Christian Eberle

Numerical Investigation of Transient Soot Evolution Processes in an Aero-Engine Model Combustor

This article presents unsteady Reynolds averaged Navier–Stokes simulations (URANS) of a well-characterized aero-engine model combustor with finite-rate chemistry (FRC). The simulations give insight into the complex formation and destruction processes of soot at technically relevant conditions. It will be shown that a recently developed PAH (polycyclic aromatic hydrocarbons) and soot model is able to predict soot under complex combustion conditions at elevated pressure. Finite-rate chemistry is employed for the gas phase, a sectional approach for PAHs and a two-equation model for soot. Thus, feedback effects, such as the consumption of gaseous soot precursors by growth of soot and PAHs, are inherently captured accurately. In agreement with the experiment a precessing vortex core (PVC) is observed in the ethylene fueled combustor. This requires that the computational grid covers swirlers. The PVC intensifies mixing of fuel, primary air, and hot burned gas from the inner recirculation zone, thereby supporting flame stabilization and subsequently influencing soot. The numerical results (velocity components, temperature, and soot volume fraction) compare well with experimental data. Details of soot evolution and remaining differences to the experiment are analyzed.

Influence of Turbulence-Chemistry Interaction Modeling on the Structure and the Stability of a Swirl-Stabilized Flame

Scale Adaptive Simulations (SAS) of the swirl-stabilized flame of the PRECCINSTA burner are presented. Within the computations a large share of velocity fluctuations as well as temperature and species fluctuations are resolved in combustion regions. With rising resolution of fluctuating quantities, the impact of the sub grid scale (SGS) turbulence and turbulence chemistry interaction (TCI) models decreases. The main topic of this paper is to investigate the residual influence of an SGS-TCI model in case of the SAS of the swirl-stabilized flame. As a first outcome, the data obtained in this work are in very good agreement with experimental and LES data from the literature. And secondly, the modeling of SGS-TCI is of minor importance for the resulting time-mean quantities due to the high share of resolved velocity, temperature and species fluctuations in the computations.Copyright

Soot Predictions in an Aero-Engine Model Combustor at Elevated Pressure Using URANS and Finite-Rate Chemistry

Unsteady numerical simulations of an aero-engine model combustor are presented. The combustor is fueled by ethylene instead of the more complex realistic fuel kerosene to provide well defined inflow conditions. Comprehensive validation data obtained by several laser diagnostics is available. Turbulence is treated by the SST model. A finite-rate chemistry model, where a separate transport equation is solved for each species, is applied in order to treat combustion accurately. Chemistry-turbulence interaction is modeled by an assumed probability density function model (APDF). A sectional model is applied for polycyclic aromatic hydrocarbons, while soot is modeled by a two-equation model. Velocity components and temperature are predicted with good to excellent agreement against measurements. Reasonable agreement between measured and calculated soot volume fraction is found, remaining differences are discussed by time-resolved data analysis. A proper resolution of unsteady motion is identified to be crucial for accurate soot predictions.

Numerical Simulations of Soot and NOx Distributions in a Full Scale Aero-Engine Combustor at Two Different Flight Altitudes

Numerical simulations of a full-scale aero-engine combustor at two different flight altitudes are presented. Turbulence is treated by the SST model. A finite-rate chemistry model, where a separate transport equation is solved for each species, is applied in order to treat combustion accurately. Chemistry-turbulence interactions are modeled by an assumed probability density function model (APDF). A sectional model is applied for polycyclic aromatic hydrocarbons, while soot is modeled by a highly efficient two-equation model. The liquid phase of the fuel is calculated by a Lagrangian spray code which is coupled to the CFD code. Comparatively good agreement between experiment and simulation is observed at the reference operating point. At a reduced flight altitude, lower temperatures and lower NOx emissions are predicted, which result from a higher air-fuel ratio. Soot emissions at the combustor exit are due to cold regions in the vicinity of the combustor walls.

Toward finite-rate chemistry large-eddy simulations of sooting swirl flames

ABSTRACT This paper presents time-resolved numerical simulations of a well-characterized sooting swirl flame at elevated pressure. Recently published unsteady Reynolds averaged Navier–Stokes simulations (URANS) are compared here to newly performed large eddy simulations (LES). Finite-rate chemistry, where transport equations are solved for each chemical species, is employed for the gas phase, a sectional approach for polycyclic aromatic hydrocarbons (PAHs), and a two-equation model for soot particles. Feedback effects such as the consumption of gaseous soot precursors by growth of soot and PAHs are inherently captured accurately by a coupled solution of the set of governing equations. The numerical results (velocity components, temperature, and soot volume fraction) compare well with experimental data. No significant differences between URANS and LES are observed for time-averaged temperatures and velocity components, while the prediction of soot is significantly improved by LES. It will be shown that an accurate description of the instantaneous flame structure (especially of the hydroxyl radical distribution) by resolution of turbulent scales is of fundamental importance for accurate soot predictions in confined swirl flames with strong secondary air injection.

Methodology for the numerical prediction of pollutant formation in gas turbine combustors and associated validation experiments

For aircraft engine manufacturers the formation of pollutants such as NOx or soot particles is an important issue because the regulations on pollutant emissions are becoming increasingly stringent. In order to comply with these regulations, new concepts of gas turbine combustors must be developed with the help of simulation tools. In this paper we present two different strategies, proposed by ONERA and DLR respectively, to simulate soot or NOx formation in combustors. The first one is based on simple chemistry models allowing significant effort to be spent on the LES description of the flow, while the second one is based on more accurate, but also more expensive, models for soot chemistry and physics. Combustion experiments dedicated to the validation of these strategies are described next: The first one, performed at DLR, was operated at a semi-technical scale and aimed at very accurate and comprehensive information on soot formation and oxidation under well-defined experimental conditions; the second one, characterized at ONERA, was aimed at reproducing the severe conditions encountered in realistic gas turbine combustors. In the third part of the paper the results of combustion simulations are compared to those of the validation experiments. It is shown that a fine description of the physics and chemistry involved in the pollutant formation is necessary but not sufficient to obtain quantitative predictions of pollutant formation. An accurate calculation of the turbulent reactive flow interacting with pollutant formation and influencing dilution, oxidation and transport is also required: when the temperature field is correctly reproduced, as is the case of the ONERA simulation of the DLR combustor, the prediction of soot formation is quite satisfactory while difficulty in reproducing the temperature field in the TLC combustor leads to overestimations of NOx and soot concentrations.