抄録
Combustion behaviors of isolated fuel droplets for ethanol, n-decane and 1-butanol were investigated in high concentrations of carbon dioxide under microgravity. Fuel droplets were anchored at the tip of a quartz fiber with the diameter of 50μm. The ambience consisted of oxygen, nitrogen and carbon dioxide. Oxygen concentration was fixed at 21 % in volume, and the concentration of carbon dioxide was varied from 0 % to 60 % in volume. Detailed measurements of the droplet surface area were conducted using a high speed video camera, and instantaneous burning rates were calculated from histories of the surface area. Droplet flames were also observed using a video camera. An estimation of the droplet diameter using an ellipsoidal approximation had large error especially at the initial stage. The behavior of the instantaneous burning rate, which was measured from the change in surface area, showed droplet combustion events as with the thermal expansion, ignition and following combustions. Instantaneous burning rates of n-decane and 1-butanol showed an increasing trend, while that for ethanol was almost constant during the droplet life time. The burning rates for n-decane and 1-butanol were influenced by the initial droplet diameter. A stepwise increase in the instantaneous burning rate was observed for large n-decane droplet in air, which occured around the soot shell collation. However, this behavior was not observed in high concentration of carbon dioxide even for large droplets. In high concentration of carbon dioxide, soot production was suppressed, and this suppression was enhanced for smaller droplets.
