Pyrolitic and oxidative structures in HODO MILD Combustion

The typical reactive structure stabilized in a diffusion layer in standard conditions can be significantly modified whether injected flows are diluted and/or pre-heated. The increase of the fuel and/or oxidant flow dilution up to extreme conditions could lead to the formation of not ignitable mixtures so that the oxidation processes could be sustained just in case the pre-heating temperature of one of the two flows is high enough to promote the auto-ignition of the system. The flows high initial enthalpy and the low fuel and/or oxygen 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 Combustion processes. More specifically a combination of both heating and dilution of oxidant and/or fuel can yields to a not premixed combustion process named Hot Diluted Diffusion Ignition (HDDI). The effect of inlet conditions on the stabilized reactive structure has been studied by analyzing the behavior of a steady, unidimensional diffusive layer. Numerical simulations have been carried out by means of OPPDIF application of ChemKin code and kinetic mechanisms available in literature, which simulates the behavior of two opposed jets. The change of the structures of the reactive region induced by a hot and diluted oxidant flow (HDDI-HODO) fed towards a fuel jet at environmental temperature has been numerically analyzed. Temperature and heat release profiles are considered as key parameters to understand the main features of the reactive region. The oxidant flow dilution causes a lower maximum temperature and more uniform temperature profiles with respect to un-diluted traditional diffusion flames even though oxidant flow is pre-heated. At the same time, higher oxidant flow dilution levels affect drastically heat release profiles and cause the disappearance of fuel pyrolitic reactions (negative heat release values). Further simulations were performed to understand the effect of water and carbon dioxide on flame structure.