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The Jet Fuel Thermal Oxidation Test (JFTOT) is a widely used ASTM test procedure that evaluates the thermal stability of a fuel. It is evidenced by the fuel's resistance to undergo any chemical changes during increased thermal oxidative stress, until such time that a thermal breakpoint temperature is reached. A characteristic trace of the fuel is then deposited onto an aluminum test coupon and the fuel is rated on a pass/fail books relative to industry standards. Current rating systems approved by the industry for evaluating these tube deposit formations often fail to generate repeatable rating results. The required visual inspection of these deposit areas produces considerable uncertainty among many reviewers (due to a host of uncontrolled variables within the human observer). This paper will focus on the needs of the industry for an improved method of evaluating jet fuel thermal oxidation deposits using laser-induced spectral flourometry (SF). A novel device for reading these tube deposit areas from the cylindrical surface of an aluminum test coupon will be described as well. The overall technique yields an array of objective criteria for the evaluation of heater tube deposit areas and shows great potential for the identification of individual jet fuels on the basis of their fluorescent profiles.
Thermal oxidation products from jet fuels will be formed in the presence of fuel and oxygen at elevated temperatures. Development of fuels that will not form solid residues depends on the development of a method to analyze the rate of oxidation of fuels. Gravimetric determination of fuel residues was imprecise and time consuming. Gas Chromatography - Mass Spectrometry of oxidation products yields a great deal of fundamental information but is too specific to be used as a rapid method for determining the rate of oxidation. Fourier Transform Infrared Spectrometry is non-specific and gives integrated signals for classes of oxidation products that will give rapid analytical results for rates of oxidation as well as the ability to study the effectiveness of chemical additives.
Thermal oxidation products from jet fuels will be formed in the presence of fuel and oxygen at elevated temperatures. Development of fuels that will not form solid residues depends on the development of a method to analyze the rate of oxidation of fuels. Gravimetric determination of fuel residues was imprecise and time consuming. Gas Chromatography - Mass Spectrometry (GC-MS) of oxidation products yields a great deal of fundamental information but is too specific to be used as a rapid method for determining the rate of oxidation. Fourier Transform Infrared (FTIR) Spectrometry is non-specific and gives integrated signals for classes of oxidation products that will give rapid analytical results for rates of oxidation as well as the ability to study the effectiveness of chemical additives.
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 ...
ASTM D3241/Jet Fuel Thermal Oxidation Tester (JFTOT) procedure, the standard method for testing thermal stability of conventional aviation turbine fuels is inherently limited due to the subjectivity in the color standard for tube deposit rating. Quantitative assessment of the physical characteristics of oxidative fuel deposits provides a more powerful method for comparing the thermal oxidation stability characteristics of fuels, especially in a research setting. We propose employing a Spectroscopic Ellipsometer to determine the film thickness and profile of oxidative fuel deposits on JFTOT heater tubes. Using JP-8 aviation fuel and following a modified ASTM D3241 testing procedure, the capabilities of the Ellipsometer will be demonstrated by measuring oxidative fuel deposit profiles for a range of different deposit characteristics. The testing completed in this report was supported by the NASA Fundamental Aeronautics Subsonics Fixed Wing Project.
The Fourier Transform Infrared Spectometry (FTIR) with a horizontal attentuated total reflectance (HATR) sample accessory has been found to rapidly give good qualitative results for the extent of oxidation. The concentration of oxidation products has been determined and compared to the deposits formed for 12 thermally stressed fuels. No correlation was found between the extent of oxidation and deposition.