Effects of Nitrogen Dioxide on the Oxidation of Levitated exo-Tetrahydrodicyclopentadiene (JP-10) Droplets Doped with Aluminum Nanoparticles

Nitrogen dioxide (NO2) can significantly improve the combustion of hydrocarbon fuels, but the effect of NO2 on the ignition of fuels with energy densities enhanced by aluminum (Al) nanoparticles has not been studied. We therefore investigated the effects of NO2 on the ignition of JP-10 droplets containing Al nanoparticles initially acoustically levitated in an oxygen–argon mixture. A carbon dioxide laser ignited the droplet and the resulting combustion processes were traced in real time using Raman, ultraviolet–visible (UV–vis), and Fourier-transform infrared (FTIR) spectroscopies simultaneously with a high-speed optical or thermal imaging camera. Temperature temporal profiles of the ignition processes revealed that a 5% concentration of NO2 did not cause measurable differences in the ignition delay time or the initial rate of temperature rise, but the maximum flame temperature was reduced from 2930 ± 120 K to 2520 ± 160 K. The relative amplitudes of the UV–vis emission bands were used to deduce how NO2 affected the composition of the radical pool during the oxidation process; for example, the radicals NO, NH, and CN were detected and the OH (A 2Σ+–X 2Π) band at 310 nm was less prominent with NO2. Localized heating from a tightly focused infrared laser beam provided sufficient energy to activate chemical reactions between the JP-10 and NO2 without igniting the droplet. Raman spectra of the residue produced give information about the initial oxidation mechanisms and suggest that organic nitro compounds formed. Thus, in contrast to previous studies of hydrocarbon combustion without Al nanoparticles, NO2 was found not to enhance the ignition of an Al-doped JP-10 droplet ignited by a CO2 laser.