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Evaluation of Optimized 3-step Global Reaction Mechanism
for CFD Simulations on Sandia Flame D
Abdallah Abou-Taouk and Lars-Erik Eriksson
Department of Applied Mechanics,
Chalmers University of Technology
Gothenburg, Sweden
Abstract. The aim of this paper is to evaluate a new optimized 3-step global reaction mechanism (opt) [1] for a methane-
air mixture for industry purpose. The global reaction mechanism consists of three reactions corresponding to the fuel
oxidation into CO and H2O, and the CO CO2equilibrium reaction. Correction functions that are dependent on the local
equivalence ratio are introduced into the global mechanism. The optimized 3-step global reaction scheme is adapted intothe Computational Fluid Dynamics (CFD) analysis of a partially-premixed piloted methane jet flame. The burner consists
of a central nozzle (for premixed fuel/air), surrounded by a premixed pilot flame, and an annular co-flow stream. Both
steady-state RANS (Reynolds Averaged Navier Stokes) and time-averaged hybrid URANS/LES (Unsteady RANS/Large
Eddy Simulation) results have been computed and compared with experimental results obtained from the Sydney burnerat Sandia National Laboratories, Sandia Flame D [2]. The CFD results with the optimized 3-step global reaction
mechanism show reasonable agreement with the experimental data based on emission, velocity and temperature profiles,
while the 2-step Westbrook Dryer (WD2) [3] global reaction mechanism shows poor agreement with the emission
profiles.
Keywords:CFD, PSR, global mechanism, combustion, methane-air mixture, WD2
PACS: 47.70.Pq
INTRODUCTION
A demand on reduced emissions and improved efficiency for the gas turbine combustors implies that reliable and
accurate modelings of chemical kinetics are crucial. In reality detailed reaction mechanisms for hydrocarbon
combustion are extremely complex since they involve around 100 species and 10000 reactions. Some detailedmechanisms of methane-air combustion involve more than 300 elementary reactions and over 30 species [4]. From a
CFD perspective it is still too expensive to include all species and reactions. The use of global reaction mechanisms
is one way to go for CFD simulations, since they are easily implemented in commercial software. Several differentreduced reaction mechanisms of methane-air mixtures exist in the literature [5-7]. The drawback of most of the
published global mechanisms is that they are not flexible enough to cope with a wide range of equivalence ratios.
The presently used optimized 3-step global reaction mechanism (opt) has been developed for equivalence ratios in
the range 0.5-1.8 and at atmospheric pressure [1].
The WD2 global reaction mechanism [3] is commonly seen in the literature and is an old industry standard for CFD
simulations. The drawback with the WD2 is due to the poor emission prediction at rich conditions since it producestoo much CO2and not enough carbon monoxide compared to the detailed reaction mechanism. LES simulations by
Pitsch et al. [8] show very good agreement with the experimental data. However, the mesh size is three times larger
and the time step is ten time smaller than the SAS-SST model, which implies that the computational time is very
expensive. The aim of the present work is to improve, validate and evaluate current standard industrial CFD tools. Itis too expensive for the industry to run LES in their daily work. In the CFX validation report [9] the conclusion is
that the RNG k- model together with the WD2 model works well for industry purpose. CFD simulation with thesame settings as in the CFX validation report [9] has been performed and used for comparisons in the present work
(RNG k- WD2). The results obtained agree well with those given in the CFX validation report. So, no
improvements or modifications have been done for the RNG k- WD2. The improvements of the predictions of the
emissions are seen by the 3-step global reaction mechanism since the optimized mechanism increases the reaction
rate on the first reaction to increase the CO production. Simultaneously, the second reaction rate is reduced, thusoxidizing less CO and therefore less CO2 is produced.
Numerical Analysis and Applied Mathematics ICNAAM 2011
AIP Conf. Proc. 1389, 66-69 (2011); doi: 10.1063/1.3636672 2011 American Institute of Physics 978-0-7354-0956-9/$30.00
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FIGURE 1.
Taouk et al.
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12].
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-Taouk and L
or Gas Turbin
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ombustion Sci
. S. Lau, C. K.
,Int. J. Heateredith and
68.
skaya, M. Bra
urbines, Jourkulakrishnan,
ulation of SyGT2006-9057
and H. Steineids 12, 2541-2
y, Piloted met
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K] plotted at c
p global reame D has be
tep global rea
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en funded b
he Royal Ins
ratefully ack
.E Eriksson, 2
s, ASME Tur
tories, Turbule and Frederic
. 1984, Vol. 1
Chan, Y. C.
ass Tran. 46, 3avid L. Black,
un-Unkhoff an
al of Engineeri. Kwon, A.J.
gas/Methane
, 2000, Larg555.
hane jet flame
. Sanjose and
73.Ansys CFX, ht
B. H. Hjertag
on. 16th Sym
ature profiles
en the experf the burner
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tic temperature
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tion mechanien modeled
ction mechan
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the Swedish
itute of Tech
owledged.
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011, Optimiz
bo Power Exp
nt Diffusion Fl L. Dryer, 19
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tp://www.ansy
r. On mathe
p. (Int'l.) on Co
and axial pr
imental dataand well p
t included in
tures the tem
[K] plotted at
t: Axial profile
NCLUSI
ism for methith the optim
ism show rea
hile the 2-
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