Combustion characteristics of high energy Ti–Al–B nanopowders in a decane spray flame

Boron-based fuels theoretically offer significantly higher volumetric energy densities than conventional liquid fuels. High-energy, amorphous, reactive mixed-metal (Ti–Al–B) nanopowders (RMNPs), produced by a sonochemical reaction, have shown exceptional air stability and greater energy density than nano-aluminum. In this work, we examine the combustion behavior of suspensions created using these high-energy RMNPs suspended in n-decane with a non-ionic surfactant to aid dispersion. Measurements were taken in a reacting spray plume, created using a flow-blurring atomizer and piloted with a surrounding annular propane-air flat-flame burner. Flame spectra were collected and used to identify chemiluminescence of relevant gas-phase intermediate species (BO2, AlO) and to calculate the temperature of reacting particles using multi-wavelength pyrometry. Fluctuations in the nanopowder content delivered to the spray flame were tracked by in situ sampling of the suspensions at the point of entry into the atomizer fuel tube and determining a correlation between additive concentration and flame luminance. This correlation was used to relate the condensed-phase temperature to additive concentration. The combustion characteristics of these suspensions were compared with neat n-decane fuel, as well as suspensions of commercially sourced aluminum nanopowder. Although temperature increased with concentration for both additives, the temperature measured for suspensions with nano-Al were shown to have a much stronger dependence on additive concentration than those with RMNPs. X-ray powder diffraction of the RMNP combustion products showed the fully oxidized TiO2, B(OH)3, and Al2O3 phases, along with TiB2 and TiC phases. Thermogravimetric analyses of the residual powders showed a mass gain corresponding to 2–4% residual active metal content, suggesting the combustion of the intermetallic RMNPs in the spray flame was nearly complete. These results show that these amorphous, sonochemically-generated, high-energy intermetallic RMNPs combust well in a hydrocarbon spray flame, but poor suspension stability and adverse effects on spray behavior remain technical barriers that must be improved.