Large-eddy simulation of a bluff-body stabilized nonpremixed flame (original) (raw)
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The coupling of turbulence and chemistry in a premixed bluff-body flame as studied by LES
Combustion and Flame, 2004
In previous work a subgrid scale fractal model for large eddy simulation of turbulent combustion was developed and validated. In the present article the fractal model applicability is tested by simulating a bluff-body premixed flame anchored in a straight channel. The model assumes that chemical reactions take place only at the dissipative scales of turbulence, i.e., near the so-called "fine structures" (eddy dissipation concept). The model estimates the local spatial dissipative scale η, considering also the growth effect due to heat release, and turns itself automatically off where the local spatial filter equals η. The premixed burner is simulated in 2 and 3 dimensions, both for cold flow and for reacting cases. Results are compared with experimental data and show three-dimensional vortex structures periodically shortening the recirculation zone downstream of the bluff body and entraining fresh mixture into the hot recirculating region. This physical mechanism is involved in flame anchoring. The effect of assuming periodic boundary conditions in the spanwise direction, instead of solid side walls, is also investigated. The analysis shows that periodic boundary conditions cannot capture various effects of side walls, such as the shortening of the recirculation zone and the flow acceleration downstream; furthermore, it also does not allow predictions of wall heat transfer. The 2D reactive case results are also compared with those using RANS κ − ε and LES-Smagorinsky models. Finally, comparing kinetic energy spectral densities in the nonreacting and reacting cases it is shown that large-scale fluctuations are damped in the latter and that fast chemical reactions cause a high-frequency energy peak.
A modified hydro-thermo-diffusive theory of laminar counterflow premixed flames
WSEAS transactions on fluid mechanics, 2006
Scale-invariant forms of conservation equations in reactive fields are discussed. The modified forms of the conservation equations at eddy-dynamic, and cluster-dynamic scales are then solved to describe the hydrothermo-diffusive structure of laminar counterflow premixed flames. The predicted temperature profiles as well as the flame thermal thicknesses are found to be in good agreement with the measurements made on lean methaneair premixed flames stabilized on a stagnation-point burner at different stretch rates reported in a previous study. The error-function type geometry of the predicted temperature profile is also in accordance with the modified hydro-thermo-diffusive theory of laminar flames introduced earlier.
Journal of Fluid Mechanics, 1982
To study effects of flow inhomogeneities on the dynamics of laminar flamelets in turbulent flames, with account taken of influences of the gas expansion produced by heat release, a previously developed theory of premixed flames in turbulent flows, that was based on a diffusive-thermal model in which thermal expansion was neglected, and that applied to turbulence having scales large compared with the laminar flame-thickness, is extended by eliminating the hypothesis of negligible expansion and by adding the postulate of weak-intensity turbulence. The consideration of thermal expansion motivates the formal introduction of multiple-scale methods, which should be useful in subsequent investigations. Although the hydrodynamic-instability mechanism of Landau is not considered, no restriction is imposed on the density change across the flame front, and the additional transverse convection correspondingly induced by the tilted front is described. By allowing the heat-to-reactant diffusivity...
Investigation of Modeling for Non-Premixed Turbulent Combustion
Flow Turbulence and Combustion, 1998
A method for predicting filtered chemical species concentrations and filtered reaction rates in Large-Eddy Simulations of non-premixed, non-isothermal, turbulent reacting flows has been demonstrated to be quite accurate for higher Damköhler numbers. This subgrid-scale model is based on flamelet theory and uses presumed forms for both the dissipation rate and subgrid-scale probability density function of a conserved scalar. Inputs to
2021
The present study investigates a model for the large-scale simulation of turbulent non-premixed flames. In these flames, fuel and oxidizer arrive separately in the reaction zone.Easier to design than premixed flames (no prior mixing of the reagents in proportions compatible with the flammability limits), these flames are also safer since there is no risk of a flashback, which motivates their use in a certain number of situations (industrial furnaces, rocket motors, etc.).On the other hand, they are generally less efficient, and the inability to control their maximum temperature favors the formation of nitrogen oxides.Numerical simulation has now become essential to help design efficient burners. Despite the continued growth in the power of computing resources, direct numerical simulations (DNS), without modeling the flame/turbulence interaction, remain impossible for combustion chambers of practical interest.Large-scale simulation (LES) represents a good compromise in terms of infor...
