Numerical and experimental studies of the NO formation in laminar coflow diffusion flames on their transition 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 1530 K. 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 and multicomponent mixture-averaged transport. Radiation effects are also included (except for Case M) in the detailed computations using an optically thin approximation. 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 agreement for Case M far downstream is quite fair in which the computations predict lower temperatures. 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 200 K. Comparison of computed heat release and formaldehyde distributions reveals that stabilization of Case NP and P occurs by an edge flame, while for Case M, it takes place by autoignition. The importance of using multicomponent transport in the computations is investigated by performing additional computation assuming constant Schmidt numbers for all species. The results indicate that this assumption has a considerable effect on the predicted flame structure, especially at the centerline. Effects of radiative heat losses have been investigated by neglecting radiation in the computations. It turns out that these losses do not influence the predictions considerably, especially at lower axial distances; however, their effect slightly increases with increasing axial distance. When the results are plotted in mixture fraction space, the experimental and computational data at different heights are very well correlated and they collapse on one line for Case NP and P within the uncertainty of the measurements. However, such a correlation for Case M is not found at lower axial distances due to the incomplete combustion of the fuel/oxidizer mixture. Further investigations on the structure of Case M are done by flamelet analyses in mixture fraction space. It is found that igniting 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.