Characteristics of non-premixed oxygen-enhanced combustion: I. The presence of appreciable oxygen at the location of maximum temperature

The presence of appreciable molecular oxygen at the location of maximum temperature has been observed in non-premixed oxygen-enhanced combustion (OEC) processes, specifically in flames having a high stoichiometric mixture fraction (Z_st) produced with diluted fuel and oxygen-enrichment. For conventional fuel-air flames, key features of the flame are consistent with the flame sheet approximation (FSA). In particular, the depletion of O₂ at the location of maximum temperature predicted by the FSA correlates well with the near-zero O₂ concentration measured at this location for conventional fuel-air flames. In contradistinction, computational analysis with detailed kinetics demonstrates that for OEC flames at high Z_st: (1) there is an appreciable concentration of O₂ at the location of maximum temperature and (2) the maximum temperature is not coincident with the location of global stoichiometry, O₂ depletion, or maximum heat release. We investigate these phenomena computationally in three non-premixed ethylene flames at low, moderate, and high Z_st, but with equivalent adiabatic flame temperatures. Results demonstrate that the location of O₂ depletion occurs in the vicinity of global stoichiometry for flames of any Z_st and that the presence of appreciable O₂ at the location of maximum temperature for high Z_st flames is caused by a shift in the location of maximum temperature relative to the location of O₂ depletion. This shifting is attributed to: (1) finite-rate multi-step chemistry resulting in exothermic heat release that is displaced from the location of O₂ depletion and (2) the relative location of the heat release region with respect to the fuel and oxidizer boundaries in mixture fraction space. A method of superposition involving a variation of the flame sheet approximation with two heat sources is shown to be sufficient in explaining this phenomenon.