Effects of droplet interaction on spontaneous ignition of an n-decane droplet pair

As a fundamental study on the droplet-interaction effect in the fuel-spray ignition, spontaneous ignition of an n-decane droplet pair rapidly inserted into hot air was experimentally studied in the ambient temperature range where the low-temperature oxidation reactions are active. Two droplets suspended on 14 lm SiC fibers initially at room temperature were inserted into a hot furnace. Droplet diameter was 1 mm. Three hot junctions of K-type thermocouples with a diameter of 25 lm were located near the droplet pair, and cool-flame and hot-flame appearances were detected. The experiments were performed in microgravity to exclude the effect of buoyancy. First, at atmospheric pressure, where only cool flame appears, cool-flame ignition delay and cool-flame temperature increased with decreasing inter-droplet distance. The former is supposed to be mainly caused by the mutual cooling effect, and the latter is by the enhanced fuel supply through duplicated fuel sources. Next, at 0.3 MPa, where two-stage ignition occurs, cool-flame ignition delay increased with decreasing inter-droplet distance in the same way as at atmospheric pressure. On the other hand, the duration between cool-flame appearance and hot-flame appearance (second induction time) decreased with decreasing inter-droplet distance. This is supposedly caused by higher cool-flame temperature through higher fuel concentration in the reaction zone at the moment of cool-flame appearance. Thus the mutual cooling effect was dominant before cool-flame appearance, while the effect of duplicated fuel sources was dominant after cool-flame appearance.