Soot formation in non-premixed counterflow flames of conventional and alternative jet fuels

With increasing concerns over fuel cost, environmental pollutions, and energy security, the development of alternative fuels from renewable sources has gained considerable attention in the past decade. To evaluate the sooting propensities of representative conventional and alternative jet fuels, the present study has investigated soot formation in non-premixed combustion for three conventional reference jet fuels, i.e., Jet-A, JP-8, and JP-5, and three selected alternative jet fuels, i.e., Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), Hydroprocessed Esters and Fatty Acids from camelina (HEFA-Camelina), and Alcohol-to-Jet (ATJ). The soot volume fraction profiles of these jet fuels in the atmospheric-pressure counterflow non-premixed flame configuration were measured using Laser-Induced Incandescence technique. In addition, the effects of global strain rate and reactant concentration on soot formation were investigated. At the same stoichiometric mixture fraction, the experimental results show that the sooting level increases with increasing fuel and O-2 concentrations, while it decreases with increasing global strain rate, as expected. It is also worth noting that the three alternative jet fuels tested demonstrated greater sensitivity to variations in global strain rate and reactant concentration, as compared to the three conventional jet fuels. Based on the measured soot volume fractions, the sooting propensity of the six jet fuels are ranked as follows: JP-5 > Jet-A > JP-8 > ATJ > FT-SPK > HEFA-Camelina. The observed ranking is consistent with the aromatics content in each jet fuel except when comparing FT-SPK and ATJ. While ATJ has a lower aromatics content than FT-SPK, it contains much heavier hydrocarbon components, thereby leading to higher sooting propensity. Furthermore, the correlations of soot volume fraction with aromatics content and hydrogen content are examined for all six jet fuels. Although such correlations work well for the three conventional jet fuels, the present results suggest that they cannot be directly transferable to the alternative jet fuels, as these alternative fuels are composed of negligible amounts of aromatics, as well as composed of distinct chemical compositions.