Abstract
As a fundamental study on the droplet-interaction effect in the fuel-spray ignition, spontaneous ignition of an n-decane droplet pair in hot air was experimentally studied in normal gravity and microgravity. Two suspended droplets initially at room temperature were brought into a hot furnace filled with air and ignited spontaneously. In the previous studies, cool-flame and hot-flame ignition delays were evaluated thorough thermocouple measurement, and their dependence on inter-droplet distance was discussed thorough the development of temperature and fuel-concentration distribution around a droplet pair. However, two-dimensional observation of cool-flame and hot-flame behavior was not possible with thermocouple measurement. In the present study, interferometry was applied in order to detect cool flame, which emits almost no visible light. Density field around a droplet pair was qualitatively observed with a high-speed camera, and the locations of cool-flame and hot-flame appearance were evaluated. Pressure was 0.3 MPa, and ambient temperature was between 560 and 680 K. The ambient conditions are in the range where two-stage ignition occurs. Initial droplet diameter was either 0.8 mm or 1 mm. In normal gravity, cool flame always appeared on the lower side of the droplet pair, and hot flame appeared on the upper side in most cases, which is obviously the effect of buoyancy. In microgravity, cool flame appeared on the outer side of the pair, and hot flame appeared on the inner side of the pair, which corresponds with the discussion in the previous studies.
