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For technical readers in the aviation and fuel industries, and in testing laboratories, explores the history and philosophy of the thermal stability of aviation fuel, and considerations during the fuel's manufacture, storage and transport, use, and assessment. The 13 papers, representing a number of
Current and planned gas turbine engines use fuel as their primary heat sink. When jet fuel is thermally stressed it will form gums and deposits. These deposits can block engine fuel nozzles, causing damage to the engine hot sections, especially the combustor region. The fuel.s thermal stability is a critical fuel property with respect to optimum performance of modern military gas turbine engines. The current standard method to rate fuel thermal stability, the Jet Fuel Thermal Oxidation Tester (JFTOT), is a subjective, pass-fail type test and is not adequate as a tool to quantitatively investigate fuel thermal stability. This report describes a program to design, construct and commission a rig capable of quantifying fuel.s thermal stability based on carbon and sulphur deposit formation in a heated metal test tube. A fuel known to be unstable both chemically and thermally, sourced from RAAF Townsville, was used as a test fuel for commissioning the rig. The rig was found capable of discriminating between differing test conditions and was successful in rating fuels. thermal stabilities based on quantification of the fuels. deposit-forming capacities. A significant finding of commissioning procedures was the high levels of sulphur deposit formed in the test fuel.
The effectiveness of various refining processes in upgrading the thermal stability of aircraft turbine engine fuels has been examined. A Jet A-1 fuel was subjected to clay-treatment, desulfurization, and hydrogenation. The thermal stability of the treated and untreated fuels was determined using the Jet Fuel Thermal Oxydation Tester (JFTOT) thermal stability method. Desulfurization increased the JFTOT breakpoint by 120 to 140F, and desulfurization followed by hydrogenation increased the JFTOT breakpoint of the fuel by more than 150F. A low-aromatic JP-4 type of fuel, blended from a hydrogenated stock and a solvent-treated stock to remove aromatics, was also tested and compared to a conventional JP-4 fuel. Desulfurization, hydrogenation, clay treatment, and aromatic solvent extraction have been shown to be effective methods for upgrading the thermal stability of jet fuels.
The focus of this study was on the autoxidation kinetics of deposit precursor formation in jet fuels. The objectives were: (1) to demonstrate that laser-induced fluorescence is a viable kinetic tool for measuring rates of deposit precursor formation in jet fuels; (2) to determine global rate expressions for the formation of thermal deposit precursors in jet fuels; and (3) to better understand the chemical mechanism of thermal stability. The fuels were isothermally stressed in small glass ampules in the 120 to 180 C range. Concentrations of deposit precursor, hydroperoxide and oxygen consumption were measured over time in the thermally stressed fuels. Deposit precursors were measured using laser-induced fluorescence (LIF), hydroperoxides using a spectrophotometric technique, and oxygen consumption by the pressure loss in the ampule. The expressions, I.P. = 1.278 x 10(exp -11)exp(28,517.9/RT) and R(sub dp) = 2.382 x 10(exp 17)exp(-34,369.2/RT) for the induction period, I.P. and rate of deposit precursor formation R(sub dp), were determined for Jet A fuel. The results of the study support a new theory of deposit formation in jet fuels, which suggest that acid catalyzed ionic reactions compete with free radical reactions to form deposit precursors. The results indicate that deposit precursors form only when aromatics are present in the fuel. Traces of sulfur reduce the rate of autoxidation but increase the yield of deposit precursor. Free radical chemistry is responsible for hydroperoxide formation and the oxidation of sulfur compounds to sulfonic acids. Phenols are then formed by the acid catalyzed decomposition of benzylic hydroperoxides, and deposit precursors are produced by the reaction of phenols with aldehydes, which forms a polymer similar to Bakelite. Deposit precursors appear to have a phenolic resin-like structure because the LIF spectra of the deposit precursors were similar to that of phenolic resin dissolved in TAM. Naegeli, David W. Glenn Research Center ...