COMBUSTION RESEARCH: Laser fluorescence probes fuel-deposit formation

Investigators from the Southwest Research Institute (San Antonio, TX) have used laser fluorescence to determine how polymer deposits form in fuels. The effort was geared to formulating a means of preventing formation of deposits that would foul fuel systems in future high-speed jet aircraft in which fuel will be used not only for propulsion but also for cooling airframe and engine components.

Such heating increases deposit formation, clogging narrow fuel-nozzle passages and reducing the heat transfer through heat ex changer walls. In future the work may also be applied to automobile engine fuel-injection nozzles in order to maintain combustion efficiency (see Laser Focus World, March 1997, p. 135).

Researcher David Naegeli heated Jet A fuel contained within oxygen-rich ampoules to "thermally stress" it at temperatures from 120°C to 180°C. Other fuels were also investigated to gain insight into deposit precursor formation. Concentrations of precursor compounds and oxygen consumption were among the measurements made. Laser-induced fluorescence (LIF) provided deposit precursor concentrations in the fuel at various temperatures. Oxygen consumption, present in hydrocarbon fuels exposed to oxygen, was determined by the pressure drop in the ampoule.

Naegeli noticed that the fluorescence spectrum of precursors in the heated fuel had a shape and peak fluorescence similar to that of phenolic resin, a polymer similar to Bakelite plastic, dissolved in a mixture of equal volumes of toluene, acetone, and methanol (TAM), a powerful solvent. Phenols and aldehydes from the decomposition of hydroperoxides in the fuel reacted together to form the phenolic resin within the oxidizing fuel.

The deposit formation is a complex process involving several chemical reactions, mass transport effects, and surface interactions. Tests on the different fuels showed that deposit precursors form in fuels that contain aromatic compounds, as evidenced by the presence of precursor LIF spectra. Because sulfur compounds are known to increase deposition, Naegeli concluded that they form strong acids that catalyze benzylic hydroperoxide decomposition to phenols, thus accelerating precursor formation. He suggests that an acid-buffer type of fuel additive, such as an amine carbonate, could arrest jet fuel deposit formation. A buffer would not make the fuel basic, causing potential base reactions, but only neutralize any acid present as needed. This investigation was funded by NASA Lewis Research Center.