Numerical simulation of fuel spray ignition through the comparison to a droplet in a cell

Spontaneous ignition of mono-disperse spray with uniform droplet distribution was simulated through the numerical calculation of a fuel droplet in a closed cell. The model of the study handles a fuel droplet positioned at the center of a closed spherical cell. It is one-dimensional with the spherical symmetry. The gasified fuel vaporized from the droplet stays inside the cell. Thus the ignition of a droplet is comparable to that of a mono-disperse spray with uniform droplet distribution. Initially the droplet is cold, and it ignites spontaneously in a hot ambient gas. The model is fully transient and derives from mass-, species- and energy-conservation. Pressure is assumed to be spatially uniform and time-dependent. Mass fraction of each species in the gas phase and temperature in the gas and liquid phase are given as functions of time and radial coordinate. All the properties are temperature- and pressure-dependent. The calculation is continued even after the droplet disappears. Thus the ignition of fine droplets whose droplet lifetime is shorter than ignition delay can be also calculated. The employed fuels are n-heptane and n-decane. Detailed reaction models were employed for the both fuels. The effects of the initial conditions of the ambient (temperature and pressure), overall equivalence ratio and the initial droplet diameter are examined. There was transition from heterogeneous ignition to homogeneous ignition with the decrease of the initial droplet diameter. Two-stage ignition behavior was observed even for fine n-heptane droplets at an elevated pressure. Increase in overall equivalence ratio showed negative influence on cool flame appearance; however, after cool flame appearance it showed positive influence on hot flame appearance. Only hot flame ignition was observed for n-decane droplets at atmospheric pressure. The dependence of the ignition delay on the initial droplet diameter was qualitatively similar to that of n-heptane droplets at an elevated pressure.