A. Kempf

Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles.

This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and non-luminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%.

Large-eddy simulation of a counterflow configuration with and without combustion

A large-eddy simulation in three dimensions was used to study the flow, mixing fields, and combustion in a counterflow burner. A non-reactive case (air/air jets) and a reactive case (methane/air jets) were investigated. Such a configuration is well suited to study and calibrate models for non-premixed flames because of its simplicity and versatility. In the numerical method, fluctuations of density in space and time were considered to depend only on chemistry, not on pressure. The effect of heat release was included by means of the mixture-fraction formulation. To represent the subgrid scale stresses and scalar flux, a Smagorinsky model was used, in which the Smagorinsky coefficient was determined by the dynamic Germano procedure. An equilibrium chemistry model was used to relate the mixture fraction to density, temperature, and species concentrations. The subgrid distribution of the mixture-fraction fluctuation was presumed to have the shape of a β-function. The computed results were found to be in overall agreement with experimental data for the non-reactive case. For the reactive case, in which a simple combustion model was used, a satisfactory agreement with measured data was achieved. A strong influence of combustion on turbulence mechanisms is apparent.

A posteriori testing of algebraic flame surface density models for LES

In the application of Large Eddy Simulation (LES) to premixed combustion, the unknown filtered chemical source term can be modelled by the generalised flame surface density (FSD) using algebraic models for the wrinkling factor Ξ. The present study compares the behaviour of the various models by first examining the effect of sub-grid turbulent velocity fluctuation on Ξ through a one-dimensional analysis and by the LES of the ORACLES burner (Nguyen, Bruel, and Reichstadt, Flow, Turbulence and Combustion Vol. 82 [2009], pp. 155–183) and the Volvo Rig (Sjunnesson, Nelsson, and Max, Laser Anemometry, Vol. 3 [1991], pp. 83–90; Sjunnesson, Henrikson, and Löfström, AIAA Journal, Vol. 28 [1992], pp. AIAA–92–3650). Several sensitivity studies on parameters such as the turbulent viscosity and the grid resolution are also carried out. A statistically 1-D analysis of turbulent flame propagation reveals that counter gradient transport of the progress variable needs to be accounted for to obtain a realistic flame thickness from the simulations using algebraic FSD based closure. The two burner setups are found to operate mainly within the wrinkling/corrugated flamelet regime based on the premixed combustion diagram for LES (Pitsch and Duchamp de Lageneste, Proceedings of the Combustion Institute, Vol. 29 [2002], pp. 2001–2008) and this suggests that the models are operating within their ideal range. The performance of the algebraic models are then assessed by comparing velocity statistics, followed by a detailed error analysis for the ORACLES burner. Four of the tested models were found to perform reasonably well against experiments, and one of these four further excels in being the most grid-independent. For the Volvo Rig, more focus is placed upon the comparison of temperature data and identifying changes in flame structure amongst the different models. It is found that the few models which largely over-predict velocities in the ORACLES case and volume averaged in a previous a priori DNS analysis (Chakraborty and Klein, Physics of Fluids, Vol. 20 [2008], p. 085108), deliver satisfactory agreement with experimental observations in the Volvo Rig, whereas a few of the other models are only able to capture the experimental data of the Volvo Rig either quantitatively or qualitatively.

Prediction of finite chemistry effects using large eddy simulation

A large eddy simulation (LES) in three dimensions is applied to study flow, mixing, and combustion ina highly turbulent jet flame. Turbulence chemistry interactions, including finite rate chemistry effects, are investigated. The hydrogen fuel has been diluted with nitrogen to allow for both accurate numerical and accurate experimental investigation. In the numerical method, fluctuations of density in time and space are considered to depend only onthe chemical state, not on pressure. This low-Mach assumption greatly improves the efficiency of the code. Mixing and the effects of heat release are included by means of the mixture-fraction formulation. To model subgrid scale stresses and scalar fluxes, the Smagorinsky model is used since the dynamic Germano procedure did not show any particular advantage for this flame. To relate mixture fraction to density, temperature, and species concentrations, a steady flamelet model is used. To evaluate the performance of LES with steady flamelet chemistry, a comparison has been made to experimental data, as well as to the results of a probability density function simulation, with a five-step mechanism considering differential diffusion effects. This is done in terms of averaged quantities, scatter plots, and conditional averages. The LES results were found to be in good agreement with the existing data. For this stable flame, the influence of differential diffusion (inherent to hydrogen flames) seems to be negligible.

A simple model for the filtered density function for passive scalar combustion LES

LES models for turbulent non-premixed combustion usually require knowledge of the filtered density function of the conserved scalar, and we propose to use a simple top-hat function. Such top-hat distributions were developed as probability density functions for RANS applications in the 1970s but were soon surpassed by the β function. We find that in the context of LES, the top-hat distribution provides an excellent alternative to the now much more common β function. The top-hat function is assessed through a phenomenological analysis of Direct Numerical Simulation (DNS) data from a planar jet and of experimental data from a turbulent opposed jet. The approach is then tested a posteriori for a piloted diffusion flame (Sandia Flame D). Advantages of the top-hat function are the ease of implementation and the reduced dimensionality of look-up tables. The present paper also discusses inconsistencies of sub-grid β-FDFs, the FDFs sensitivity on implicit filtering, and the regime in which a β assumption can be a valid filtered density function for LES.

