Numerical analysis of ignition of fuel droplet array in hot stagnant air

Numerical analysis was conducted to simulate the ignition phenomena of a fuel droplet array in hot stagnant air. Previous experimental results showed that ignition times of an n-heptane droplet array quickly put into quiescent hot air were less than the ignition time of a single droplet. The objective of the present study was to clarify this interesting behavior of ignition time by numerical analysis. We assumed that a heptane droplet array with a droplet diameter of 0.75 to 1.25 mm and spacing of 4 to 20 mm was immersed in hot air with a temperature of 1123 K at time zero. The unsteady equation set for the array system was solved numerically by means of the finite-difference method. The results showed that ignition times became shorter than that of a single droplet as the droplet spacing decreased and that ignition times increased rapidly when the spacing further decreased. These ignition time behaviors were consistent with experimental results. Time-dependent temperature distributions indicated that the first ignition position(s) was located between droplets when the ignition time was less than that of a single droplet. When the spacing was smaller, an intense reaction region surrounded the array as a cylindrical tube. The basic mechanism of the shorter ignition time of a droplet array is a slight decrease of the vaporized fuel mass flux due to the suppression of the increase in droplet surface temperature in the array.