The effect of initial diameter in spherically symmetric droplet combustion of sooting fuels

Abstract
The effect of initial droplet diameter on the burning rate of sooting fuels - nheptane and 1-chloro-octane - was examined experimentally at low gravity. A 1.2 s drop tower provided a low gravity environment to minimize buoyancy and achieve spherically symmetric flames for stationary droplets. Free-floating and fibre-supported droplets were burned, and both techniques gave matching results for droplets of similar initial diameter. Burning rate constants for both fuels were measured for a large number of droplets ranging from 0.4 to 1.1 mm in initial diameter. Results showed that burning rate constants decreased monotonically as the initial droplet diameter was increased above 0.6 mm for both fuels. This decrease was considered to be due to the observed increase in soot formation and accumulation in a shell-like structure inside the flame of the larger droplets. The increased collection of soot inside the larger droplet flames reduced the proportional heat release from the flame and may have acted as a barrier to heat transfer from the flame to the droplet. Flame-to-droplet diameter ratio increased monotonically with time, thus suggesting that quasi-steady combustion was not achieved. The flames and soot shells for 1-chloro-octane droplets with their lower burning rates remained closer to the droplet surface than similarly sized n-heptane droplets.

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