Transient inception of MILD combustion in hot diluted diffusion ignition (HDDI) regime: A numerical study

Unsteady 1-D reactive diffusive layers have been analyzed under two conditions: 1. Hot oxidant diluted fuel (HODF) conditions, which are representative of low-heat-value fuel burning in a high-temperature air flow. 2. Hot fuel diluted fuel (HFDF) conditions, which are representative of fuel injected in a high-temperature recirculated flow burning, in turn, in low-temperature air flow. The analysis has been performed by means of numerical detailed simulations of thermochemical patterns in a dense grid of input parameters synthesized in regime diagrams in which the main characteristic times of the autoignition and stabilization processes of the reactive mixing layers are reported. The analysis is focused on the comparison between unsteady and steady regimes under MILD combustion conditions and between the two limit cases presented in the paper. The main conclusions are: • The domains of MILD combustion of an autoigniting mixing layer shrink with respect to the corresponding domains under steady HODF/HFDF conditions. Higher inlet temperatures are needed in autoignition regimes to make a combustion process possible. • The most reactive mixture fraction is always placed on the high-temperature side with respect to the stoichiometric mixture fraction under both HODF and HFDF conditions. • The process evolution rates of unsteady MILD combustion are not universal. The rates are relatively slow under HODF conditions for either the autoignition or the subsequent oxidation process. In contrast, the rates are relatively fast for the autoignition and for the first oxidation processes under HFDF conditions. • The autoignition delay time is strictly correlated to the inlet temperature only for HFDF conditions, whereas they also depend on dilution level under HODF conditions.