Isolated n-decane droplet combustion – Dual stage and single stage transition to “Cool Flame” droplet burning

Abstract

Observations of “Cool Flame” burning for large diameter isolated droplets on board the International Space Station have stimulated interest in combustion initiation/generation of non-premixed combustion modes. For a number of n-alkane fuels at large initial droplet diameters, the initiation process was observed to first establish a hot flame condition that radiatively extinguished, followed by a quasi-steady, “Cool Flame” droplet burning mode. However, recent large diameter n-decane experiments show that depending on the ignition energy supplied, the first stage hot flame condition was absent, with an apparent, direct establishment of a “Cool Flame” burning mode that continued to diffusive extinction. Here we report these experimental observations and elucidate the underlying parameters resulting in dual and single stage “Cool Flame” burning. Detailed, transient sphero-symmetric droplet combustion modeling is applied to interpret the experiments. The simulations indicate that the balance and duration of the ignition energy applied, the energy release associated with reaction of partially premixed fuel vapor surrounding the droplet, heat flux to the drop surface, and far field diffusive heat loss all play key roles as to whether a dual stage, radiatively extinguished hot-flame-to-Cool-Flame-transition for only large droplets or direct establishment of “Cool Flame” burning for all droplet sizes occurs. The rate at which the reactive partially premixed vapor layer surrounding the droplet is formed, its volume, and its subsequent reaction significantly influence the observed transition to “Cool Flame” burning. The initial droplet temperature relative to saturation and flash point temperatures of the fuel and the liquid phase heat capacity contribute to the thermal transport requirement at the droplet surface for establishing the partially premixed reactive layer surrounding the droplet, which through its reaction history defines whether a transition to “Cool Flame” burning can be initiated without a requirement for radiative extinction of a hot flame burning mode.