Small aircraft turbine noise from combustion of synthetic kerosene fuels

In this paper the use of alternative fuels - synthetic kerosene based - were investigated in a small aircraft gas turbine engine to determine their sound and vibrations signature at Georgia Southern University's Aerospace Engine Laboratory. Three types of fuels were used: a natural gas, a coal derived synthetic kerosene and Jet-A as a reference fuel. The alternative fuels are Fischer-Tropsch process fuels. Synthetic fuels are attractive in the aviation industry because of their potential for reducing energy dependence and the growing need for higher efficiencies, while reducing emissions. The research SR-30 gas turbine was used to evaluate combustion noise from the selected fuels. The turbine can operate at a maximum speed of 80,000 rpm, produce a maximum thrust of 40 lbf, has a pressure ratio of 3.4 to 1, and a specific fuel consumption of 1.22 lb_fuel/th_rust per hour. The measurement transducers were state of the art Brüel & Kjaer 1/4 inch microphone and a triaxial accelerometer which were interfaced with the analysis software, Brüel & Kjaer Pulse 21. Data was taken at 70,000 rpm for a time span of five minutes at stabilized conditions. One-third octave analysis indicated highest sound difference between the fuels at 400 Hz, and a more pure sound level was obtained. The difference for Jet A was as much as 6 dBA higher for exhaust measurement at 400 Hz, and for FT-NG the exhaust had a higher decibel reading from the exhaust side by as much as 4.4 dBA at the 400 Hz frequency, and FT-CG had as much as 7.3 dBA difference at 315 Hz. The sound from the combustion of these fuels produced the differences of sound when the positions of the microphones changed. However, with this new introduction of sound measurement locations and angles, the combustion differences are still distinct and show a more in depth view of the sound quality that is being received by the microphone.