Numerical Study of Mild Combustion in Hot Diluted Diffusion Ignition (HDDI) regime

Mild Combustion is a process defined on the ground of well-identified external parameters, namely the temperature of reactants and the maximum allowable temperature increase. Although the definition is rigorous, it is necessary to identify unique, intrinsic structural properties of processes evolving in such conditions in the different, basic configurations that make Mild Combustion relevant from practical point of view. In this paper the configuration of opposed jets of hot air versus cold fuel/diluent mixture, here referred as Hot Diluted Diffusion Ignition (HDDI), has been considered. 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, fuel dilution, pressure and strain rate. 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 the asymptotic condition with very diluted fuel where the pyrolysis region, typical of standard diffusion flames, is no longer present in the heat release profiles, the sub-domain Mild-HDDI Combustion is identified. In this regime the position of the maximum heat release is completely uncorrelated with the stoichiometric mixture fraction. In this case the oxidation takes place only where the autoignition can develop inside constrain of the residence time of the fuel in the system.