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Title: Numerical simulation of turbulent Bunsen flames with a level set flamelet model

Abstract

An extension to the level set flamelet model of Peters [Turbulent Combustion, Cambridge Univ. Press, Cambridge, UK, 2000] for premixed turbulent combustion that takes the effects of cold ambient air entrainment into account is presented. The model is valid in the flamelet regime, where the reaction zone thickness is smaller than the Kolmogorov length scale. The inner structure of the instantaneous flame front is taken into account by a presumed pdf approach of resolved laminar flamelets. To ascertain the performance of the level set flamelet model in an engineering context, the model is coupled to a standard k-{epsilon} model describing the turbulent flow field. This RANS level set flamelet model is then applied to the simulation of three turbulent Bunsen flames, F1, F2, and F3, different only in their respective Reynolds numbers. The predicted simulation results are compared to measurements of these flames showing good overall agreement. The results show that all three flames primarily fall within the thin reaction zone regime and that although a significant amount of cold ambient air is entrained close to the turbulent flame front, thereby significantly modifying the composition and the mean temperature of the burned gas flow, the flame front itself burns eithermore » in a fully (F3) or almost fully (F1 and F2) stoichiometric premixed environment. Furthermore, it is shown that resolving the inner structure of the instantaneous flamelet is a necessity for predicting the mean distribution of certain species, specifically radicals, in the turbulent flame. (author)« less

Authors:
 [1]
  1. Center for Turbulence Research, Stanford University, Stanford, CA 94305 (United States)
Publication Date:
OSTI Identifier:
20727310
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 145; Journal Issue: 1-2; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; COMPUTERIZED SIMULATION; FLAMES; TURBULENCE; COMBUSTION; REYNOLDS NUMBER; AIR; AMBIENT TEMPERATURE

