The Effects of Oxygen Dilution on Two-Stage Autoignition of Isolated N-Dodecane Droplets in Microgravity

Abstract

ABSTRACT Two-stage autoignition of n-dodecane droplets with varying ambient oxygen concentrations in oxygen-nitrogen mixtures are investigated experimentally under microgravity conditions using high-speed shadowgraphy. The ambient pressure and temperature are held constant at 3 atm and 650 K, respectively, while the droplet initial diameter is fixed approximately at 1.2 mm. During the two-stage autoignition process, first a cool-flame front forms in the leaner regions farther away from the evaporating droplet, and it then propagates toward the fuel-rich region closer to the droplet surface, eventually encompassing the droplet. A hot-flame kernel is then initiated in the wake of the cool flame and very quickly expands, establishing a classical diffusion flame around the droplet. The first and second ignition delay times are measured from shadowgraphic images captured at 3000 frames per second. The first induction time is found to be insensitive to the ambient oxygen concentration, while the second induction time varies approximately as the negative 2 power of the oxygen mole fraction.