Numerical simulation of diluent effects on flame balls

The effects of various diluent gases on steady, source-free spherical premixed flames (“flame balls”) were modeled using a time-dependent numerical code with detailed chemical, transport, and radiation submodels. The diluent gas affects the Lewis number ( Le ) and radiative properties of the flames. Numerical solutions for the steady properties and stability limits were obtained for lean H 2 -air, H 2 −O 2 , H 2 −O 2 −CO 2 , and H 2 −O 2 −SF 6 mixtures. When results were nondimensionalized by the properties at the dynamic stability limit, all results collapsed onto one of two curves, depending only on whether the dominant source of radiative loss is from the product H 2 O, whose concentration decays to zero in the far-field, or diluent gas, whose concentration is constant in the far-field. No significant Le effect was found. Numerical predictions were compared with recent Space Shuttle experiments. For CO 2 and SF 6 diluents, experimental results lie between computational predictions obtained with diluent radiation included and with diluent radiation artificially suppressed, indicating that radiation models including reabsorption effects are needed in these cases. Significant influences of the chemical mechanism were found, even for mechanisms that properly predict the burning velocities of H 2 -air mixtures away from extinction limits. Sensitivity analysis showed that the discrepancies are due mainly to differences in the H+O 2 +H 2 O→HO 2 +H 2 O rate parameters for these mechanisms.