Flamelet-Modeling of Inverse Rich Diffusion Flames

An experimental and numerical investigation of a confined laminar inverse diffusion flame (IDF) with pure oxygen as oxidizer and carbon dioxide diluted methane as fuel with a global stoichiometry of partial oxidation processes (equivalence ratio of 2.5) is presented. The present burner setup allows studying both the flame and the post-flame zone in a simplified geometry considering typical operating conditions as found in large-scale gasifiers. This partial oxidation flame setup is characterized by very high temperatures close to the stoichiometric oxidation zone due to oxy-fuel combustion, whereas lower temperatures and slow endothermic post-flame conversion reactions with long residence times are found in the fuel rich post-flame region. The scope of this paper is to investigate different modeling approaches suitable for both regimes by comparing the simulation results to detailed experimental data. Planar OH laser-induced fluorescence (OH-LIF) was performed for measuring the hydroxyl radical in the reaction zone and the results are compared to CFD calculations. Based on this comparison, the necessary level of detail of diffusion flux modeling, which includes Soret and Dufour effects, is analyzed and established. Finally, steady and unsteady non-premixed flamelet approaches based on a single mixture fraction are used in order to study their applicability for both the oxidation and post-flame zone. Significantly different time scales are obtained using different flamelet paths. Their influence on the results is investigated in the steady flamelet and the Lagrangian flamelet approach.