2-STAGE IGNITION OF N-HEPTANE ISOLATED DROPLETS

Experimental and theoretical studies on the spontaneous ignition proce ss of isolated fuel droplets were carried out. Time dependent temperat ure fields around the igniting droplets were observed by interferometr y so that two step temperature rise can be detected. Some experiments are performed under microgravity to obtain reference data. Induction t imes are examined as a function of ambient temperature. As a result, a zero temperature coefficient region is found, which is equivalent to the NTC (negative temperature coefficient) region for the ignition of premixed gas. A numerical model is developed applying a simplified che mical reaction model that includes the low and the high temperature re actions. The model is able to reproduce the two step temperature rise and the roles of the two kinds of reactions on the ignition process up to the establishment of a diffusion flame around the droplet are exam ined.