Aviation fuel is used as a coolant to remove waste heat loads from an aircraft in addition to its conventional role as an energy source through combustion. As the fuel temperature increases, several reactions involving hydrocarbon molecules, dissolved oxygen, and impurities take place, which ultimately lead to the formation of gums and solid deposits. Ultraviolet-visible light absorption and fluorescence spectroscopy were used to study the degradation of commercial Jet A-1 as it was thermally stressed. Both techniques were shown to be highly sensitive to the chemical reactions occurring in the thermally stressed fuel that led to the formation of oxygenate products.

In this study, the thermal oxidative stability of a kerosene-type Jet A-1 commercial aviation fuel has been investigated using a three-dimensional (3D) excitation/emission matrix fluorescence (EEMF) method. The fuel was thermally stressed in flow test conditions over a range of temperatures in the autoxidative regime. To determine the effect of dissolved oxygen on aviation fuel thermal stability, the measurements were conducted for both air-saturated and fully deoxygenated fuel samples. The increase in the fuel temperature results in a large red shift of the fluorescence signals for the air-saturated fuel; however, fully deoxygenated fuel showed no difference in the fluorescence spectra with respect to neat fuel. The observed increase in the emission wavelengths of the collected spectra may be attributed to the formation of high-molecular-mass compounds within the liquid fuel. These species are formed as a consequence of the chemical reactions activated during the thermal stressing process. The use of 3D fluorescence spectra for aviation fuel analysis is shown to be a fast, suitable, and easily implementable tool for establishing and verifying fuel quality. Also, the level of fuel thermal degradation may be ascertained using this method, which should be of great interest for a better thermal management control of turbine engines. ■ INTRODUCTIONThe use of onboard aviation fuel as a coolant to remove waste heat loads produced by turbine engines and avionic components is of great interest for the aircraft industry. However, kerosene-type aviation fuels, as well as other middle distillates, have the tendency to degrade at high temperatures and form solid deposits. Such phenomena are generally referred to as the thermal stability of the fuel, which is one of the specifications required for commercial aviation fuel and one of the most critical fuel properties.Currently, aviation fuel thermal stability, for fuel certification, is determined using the Jet Fuel Thermal Oxidation Tester (JFTOT), American Society for Testing and Materials (ASTM) D3241 Standard. 1 It is worth noting, that the JFTOT, being a simple pass/fail verification procedure, does not offer a quantitative assessment of the amount of gums and solid deposits formed within the fuel lines.The main problem concerning the fuel instability at elevated temperatures is the formation of solid deposits and gums within the fuel lines, thus constricting the fuel flow and increasing pressure drops. Moreover, if the fuel restriction is left unchecked, it decreases the effectiveness of the surfaces to act as heat exchangers and ultimately causes engine shutdown. 2−4 Dependent upon the temperature experienced by the liquid fuel during the heat transfer, two different mechanisms were shown to be responsible for the fuel thermal degradation process and the formation of deposits. At high fuel temperatures, generally above 400°C, the fuel instability mechanism is characterized by the breakdown of the hydrocarbon chemical bonds by pyrolytic thermal cracking reactions. These ...