F. Hahn

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.

LES of Vortex Breakdown in Swirled Bluff-Body Flows

In this work the large-eddy simulation (LES) technique is used to investigate swirl configurations with different swirl intensities. The main aim is to shed light on the formation of swirl induced flow instabilities which affect the mixing of fuel and oxidiser in the nozzle region. LES is applied to the isothermal, unconfined Sydney Bluff-Body flow cases, with swirl numbers of 0.54 (low-swirl case, N29S054) and 1.59 (high-swirl case, N16S159). All computations are performed with the second order accurate, finite-volume CFD code, FASTEST, on an elliptically-smoothed, boundary-fitted, multi-block, hexahedral grid. For the low-swirl case, numerical results with different turbulence models (Smagorinsky and Germano) are compared with experimental data; both models perform very well. The prediction of different flow features had varying success. The computation of the high-swirl case is performed with the Germano turbulence model and compared to experimental data. A study of flow structures is carried out using a vortex identification technique. The onset of vortex breakdown was obtained for the low-swirl case only.Copyright

Study of Various Configurations Under Variable Density Mixing Conditions Aiming on Gas Turbine Combustion Using LES

In most technical applications involving mixing, density variation is of importance, while acoustic effects are normally considered to be unimportant. CFD models of such of such applications benefit from a low Mach number based formulation, which significantly reduces the computational effort when compared to fully compressible, variable density approaches. Within LES, which relies on accurate high-order numerics, problems arise due to the tendency of these pressure correction based formulations to oscillate for variable density flows. Damping of these oscillations is contrary to accuracy and predictability, regardless of wether it is done by numerics or adopted models. In this work, a high order approach which prevents the development of unphysical oscillations is described. This approach is then applied to several test cases of increasing complexity. Therefore the geometry flexible flow solver, FASTEST, is extended and used as a research tool. As test cases, several configurations were used. These are, in order of increasing flow complexity: a convected density wave, a mixing layer, a multi jet in cross flow, and, as a swirl configuration, a model gas turbine combustor. The results of the different test cases are discussed in detail. Where available, comparison between experiment and simulation will be made.Copyright

Flow and Mixing in a Model GT Combustor Investigated by LES and Monte-Carlo Filtered PDF Methods

In advanced gas turbines prediction and understanding of mixing and combustion dynamics become increasingly important to achieve higher efficiency and lower emissions. Therefore suitable information on the small scales is required due to their influence on a vast number of process factors such as micromixing efficiency, chemical reaction rate, turbulence-chemistry interaction, etc.. Here the subgrid probability density function (pdf) of a transported scalar is obtained by an Eulerian Monte-Carlo approach. In this context the evolution of the pdf is represented by ensembles of stochastic particles. The macromixing is provided by LES resolving the large scales. Dealing with confined configurations, the ability of LES to enable accurate temporal and spatial analysis of flow and mixing process is first demonstrated within a swirl stabilized model gas turbine combustor. Then LES is coupled to the Eulerian Monte Carlo method to provide an extensive study of multi-scale mixing processes. Therefore a feature of realistic combustors is investigated by means of two opposite rows of jets penetrating a cross flow. It turns out that the hybrid LES-MC method is a reliable tool for this purpose.Copyright

Hybrid LES/CAA Simulation of a Turbulent Non-Premixed Jet Flame

In this work, a numerical study of combustion induced noise is performed. For this purpose, a hybrid LES/CAA approach is applied to the simulation of a turbulent jet flame. The approach is based on a low-mach number LES and linearized acoustic equations. Both LES and CAA computations are compared to experimental investigations.