Unsteady Flamelet Modeling of Turbulent Hydrogen-air Diffusion Flames

The unsteady flamelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion flame. An unsteady flamelet is solved interactively with a CFD solver for the turbulent flow and the mixture fraction field. Transient effects occurring in steady jet diffusion flames are discussed in terms of the relevant timescales. It is shown that radiation can be neglected and that the flame structure is hardly influenced 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 flamelet libraries yields good results for flame 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 flamelet as a postprocessing mode.