methane swirl stabilized lean burn flames assessment of scale resolving simulations

Abstract The reliable prediction of the turbulent combustion process in lean flames is of paramount importance in the design of gas turbine combustors. The present work presents an assessment of the capabilities of Flamelet Generated Manifold (FGM) in the framework of Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) At this purpose the TECFLAM swirl burner consisting of a strongly swirling, unconfined natural gas flame was chosen. Results highlight that RANS-FGM succeeds in predicting the main characteristics of the reacting flow field and species concentrations. However, only LES is capable of reproducing the actual turbulent mixing between swirling flow and co-flow, thus leading to appreciable enhancements with respect to RANS results

Computational Strategies for Faster Combustion Simulations with Detailed Chemistry

Combustion of fossil fuels is still the biggest source of power generation in the world. However, pollutants released to the atmosphere from combustion represent a risk for human health and the environment. Hence it is desirable to design a combustor that produces the maximum useful thermal power output while keeping low concentration levels of harmful emissions such as CO, P.M., NOx, and SOx. In the past, combustor design was aided by the compilation of large sets of experimental data and the development of empirical correlations which is an expensive process. Nowadays numerical simulations have become an important tool in the research and design of combustors. Numerical simulations allow the study of combustion systems under hazardous conditions and beyond their performance limits, and they are usually inexpensive and fast (compared to experiments). The main bottle-neck in combustion simulations is the accurate prediction of the concentration of the many species involved in combustion. Current computational fluid dynamic (CFD) simulations commonly use simplified versions of the chemical reaction mechanisms. But utilization of simplified chemical models comes with the associated inaccuracy while saving computational time.;In the present study the virtues of the chemical reactor network (CRN) approach are investigated and a new integration method is proposed to accelerate the calculation of species concentrations using reduced and detailed chemical mechanisms. Utilization of the CRN approach enabled the implementation of a detailed methane-air chemical mechanism that incorporates 53 chemical species and 325 reactions. The CRN approach was applied to two combustor configurations: a premixed methane-air swirl burner, and a non-premixed methane-air swirl burner. The CRN was built using results from the CFD simulations that were obtained using simplified chemical mechanisms with just one or two reactions. Numerical predictions of the premixed combustor behavior obtained using CRN simulations were compared with other CFD simulations that used mechanisms with more reactions and chemical species. The CRN results closely matched the CFD simulations with larger chemical mechanisms, the maximum relative difference of the predicted concentration for the major species (i.e. O 2, CO2, H2O, and N2) was 2.82% when compared to the CFD simulations. The calculation time of the CRN was greatly reduced, the maximum reduction of the CRN simulation took only one seventh of the computational time when compared with a CFD simulation. The CRN simulations of the non-premixed burner were also compared with experiments. Predicted spatial profiles of velocity, temperature, and mass fraction concentrations were compared with measurements. Results showed that the velocity and some mass fraction profiles matched the experimental measurements near the dump plane but it was found that downstream of the dump plane the temperature was overpredicted. Due to the temperature overprediction, the maximum difference was 250 [K], the nitrogen oxide (NO) concentration was overpredicted by 30 [ppm]. The relative difference of the predicted NO at the outlet of the combustor is 150% when compared with the experimental value.;Further, a novel integration method named log-time integration method (LTIM) was developed to calculate the solution of ideal reactors used in the CRN simulations. The integration method consists of the transformation of the time variable to the logarithmic space along with the use of variable time steps. The LTIM approach was applied to the solution of a perfectly stirred reactor (PSR) using a detailed chemical mechanism. PSR-LTIM results were compared with a commercial PSR code which is available in the CHEMKIN software package. The maximum relatively difference of the concentration of the species of interest was only 1%. Calculated species concentration using the PSR-LTIM matched the results from CHEMKIN with comparable computational time, the computational time of the PSR-LTIM was 5.3 [s] and for CHEMKIN was 3 [s]. The integration method was compared to higher order integration methods available in the literature producing satisfactory results with less CPU time, the LTIM approach took one fifth of the computational time of a higher order integration method. The LTIM was also applied to the solution of a premixed one dimensional methane-air flame, FLAME-LTIM, where a mechanism incorporating nine chemical species and five global reactions mechanism was used. Calculated temperature and mass fraction profiles matched closely the results obtained using the equivalent commercial code CHEMKIN PREMIX. The relative temperature difference at the outlet of the domain was 0.5% and the maximum difference in the chemical specie concentration at the outlet of the domain was 13.2%.;The outcome of the present research can be used to perform a rapid design analysis of gas turbines and similar combustors to achieve low levels of emissions

Burning syngas in a high swirl burner: effects of fuel composition

Flame characteristics of swirling non-premixed syngas fuel mixtures have been simulated using large eddy simulation and detailed chemistry. The selected combustor configuration is the TECFLAM burner which has been used for extensive experimental investigations for natural gas combustion. The large eddy simulation (LES) solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The predictions for and flames show considerable differences between them for velocity and scalar fields and this demonstrates the effects of fuel variability on the flame characteristics in swirling environment. In general, the higher diffusivity of hydrogen in fuel is largely responsible for forming a much thicker flame with a larger vortex breakdown bubble (VBB) in a swirling flame compare to the but syngas flames

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

Proper orthogonal decomposition analysis of a turbulent swirling self-excited premixed flame

