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

The spontaneous ignition of single n-heptane droplets in a closed constant volume filled with air was numerically calculated with a model that predicts pre-vaporized ignition. Initial pressure and initial air temperature were fixed at 3 MPa and 773 K, respectively. When the overall equivalence ratio (φ) was fixed to be sufficiently large, there existed 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 investigated with this model. Ignition delay of a cool flame had the dependence on d₀ that is similar to that of ignition delay of a hot flame when φ was 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).