Laser-Induced Fluorescence for Real-Time Monitoring of Jet-Fuel Thermal Stability, 08-R9432

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Principal Investigators
George R. Wilson III
Clifford A. Moses (SwRI retiree)
David W. Naegeli (SwRI retiree)

Inclusive Dates:  10/01/03 – 01/03/06

Background - Thermal stability is a characteristic that relates to the fuel’s ability to resist the formation of deposits when passed over a heated surface. This fuel property is important because the fuel is used to cool the engine oil as well as electronics, and any deposit would reduce heat transfer efficiency. Additionally the fuel must pass through the fuel nozzle without leaving deposits that would increase the pressure drop or partially block the exit orifice; such deposits could then lead to hot streaks, nonuniform temperature profiles, ignition difficulties or all the above. Thermal stability is becoming a greater issue with modern military and civilian aircraft engines as the cycle temperatures increase to achieve greater efficiency.

Thermal stability is not actually a measured property in jet fuel. Thermal stability quality is judged by whether the fuel can pass over a heated surface in a standardized test without leaving more than a very thin deposit as evidenced by a color change on the heated surface. All jet fuel leaves the refinery with acceptable thermal stability as defined by the fuel specification. However, thermal stability can be degraded by contamination during transport to the airport or as a result of chemical reactions in the fuel during prolonged storage. Strangely enough, only two or three airports in the United States routinely check the thermal stability of the fuel upon or after delivery. The current standardized test for thermal stability is not practical to general airport use; it is expensive, time consuming, and requires a trained operator. Airlines and engine manufacturers are very concerned over the lack of control over thermal stability at a time when the demand for thermal stability is increasing. SwRI researchers have previously determined that the gums that are the precursors to deposit formation will fluoresce when illuminated with laser light. The purpose of this research was to determine if laser-induced fluorescence (LIF) can be used to develop an in-line, real-time system that can signal when the thermal stability of a jet fuel has become degraded to the point it will not pass the specification test.

Approach - The research was conducted in two phases. The first phase used static cells, i.e., nonflowing, to demonstrate that LIF can distinguish “off-spec” fuels from good fuels and to develop design guidance for a flow system. In the second phase, a flow system with isothermal heating and an optical cell was used to demonstrate real-time measurements of thermal stability as a fuel property.

Accomplishments - Final experiments were conducted with five jet fuels of different origin and having a broad range of thermal stability. Correlations between the fluorescence intensity, a measure of gum concentration, and quantitative data from the JFTOT, the current industry standard for jet-fuel thermal stability, had r2 values between 0.98 and 0.99. These results demonstrate that LIF can be used as a real-time measure of fuel thermal stability both for quality control at the airport and as a data source to incorporate fuel thermal stability into deposition models.

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