Thermoacoustic oscillations constitute a serious threat to the integrity of combustion systems. The goal of the present work is to determine the effect of the equivalence ratio (φ), inlet flow velocity (U), and burner geometry on the characteristics of the self-excited oscillations and to reveal the dominant mechanisms. It also focuses on the data post-processing aiming at extracting information about the dynamics that are not captured through classical ensemble-averaging, and hence the Proper Orthogonal Decomposition technique is used. Experiments were conducted with a fully-premixed air/methane flame stabilized on a conical bluff body. Self-excited acoustic instabilities were induced by extending the length of the combustion chamber downstream of the bluff body. The flame was visualised using OH* chemiluminescence and OH PLIF at 5 kHz. Proper Orthogonal Decomposition (POD) and Fast Fourier Transform analysis were conducted on the imaging data. A strong effect of the chamber length was found, which primarily drove the generation of acoustic oscillation and flame-vortex interaction. Significant differences in the flame roll-up were found when either the burner geometry or the equivalence ratio was altered. Changes were detected in the frequency of oscillations, which showed a general trend to increase with φ and U and decrease with the length of the duct. Analysis of the POD modes allowed an estimate of the convection speed of the flame structures associated with the dominant frequency and it was found that this convection speed was about 1.5 U for most conditions studied

Simulations of turbulent swirl combustors

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.This work was supported by the European Commission through the Marie Curie Project MYPLANET

Computational Modeling of Turbulent Swirling Diffusion Flames

Schopnost predikovat tepelné toky do stěn v oblasti spalování, konstrukce pecí a procesního průmyslu je velmi důležitá pro návrh těchto zařízení. Je to často klíčový požadavek pro pevnostní výpočty. Cílem této práce je proto získat kvalitní naměřená data na experimentálním zařízení a využít je pro validaci standardně využívaných modelů počítačového modelování turbulentního vířivého difúzního spalování zemního plynu. Experimentální měření bylo provedeno na vodou chlazené spalovací komoře průmyslových parametrů. Byly provedeny měření se pro dva výkony hořáku – 745 kW a 1120 kW. Z měření byla vyhodnocena data a odvozeno nastavení okrajových podmínek pro počítačovou simulaci. Některé okrajové podmínky bylo nutné získat prostřednictvím dalšího měření, nebo separátní počítačové simulace tak jako například pro emisivitu, a nebo teplotu stěny. Práce zahrnuje několik vlastnoručně vytvořených počítačových programů pro zpracování dat. Velmi dobrých výsledků bylo dosaženo při predikci tepelných toků pro nižší výkon hořáku, kde odchylky od naměřených hodnot nepřesáhly 0.2 % pro celkové odvedené teplo a 16 % pro lokální tepelný tok stěnou komory. Vyšší tepelný výkon však přinesl snížení přesnosti těchto predikcí z důvodů chybně určené turbulence. Proto se v závěru práce zaměřuje na predikce vířivého proudění za vířičem a identifikuje několik problematických míst v použitých modelech využívaných i v komerčních aplikacích.The ability to predict local wall heat fluxes is highly relevant for engineering purposes as these fluxes are often the main results required by designers of fired heaters, boilers and combustion chambers. The aim of this work is to provide reliable data measured by an innovative method for the case of swirling diffusion natural gas flames and consequently utilize the data for validation of Computational Fluid Dynamic simulations represented by commercial solver ANSYS Fluent® 12.1. The subject is a large-scale combustion chamber with a staged-gas industrial type low-NOx burner at two thermal duties, 745 kW and 1120 kW. Attention is paid to the evaluation of boundary conditions via additional measurement or simulation, such as wall emissivity and wall temperature. Several in-house software codes were created for computational support. Remarkable results were obtained for low firing rate where prediction reached accuracy up to 0.2 % in total extracted heat and better than 16 % in local wall heat flux in individual sections. However, for high firing rate the accuracy significantly decreases. Consequently close attention was paid to the confined swirling flow phenomena downstream of the swirl generator. There were identified several problematic points in the prediction capabilities of utilized computationally capable, industry-standard models.

Design and Numerical Simulation of a Micro-Gas Turbine Combustor

A cannular combustor with a 100-KW thermal power was designed with a swirler, primary holes, dilution holes, and cooling holes based on an original gas turbine of a practical application. Further, the combustion process in this combustor was numerically simulated by using Computational Fluid Dynamics (CFD). A methane-reduced chemical mechanism was applied to CFD to simulate the combustion process. The combustion performance, product concentrations, and flow field were analyzed. Experimental data of airflow distribution obtained in previous study were applied in the design process. The present work was reported to verify that the experimental data can be regarded as a guide and optimization basis in the aerodynamic design process. Additionally, the consistency of numerical results and design data indicates that the design in this paper could satisfy the design requirements

Simulation numérique à trois dimensions d'une flamme de diffusion en utilisant un schéma réactionnel quasi-global et détaillé

Dans le présent travail, on s'intéresse à la simulation numérique en trois dimensions (3D) des flammes de diffusion d'un écoulement turbulent et réactif dans une chambre de combustion d'une turbine à gaz. Un des objectifs est d'étudier l'influence des modèles de combustion et les mécanismes réactionnels sur la prédiction de l'écoulement du champ de température et la modélisation des émissions polluantes NOx. Les résultats numériques obtenus sont comparés aux résultats antérieur