By Graham M. Goldin, Fluent Inc.

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The focus of FLUENT 6.1 gas-phase combustion modeling is to provide affordable, detailed, finite-rate chemistry. With the new models, kinetically controlled processes such as pollutant formation (NOx, CO, etc.) and flame ignition/extinction can be simulated with high fidelity.

The difficulty in including detailed kinetics is the extreme non-linearity of the chemical mechanism. Large computational times are required to integrate the equation set, and special care is required to properly couple the chemistry with the turbulent flow. For these two reasons, most commercially available chemistry codes are limited to physical dimensions of zero or one.

To overcome the massive computational demands of detailed chemistry simulation in 2D and 3D domains, FLUENT 6.1 incorporates ISAT (In-Situ Adaptive Tabulation¹), which can accelerate chemistry calculations up to a thousand- fold. For a chemical mechanism with N species, ISAT builds N-dimensional chemistry tables during the simulation. The expensive kinetic integrations are mitigated by retrieving the appropriate values from the table. ISAT can be used with two turbulence-chemistry interaction models in FLUENT 6.1: the Eddy Dissipation Concept (EDC) model and the PDF Transport model.

Contours of NO on a center plane near the quarl

Comparison of FLUENT predictions of NOx with experimental data for radial scans 27mm and 432mm downstream of the quarl

To demonstrate the power of ISAT, a FLUENT 6.1 simulation of Sandia's Burner Engineering Research Laboratory (BERL) industrial combustor has been performed using the EDC model. The BERL combustor consists of an annulus with swirling air, into which 24 fuel jets inject natural gas. The mixture then enters a quarl, which expands to a hexagonal combustion chamber. Because of its complex physics and ample supply of experimental data, the BERL combustor has served as a benchmark test case for combustion models in FLUENT for many years.

Since the fuel jets in the cross-flow air-stream cannot be accurately modeled in 2D, a 3D sector representing 1/24th of the burner is modeled. The simulation makes use of the realizable k-e turbulence model, and the P1 radiation model. The chemistry is described by a 9 species Augmented Reduced Mechanism (ARM), which was derived from the detailed natural gas mechanism by making steady-state assumptions for certain species.²

Results for radial NO predictions are in good agreement with experimental measurements at 27mm and 432mm downstream of the quarl, despite the many assumptions made in modeling the turbulence, chemistry, radiation, and their interactions with each other. Radial profiles of temperature and other species concentrations follow the same trends. In addition, ISAT provides a net speed-up of 65 for this case. Without ISAT, a simulation that can be completed overnight would require a month of run-time!

References

1 Pope S.B., "Computationally Efficient Implementation of Combustion Chemistry Using In-Situ Adaptive Tabulation", Combustion Theory and Modeling, 1, pp. 41-63, 1997.

2. http://www.et.byu.edu:8080/~tom/Papers/ Hemant-WSS96/WSS.html

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