Experimental study of the combustion dynamics of jet fuel droplets with additives in the absence of convection

Abstract

Combustion of stationary JP8 fuel droplets mixed with additives is examined in a low gravity environment to reduce the influence of convection and promote spherical droplet flames. Both qualitative information on flame structure and sooting tendencies and quantitative information on evaporation rates and flame and soot shell diameters are reported. The additives were hexanol (C 6H 14O) and a commercial blend of a detergent/dispersant termed “+100” that is widely used as an additive in military transport systems. Experiments for pure nonane (C 9H 20) and hexanol droplets are also reported for comparison with single component fuels that have boiling points close to JP8. Nonane was also used for calibrating the experimental design. +100 is a commercial additive developed for JP8 to improve its thermal stability in trace quantities but its influence on combustion is essentially unknown. The hexanol concentration in JP8 was fixed to correspond to about 0.078 mass fraction of oxygenate in the JP8+hexanol mixture and the +100 concentration in JP8 was set at the commercial value (256 ppm). The initial droplet diameters for quantitative analyses were kept within a narrow range (0.40 to 0.52 mm) to eliminate consideration of the effect of initial droplet diameter in the interpretation of the results. Experiments were carried out in an oxidizing ambience containing either 79% nitrogen inert or 70% helium inert, the latter to compare results for burning under conditions where soot formation was suppressed. The lower concentration of helium inert was necessitated to allow ignition by the multiple spark arrangement used in the present investigation. Extensive droplet heating was observed for JP8 and JP8+100 in air. JP8+hexanol, along with pure nonane and hexanol, showed no evidence of droplet heating due to the lower thermal diffusion time associated with the higher thermal diffusivity of hexanol and nonane compared to JP8. The burning rate was dependent on time for burning in both air and the 70% He/30% O 2 mixture in which soot formation was virtually eliminated. Some evidence for sooting propensities in the approximate order JP8>JP8+100>JP8+hexanol was found by comparing backlighted and flame-illuminated photographs. A scaling relationship is presented that consolidates the droplet, flame, and soot shell diameters into a single curve with a power law relationship of the nondimensional burning time.