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- Yu, Jiangfei, et al.
(författare)
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Onset of cellular flame instability in adiabatic CH4/O-2/CO2 and CH4/air laminar premixed flames stabilized on a flat-flame burner
- 2013
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 160:7, s. 1276-1286
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents numerical and experimental studies on the onset and evolution of laminar cellular flames of CH4/O-2/CO2 (oxy-fuel) and CH4/air mixtures under adiabatic conditions, stabilized in the proximity of a flat-flame burner at atmospheric pressure. In the numerical simulations, a two-dimensional domain with periodicity at the transverse far field boundaries is resolved using a high accuracy finite difference method and employing a detailed chemical kinetic mechanism and detailed transport properties. In the experiments a specially designed adiabatic flat-flame burner, a so-called heat flux burner, is employed. A key parameter, the standoff distance between the flame front and the burner exit plate, is identified. A critical standoff distance is found, above which cellular flame instability is observed. It is shown that the critical standoff distance is closely related to the density ratio and the laminar flame thickness for each flame studied. The observed onset of cellular flames is governed by the hydrodynamic instability mechanism, which is generally suppressed by the burner when the flame is very close to the burner plate. Diffusive-thermal effects play an important role in the flame instability when the flame is far from the burner. The critical standoff distance has no clear correlation with the Lewis number, indicating a less significant effect of diffusive-thermal instability on the flames near the burner. (c) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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- Yu, Jiangfei, et al.
(författare)
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Onset of cellular instability in adiabatic H-2/O-2/N-2 premixed flames anchored to a flat-flame heat-flux burner
- 2013
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 1879-3487 .- 0360-3199. ; 38:34, s. 14866-14878
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Tidskriftsartikel (refereegranskat)abstract
- The onset of cellular instability in adiabatic H-2/O-2/N-2 premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H-2/O-2/N-2 flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%-30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2-3 times of the burning velocity of the corresponding planar flame. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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- Yu, Rixin, et al.
(författare)
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An improved high-order scheme for DNS of low Mach number turbulent reacting flows based on stiff chemistry solver
- 2012
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Ingår i: Journal of Computational Physics. - : Elsevier BV. - 0021-9991. ; 231:16, s. 5504-5521
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Tidskriftsartikel (refereegranskat)abstract
- We present an improved numerical scheme for numerical simulations of low Mach number turbulent reacting flows with detailed chemistry and transport. The method is based on a semi-implicit operator-splitting scheme with a stiff solver for integration of the chemical kinetic rates, developed by Knio et al. [O.M. Knio, H.N. Najm, P.S. Wyckoff, A semi-implicit numerical scheme for reacting flow II. Stiff, operator-split formulation, Journal of Computational Physics 154 (2) (1999) 428-467]. Using the material derivative form of continuity equation, we enhance the scheme to allow for large density ratio in the flow field. The scheme is developed for direct numerical simulation of turbulent reacting flow by employing high-order discretization for the spatial terms. The accuracy of the scheme in space and time is verified by examining the grid/time-step dependency on one-dimensional benchmark cases: a freely propagating premixed flame in an open environment and in an enclosure related to spark-ignition engines. The scheme is then examined in simulations of a two-dimensional laminar flame/vortex-pair interaction. Furthermore, we apply the scheme to direct numerical simulation of a homogeneous charge compression ignition (HCCI) process in an enclosure studied previously in the literature. Satisfactory agreement is found in terms of the overall ignition behavior, local reaction zone structures and statistical quantities. Finally, the scheme is used to study the development of intrinsic flame instabilities in a lean H-2/air premixed flame, where it is shown that the spatial and temporary accuracies of numerical schemes can have great impact on the prediction of the sensitive nonlinear evolution process of flame instability. Crown Copyright (C) 2012 Published by Elsevier Inc. All rights reserved.
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| 7. |
- 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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| 10. |
- Carlsson, Henning, et al.
(författare)
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Direct numerical simulation of lean premixed CH4/air and H-2/air flames at high Karlovitz numbers
- 2014
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 1879-3487 .- 0360-3199. ; 39:35, s. 20216-20232
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH4/air and H-2/air flames at high Karlovitz numbers (Ka similar to 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (similar to 500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O-2+M = HO2 + M in lean H-2/air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH4/air and H-2/air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H-2/air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection-diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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