Soot volume fraction and morphology of conventional and surrogate jet fuel sprays at 1000-K and 6.7-MPa ambient conditions

Abstract

This paper presents soot processes of a blend of 23% m-xylene and 77% n-dodecane, which has been selected by several working groups as a surrogate for jet fuel. Fuel sprays were injected into high-temperature, high-pressure ambient conditions that are representative of practical engine combustion. Simultaneous laser extinction ( KL) measurement and planar laser-induced incandescence imaging were performed to derive the in situ soot volume fraction. Also, soot particles were extracted from different positions within the reacting jet by means of a thermophoretic probe, and analyzed using transmission electron microscopy (TEM) to clarify the soot structure and its correlation with the measured soot volume fraction. The same measurements were repeated for the conventional jet fuel to understand the overall performance of the selected surrogate fuel. The soot volume fraction results show that, at fixed ambient conditions, the surrogate fuel produces more soot than the conventional jet fuel. The TEM images show that the soot aggregates are more agglomerated, which may not be easily eliminated by in-cylinder oxidation. The total number of primary particles and the mean primary particle size are higher for the surrogate fuel, consistent with the soot volume fraction trend. Considering that there is similar lift-off length between fuels, the differences in soot level and morphology are caused by molecular structure effects, such as a higher aromatic content. The quantitative soot database obtained from the present study offers data for the validation of soot kinetic models, particularly at high temperature and pressure conditions where little fundamental data exist.