Numerical and experimental investigations on the influence of preheating and dilution on transition of laminar coflow diffusion flames to Mild combustion regime

A numerical and experimental study has been carried out to acquire knowledge about the structure and stabilization mechanism of coflow flames in their transition to the Mild combustion regime. In total, three CH₄/N₂/oxidizer coflow flames have been studied with a systematic dilution and preheating of the fuel and coflow streams. These flames comprise the non-preheated case (Case NP), preheated case (Case P) and Mild case (Case M), diluted and preheated from ambient temperature up to 1530K. Radial profiles of temperature and species concentrations have been measured using spontaneous Raman scattering. Detailed computations have been performed by steady-state simulations of these cases using detailed chemistry with the GRI 3.0 mechanism, multi-component mixture-averaged transport and an optically thin approximation for radiative heat losses. An overall good agreement has been found between results of the detailed computations and experiments for Case NP, Case P and at lower axial distances for Case M. The importance of using multicomponent transport and radiative heat losses in the computations has been investigated by performing additional computations with more simplified models for Case NP. A comparison of computed temperature distributions indicates that the progressive preheating and dilution of the oxidizer and fuel leads to a reduction of the temperature rise in the reaction zone with respect to a non-reacting case; this rise in Case M is less than 200K. Comparison of computed heat release and CH₂O distributions reveals that stabilization of Case NP and P occurs by an edge flame, while for Case M, it takes place by autoignition. Further investigations on the structure of Case M has been done by flamelet analyses in mixture fraction space. It is found that igniting flamelets, in contrast to steady flamelets, represent very well the structure of Case M at lower axial distances. This observation further emphasizes the stabilization of the Mild case by the autoignition phenomena.