Abstract
This work investigates the validity of the droplet ignition criteria that were derived based on the quasi-steady-gas-phase (QSGP) assumption without considering transient fuel vapor accumulation (FVA) effects. Numerical results of the ignition models with transient gas phase and quasisteady gas phase without considering FVA effects are compared with respect to the preignition distribution of fuel vapor concentration, the ignition delay, and the minimum ignitable temperature. Calculations are made for n-heptane and n-hexadecane, which have very different volatilities, at various ambient temperatures, initial droplet temperatures, and initial droplet diameters. Without considering the transient FVA effects (i.e., ignoring the transient mass conservation equation), the QSGP ignition model considerably underpredicts ignition delays, with errors increasing sharply with increasing fuel volatility. As far as the minimum ignitable gas temperatures are concerned, for both heptane and hexadecane droplets the QSGP results are qualitatively correct, but appear quantitatively underestimated. These results indicate that the existing QSGP droplet ignition criteria derived without considering FVA effects, which have been widely used in droplet and spray ignition analyses, are unsuitable for the determination of ignition delays. They may be used for determining the minimum ignitable temperatures, but significant underestimations may exist.