Characterization of Particulate Resulting from Oil Contamination of Aircraft Bleed Air

Possible oil contamination of aircraft bleed air is an ongoing operational issue for commercial aircraft. A sensitive and reliable method to detect contamination, especially at very low levels, has been elusive due, in part, to the lack of information about the physical nature of oil that results when entrained in the bleed air by an engine compressor. While it was expected that high shear rates in the compressors would result in very finely dispersed particles, detailed data on the size characteristics of these droplets were not available, making it difficult to develop reliable detection techniques. The goal of the reported research was to collect experimental data to provide this information. The concentration and size distribution of particles were measured for bleed air with different rates of controlled oil contamination under various engine operating conditions. Multiple contamination levels were generated by injecting jet engine lubricating oil into the air intake of two different turbine engines (Allison 250 C18 and Allison 250 C28B). The resulting contaminated bleed air was cooled and sampled. In addition, another series of tests was conducted on a Pratt and Whitney F117-PW-100 engine on board an Air Force C17 aircraft with oil injected into the first stage of the compressor. Test results show that oil contamination in the compressor will result in a fog of very fine droplets in the bleed air under most operating conditions. With moderately high contamination rates at elevated power levels, the concentration distribution and particle size does not vary much with power and generally depends on the rate of contamination. Moreover, at elevated power levels, the peak particle concentration takes place in the range of 50 nanometers (nm) to 70 nm and the bulk of the particles form at less than 150 nm. At very low contamination rates, the ultrafine particles generated can be 10 nm or less. This research shows that detection should focus on the particles less than 100 nm and detection of particles as small as 10 nm may be required for detection of very low contamination levels.