No abstract

In the formative years of gas turbine engine development, it was thought that the engine could run on most liquid fuels. It was soon realized, however, that much better performance could be obtained if fuel properties were restricted. The number of properties specified increased over the years, and today's aviation gas turbine fuels must meet a long list of requirements. In fact, our current jet engine fuels have the most extensive specifications of all hydrocarbon-based fuels.

The effects of trace organic components on the thermal stability of fuels have been demonstrated in previous chapters. This chapter will deal with materials which exert effects at much lower concentrations, parts per billion (ppb) rather than parts per million (ppm). Metals dissolved in jet fuel are effective at these low concentrations because they are acting as catalysts for one or more of the chemical reactions involved in the sequence of insolubles formation. Metals are a fact of life for jet fuels, however, since refinery equipment, transportation equipment, storage tanks, and aircraft fuel systems are constructed from metals. In many cases the metals are chosen for these uses on the basis of material properties and economy rather than possible degradation of thermal oxidation stability or other important fuel properties.

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 play a major role in the phenomena involved in the formation and deposition of fuel-insoluble material. Temperature is the most important of these factors and will be dealt with in some depth. Other physical parameters to be discussed include system pressure, flow regime, heat transfer, deposit morphology and physical characteristics, and surface roughness of the test section. In addition, some recent efforts in modeling the overall process of thermal oxidation stability will be addressed in this chapter although the models include chemical as well as physical effects. The variety of equipment used in thermal stability work and the multiplicity of goals behind this type of research may explain the different results and conclusions reached by different investigators. If a reasonable explanation can resolve a difference in results, this will be presented.