Numerical simulations of spontaneous ignition of mono-disperse fuel spray in lean premixed gas

Spontaneous ignition of mono-disperse fuel spray with uniform droplet distribution in hot fuel/air premixed gas was simulated through one-dimensional numerical model of an isolated fuel droplet in a closed constant-volume cell. Namely, the inter-droplet interaction in a spray was expressed through the interference of the outer boundary of the cell. Equivalence ratio calculated from initial amounts of fuel in the liquid phase and oxygen in the gas phase was defined as liquid-phase equivalence ratio φ_l. Since the premixed gas was fuel-lean, the existence of the droplet might either promote the ignition through a role as fuel source or hinder the ignition through a role as heat sink, which could depend on initial droplet diameter d₀, φ_l, initial gas-phase temperature T_g0, initial liquid-phase temperature T_l0, gas-phase equivalence ratio φ_g and initial pressure P₀. The effects of the first three conditions were examined. d₀ was less than 200 μm and typically around 30 μm. T_l0, φ_g and P₀ were 300 K, 0.4 and 3 MPa, respectively. Fuel was nheptane. Detailed chemical kinetics including the low-temperature oxidation reactions were employed, and therefore cool-flame ignition delay τ_cf and hot-flame ignition delay τ_ig were evaluated. First, only d₀ was varied. When d₀ was relatively large, both τ_cf and τ_ig were not different from those of only premixed gas with the same φ_g. It means that such large droplet required relatively long time for vaporization compared with chemical characteristic time, and the ignition occurred at the outer boundary of the cell that the fuel vapor from the droplet did not reach. However, as d₀ decreased, τ_cf and τ_ig increased and decreased, respectively, first. Thus interaction between spray and premixed gas was recognized. Next, only φ_l was varied. With increasing φ_l, τ_ig either increased monotonically or had minimal value depending on T_g0