Numerical simulations of two-stage ignition of fine single fuel droplets in a closed constant-volume cell

Spontaneous ignition of fine single fuel droplets whose diameters were less than 100 μm was numerically simulated. Each droplet was placed in a closed cell so that single droplets could be compared to sprays. The numerical model is fully transient, and it can continue calculation even after the droplet has completely vaporized. Fuel was n-heptane, and ambient gas was air. Initial pressure and initial air temperature were fixed at 3 MPa and 773 K, respectively. The Droplet was initially at room temperature. First, a relatively simple reaction model was employed, and the followings were shown. When overall equivalence ratio φ_overall was fixed to be sufficiently large, ignition limit was not observed in terms of initial droplet diameter d₀, and ignition delay took a minimum value at certain d₀. There was transition from heterogeneous ignition to homogeneous ignition with decreasing d₀. Next, a qualitatively and quantitatively reliable reaction model was employed, and two-stage ignition behavior was observed even for fine droplets. Increase in φ _overall showed negative influence on cool flame appearance; however, after cool flame appearance it showed positive influence on hot flame appearance.