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

Unsteady, sphere-symmetric n-heptane droplet burning behaviour is investiga ted by performing a numerical model simulation from deployment of the dropl et to quasi-steady burning. The numerical model used in the present study i ncorporates complex chemistry consisting of 96 forward and backward reactio n steps and a detailed molecular transport mechanism such that droplet burn ing behaviour can be simulated with almost no simplification. Unsteady burn ing behaviour such as droplet heating before and after the ignition, and va riation of the gasification rate and the flame stand-off ratio were investi gated 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 dame stand-off ratio. This behavio ur is consistent with previous experimental observations. The calculated fu el accumulation effect is not significant enough to account for this prolon ged unsteadiness of the flame stand-off ratio, Furthermore, it was shown nu merically that the observed unsteadiness does not stem either from the init ial 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 igniti on was delayed over 10 times compared to the heat-conduction-limited case.