Aircraft turbine sound and vibrations signatures for a synthetic kerosene fuel

Aero gas turbine engines generate high levels of sound across a wide frequency spectrum. The total sound energy produced by the engine is composed of multiple thermodynamic sources, including the air intake, combustion, and exhaust. The focus in this research is the investigation of the combustion noise. The combustion process is complex and dependent upon the properties of the fuel being used. Different fuels have different reaction and evaporation times, indicating that the noise may increase or decrease between each type of fuel. Fuels can affect how a turbine produces a sound output that will eventually be perceived by the human ear. In this study, two fuels were used in the operation of an aircraft gas turbine in the Georgia Southern University's Aerospace Engine Laboratory. The SR-30 gas turbine is capable of operating 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 lbfuel/lbthrust per hour. The fuels used were Jet-A fuel and two synthetic kerosene 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. While synthetic fuels show multiple benefits for their use over traditional jet fuels, the sound and vibration signatures were less investigated and this brought the need of this paper. This is especially important if the sound shows a noticeable decibel difference of three decibels or more. The three decibels difference is key, since humans can perceive a difference in sound, based on the logarithmic scale for decibel, of three decibels or more. Hence, A-weighting would be used for the determination of a noticeable difference in sound. This study investigates the vibrations characteristics within the 1/3 octave band of 400 Hz. The sound and vibrations of the engine were measured with an advanced Bruel and Kjaer condenser microphone and piezoelectric tri-axial accelerometer. The sound and vibration characteristics in the mid frequency range is of particular interest regarding combustion of the gas turbine. It has been determined that a 7 dB(A) difference between the reference fuel and the synthetic fuel was achieved at 400 Hz on a 1/3 octave band analysis. Overall, sound signals coming from one of the synthetic kerosene fuels was higher than Jet-A. The highest fuel throughout the vibrations signals overall was Jet-A and at least one of the synthetic kerosene fuels. All data was processed using a Constant Percentage Bandwidth analysis. Understanding the combustion from the sound and vibrations point-of-view can help to foresee the potential danger to the components of the engine, understand the potential effects of sound on the human passenger, and work towards a design to mitigate these phenomena, if necessary.