Numerical Study on Inter-Droplet Interaction in Spontaneous Ignition of Fuel Droplet Clouds

Abstract

Spontaneous ignition of fuel droplets was numerically studied as a fundamental study for fuel spray. First, spontaneous ignition of an isolated droplet in a closed cell filled with hot fuel/air premixed gas was calculated. Namely, the inter-droplet interaction in a spray was expressed through the interference of the outer boundary of the cell. Fuel was n-heptane. Detailed chemical kinetics including the low-temperature oxidation reactions were employed, and therefore cool-flame ignition delay τcf and hot-flame ignition delay τig were evaluated. Initial liquid-phase temperature, gas-phase equivalence ratio and initial pressure were 300 K, 0.4 and 3 MPa, respectively. Since the premixed gas was fuel-lean, the existence of the droplet might either promote the ignition through a role as fuel source or hinder the ignition through a role as heat sink. When initial droplet diameter do was relatively large, both τcf and τig were not different from those of only premixed gas. It means that such large droplet required relatively long time for vaporization compared with chemical characteristic time, and the ignition occurred at the outer boundary of the cell that the fuel vapor from the droplet did not reach. However, as d0 decreased, τcf and τig increased and decreased, respectively, first. Thus interaction between spray and premixed gas was recognized. With increasing liquid-phase equivalence ratio, τig either increased monotonically or had minimal value depending on initial gas-phase temperature. Next, spontaneous ignition of a droplet pair was studied both experimentally and numerically. The experiments were performed in microgravity in order to get rid of the effects of natural convection using n-decane as fuel. Both τcf and τig increased with decreasing inter-droplet distance, while the duration between coolflame appearance and hot-flame appearance decreased, which is supposedly caused by higher cool-flame temperature. The experimental results can be explained thorough fuel vaporization behavior simulated by numerical calculations. Thus, droplet number density of spray can have the opposite effects on spontaneous ignition depending on conditions.