Modeling chemical flame structure and combustion dynamics in LES
Proceedings of the Combustion Institute, 2011
In turbulent premixed combustion, the instantaneous flame thickness is typically thinner that the grid size usually retained in Large Eddy Simulations (LES), requiring adapted models. Two alternatives to couple chemical databases with LES balance equations, the Thickened Flame (TFLES) and the Filtered Tabulated Chemistry (F-TACLES) models, are investigated here and compared in terms of chemical flame structure and dynamics. To avoid the uncertainties related to the modeling of sub-grid scale turbulence / flame interactions, this comparison is conducted in situations where the flame front is not wrinkled at sub-grid scale levels. The thinner quantity requiring an accurate discretization on the numerical grid mesh is the reaction rate of the thickened or filtered progress variable. The thermal flame structure is found to be considerably thicker in TFLES than when using F-TACLES. The simulation of a 2D unsteady Bunsen burner flame shows that the thermal thickness spreading strongly affects the flame dynamics giving a decisive advantage to F-TACLES compared to TFLES.
Flow, Turbulence and Combustion
The statistical behaviour of turbulent scalar flux and modelling of its transport have been analysed for both major reactants and products in the context of Reynolds Averaged Navier Stokes simulations using a detailed chemistry Direct Numerical Simulation (DNS) database of freely-propagating H 2 −air flames (with an equivalence ratio of 0.7) spanning the corrugated flamelets, thin reaction zones and broken reaction zones regimes of premixed turbulent combustion. The turbulent scalar flux in the cases representing the corrugated flamelets and thin reaction zones regimes of combustion exhibit predominantly countergradient transport, whilst a gradient transport has been observed for the broken reaction zones regime flame considered here. It has been found that the qualitative behaviour of the various terms of the turbulent scalar flux transport equation for the major species such as H 2 , O 2 and H 2 O in the cases representing the corrugated flamelets and thin reaction zones regimes of combustion are mostly similar, whilst the behaviour is markedly different for the case representing the broken reaction zone regime. However, the terms for the scalar flux transport equation for H 2 and O 2 show same signs whereas the corresponding terms for H 2 O show signs opposite to those for H 2 and O 2. The performances of the well-established existing models for the unclosed terms of the turbulent scalar flux transport equation have been found to be similar for H 2 , O 2 and H 2 O Some of the existing models for turbulent flux, pressure gradient, molecular diffusion and reaction contributions have been found to yield reasonable performance for the cases representing the corrugated flamelets and thin reaction zones regimes but the existing closures for these terms have been found to be mostly inadequate for the broken reaction zones regime flames.
Eulerian particle flamelet modeling of a bluff-body CH4/H2 flame
Combustion and Flame, 2007
In this paper an axisymmetric RANS simulation of a bluff-body stabilised flame has been attempted using steady and unsteady flamelet models. The unsteady effects are considered in a post-processing manner through the Eulerian Particle Flamelet Model (EPFM). In this model the transient history of scalar dissipation rate, conditioned at stoichiometric mixture fraction is required to generate unsteady flamelets and obtained by tracing Eulerian particles. In this approach unsteady convective-diffusive transport equations are solved to consider the transport of Eulerian particles in the domain. Comparisons of the results of steady and unsteady calculations show that transient effects do not have much influence on major species, including OH and the structure of the flame therefore can be successfully predicted by steady or unsteady approaches. However, it appears that slow processes like NO formation can only be captured accurately if unsteady effects are taken into account while steady simulations tend to overpredict NO. In this work turbulence has been modelled using the Reynolds Stress Model (RSM). Predictions of velocity, velocity rms, mean mixture fraction and its rms show very good agreement with experiments. Performance of three detailed chemical mechanisms, the GRI Mech 2.11, the San Diego mechanism and the GRI Mech 3.0 has also been evaluated in this study. All three mechanisms performed well with both steady and unsteady approaches and produced almost identical results for major species and OH. However, the difference between mechanisms and flamelet models becomes clearly apparent in the NO predictions. The unsteady model incorporating the GRI Mech 2.11 provided better predictions of NO compared to steady calculations and showed close agreement with experiments. The other two mechanisms showed overpredictions of NO with both unsteady and steady models. The level of overprediction is severe with the steady approach. The GRI Mech 3.0 appears to overpredict NO by a factor of two compared to GRI Mech 2.11. The NO predictions by the San Diego mechanism fall between the two GRI mechanisms. The present study demonstrates the success of the EPFM model and when used with the GRI 2.11 mechanism predicts all flame properties, major and minor species very well and most importantly the correct NO levels.
2017
A premixed propane-air flame stabilised on a triangular bluff-body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported Flame Surface Density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out for nonreacting and reacting cases. The key physics of the flow field and the flame structure are also investigated in detail. For the non-reacting flow, good agreement for both the time-averaged and RMS velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. Using the dense mesh, a larger fraction of turbulent kinetic energy is resolved and there is better resolution of local flame wrinkling, as manifested by a thinner flame front with higher localised FSD. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and destruction in the model afterburner.