DNS Study of the Optimal Chemical Markers for Heat Release in Syngas Flames

The purpose of this study is to identify a quantitative marker of the heat release rate (HRR) distribution using experimentally measurable species. Turbulent syngas (CO/H-2/air) flames with different equivalence ratios, H-2/CO ratios, and turbulence intensities are computed by Direct Numerical Simulations (DNS) in order to obtain an indirect but accurate estimation of heat release profiles. To check the robustness of the estimation, two different kinetic mechanisms have been considered. Based on a direct image analysis of the DNS results, normalized species concentrations combined with exponents are systematically tested in an attempt to reconstruct as accurately as possible the field of heat release rate. A systematic comparison is used to identify the best possible exponents associated with each species combination. Differing from previous studies, the present analysis takes into account the local thickness of the turbulent heat release zone. As a consequence, the obtained optimal species combinations represent not only the position of peak heat release but also local changes in the topology of the reaction zone (thickness, curvature). In the end, the heat release rate of atmospheric syngas flames can, in general, be best approximated using the concentrations of HCO and OH, using c(HCO) (-1.5)xc(HCO) (-7.5), when considering only species that are measurable by Laser-Induced Fluorescence. Another excellent reconstruction would be c(CH2O)(-0.32) x c(OH)(-0.8) for cases where CH2O is preferred to HCO.