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Title: Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames

Abstract

A transported joint probability density function (JPDF) approach closed at the joint scalar level has been applied to investigate two nonpiloted CH{sub 4}/H{sub 2}/N{sub 2} turbulent (Re 15200 and 22800) jet diffusion flames. The flames have been studied experimentally at the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) and at Sandia National Laboratories and are well characterized experimentally through extensive velocity and scalar measurements. The flames offer the opportunity of computational investigations of turbulence-chemistry interactions in CH{sub 4}/H{sub 2} flames in the absence of both partial premixing with air and with a smaller stoichiometric mixture fraction (Z{sub st}=0.167) than in the corresponding piloted Sandia flames. The two flames also offer different levels of local extinction. Comparatively few theoretical studies have been performed of these flames and the present work provides an assessment of the ability of the transported PDF approach to reproduce their detailed thermochemical structure. The chemical closure is obtained through a systematically reduced C/H/O/N mechanism featuring 16 independent, 4 dependent, and 28 steady-state scalars. The velocity field is computed using the second moment closure of Speziale et al. and molecular mixing is modeled using the modified Curl's model. It is shown that velocity and scalar fields are generallymore » well reproduced for 10=<x/D=<80 though uncertainties in boundary conditions have an impact closer to the burner exit.« less

Authors:
;  [1]
  1. Thermofluids Division, Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2AZ (United Kingdom)
Publication Date:
OSTI Identifier:
20681465
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 143; Journal Issue: 4; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; FLAMES; MATHEMATICAL MODELS; METHANE; HYDROGEN; NITROGEN; TURBULENCE; COMBUSTION KINETICS

