O.T. Stein

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.

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.

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.

Direct Numerical Simulation of Non-premixed Syngas Combustion Using OpenFOAM

A direct numerical simulation (DNS) solver for turbulent reacting flows is developed using libraries and functions from the open-source computational fluid dynamics package OpenFOAM. The solver serves as a reference for developing sub-grid scale models for the large eddy simulation (LES) of turbulent flames. DNS typically requires spatial and temporal discretisation schemes of high order, which are not readily available in OpenFOAM. We validate our OpenFOAM solver by performing direct numerical simulations of a well-defined DNS case featuring non-premixed syngas combustion in a double shear layer. This configuration has previously been studied by Hawkes et al. (Proc Combust Inst 31:1633–1640, 2007) using a purpose-built, high-order DNS solver. Despite the lower discretisation schemes of OpenFOAM, simulation results agree very well with the reference DNS data. Local extinction and re-ignition of the syngas flame are captured and effects of differential diffusion are highlighted. Parallel scaling results using the HazelHen architecture of HLRS Stuttgart are reported.

Certain Aspects of Conditional Moment Closure for Spray Flame Modelling

Large-eddy simulations (LES) have been coupled with a conditional moment closure (CMC) method for the improved modelling of small scale turbulence-chemistry interactions in turbulent spray flames. Partial pre-evaporation of the liquid fuel prior to exiting the injection nozzle requires a modified treatment for the boundary conditions in mixture fraction space and mixture fraction subgrid distribution and conditionally averaged subgrid dissipation need to be known. Different modelling approaches for the subgrid distribution of mixture fraction have been assessed, but the modelling of subgrid scalar dissipation that is responsible for the subgrid fuel transport from the droplet surface towards the cell filtered mean has not been forthcoming. Instead, we introduce a new conditioning method based on two sets of conditional moments conditioned on two differently defined mixture fractions: the first mixture fraction is a fully conserved scalar, the second mixture fraction is based on the fluid mass originating from the liquid fuel stream and is strictly not conserved due to the evaporation process. The two-conditional moment approach is validated by comparison with measurements from a turbulent ethanol spray flame and predicted temperature and velocity profiles could significantly be improved when compared to conventional LES-CMC modelling.

Large Eddy Simulation of non-reacting gas flow in a 40 MW pulverised coal combustor

The Large Eddy Simulation (LES) of non-reacting ?ow in a full-scale single coal burner test facility is presented. The LES was run with the in-house code PsiPhi of Imperial College, using immersed boundary conditions and the Smagorinsky model. The burner quarl and upstream furnace were discretised with 45 million uniform cubic cells. The LES reveals highly unsteady ?ow and identi?es major recirculation zones crucial for coal ?ame stabilisation. LES results show a good accordance with available Reynolds-Averaged Navier?Stokes (RANS) data from FLUENT. The cost of the LES is reasonable (2 weeks CPU time, 4 nodes) given the wealth of time-resolved data it provides.

Quality Issues in Combustion LES

Combustion LES requires modelling of physics beyond the flow-field only. These additional models lead to further quality issues and an even stronger need to quantify simulation and modelling errors. We illustrate stability problems, the need for consistent modelling in premixed and non-premixed combustion, and show how RANS models that have frequently been applied in an LES context can lead to strong conceptual errors. We outline the application of the error landscape approach to a complex non-premixed flame, and investigate several error indicators that have been developed for situations where no experimental reference data is available.

A Resolved Simulation Study on the Interactions Between Droplets and Turbulent Flames Using OpenFOAM

This study presents direct numerical simulations (DNS) of turbulent reacting flows around evaporating single fuel droplets and droplet arrays. Statistical analysis of interactions between the droplets and the turbulent flames are used to develop sub-grid scale models for mixture fraction based approaches such as flamelet or conditional moment closure (CMC) methods. The specific challenges are posed by the effects of the evaporating spray on the composition field in inter-droplet space and by the presence of combustion. Here, we analyse the best possible setup for such a fully resolved DNS. The numerical constraints are given by (1) the need to resolve all small scale effects, i.e. the smallest turbulent eddies and the boundary layer thickness around the droplets, and (2) the desire to include scales covering the entire turbulence spectrum to ensure a realistic interaction between the large and small scales. The largest scales are typically limited by the size of the computational domain, and these two demands (high resolution and large domain size) can easily lead to extensive computational requirements. We suggest an optimal setup for fully resolved DNS that ensures a good balance between computational cost and solution accuracy. The optimal mesh resolution and domain size do not introduce any bias for the analysis of characteristic quantities such as mixture fraction, its PDF and conditional scalar dissipation. Further, adequate scalability of OpenFOAM for the different setups is reported.