Mild Combustion in Homogeneous Charge Diffusion Ignition (HCDI) regime

The paper deals with a subcategory of Mild Combustion, which has been named Homogeneous Charge Diffusion Ignition (HCDI). It is analyzed through characterization of processes that take place in counter-diffusion configuration obtained by two opposed jets of the fresh air–fuel mixture and inert flow. The reactive field is modeled through a numerical code exploiting a detailed chemical kinetics scheme of O2/CH4/N2 system. Distributions of temperature and enthalpy as a function of the spatial coordinate and the mixture fraction are obtained for different values of the external parameters, i.e. the maximum temperature increase, the fresh mixture temperature, the inert gas mixture, the reference strain rate. They can be classified in four main categories on the ground of location and broadness of the heat release and occur in contiguous exclusive regions of external parameters plots. Continuous transitions between regions are characterized by single solutions, whereas extinction/ignition discontinuous mechanisms are obtained in transition regions where double solutions are allowed. Unique characteristics, different from those of deflagration, emerged for well-identified ranges of external parameters. They are related to a distribution of the heat release in a broad spatial and mixture fraction range. The operative conditions, for which such peculiar behavior occurs, are coincident with those where Mild Combustion is expected to take place on the ground of its “a priori” definition.