VALIDITY OF DROPLET IGNITION CRITERIA DERIVED ASSUMING GAS-PHASE QUASI-STEADINESS

This work investigates the validity of the droplet ignition criteria t hat 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 wit h respect to the preignition distribution of fuel vapor concentration, the ignition delay, and the minimum ignitable temperature, Calculatio ns are made for n-heptane and n-hexadecane, which have very different volatilities, at various ambient temperatures, initial droplet tempera tures, and initial droplet diameters, Without considering the transien t 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 fa r as the minimum ignitable gas temperatures are concerned, for both he ptane and hexadecane droplets the QSGP results are qualitatively corre ct, but appear quantitatively underestimated, These results indicate t hat the existing QSGP droplet ignition criteria derived without consid ering FVA effects, which have been widely used in droplet and spray ig nition analyses, are unsuitable for the determination of ignition dela ys, They may be used for determining the minimum ignitable temperature s, but significant underestimations may exist.