Numerical simulations of the ignition of n-heptane droplets in the transition diameter range from heterogeneous to homogeneous ignition

Spontaneous ignition of single n-heptane droplets in a constant volume filled with air was numerically simulated with the spherical symmetry. The numerical model was fully transient. It continues calculation even after the droplet has completely vaporized, and therefore can predict pre-vaporized ignition. Droplet was initially at room temperature, and its diameter was 1-100 μm. When the overall equivalence ratio (φ) was fixed to be sufficiently large, there was no ignition limit in terms of initial droplet diameter (d₀), and the ignition delay took a minimum value at certain d₀. In such a case, transition from the heterogeneous ignition to the homogeneous ignition with decreasing d₀ was observed. When d₀ was fixed to be so small that the ignition would not occur in an infinite volume of air, the ignition delay took a minimum value at certain φ, which was less than unity. Two-stage ignition behavior was studied with this model. Ignition delay of a cool flame had the dependence on d₀ that was similar to that of ignition delay of a hot flame when φ is unity. When φ was almost zero, the ignition limit for cool flame in terms of d₀ was not identified unlike that for hot flame. This is an abstract of a paper presented at the 30th International Symposium on Combustion (Chicago, IL 7/25-30/2004).