Numerical investigation of the steady flamelet approach under different combustion environments

In the present work a CFD study of a natural gas fired lab-scale furnace under different oxygen enrichments was done. Results were compared to measured temperatures and heat fluxes on a thermal sink inside the furnace. The main emphasis was to apply the steady flamelet model (SFM) with a detailed chemical mechanism to predict the flow field, temperature and species concentrations in high temperature applications. A skeletal mechanism, which considered 25 reversible reactions and 17 species, showed close agreement with the measurement for all oxygen concentrations in the oxidizer. Turbulence modelling was done by the standard and realizable k-epsilon models as well as the Reynolds stress model (RSM). It was found, that standard k-epsilon model calculates too high temperatures in the vicinity of the burner. Results from realizable k-epsilon model and RSM were in close accordance with marginal differences in flame length and maximum temperatures. The CFD calculation with the SFM was also compared with a time expensive eddy dissipation concept (EDC) simulation with 46 reactions and 17 species. Both approaches calculated similar results for temperature and species concentrations. The calculation time was reduced significantly from 4 weeks (EDC) to 6 days with the SFM although dissociation effects and forming of radicals were taken into account. Considering melting and annealing furnaces, the heat flux to a thermal sink constitutes a major role for combustion processes. Heat flux to the thermal sink was determined by measurement and CFD predictions for different O2/N2 ratios in the oxidizer. CFD calculations revealed a maximum relative error to the measurement for the heat flux of 5% for 25, 30, 45 and 100 Vol% O2 in the oxidizer. The numerical model and experiments highlighted a major increase on the furnace efficiency due to oxygen enrichment. A maximum efficiency of 67% was detected for natural-gas combustion with pure oxygen compared to 44% in case of 25 Vol% O2. It was also found, that high oxygen concentrations lead to a homogeneous temperature distribution on the sink which is desired for melting and annealing applications.