Effect of Varying the Initial Diameter of n-Octane and n-Decane Droplets over a Wide Range on the Spherically Symmetric Combustion Process: International Space Station and Ground-based Experiments
Yu Cheng Liu;Koffi N. Trenou;Jeff Rah;Michael C. Hicks; C. Thomas Avedisian (2013). "Effect of Varying the Initial Diameter of n-Octane and n-Decane Droplets over a Wide Range on the Spherically Symmetric Combustion Process: International Space Station and Ground-based Experiments." 8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah
This study reports on an investigation of varying the initial droplet diameter (Do) over a very wide range (from 0.5 mm to 5 mm) on droplet combustion. The droplet burning history is examined in an environment of reduced convection as promoted by low gravity to achieve spherical droplet flames. The fuels examined are n-octane and n-decane. The long burning times for Do > 1.2 mm were accommodated in the Multi-user Droplet Combustion Apparatus (MDCA) onboard the orbiting International Space Station (ISS), while experiments for Do < 1 mm were carried out in a ground-based drop tower. The results reported encompass the widest range of Do examined in the history of droplet combustion experimentation for a given fuel. Both free floating (unsupported) and fiber-supported droplets are deployed and ignited. Quantitative data are obtained from digital analysis of the individual video images of the burning process for the droplet, flame and soot shell diameters. Results show that the droplet burning rate decreases with increasing Do throughout the Do range investigated. The mechanisms responsible include a combination of fuel molecule residence time effects and radiative losses from the flame, both of which influence soot formation to varying degrees. Assuming that increasing soot formation (e.g., from increasing residence times) would lower heat transfer to the droplet, "small" droplets (Do < 1 mm, with negligible radiation losses) will burn slower as Do increases in this initial droplet diameter range, which is consistent with the experimental results. For Do > 1.5 mm the droplet flames appeared less luminous and therefore less sooty, yet the droplets continued to burn progressively slower as Do increased. This effect is conjectured to be the result of increased radiative losses that would tend to reduce sooting that outweigh the longer residence times of the larger droplets that would tend to increase sooting. The experimental results reported also include the evolution of relative distances between the flame, soot shell, and droplet diameters, all of which are influenced by Do.