Flamelet-based modeling of NO formation in turbulent hydrogen jet diffusion flames

The potential of using the laminar flamelet model for predictions of NOx emissions from turbulent hydrogen jet diffusion flames with various amounts of helium dilution is explored in this study. The flamelet approach treats turbulent flames as an ensemble of laminar flames subject to local fluid dynamic stretch which causes chemical kinetics to deviate from equilibrium. This flame stretch rate can be described as the scalar dissipation rate or as the strain rate. Numerical modeling is performed to assess the merits of these two choices by comparing predicted scaling behavior of NOx emission index with the experimental data of Driscoll and co-workers [1,2]. The present study reveals that only with the scalar dissipation rate as the nonequilibrium parameter will the predicted NOx emission indices exhibit a clear scaling relation vs the Damkohler number. The flamelet predictions are found to improve when the Damkohler number increases. However, the absolute NOx levels are overpredicted, which is attributed to differential diffusion effects. (C) 1997 by The Combustion Institute.