A numerical study of the unsteady burning behaviour of n-heptane droplets

Abstract

Unsteady, sphero-symmetric n-heptane droplet burning behaviour is investigated by performing a numerical model simulation from deployment of the droplet to quasi-steady burning. The numerical model used in the present study incorporates complex chemistry consisting of 96 forward and backward reaction steps and a detailed molecular transport mechanism such that droplet burning behaviour can be simulated with almost no simplification. Unsteady burning behaviour such as droplet heating before and after the ignition, and variation of the gasification rate and the flame stand-off ratio were investigated by employing three different heat transfer modes inside the droplet, i.e. no droplet heating, conduction limit and infinite heat conductivity.The model simulation results showed that the gasification rate reaches the quasi-steady state much earlier than the flame stand-off ratio. This behaviour is consistent with previous experimental observations. The calculated fuel accumulation effect is not significant enough to account for this prolonged unsteadiness of the flame stand-off ratio. Furthermore, it was shown numerically that the observed unsteadiness does not stem either from the initial droplet heating or from the droplet surface regression.When the heat conduction limit model was employed, the droplet surface was heated rapidly and no significant ignition delay was observed. In contrast, when the infinite heat conductivity model was employed, the droplet ignition was delayed over 10 times compared to the heat-conduction-limited case.