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CECR Research:

Skeletal and Reduced Mechanisms for Methane-Air Flames including NOx Chemistry

Abstract

Skeletal and reduced mechanisms have been developed for modeling nonpremixed methane-air flames including nitrogen chemistry. The skeletal mechanism includes 61 reactions with 25 species for methane combustion and an additional 52 reactions with 15 additional species to model the nitrogen chemistry. Techniques of systematic reduction were used to develop a five-step reduced mechanism for the hydrocarbon chemistry. Six-step and one-step reduced mechanisms are available for use with the hydrocarbon mechanism to model nitrogen chemistry; the one-step mechanism models only NOx while other mechanisms model most nitrogen containing species accurately.

Range of valididty and accuracy of the mechanisms

The skeletal mechanism is derived from a detailed mechanism considering both sensitivity and flux analyses for a variety of conditions. These conditions include nonpremixed counterflow configurations at pressures from 1 bar to 40 bar, with boundary temperatures from 298 K to 860 K, and with scalar dissipation rates ranging from 0.1/sec to near extinction. The skeletal mechanism maintains excellent agreement with the detailed mechanism for the retained species.

The following plots, taken from [1], compare the calculations using the reduced mechanisms and the skeletal mechanism. Calculations are performed for methane-air counterflow diffusion flames at (RED) p = 1 bar, Tf = To = 298 K, ((CYAN) p = 40 bar, Tf = To = 298 K, and (BLACK) p = 40 bar, Tf = 700 K, To = 860 K. These cases roughly correspond to ambient conditions, isothermal compression to 40 bar, and adiabatic compression to 40 bar. In each case, the solid line represents the detailed mechanism, the (long) dashed line represents the most comprehensive reduced mechanism, and the dotted line represents the reduced mechansism with one-step nitrogen chemistry (only relevant for the last plot). The units for the scalar dissipation rate on the abscissa are (1/s).

Peak Flame Temperatures

Maximum CH Mass Fraction

NOx Emission Index

The reduced mechanism is seen to consistently overpredict the temperature by 10 to 50 K. This results in the overprediction of production by the thermal mechanism; since the thermal mechanism is of greater importance for longer residence times, this is evident primarily for lower scalar dissipation rates. Prediction of CH is best at high scalar dissipation rates which is where the prompt mechanism is most important. With the one-step nitrogen chemistry, poor steady-state descriptions of intermediates important in reburn (specifically NH2) results in the underprediction of reburn relative to the six-step nitrogen chemistry at high pressures.

References

  1. J. C. Hewson and M. Bollig, Twenty-Sixth Symposium (International) on Combustion, 1996.


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International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames
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email: jhewson@ames.ucsd.edu
J. Hewson