Spontaneous ignition of fuel droplets in lean fuel-air mixtures

Experiments have been carried out on the spontaneous ignition of single fuel droplets in lean fuel-air mixtures. The residence time, which is defined as the elapsed time from the introduction of the mixture into the furnace to the insertion of the droplet, is 15 s. The change in the chemical species concentration in fuel-air mixtures before the insertion of the droplet is predicted by the calculation of chemical reaction. In the case of lower ambient temperatures, the methane-air mixture does not react so much and most of methane and oxygen in the ambience survives for the residence time of 15 sec. In this case, the ignition delay time in mixtures is almost the same as that in air, which indicates that the existence of fuel in the ambience has little effect on the spontaneous, ignition behavior. On the other hand, the ignition delay time in mixtures becomes larger than that in air for higher ambient temperature. This is mainly due to the decrease in the oxygen concentration in the ambience caused by chemical reaction of fuel-air mixtures during the residence time. However, the quantitative comparison suggests that the increase in the ignition delay time in fuel-air mixtures is not explained by only reduced oxygen concentration.