Sooting tendencies of diesel fuels, jet fuels, and their surrogates in diffusion flames

Abstract

Surrogates of transportation fuels are useful design tools for developing engines with cleaner and more efficient combustion. It is important that information about the sooting tendency of the target fuel be incorporated in the formulation of these surrogates. Toward that end, in this work we report experimentally measured sooting tendencies of several diesel and jet fuels, surrogates of these fuels, and pure compounds which are components of these surrogates. These fuels, surrogates, and compounds were separately doped in trace quantities (0.5% by mass) into the fuel of a methane-air laminar coflow flame. The sooting tendencies, expressed as Yield Sooting Indices (YSIs), were quantified from spatially resolved two-dimensional soot volume fraction distributions of these flames as measured with color ratio pyrometry. Previous means of matching the sooting behavior of the surrogate to the fuel, such as by matching the fuel’s hydrogen content or its double bond equivalent, were found to be inadequate because these approaches do not fully account for the effects on the fuel’s sooting tendency of branching in aliphatics and alkyl substitutions in aromatics. A strong correlation was observed between the YSI and the relative abundance of different types of carbon atoms (as quantified by 1H and 13C NMR spectroscopy) in these fuels and compounds, indicating that such NMR characterization can capture essential structure-specific sooting information of relevance to surrogate formulation. Based on these YSI measurements, we demonstrate two approaches to predict the sooting tendency of a fuel surrogate. These approaches, based on (1) the choice of pure compounds that are blended to create a surrogate, or (2) the distribution of carbon atom types in a surrogate, can be used to tune the surrogate’s formulation to match the sooting tendency of its target fuel.