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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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