Numerical analysis of the cool flame behavior of igniting n-heptane droplets

For many technical combustion applications like Lean Prevaporized Premixed (LPP) Systems the knowledge of droplet self-ignition processes as the basic element of spray ignition is necessary. Especially the two-stage ignition behavior due to a cool flame and therefore the Zero Temperature Coefficient Behavior (ZTC) are hereby important. Detailed Numerical Simulations give the opportunity to analyze these processes leading to droplet self-ignition, to get a deeper insight into the existing phenomena. The present study is carried out with a former validated, detailed numerical model including a substantial chemical reaction mechanism for n-Heptane with 437 reactions and 92 species. The Temperature Profiles as a function of time and as a function of radius as well as the profiles of the temperature gradient as a function of time and as a function of radius clarify the characteristics of the ignition process. The mixture fraction due to evaporation and diffusion and therefore the pool on the for the cool flame appearance important species OQ'OOH determines the activity of the low temperature reactions and thus the maximum possible cool flame temperature. The first local temperature rise, which is produced by the low temperature reactions, takes place below ambient temperature at very low fuel / air mixture ratios. It can be found, that the place of the maximum temperature and of the maximum temperature gradient as well, do not coincide during the first stage of ignition. The spatial in-homogeneity of the temperature and species concentration field around the droplet allows for a parallel burning of a cool flame caused by low temperature reactions, even when hot flame ignition has already occurred.