Experimental Study of Initial Diameter Effects on Convection-free Droplet Combustion in the Standard Atmosphere for n-Heptane, n-Octane, and n-Decane: International Space Station and Ground-based Experiments
Published on Jan 13, 2014
· DOI :10.2514/6.2014-1019
A comprehensive investigation is reported on varying the initial droplet diameter (Do) over a wide range on the burning characteristics of three normal alkane fuels that are representative of components found in practical fuel systems. The droplet burning characteristics of n-heptane, n-octane and n-decane, were studied experimentally in a low gravity ambience to minimize the influence of convection and promote spherical droplet flames as well as formation of a shell-like structure of soot aggregates that reside between the droplet and flame. Initial droplet diameters ranged from about 0.5 mm to 5.0 mm, and the experiments were carried out in the standard atmosphere (room temperature and normal atmospheric pressure) in a ground-based (drop tower) and a spaced-based (the International Space Station) facility. The range of Do investigated influences mechanisms related to radiative transport and sooting dynamics on the droplet burning process that determine the droplet burning rate, sooting dynamics and flame extinction mechanisms. The results show that the burning rate monotonically decreases with increasing Do. Varying Do over the range investigated promotes a transition from a soot-dominated process, with a minimal influence of luminous radiative affects, for small droplets to increased radiative losses for larger droplets that reduce heat transfer to the droplet surface. At a given time after ignition, the relative position of the flame to the droplet decreased with increasing Do. A rather abrupt increase in flame diameter was noted for Do ~ 1 mm followed by a monotonic decrease with further increases of Do for all of the fuels examined. The relative position of the soot shell to the droplet increased with time, while it also increased with Do for a given time after ignition. A three-staged burning process was found for Do > 3 mm suggesting several extinction modes. An early extinction mechanism is speculated to be the result of radiation losses from the flame rather than more diffusively controlled processes. Evaporation continues after the first extinction until reaching a second limit with a rather abrupt decrease in the droplet burning rate – which is speculated to be a “cool-flame” extinction. The morphology of the extinction process showed an oscillatory dynamic in which the flame would peel away from the droplet then re-appear before completely disappearing.