Structure of the ethanol spray flame under conventional and MILD conditions

• Triple flame was observed in atmospheric air co-flow condition. • Triple flame structure is weakened by increasing T and decreasing O2%. • Premixed flame was observed in hot co-flow condition. • NO pollutant generation shows more sensitivity to dilution rather than preheating. A numerical examination is carried out on the experimental MILD turbulent spray burner of Delft Spray in Hot Co-flow, DSHC. The RANS approach is combined with the Lagrangian tracking of droplets for modeling the pressure-swirl atomizer. The EDC combustion model is employed, to take into account detailed chemical mechanisms. Predictions show that the structure of the triple-flame of liquid fuel in the atmospheric condition is different from the conventional gaseous triple-flames. Furthermore, the structure of the MILD spray flame is compared to the non-MILD spray flame. The flame index shows that the triple-flame structure in the conventional spray flame that consists of two outer and middle diffusion branches, and a premixed inner branch converts to a double-flame structure under MILD condition, including an outer diffusion and a premixed inner branch. In another step, sensitivity analysis relative to the temperature, and oxygen concentration of the co-flow stream is performed independently, which was not possible in the experiment. The parameter of ignition delay time is used for the selection of parametric study cases. Moderate preheating could lead to deviation from the MILD combustion regime, while high preheating is beneficial. NO pollutant generation shows more sensitivity to dilution rather than preheating in MILD spray flame. This study showed that increasing or decreasing each of the temperature and oxygen concentration doesn’t improve all the effective aspects toward achieving the MILD condition. So, the effective parameters should be changed according to a specific and more precise goal.