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Physical and chemical parameters effect significant control over thermal stability. Temperature is the most important of the physical factors as deposition of solids increases as the temperature rises. Other physical factors which play an important role in thermal stability are system pressure, flow regime, test duration, and characteristics of the heated surface. Chemical aspects which are important in thermal stability are oxidation, fuel composition, and metal catalysis. Free-radical autoxidation is the trigger which initiates the reactions that ultimately form insoluble material. Hydroperoxides play a crucial role in deposition. Sulfur, nitrogen, and oxygen are found in large concentrations in fuel system deposits compared to their presence in unstressed fuel. Compounds containing these elements are more readily oxidized than hydrocarbons and appear to be instrumental in increasing polarity and thus reducing solubility of oxidation products in the low-polarity jet fuel. Our understanding of the chemistry of thermal stability is complicated by the fact that a very small fraction (<0.1 ppm) of fuel is converted into deposits and filterable solids.
physical factors, temperature, chemical aspects, oxidation, fuel composition, hetero-compounds
consultant, Hughes Associates, Inc., Wheaton, MD