LES OF THE SYDNEY SWIRL FLAME SERIES: AN INITIAL INVESTIGATION OF THE FLUID DYNAMICS

The non-premixed turbulent Sydney Swirl Burner is a target of the workshop series on turbulent non-premixed flames (TNF). Thorough experimental investigations in Sydney and Sandia provided experimental data for 10 different configurations. This flame series allows for the examination of various fuel compositions, flow rates and swirl numbers and its computation by Large Eddy Simulation (LES) promises detailed insight into turbulent non-premixed swirl combustion. To improve the reliability of these simulations elaborate preliminary studies must be carried out, which are detailed in this paper. Spatial resolution requirements are discussed and an appropriate sampling period for statistics is determined. The paper presents first and second moments for the velocity components in the non-reactive flow and preliminary mixture fraction results for one flame of the series. Results are in reasonable agreement with experimental data, while some deficiencies indicate the need for further grid refinement and more accurate inflow data before the simulation of the entire flame series can be attempted.

Large eddy simulation of combustion processes under gas turbine conditions

The method of large eddy simulation (LES) has a high potential to accurately predict complex turbulent flows. Gas turbine combustion systems feature a number of phenomena interacting with each other such as swirl with recirculation, complex turbulent mixing and combustion of premixed, non-premixed as well as partially premixed nature. We therefore tend to approach the simulation of real gas turbine combustors step by step. The aim of this paper is to document some of the progress made at EKT in assessing the capability of LES in flows separately exhibiting distinct features of gas turbine combustors. Some results from two configurations are presented and discussed: a non-confined isothermal swirl flow with precessing vortex core and a non-premixed bluff-body flame.

Combustion LES for Premixed and Diffusion Flames

This paper demonstrates the ability of large-eddy simulation (LES) to accurately predict turbulent combustion. A non-premixed bluff-body flame was simulated, relying on the conserved scalar approach and the laminar flamelet concept. The results were found to agree well with experimental data. A premixed flame, stabilised behind a rearward-facing step, was investigated with LES and a filtered G-equation model. The results were compared to experimental data to assess the accuracy of the approach. The filtered G-equation model was chosen based on the generalised regime diagram for turbulent combustion, which considers the influence of the LES filter width. A combination of both simulation methods introduced here gives the possibility to simulate partially premixed combustion.

Multi-directional 3D flame chemiluminescence tomography based on lens imaging

Flame chemiluminescence tomography (FCT) has been widely used in flame diagnostics for three-dimensional (3D), spatially resolved measurements of instantaneous flame geometry and, to some extent, of species concentrations. However, in most studies, tomographic reconstructions are based on a traditional parallel projection model. Due to the light collection characteristics of a lens, a parallel projection model is not appropriate for the practical optical setups that are used for emission imaging, particularly at small F-numbers. Taking the light collection effect of the lens into account, this Letter establishes a complete and novel tomographic theory for a multi-directional tomography system consisting of a lens and CCD cameras. A modified camera calibration method is presented first. It determines the exact spatial locations and intrinsic parameters of the cameras. A 3D projection model based on the lens imaging theory is then proposed and integrated into the multiplicative algebraic reconstruction technique (MART). The new approach is demonstrated with a 12-camera system that is used to reconstruct the emission field of a propane flame, thereby resolving space and time.

A posteriori testing of the flame surface density transport equation for LES

Flame Surface Density (FSD) models for Large Eddy Simulation (LES) are implemented and tested for a canonical configuration and a practical bluff body stabilised burner, comparing common algebraic closures with a transport equation closure in the context of turbulent premixed combustion. The transported method is expected to yield advantages over algebraic closures, as the equilibrium of subgrid production and destruction of FSD is no longer enforced and resolved processes of strain, propagation and curvature are explicitly accounted for. These advantages might have the potential to improve the ability to capture large-scale unsteady flame propagation in situations with combustion instabilities or situations where the flame encounters progressive wrinkling with time. The initial study of a propagating turbulent flame in wind-tunnel turbulence shows that the Algebraic Flame Surface Density (FSDA) method can predict an excessively wrinkled flame under fine grid conditions, potentially increasing the consumption rate of reactants to artificially higher levels. In contrast, the Flame Surface Density Transport (FSDT) closure predicts a smooth flame front and avoids the formation of artificial flame cusps when the grid is refined. Five FSDA models and the FSDT approach are then applied to the LES of the Volvo Rig. The predicted mean velocities are found to be relatively insensitive to the use of the FSDT and FSDA approaches, whereas temperature predictions exhibit appreciable differences for different formulations. The FSDT approach yields very similar temperature predictions to two of the tested FSDA models, quantitatively capturing the mean temperature. Grid refinement is found to improve the FSDT predictions of the mean flame spread. Overall, the paper demonstrates that the apparently complicated FSD transport equation approach can be implemented and applied to realistic, strongly wrinkled flames with good success, and opens up the field for further work to improve the models and the overall FSDT approach.