Anja Ketelheun

A Numerical Study of the Flame Stabilization Mechanism Being Determined by Chemical Reaction Rates Submitted to Heat Transfer Processes

Abstract Large Eddy Simulations of a turbulent lean premixed stratified burner are conducted in order to determine the physical mechanisms that dominate the flame stabilization close to burner walls. The purpose of this work is both to provide insight into the underlying physics as well as to check whether the deficiencies found in previous simulations are related to an inappropriate heat transfer treatment. The simulation utilizes a three-dimensional detailed chemistry database in order to capture the chemical reaction rates based on local mixing and thermal conditions. The study is supplemented by very accurate wall temperature measurements to remove the large uncertainty revealed in the past for this configuration. The results obtained from the simulations are evaluated by means of a qualitative illustration of the different flame stabilizations and comparisons with experimental data.

High performance computing of the Darmstadt stratified burner by means of large eddy simulation and a joint ATF-FGM approach

Current trend in design and operation of industrial gas turbines or internal combustion engines implies using the lean-fuel and stratified conditions aiming at the reduction of the harmful emissions and efficiency improvement. This has led to an increasing use of computational methodology, which allows detailed insight into combustion physics and processes controlling the emission formation. In the present work, the Darmstadt stratified burner is investigated by means of Large Eddy Simulation, implemented into the in-house, finite-volume-based numerical code FASTEST. The code solves the incompressible, variable-density Navier–Stokes equations coupled with the species transport equations. It is parallelized via domain decomposition technique using message passing interface (MPI). The complex chemical mechanisms are described by tabulated detailed chemistry utilizing the Flamelet Generated Manifolds (FGM) approach combined with the Artificially Thickened Flame model (ATF). The results obtained are comparatively assessed along with the complementary measurements. In-depth analysis of the flow field is conducted based on numerical simulations. Further studies have been carried out with respect to grid resolution and scalability.

Strategies for presumed PDF modeling for LES with premixed flamelet-generated manifolds

Large Eddy Simulation is applied to a non-premixed bluff-body stabilized swirled methane-air flame of the Sydney flame series. The combustion chemistry is included via the so-called premixed flamelet-generated manifolds, being a progress variable approach based on steady premixed laminar flamelets. As turbulent mixing and chemistry interact on the subgrid scales, additional modeling of the probability density function of the mixture fraction and the progress variable is required. Two different approaches considering the statistical independence of these two quantities are presented. No large differences between these approaches were observed in a first computation, therefore, only one method was investigated in more detail. Radial profiles of velocity components and species obtained in the latter computation are compared with experimental data.

CO Prediction in LES of Turbulent Flames With Additional Modeling of the Chemical Source Term

The prediction of combustion processes using Large Eddy Simulation (LES) combined with tabulated chemistry has proven to be very successful and become very popular during the last years in both academia and industry. Technical combustion systems feature a wide range of time and length scales which need to be resolved. The LES describes the rather slow, but turbulent and unsteady flow field very well, while the fast chemical reactions can be represented by tabulated chemistry models like Flamelet Generated Manifolds. Pollutants, being only present at lower concentrations and developing slowly are not easy to capture with the standard manifold defined by the fast major combustion products. Therefore, additional modeling in order to predict the carbon monoxide emissions is presented in this paper. The choice of the reaction progress variable and the solution of an additional transport equation with and without extra modeling for the post flame zone was investigated. These models are applied to a standard test case and compared to experimental data and the standard tabulation approach.Copyright

Large Eddy Simulation of Combustion Systems at Gas Turbine Conditions

Three different aspects of Large Eddy Simulation (LES) of combustion processes are covered in this chapter. All three are based on using tabulated chemistry models to cover the chemical reactions occurring in gas turbines. The chosen approaches are all based on flamelet models. One part of the work deals with the investigation of subgrid scale models using Transported Eulerian Monte Carlo Probability Density Function (PDF) methods. The chemistry is dealt with using non-premixed flamelets and premixed Flamelet Generated Manifolds (FGM). The second aspect covered is the extension of the FGM method towards premixed flames where unresolved subgrid flame structures need to be handled. Therefore, the FGM approach was coupled with the Artificially Thickened Flames (ATF) model. The third aspect of combustion LES discussed here deals with the inclusion of more detailed reaction kinetics in the FGM approach in order to better predict minor species like nitric oxides or carbon monoxide, which are important development goals in today’s gas turbine industry. As all three aspects discussed in this chapter are located on the smallest scales in a combustion system, either the flow or flame subgrid structures, they are closely related to each other.

Numerical Analysis of Thermoacoustic Instabilities Using LES and Computational Aeroacoustics

Abstract Many technical combustion devices are susceptible to thermoacoustic instabilities. A deeper understanding of this transient phenomenon as well as the underlying physical processes is desirable. In this context, a turbulent enclosed flame is investigated numerically within this work by means of a hybrid LES/CAA (Large Eddy Simulation/Computational Aero Acoustics) approach as a step towards a numerical prediction of combustion instability. A thermoacoustically stable as well as a thermoacoustically unstable operating point are investigated. The LES computations reveal good overall agreement between simulation and experimental data. However, the recirculation zone, which is created by a backward facing step, is underestimated for the unstable case. The amplitudes as well as the frequencies of the simulated acoustic pressure are in good agreement with experimental data for the stable case. Considering the unstable case, the frequency of the oscillation is captured accurately, while the amplitudes are underpredicted.