Some New Observations on the Combustion of Hydrocarbon Droplets at Elevated Pressures

Abstract

Evaporation and combustion of cold n-pentane droplets injected into a hot oxidizing gas at moderate pressures was observed with high speed motion picture photography. High-quality photographs showed details of the flame structure. Droplets were clearly seen even above the critical pressure. Droplet wake flames at all pressures resembled a candle flame. A zone of carbon growth in the near wake was followed by a luminous zone of carbon oxidation in the far wake.

Evaporation rates of n-pentane droplets measured at moderate pressures up to 615 psia (n-penlane reduced pressure 1.26) have been correlated by the relation

for 10 < Re_m < 2 × 10⁴. Data for this correlation were obtained for pressures from 65 to 615psia, O₂ mole fractions 0.25 and 0.4 with He and N₂ diluent, droplet diameters from 200 to 2000 μ, with and without combustion. Large scatter arises mainly from assumptions about droplet temperature.

Standard drag coefficients evaluated at approach conditions fit the data well in the mean.

Transient effects predicted for high-pressure droplet combustion appear not to be significant in the range of present experiments.

The oxidation processes in the droplet wake are very sensitive to vitiation of ambient oxidizing gas. These processes might limit the combustion rate in vitiated atmospheres at moderate pressures.