| 1. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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Large Eddy Simulation and Experiments of Stratified Lean Premixed Methane/Air Turbulent Flames
- 2007
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 31:1, s. 1467-1475
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a joint large eddy simulation and laser diagnostic investigation of premixed turbulent low swirl flames. A lean premixed methane/air mixture, of the equivalence ratio 0.60-0.66, is injected from a 50 mm diameter low swirl burner to a low speed co-flowing air at room temperature and pressure. The level-set G-equation is employed to simulate the inner layer flame front. Flamelet chemistry is used to determine the flame properties in the reactive zones. Mixing and heat transfer in the post-flame zone down-stream are modeled using transport equations. In addition to large eddy simulation, simultaneous 2-D laser induced fluorescence of OH and 2-D particle image velocimetry are used to characterize the basic flame structure. Laser Doppler velocimetry is employed to further analyze the flow velocity along the central axis above the burner, and 2-D filtered Rayleigh scattering is used to measure the temperature field in the lower part of the flame. A bowl-shaped, highly wrinkled turbulent flame is stabilized at a position about one-half diameter above the burner. The flame consists of two distinct parts; around the burner axis, a premixed flame with uniform mixture fraction is stabilized in the low speed flow region induced by the inflow swirl; off the axis of the burner, a stratified lean premixed flame is found in the shear layer of the flow field. Flame holes (local extinction) owing to overly lean mixtures are observed in the off-axis lean stratified part of the flame. A unified level-set G-equation is developed to model the flame holes. The basic flow and flame structure from the model simulations are compared to the laser diagnostic measurements; the height of flame stabilization (lift-off height), the mean temperature profile, and the mean axial and radial velocity components together with rms velocity components are in fairly good agreement with measurement data. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Baudoin, Eric, et al.
(författare)
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Comparison of LES Models Applied to a Bluff Body Stabilized Flame
- 2009
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Ingår i: Conference Proceeding Series, Digital.
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Konferensbidrag (refereegranskat)abstract
- Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large Eddy Simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion calculations as the transient nature of the flow is resolved. In most situations, however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here, we examine the predictive capabilities and the theoretical links between LES flamelet models, such as the G-equation model (G-LES), and LES finite rate chemistry models, such as the Thickened Flame Model (TFM-LES), the Partially Stirred Reactor model (PaSR-LES), the Eddy Dissipation Concept (EDC-LES) model, a Presumed Probability Density Function (PPDF-LES) model and the Implicit LES (QL-LES) model. The models are described, and theoretical links between these are discussed in terms of the turbulent flame speed and flame thickness. In addition, the different models are used to study a bluff-body stabilized flame and the resulting predictions are compared with experimental data for two operating conditions.
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| 3. |
- Duwig, Christophe, et al.
(författare)
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Large Eddy Simulations of a piloted lean premix jet flame using finite-rate chemistry
- 2011
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Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 15:4, s. 537-568
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Tidskriftsartikel (refereegranskat)abstract
- A Large Eddy Simulation (LES) model capable of accurately representing finite-rate chemistry effects in turbulent premixed combustion is presented. The LES computations use finite-rate chemistry and implicit LES combustion modelling to simulate an experimentally well-documented lean-premixed jet flame stabilized by a stoichiometric pilot. The validity of the implicit LES assumption is discussed and criteria are expressed in terms of subgrid scale Damkohler and Karlovitz numbers. Simulation results are compared to experimental data for velocity, temperature and species mass fractions of CH4, CO and OH. The simulation results highlight the validity and capability of the present approach for the flame and in general the combustion regime examined. A sensitivity analysis to the choice of the finite-rate chemistry mechanism is reported, this analysis indicates that the one and two-step global reaction mechanisms evaluated fail to capture the reaction layer with sufficient accuracy, while a 20-species skeletal mechanism reproduces the experimental observations accurately including the key finite-rate chemistry indicators CO and OH. The LES results are shown to be grid insensitive and that the grid resolution within the bounds examined is far less important compared to the sensitivity of the finite-rate chemistry representation. The results are analyzed in terms of the flame dynamics and it is shown that intense small scale mixing (high Karlovitz number) between the pilot and the jet is an important mechanism for the stabilization of the flame.
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| 4. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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A Comparative Study of LES Turbulent Combustion Models Applied to a Low Swirl Lean Premixed Burner
- 2008
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Ingår i: AIAA 2008-513. - Reston, Virigina : American Institute of Aeronautics and Astronautics.
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Konferensbidrag (refereegranskat)abstract
- In this study we compare two types of Large Eddy Simulation (LES) turbulent combustion models with experimental data for a low swirl stabilized turbulent lean premixed flame. Such flames are a great challenge to numerical simulations since they are unsteady and sensitive to boundary conditions, and details of the experimental set-up. The two classes of LES turbulent combustion models considered are the flamelet and finite rate chemistry models. Individual models of each category may be very different, but in the former the flame is considered infinitely thin, whereas in the latter the chemical kinetics and the diffusion governs the flame behavior. As representative of the flamelet models we here use a G-equation model, and as representative of the finite rate chemistry models we use the thickened flame model and the partially stirred reactor model. Predictions are being compared with measurement data for an atmospheric low-swirl methane/air flame. The experimental measurement data include data from stereoscopic PIV, filtered Rayleigh scattering and acetone LIF, providing information about the velocity, temperature and fuel distribution. All LES show reasonable agreement with the experimental data, predicting a lifted weakly swirling, flame oscillating back and forth just above the rim of the burner. A more detailed comparison of the predictions with the experimental data show that best quantitative agreement is obtained by one of the finite rate chemistry models, whereas the best qualitative comparison is obtained by the flamelet model. Causes for the difference in qualitative and quantitative behavior are elaborated on in the concluding remarks section.
