Numerical study of flame structure and soot inception interpreted in carbon-to-oxygen atom ratio space

The utility of interpreting flame results in carbon-to-oxygen atom ratio (C/O ratio) space, as opposed to physical space or mixture fraction space, is evaluated. Flame and soot zone structures of counterflow diffusion flames have been studied for C₂H₄ and C₃H₈ and interpreted in C/O ratio space as a function of stoichiometric mixture fraction (Zst). The Burke-Schumann results expressed in C/O ratio space demonstrate how a clear and direct understanding of how structure is affected by Zst can be realized in C/O ratio space because, unlike physical or mixture fraction space, the flame location is independent of stoichiometric mixture fraction. Numerical results with detailed chemical kinetics also indicate that C/O ratio space is a fundamental variable in the sense that, for a given fuel, the location of the flame zones and critical reactions is invariant with Zst and strain rate. Two zones are clearly observed, namely the radical pool and the soot precursor zone located on the fuel side of the flame. The onset threshold of soot precursors (C₆H5 and C₆H₆) for the high temperature side of the soot precursor zone is characterized by the depletion of radicals. The role of the hydrogen radical in flame structure and soot inception is demonstrated by studying its production and consumption channels in C/O ratio space. Finally, a modified (C/O) is given to interpret the physical meaning of C/O ratio. The numerical results in this work indicate and explain the advantages of applying C/O ratio space in the analysis of flame structure and soot precursor chemistry.