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

Abstract

Spontaneous ignition of single n-heptane droplets in a constant volume filled with air is numerically simulated with the spherical symmetry. The volume is closed against mass, species, and energy transfer. The numerical model is fully transient. It continues calculation even after the droplet has completely vaporized, and therefore can predict pre-vaporized ignition. Initial pressure and initial air temperature are fixed at 3 MPa and 773 K, respectively. The droplet is initially at room temperature, and its diameter is between 1 and 100 μm. When the overall equivalence ratio ϕ is fixed to be sufficiently large, there exists no ignition limit in terms of initial droplet diameter d 0, and the ignition delay takes a minimum value at certain d 0. In such a case, transition from the heterogeneous ignition to the homogeneous ignition with decreasing d 0 is observed. When d 0 is fixed to be so small that the ignition would not occur in an infinite volume of air, the ignition delay takes a minimum value at certain ϕ, which is less than unity. Two-stage ignition behavior is investigated with this model. Ignition delay of a cool flame has the dependence on d 0 that is similar to that of ignition delay of a hot flame when ϕ is unity. When ϕ is almost zero, the ignition limit for cool flame in terms of d 0 is not identified unlike that for hot flame.