On the suppression of negative temperature coefficient (NTC) in autoignition of n-heptane droplets

Autoignition of n-heptane droplets under microgravity is investigated numerically. The comprehensive model, considering the transience in both the gas and liquid phases and non-ideal thermophysical properties, includes the 116-step reaction mechanism of Griffiths. Two-stage ignition manifests for ambient temperature less than 900 K at elevated pressures of 0.5 and 1.0 MPa. The predicted first delays and total delays agree well with the experimental data in the literature. The second delay decreases greatly with increasing pressure because a stronger Stefan flow supplies more fuel vapor for reaction as the cool flame shifts closer to the droplet to enhance evaporation. The Stefan flow effect, in combination with the inhomogeneous temperature and fuel vapor distributions, explains why the NTC (negative temperature coefficient) present in homogeneous mixtures is not observed in droplet ignition experiments. Near the minimum ignition diameter, the ignition delay increases for smaller droplets at T_∞ = 700 K, P_∞ = 1.0 MPa. For a droplet smaller than the minimum ignition diameter, only first ignition with cool flame is reached. The absence of ZTC (zero temperature coefficient) in our simulations may be attributed to the weaker inverse temperature dependence of the reaction mechanism adopted.