Combustion of Nonane and JP8 Droplets With Additives in Low Convection

Abstract

The combustion of nonane and JP8 droplets was studied in an environment to promote spherically symmetric droplet flames. Measurements of the droplet, flame and soot ‘shell’ or ‘cloud’ diameters were made to examine how the burning process and sooting dynamics were influenced by three miscible additives: hexanol, tripropylene glycol methyl ether (TPM), and an additive that had been previously developed to improve thermal stability of JP8, called as ‘+100’. The experimental results presented for nonane were used to compare with JP8 for a comparatively simple fuel. The initial droplet diameters ranged from 0.4mm to 0.7mm to allow quantitative measurement of the influence of droplet diameter on the droplet burning process. Spherical symmetry was promoted by carrying out the experiments in microgravity. The burning conditions were room temperature air at atmospheric pressure. Soot formation was found to be massive for JP8 compared to nonane, with thick dark soot clouds that accumulated significant soot as burning progressed. With hexanol and TPM mixed with JP8, soot formation was noticeably reduced. Soot trends in the approximate order of JP8 > JP8+100 > JP8+100 / TPM (90/10, v/v) > JP8+100 / TPM (80/20) > JP8+100 / hexanol (50/50) > nonane are noted for the fixed initial droplet diameter. Significant droplet heating was found for JP8 compared to nonane, due to the higher liquid thermal diffusivity of nonane compared to JP8 and the lower product of density and specific heat of nonane compared to JP8. A distinct influence of initial droplet diameter on the subsequent evolution of droplet diameter after ignition was found for nonane in that larger droplets burned slower than smaller droplets for the range of initial droplet diameters examined. The evolution of soot shell diameter was independent of additive concentrations for JP8 and still distinct from nonane. A new scale variable is presented which collapses the various ‘standoff’ diameters (soot shell and flame) onto a single curve for a given fuel.