Spacelab and Drop-Tower Experiments on Combustion of Methanol/Dodecanol and Ethanol/Dodecanol Mixture Droplets in Reduced Gravity

Abstract

This paper describes reduced-gravity experiments on the combustion of droplets composed of metha-nol/dodecanol and ethanol/dodecanol mixtures. The experiments used the NASA Glenn Research Center 2·2 s drop tower as well as facilities in Spacelab during the USML-2 flight. In the drop-tower experiments, initial droplet diameters ranged from 0·8 mm to 1.2 mm, initial dodecanol mass fractions, Y, (in both mixtures) were 0·1, 0·25 and 0·5, and the ambient gas mixture was either an 02/He mixture (with a molar O2 concentration of 0·5) or atmospheric air. Most drop-tower experiments were at 0·1 MPa ambient pressure, although two tests were at 0·04 MPa. In the Spacelab experiments, the methanol/dodecanol droplets burned in ambient shuttle air (0·21 oxygen mole fraction) at 0·1 MPa, the initial droplet sizes were in the range 4·2 mm to 5·4 mm, and the initial values of Y were 0·2 and 0·4. Burning rates varied with experimental conditions, and disruptive burning was very frequent. One of the largest methanol/dodecanol droplets extinguished in an oscillatory fashion. Data on flame contractions, in combination with existing theoretical analyses, were used to infer effective liquid species diffusivities, D, that apply to these experiments. The D values varied slowly with composition for 0·25 SYS 0·5. In this range, the theoretical values for liquid species diffusivities compare well with experimental values. The D values for Y < 0·25 varied rapidly with composition, and D values for Y = 0·1 were substantially smaller than for Y > 0·25. These variations in D are attributed to changes in liquid-liquid phase-equilibrium behaviors as liquid compositions change during combustion. Sooting was visually observed above threshold values of the initial dodecanol mass fraction (this threshold depended on the ambient conditions, and was different for methanol and ethanol), but only after flame contractions.