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| 5. |
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| 6. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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Large eddy simulation and laser diagnostic studies on a low swirl stratified premixed flame
- 2009
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 156:1, s. 25-36
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents numerical simulations and laser diagnostic experiments of a swirling lean premixed methane/air flame with an aim to compare different Large Eddy Simulations (LES) models for reactive flows. An atmospheric-pressure laboratory swirl burner has been developed wherein lean premixed methane/air is injected in an unconfined low-speed flow of air. The flame is stabilized above the burner rim in a moderate swirl flow, triggering weak vortex breakdown in the downstream direction. Both stereoscopic (3-component) PIV and 2-component PIV are used to investigate the flow. Filtered Rayleigh scattering is used to examine the temperature field in the leading flame front. Acetone-Planar Laser Induced Fluorescence (PLIF) is applied to examine the fuel distribution. The experimental data are used to assess two different LES models: one based on level-set G-equation and flamelet chemistry, and the other based on finite rate chemistry with reduced kinetics. The two LES models treat the chemistry differently, which results in different predictions of the flame dynamic behavior and statistics. Yet, great similarity of flame structures was predicted by both models. The LES and experimental data reveal several intrinsic features of the low swirl flame such as the W-shape at the leading front, the highly wrinkled fronts in the shear layers, and the existence of extinction holes in the trailing edge of the flame. The effect of combustion models, the numerical solvers and boundary conditions on the flame and flow predictions was systematically examined. (c) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 7. |
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| 8. |
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| 9. |
- Nogenmyr, Karl-Johan, et al.
(författare)
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Numerical computations and optical diagnostics of unsteady partially premixed methane/air flames
- 2010
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 157:5, s. 915-924
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Tidskriftsartikel (refereegranskat)abstract
- The structures and dynamics of unsteady laminar partially premixed methane/air Bunsen flames are studied by means of numerical simulations, OH and CH PLIF imaging, and high speed chemiluminescence imaging employing a high framing speed intensified charge coupled device camera. The Bunsen burner has a diameter of 22 mm. Rich methane/air mixtures with an equivalence ratio of 1.5 are injected from the burner into atmosphere at different flow speeds ranging from 0.77 to 1.7 m/s, with Reynolds numbers based on the nozzle flow ranging from I 100 to 2500. The numerical simulations are based on a two-scalar flamelet manifold tabulation approach. Detailed chemistry is used to generate the flamelet manifold tabulation which relates the species concentrations, reaction rates, temperature and density to a distance function G and mixture fraction Z. Two distinct reaction zones are identified using CH and OH PLIF imaging and numerical simulations; one inner reaction zone corresponds to premixed flames on the rich side of the mixture and one outer reaction zone corresponds to mixing controlled diffusion flames on the lean side of the mixture. Under normal gravity conditions both the inner premixed flames and the outer diffusion flames are unsteady. The outer diffusion flames oscillate with a flickering frequency of about 15 Hz, which slightly increases with the burner exit velocity. The inner premixed flames are more random with much more small-scale wrinkling structures. Under zero gravity conditions the outer diffusion flames are stable whereas the inner premixed flames are unstable and highly wrinkled. It appears that the outer diffusion flames are governed by the Rayleigh-Taylor instability whereas the inner premixed flames are dictated by Landau-Darrieus instability. The two-scalar flamelet approach is shown to capture the basic structures and dynamics of the investigated unsteady partially premixed flames. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 10. |
- Nogenmyr, Karl-Johan
(författare)
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On the Modeling of Premixed Combustion under Varying Equivalence Ratios
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with numerical simulation of reactive flows. The numerical method is three dimensional and unsteady, which allows for spatial and temporal resolution of most of the scales of the fluid motion and the reaction process. In the turbulent cases studied, scales down to Taylor scales are resolved. Hence, it can be considered to be a large eddy simulation, LES. In the laminar cases studied, all fluidic motions are resolved. The reaction modeling is based on the flamelet concept where the scale separation of the flow and chemistry is exploited. This gives the opportunity to use the computational resources to capture the fluid motion in three dimensions, without computing the transport of the vast amount of species that are present in the flame. The behavior of swirling flows in confined geometries is complicated. In this thesis, a highly swirling (with a swirl number, S=1.5) confined flame is studied in a model combustor. The combustor has a sudden contraction at the outlet, which alters the complete flow field structure. The influence of this contraction is investigated. In addition, the effects of heat release are studied. For all these cases the flow field structure is carefully examined and a mechanism describing the behavior is suggested. The numerical computations are validated against experimental data. The second flame studied is situated in low swirling unconfined flow. The flame is fully detached from the burner allowing for entrainment of the surrounding air into the fuel/air mixture ahead of the reaction zone. This, in turn, introduces several challenges regarding the modeling of the flame. A model is proposed in this thesis, and after being validated, the model is used to study the dynamics and the stabilization mechanism of the low swirl flame. All computations and conclusions are backed up with experimental data. The third flame studied in this thesis is an unsteady laminar jet flame. A rich methane/air mixture is issued into the surrounding air at a low velocity. The mixture is within the flammability limit and a conical, Bunsen flame similar, premixed flame front is found. The incompletely oxidized products from this flame mixes with the surrounding air and are completely oxidized in a reaction layer surrounding the premixed flame. The structure and the instability of this partially premixed flame are investigated. As for the previous cases, the numerical results are validated against experimental data.
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