Citation Formats

Herrmann, M. Numerical simulation of turbulent Bunsen flames with a level set flamelet model. United States: N. p., 2006. Web. doi:10.1016/j.combustflame.2005.09.016.
Herrmann, M. Numerical simulation of turbulent Bunsen flames with a level set flamelet model. United States. doi:10.1016/j.combustflame.2005.09.016.
Herrmann, M. Sat . "Numerical simulation of turbulent Bunsen flames with a level set flamelet model". United States. doi:10.1016/j.combustflame.2005.09.016.
@article{osti_20727310,
title = {Numerical simulation of turbulent Bunsen flames with a level set flamelet model},
author = {Herrmann, M.},
abstractNote = {An extension to the level set flamelet model of Peters [Turbulent Combustion, Cambridge Univ. Press, Cambridge, UK, 2000] for premixed turbulent combustion that takes the effects of cold ambient air entrainment into account is presented. The model is valid in the flamelet regime, where the reaction zone thickness is smaller than the Kolmogorov length scale. The inner structure of the instantaneous flame front is taken into account by a presumed pdf approach of resolved laminar flamelets. To ascertain the performance of the level set flamelet model in an engineering context, the model is coupled to a standard k-{epsilon} model describing the turbulent flow field. This RANS level set flamelet model is then applied to the simulation of three turbulent Bunsen flames, F1, F2, and F3, different only in their respective Reynolds numbers. The predicted simulation results are compared to measurements of these flames showing good overall agreement. The results show that all three flames primarily fall within the thin reaction zone regime and that although a significant amount of cold ambient air is entrained close to the turbulent flame front, thereby significantly modifying the composition and the mean temperature of the burned gas flow, the flame front itself burns either in a fully (F3) or almost fully (F1 and F2) stoichiometric premixed environment. Furthermore, it is shown that resolving the inner structure of the instantaneous flamelet is a necessity for predicting the mean distribution of certain species, specifically radicals, in the turbulent flame. (author)},
doi = {10.1016/j.combustflame.2005.09.016},
journal = {Combustion and Flame},
number = 1-2,
volume = 145,
place = {United States},
year = {Sat Apr 15 00:00:00 EDT 2006},
month = {Sat Apr 15 00:00:00 EDT 2006}
}
  • Based on the flamelet concept, a numerical model has been developed for fast predictions of NO{sub x} and CO emissions from laminar flames. The model is applied to studying NO formation in the secondary nonpremixed flame zone of fuel-rich methane Bunsen flames. By solving the steady-state flamelet equations with the detailed GR12.1 methane-air mechanism, a flamelet library is generated containing thermochemical information for a range of scalar dissipation rates at the ambient pressure condition. Modeling of NO formation is made by solving its conservation equation with chemical source term evaluated based on flamelet library using the extended Zeldovich mechanism andmore » NO reburning reactions. The optically-thin radiation heat transfer model is used to explore the potential effect of heat loss on thermal NO formation. The numerical scheme solves the two-dimensional Navier-Stokes equations as well as three additional equations: the mixture fraction, the NO mass fraction, and the enthalpy deficit due to radiative heat loss. With an established flamelet library, typical computing times are about 5 hours per calculation on a DEC-3000 300LX workstation. The predicted mixing field, radial temperature profiles, and NO distributions compare favorably with recent experimental data obtained by Nguyen et al. The dependence of NO{sub x} emission on equivalence ratio is studied numerically and the predictions are found to agree reasonably well with the measurements by Muss. The computed results show a decreasing trend of NO{sub x} emission with the equivalence ratio but an increasing trend in the CO emission index. By examining this trade-off between NO{sub x} and CO, an optimal equivalence ratio of 1.4 is found to yield the lowest combined emission.« less
  • An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (PDF), in which the marginal PDF of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in themore » first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed PDF. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional PDFs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the marginal PDF of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed PDF of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional PDFs and burning indices are in good agreement with measurements. (author)« less
  • The aim of this work is to analyze the application of flamelet models based on the mixture fraction variable and its dissipation rate to the numerical simulation of partially premixed flames. Although the main application of these models is the computation of turbulent flames, this work focuses on the performance of flamelet concept in laminar flame simulations removing, in this way, turbulence closure interactions. A well-known coflow methane/air laminar flame is selected. Five levels of premixing are taken into account from an equivalence ratio {phi}={infinity} (nonpremixed) to {phi}=2.464. Results obtained using the flamelet approaches are compared to data obtained frommore » the detailed solution of the complete transport equations using primitive variables. Numerical simulations of a counterflow flame are also presented to support the discussion of the results. Special emphasis is given to the analysis of the scalar dissipation rate modeling. (author)« less
  • Although the Eulerian particle flamelet model (EPFM) recently proposed by Barths et al. [Proc. Combust. Inst. 27 (1998) 1841-1847] has shown the potential capabilities to realistically predict detailed pollutant (NO{sub x}, soot) formation in a turbulent reacting flow occurring within practical combustion devices, there still exists room to improve the predicative capability in terms of local flame structure and turbulence-chemistry interaction. In this study, the EPFM approach was applied to simulate two turbulent nonpremixed jet flames of CO/H{sub 2}/N{sub 2} fuel having the same jet Reynolds number but different nozzle diameters, and the capability of predicting the NO{sub x} formationmore » as well as both similarity of major species and sensitivity of minor species to fluid-dynamic scaling for the two flames has been assessed deeply in terms of both conditional and unconditional mean structures. The present results indicate that the original EPFM substantially overpredicts the conditional scalar dissipation rate at the downstream region and consequently underpredicts the streamwise decay of superequilibrium radical concentrations to the equilibrium state. In this study, in order to correctly estimate the averaged conditional scalar dissipation rate, a new modeling of the conditional scalar dissipation rate based on a least-squares fit through a mass weighted spatial distribution has been devised. In terms of both conditional and unconditional means, the EPFM utilizing this new procedure yields nearly the same results as the Lagrangian flamelet model, and provides closer agreement with experimental data than the original EPFM approach. (author)« less
  • Previously conducted studies of the flamelet/progress variable model for the prediction of nonpremixed turbulent combustion processes identified two areas for model improvements: the modeling of the presumed probability density function (PDF) for the reaction progress parameter and the consideration of unsteady effects [Ihme et al., Proc. Combust. Inst. 30 (2005) 793]. These effects are of particular importance during local flame extinction and subsequent reignition. Here, the models for the presumed PDFs for conserved and reactive scalars are re-examined and a statistically most likely distribution (SMLD) is employed and tested in a priori studies using direct numerical simulation (DNS) data andmore » experimental results from the Sandia flame series. In the first part of the paper, the SMLD model is employed for a reactive scalar distribution. Modeling aspects of the a priori PDF, accounting for the bias in composition space, are discussed. The convergence of the SMLD with increasing number of enforced moments is demonstrated. It is concluded that information about more than two moments is beneficial to accurately represent the reactive scalar distribution in turbulent flames with strong extinction and reignition. In addition to the reactive scalar analysis, the potential of the SMLD for the representation of conserved scalar distributions is also analyzed. In the a priori study using DNS data it is found that the conventionally employed beta distribution provides a better representation for the scalar distribution. This is attributed to the fact that the beta-PDF implicitly enforces higher moment information that is in excellent agreement with the DNS data. However, the SMLD outperforms the beta distribution in free shear flow applications, which are typically characterized by strongly skewed scalar distributions, in the case where higher moment information can be enforced. (author)« less