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ICLASS 94<br>Proceedings of the Sixth International Conference on Liquid Atomization and Spray Systems

ISBN:
978-1-56700-019-1 (Print)
978-1-56700-445-5 (Online)

IGNITION OF DROPLETS: DISCUSSION ON THE VALIDITY OF THE IGNITION CRITERIA DERIVED ASSUMING GAS-PHASE QUASI-STEADINESS WITHOUT CONSIDERING FUEL VAPOR ACCUMULATION EFFECTS

S.C. Wong
National Tsing Hua University, Hsinchun, Taiwan, Republic of China

T.S. Hsu
National Tsing Hua University, Hsinchun, Taiwan, Republic of China

J.C. Chang
National Tsing Hua University, Hsinchun, Taiwan, Republic of China

Abstract

This work investigates the validity of the droplet ignition criteria which were derived based on the quasi-steady gas phase assumption without considering transient fuel vapor accumulation (FVA) effects. Numerical results of the ignition models with transient gas phase and quasi-steady gas phase without considering FVA effects are compared with respect to the pre-ignition distribution of fuel vapor concentration, the ignition delay, and the minimum ignition 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, the quasi-steady gas phase (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 ignition model yields somewhat lower limits than those of the transient model. 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 exist.