抄録
A fully transient numerical model of spontaneous ignition of single fuel droplets was developed. A physical part of the model is one-dimensional, and therefore it can easily employ a detailed chemical reaction model, which is necessary to reproduce complicated spontaneous ignition process of hydrocarbon fuels. The model simulates an isolated droplet in an open ambient at constant pressure. It was verified through microgravity experiments for relatively large (~0.7 mm) droplets, and was successful in quantitative reproduction of ignition delays of cool and hot flames. With the verified numerical model, ignition of relatively small (＜ 100 μm) droplets can be numerically observed. However, an isolated droplet in an open ambient that is smaller than a certain initial diameter does not ignite unlike droplets in a spray. The model was modified to handle a droplet in a closed cell so that single droplets could be compared to sprays. Two-stage ignition behavior (cool- and hot-flame ignitions) was observed even for such fine droplets. There was transition from heterogeneous ignition to homogeneous ignition with decreasing initial droplet diameter. The effect of ambient temperature on two-stage ignition was examined in terms of ignition delays, total heat release and cool flame temperature.
