Reactive structures of diluted heptane burning in a high-temperature air flow in MILD Combustion conditions

The typical reactive structure stabilized in a diffusion layer in standard conditions can be significantly modified whether injected flows are diluted and/or preheated. The airflow high initial enthalpy and the low fuel concentration can drastically modify the structure of the oxidative and pyrolytic region due to change of the physical and chemical kinetics respect to conventional diffusion flame. Such operative conditions are typical of MILD (Moderate or Intense Low-oxygen Dilution) combustion processes. A numerical study of n-heptane stream autoigniting at intermediate air temperature has been performed to investigate the effect of inlet conditions on the stabilized reactive structures in MILD conditions. The analysis of the reaction zones in mixture fraction space has been performed by numerically investigating a steady, one-dimensional diffusive layer under conditions where low-temperature chemistry is expected to be important. The change of the structures of the reactive region induced by a diluted fuel flow fed towards a hot airflow jet (HODF) was analyzed. Distributions of temperature and heat release rate as a function of the mixture fraction are evaluated for different values of the external parameters, i.e. pre-heating temperature of the air and fuel dilution. They have been used in order to identify combustion regimes on the ground of their location and broadness. In particular, it has been shown that a significant broadening of heat release distribution is associated to oxidant temperatures higher than the auto-ignition temperature of homogeneous charge for a characteristic time comparable to the convective characteristic time of the system, supporting the conceptual model of "distributed oxidation". In general, the results obtained in these Hot-Oxidant-Diluted-Fuel conditions are consistent with and extend those reported in the literature for methane stream, supporting the fuel flexibility of MILD Combustion processes.