Citation Formats

Lindstedt, R.P., and Ozarovsky, H.C. Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames. United States: N. p., 2005. Web. doi:10.1016/j.combustflame.2005.08.030.
Lindstedt, R.P., & Ozarovsky, H.C. Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames. United States. doi:10.1016/j.combustflame.2005.08.030.
Lindstedt, R.P., and Ozarovsky, H.C. Thu . "Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames". United States. doi:10.1016/j.combustflame.2005.08.030.
@article{osti_20681465,
title = {Joint scalar transported PDF modeling of nonpiloted turbulent diffusion flames},
author = {Lindstedt, R.P. and Ozarovsky, H.C.},
abstractNote = {A transported joint probability density function (JPDF) approach closed at the joint scalar level has been applied to investigate two nonpiloted CH{sub 4}/H{sub 2}/N{sub 2} turbulent (Re 15200 and 22800) jet diffusion flames. The flames have been studied experimentally at the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) and at Sandia National Laboratories and are well characterized experimentally through extensive velocity and scalar measurements. The flames offer the opportunity of computational investigations of turbulence-chemistry interactions in CH{sub 4}/H{sub 2} flames in the absence of both partial premixing with air and with a smaller stoichiometric mixture fraction (Z{sub st}=0.167) than in the corresponding piloted Sandia flames. The two flames also offer different levels of local extinction. Comparatively few theoretical studies have been performed of these flames and the present work provides an assessment of the ability of the transported PDF approach to reproduce their detailed thermochemical structure. The chemical closure is obtained through a systematically reduced C/H/O/N mechanism featuring 16 independent, 4 dependent, and 28 steady-state scalars. The velocity field is computed using the second moment closure of Speziale et al. and molecular mixing is modeled using the modified Curl's model. It is shown that velocity and scalar fields are generally well reproduced for 10=<x/D=<80 though uncertainties in boundary conditions have an impact closer to the burner exit.},
doi = {10.1016/j.combustflame.2005.08.030},
journal = {Combustion and Flame},
number = 4,
volume = 143,
place = {United States},
year = {Thu Dec 01 00:00:00 EST 2005},
month = {Thu Dec 01 00:00:00 EST 2005}
}
  • Numerical simulation results are presented for turbulent jet diffusion flames with various levels of turbulence-chemistry interaction, stabilized behind a bluff body (Sydney Flames HM1-3). Interaction between turbulence and combustion is modeled with the transported joint-scalar PDF approach. The mass density function transport equation is solved in a Lagrangian manner. A second-moment-closure turbulence model is applied to obtain accurate mean flow and turbulent mixing fields. The behavior of two micromixing models is discussed: the Euclidean minimum spanning tree model and the modified Curl coalescence dispersion model. The impact of the micromixing model choice on the results in physical space is small,more » although some influence becomes visible as the amount of local extinction increases. Scatter plots and profiles of conditional means and variances of thermochemical quantities, conditioned on the mixture fraction, are discussed both within and downstream of the recirculation region. A distinction is made between local extinction and incomplete combustion, based on the CO species mass fraction. The differences in qualitative behavior between the micromixing models are explained and quantitative comparison to experimental data is made. (author)« less
  • Turbulent CO/H{sub 2}/N{sub 2} (“syngas”) flames are simulated using a transported composition probability density function (PDF) method. A consistent hybrid Lagrangian particle/Eulerian mesh algorithm is used to solve the modeled PDF transport equation. The model includes standard k–ϵ turbulence, gradient transport for scalars, and Euclidean minimum spanning tree (EMST) mixing. Sensitivities of model results to variations in the turbulence model, the treatment of radiation heat transfer, the choice of chemical mechanism, and the PDF mixing model are explored. A baseline model reproduces the measured mean and rms temperature, major species, and minor species profiles reasonably well, and captures the scalingmore » that is observed in the experiments. Both our results and the literature suggest that further improvements can be realized with adjustments in the turbulence model, the radiation heat transfer model, and the chemical mechanism. Although radiation effects are relatively small in these flames, consideration of radiation is important for accurate NO prediction. Chemical mechanisms that have been developed specifically for fuels with high concentrations of CO and H{sub 2} perform better than a methane mechanism that was not designed for this purpose. It is important to account explicitly for turbulence–chemistry interactions, although the details of the mixing model do not make a large difference in the results, within reasonable limits.« less
  • Numerical simulation results are presented for a turbulent nonpremixed flame with local extinction and reignition. The transported scalar PDF approach is applied to the turbulence-chemistry interaction. The turbulent flow field is obtained with a nonlinear two-equation turbulence model. A C{sub 1} skeletal scheme is used as the chemistry model. The performance of three micromixing models is compared: the interaction by exchange with the mean model (IEM), the modified Curl's coalescence/dispersion model (CD) and the Euclidean minimum spanning tree model (EMST). With the IEM model, global extinction occurs. With the standard value of model constant C{sub f}=2, the CD model yieldsmore » a lifted flame, unlike the experiments, while with the EMST model the correct flame shape is obtained. However, the conditional variances of the thermochemical quantities are underestimated with the EMST model, due to a lack of local extinction in the simulations. With the CD model, the flame becomes attached when either the value of C{sub f} is increased to 3 or the pilot flame thermal power is increased by a factor of 1.5. With increased value of C{sub f} better results for mixture fraction variance are obtained with both the CD and the EMST model. Lowering the value of C{sub f} leads to better predictions for mean temperature with EMST, but at the cost of stronger overprediction of mixture fraction variance. These trends are explained as a consequence of variance production by macroscopic inhomogeneity and the specific properties of the micromixing models. Local time stepping is applied so that convergence is obtained more quickly. Iteration averaging reduces statistical error so that the limited number of 50 particles per cell is sufficient to obtain accurate results. (author)« less
  • The joint velocity-turbulence frequency-composition PDF method is applied to a lifted turbulent jet flame with H{sub 2}/N{sub 2} fuel issuing into a wide coflow of lean combustion products, which are at a temperature of 1045 K. Model calculations with detailed chemistry are performed using three existing mixing models (IEM, MC, and EMST) and two chemistry mechanisms (the Mueller and Li mechanisms). Numerically accurate results are obtained and compared with the experimental data. Recent experiments have shown that the stabilization height of this lifted flame is very sensitive to the coflow temperature, much more than to the inlet velocity profile ormore » the initial temperature of the fuel. One percent (i.e., 10 K) change in the coflow temperature (which is well within the experimental uncertainty) can double the lift-off height. The joint PDF calculations capture this sensitivity very well and are in good agreement with the measurements for the velocity, mixture fraction, and species. The three mixing models give relatively similar results, implying that the cases studied here are mainly controlled by chemical kinetics. The Li mechanism results in earlier ignition than the Mueller mechanism and hence gives shorter lift-off heights over the whole test range. The joint PDF calculations generally give better agreement with the measurements than previous composition PDF calculations [A.R. Masri et al., Combust. Theory Modelling 8 (2004) 1-22]. A new parallel algorithm, involving domain partitioning of particles, has been implemented to facilitate these computations.« less