DNS-studies on flame front markers for turbulent premixed combustion

Flame front identification and analysis in premixed turbulent combustion often relies on some scalar iso-surface as a flame marker, and the quest for a suitable scalar, and an appropriate flame-identifying iso-value, then becomes important. In an effort to contribute to this field, we study the suitability of various flame front identification approaches using the concept ofridge analysis. This serves to mark the central region of a given scalar field. In combustion, an essential scalar field is the heat release rate (HRR). Ridge analysis applied to the HRR-field therefore delivers a central region of combustion, which we identify with the flame front. Similar to iso-surfaces, ridges can define sheet-like objects that track the flame front in a given combustion data set. Ridges bear the advantage of being universal, intrinsic flame identifiers, because they do not depend on user-defined parameters like thresholds, iso-values or similar, and they can be applied to premixed as well as to diffusion flames. We determine flame fronts by ridge analysis of the heat release rate field in turbulent premixed combustion data sets obtained by 3D Direct Numerical Simulation (DNS). The suitability of other, iso-surface based flame identifiers is evaluated using the ridge-based, intrinsic flame front as a reference. Iso-surfaces of various scalars (several species and temperature) are compared to the ridges. The study shows that for a methane flame, iso-surfaces of several scalars can serve as good flame front markers, and suitable iso-values for these scalars can be derived from ridge-conditioned statistics. However, different fuel/air mixtures (we study two fuels, hydrogen and methane) exhibit different characteristics. For the hydrogen/air flame, none of the studied scalars was a suitable flame marker. This flame featured 'holes' (regions of vanishing chemical heat release). These are correctly detected as non-flame regions by the ridge method, but not by the iso-surfaces of classical flame marker scalars like temperature.