Reduced Mechanisms based on GRI-Mech 1.2

W.-C. Chang and J.-Y. Chen

     Department of Mechanical Engineering
     University of California at Berkeley
     Berkeley, CA 94720, USA

     Telephone: (510) 642-3286
     Fax: (510) 642-6163
     e-mail: jychen@newton.me.berkeley.edu


Introduction

With an automatic computer code (CARM), two sets of reduced mechanisms have been developed from GRI-Mech 1.2 and their performances are assessed. CARM is used to post-process the steady state solution of a perfectly-stirred-reactor (PSR) with the original GRI-Mech at equivalence ratio (Phi) equal to 1.0 and residence time (Tau) at 1 second. Quasi-steady-state assumptions for reactive species are assessed and judicially applied for eliminating minor/intermediate species.

These are ASCII files used to replace the subroutine CKWYP in the CHEMKIN-II software package. For the thermodynamics data one must use thermo12.dat, the thermodynamics data of GRI-Mech. The thermo12.dat file can be downloaded from the GRI-Mech Home Page.
Two reduced mechanisms have been developed:


To reference these reduced mechanisms, please use:
W.-C. Chang and J.-Y. Chen, http://firebrand.me.berkeley.edu/griredu.html

The rest of this document describes the reduction technique used to develop these reduced mechanisms and their performance tested by the authors.

We will be glad to learn about your experiences with these reduced mechanisms (and include your results in this document as well).


Reduction Technique

The Sandia's Perfectly Stirred Reactor (PSR) code is run under one or several conditions of interest with the detailed mechanism, here GRI-Mech 1.2. The solutions and sensitivity data of this PSR run are read by CARM for developing reduced mechanisms. CARM assess the validity of quasi-steady-state for each species and ranks them in order. Based on the ranking, users can choose species that can be approximated by the steady state assumption. An independent set of elementary reactions is chosen for eliminating those steady-state species. Next, CARM writes out the global rates of the reduced mechanism expressed in terms of the elementary rates. Finally, CARM writes out source codes for evaluation of global reaction rates. These codes, namely 'ckwyp' or 'ckwyr', are comptaible with CHEMKIN but indepdent subroutines. The condition chosen is equivalence ratio (Phi) = 1.0 and residence time (Tau) = 1.0 second at 1 atm.


Evaluation of Performances of Reduced Mechanisms

Two reduced mechanisms, 8-step and 6-step, were tested against GRI-Mech 1.2 under three flame simulation codes: Perfectly-Stirred-Reactor, Steady Laminar 1-D Premixed Flame, and Counterflow Diffusion Flame in Tsuji Geometry.


Perfectly-Stirred-Reactor (PSR)

The reduced mechanisms were tested using PSR code under four different equivalence ratios at ambient pressurere. The comparisons are presented in Figures 1 to 4. Each PSR result contains steady state solutions for residence times ranging from 0.1 second to near flame extinction. As shown in these figures, the 8-step reduced mechanism gives solutions in excellent agreement with those obtained from GRI-Mech 1.2 under all conditions. The 6-step reduced mechanism also gives satisfactory predictions for Phi = 0.7, 1.0 and 1.3. However, the performance of the 6-step reduced mechanism at Phi = 2.0 is not satisfactory. Therefore, the 6-step reduced mechanism appears to work best for lean to slightly rich mixtures. The 8-step reduced mechanism does a much better job at rich conditions as two more species are included: CH3 and C2H2.

Phi=0.7, P=1atm

Figure 1. Predicted temperature and species mole frations for PSR at Phi = 0.7 and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures.

Phi=1.0, P=1atm

Figure 2. Predicted temperature and species mole frations for PSR at Phi = 1.0 and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures.

Phi=1.3, P=1atm

Figure 3. Predicted temperature and species mole frations for PSR at Phi = 1.3 and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures.

Phi=2.0, P=1atm

Figure 4. Predicted temperature and species mole frations for PSR at Phi = 2.0 and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures.


Steady Laminar 1-Dimensional Premixed Flame

Using Sandia's premixed flame code, the reduced mechanisms have been tested under steady laminar 1-D premixed flame at Phi = 1.0 at ambient pressure. Figure 5 presents the comparisons. As seen in the figure, the temperature and density profiles predicted by both reduced mechanisms agree quite well with those by GRI-Mech 1.2. Both reduced mechanisms give satisfactory predictions of species profiles and flame speed. As shown in the plots, the performance of the 8-step reduced mechanism is slightly better than the 6-step reduced mechanism.

Phi=1.0, P=1atm

Figure 5. Predicted temperature, velocity, density, and species mole frations for steady laminar 1-D Premixed Flame at Phi = 1.0 and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures. 

Counterflow Diffusion Flame in Tsuji Geometry

The reduced mechanisms were also tested using Sandia's counterflow diffusion flame code at strain rate = 100 (1/sec) at ambient pressure and the results are presented in Figure 6. The temperature profiles predicted by both reduced mechanisms are seen to agree well with those by GRI-Mech 1.2. The 8-step reduced mechanism gives satisfactory predictions of species profiles. The performance of the 6-step reduced mechanism is much worse than the 8-step in predicting the species profiles. The deviations can be as high as 100% for CO and H2 near the flame zone.

Strain Rate=100 (1/sec), P=1atm

Figure 6. Predicted temperature and species mole frations for Counterflow Diffusion Flame in Tsuji Geometry at strain rate = 100 (1/sec) and P = 1 atm with the reduced mechanisms and GRI-Mech for methane-air mixtures. 

References

  1. Kee, R. J., Rupley, F. M., Miller, J. A., "Chemkin-II: A FORTRAN Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics ", Sandia Rep. SAND 89-8009, (1989)
  2. Glarborg, P., Kee, R. J., Grcar, J. F., Miller, J. A., "PSR: A FORTRAN Program for Modeling Well-Stirred Reactors", Sandia Rep. SAND 86-8209, (1986)
  3. Kee, R. J., Grcar, J. F., Smooke, M. D., Miller, J. A., " A FORTRAN Program for Modeling Steady Laminar One-DimensionalPremixed Flames", Sandia Rep. SAND 85-8240, (1985)
  4. Miller, J. A., Kee, R. J., Smooke, M. D., Grcar, J. F., "The Computation of the Structure and Extinction Limit of a Methane-AirStagnation Point Diffusion Flame", Paper # WSS/CI84-10, University of Colorado, Boulder, CO, April 2-3, (1984)

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