* Corresponding author: Sandra.Richter@dlr.de Proceedings of the European Combustion Meeting 2019 Measurement of the sooting propensity of aviation fuel mixtures S. Richter *,1 , T. Kathrotia 1 , C. Naumann 1 , S. Scheuermann 2 , U. Riedel 1 1 German Aerospace Center (DLR), Institute of Combustion Technology, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany 2 Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB), Institutsweg 1, 85435 Erding, Germany Abstract The influence of molecular structure and concentration of aromatic compounds on soot formation was investigated experimentally using a premixed laminar planar flame. For the experiments, a synthetic paraffinic kerosene (SPK) surrogate was prepared as a base for the addition of the aromatics to be investigated. The aromatics that were either added individually or in mixtures were toluene, n-propylbenzene, indane, 1-methylnaphthalene and biphenyl. Fur- thermore, four synthetic fuels, a fossil Jet A-1 and four neat fuel components – n-dodecane, n-octane, iso-octane and cyclohexane – were studied. In addition to the experiments, a numerical study was performed where besides the dependency on the aromatic’s molecular structure and content also the influence of temperature, pressure, fuel stoi- chiometry, and residence time on soot volume fraction was investigated. Both the experimental as well as the nu- merical investigations point out that the individual aromatic compound’s molecular structure exerts a larger influ- ence on soot formation than the content of the aromatic compound. Introduction The combustion of a fuel in an aircraft in cruising al- titude leads to the direct emission of different pollutants in the upper troposphere and lower stratosphere. Here, nearly all exhaust gas components, especially carbon dioxide (CO 2 ), nitrogen oxides (NO x ) and soot particles but also sulfur oxides (SO x ), unburned hydrocarbons and water vapor, contribute to climate change by inter- acting with atmospheric processes affecting the ele- ments of radiative forcing [1, 2]. The percentage of anthropogenic radiative forcing caused by aviation is between 3.5% and 5.0% with contrail cirrus as major factor. The formation of cirrus clouds due to contrails is catalyzed by soot emission since soot particles act as nuclei for the condensation of water vapor [1, 3]. It is well-known that aromatics, representing an im- portant constituent of crude-oil based fuel, promote the formation of soot particles [2-4]. For this reason the percentage of aromatics is limited to max. 25 vol-% in conventional crude-oil based jet fuels [5] as well as in synthetic (alternative) jet fuels [6]. Whereas the exist- ence of aromatics in conventional fuels results naturally from their occurrence in crude oil, the majority of syn- thetic fuels lack aromatics. Due to safety reasons – it is known that aromatics contribute to the swelling of cer- tain elastomers (seals) [4, 7] – a minimum aromatic content of 8 vol-% is required for each jet fuel contain- ing synthetic components [6]. Since alternative aviation fuels provide potential to reduce emissions a more pro- found knowledge of the influence of aromatics on soot formation is necessary. For this reason the project Syn- TreAmR (Synthetische Treibstoffe – Einfluss des Aro- matengehaltes auf die Rußbildung) was initiated to study the correlation between the aromatic’s molecular structure and soot formation in synthetic fuels. In detail the sooting propensity of jet fuel surrogates with differ- ent aromatic compounds characteristic for those occur- ring in conventional and synthetic fuels as well as of different alternative jet fuels and a conventional jet fuel was determined experimentally. In contrast to the commonly used threshold sooting index (TSI) [8-10] the sooting propensity was estimated by the definition of a soot threshold as a fuel-air- equivalence ratio (). Here the particle concentrations were measured continuously in the exhaust gas of a premixed flame during varying the -value. With the significant increase of the particle concentration the sooting threshold was determined. In addition to the experimental work, a numerical study was carried out where the soot volume fractions were calculated to investigate not only the dependency of soot formation on the aromatic’s molecular structure and concentration but also on temperature, pressure, fuel stoichiometry, and residence time. Experimental approach The experimental set-up consists of four parts: (I) the fuel-air-mixture preparation unit, (II) the burner, (III) the sampling probe, and (IV) the particle detection unit. A scheme of the set-up is presented in Fig. 1. For the preparation of the fuel-air-mixture the vaporized fuel is first mixed with preheated nitrogen (N 2 ), second- ly conditioned to the set temperature of 473 K and final- ly oxygen (O 2 ) is added according to the natural N 2 /O 2 - ratio in air. The liquid fuel is carried using a HPLC- pump (LC-20AD, Shimadzu); the gas flows are con- trolled with mass flow controllers (mini Cori-Flow, Bronkhorst). During the whole measurement the gas velocity of the unburned fuel-air-mixture is kept con- stant at 35 cm/s.