Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 1057±1064 UNSTEADY FLAMELET MODELING OF TURBULENT HYDROGEN-AIR DIFFUSION FLAMES

The unsteady ¯amelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion ¯ame. An unsteady ¯amelet is solved interactively with a CFD solver for the tur-bulent ¯ow and the mixture fraction ®eld. Transient effects occurring in steady jet diffusion ¯ames are discussed in terms of the relevant timescales. It is shown that radiation can be neglected and that the ¯ame structure is hardly in¯uenced by transient effects for the present case. However, for predictions of slow processes, like the formation of NO, unsteady effects have to be considered. The results predicted by the model are in reasonable agreement to experimental data for temperature, major species mass fractions, OH, and NO mole fractions. On the contrary, the use of steady ¯amelet libraries yields good results for ¯ame structure and even OH concentrations, but NO is overpredicted by an order of magnitude. However, reasonably well-predicted NO concentrations can also be obtained by solving an unsteady ¯amelet as a postprocessing mode