaise car elle ne conserve pas l’intégrale du terme source dans une flamme prémélangée. Un nouveau modèle de chimie tabulée nommé F-TACLES (Filtered Tabulated Chemistry for Large Eddy Simulation) est alors développé spécifiquement pour la simulation aux grandes échelles de la combustion parfaitement prémélangée. Le modèle est basé sur le filtrage de flammes laminaires de prémélange mono-dimensionelles

A Turbulent Combustion Model for Jet Flames Issuing in a Vitiated Coflow

International audienc

Controlling bulk Reynolds number and bulk temperature in channel flow simulations

International audienceBulk Reynolds number and bulk temperature are key quantities when reporting results in channel flow simulations. There are situations are when one wishes to accurately control these parameters while changing some numerical or physical conditions. A method to control the bulk Reynolds number and the bulk temperature in channel flow simulations is detailed. An ordinary differential equation is prescribed for the additional source term in the momentum balance equation so that the transient regime of the simulation is thoroughly tuned in order to efficiently and accurately reach the target Reynolds number value. A similar treatment is applied for the additional volume heat source term in the energy balance equation. The proposed method is specifically interesting when studying complex multi-physics in channel flow configurations when non-dimensionalization of the equations is no longer practical