Development of processing methods for measuring combustion noise in turbofan jet engines

Reducing the noise produced by civil aircraft is a key issue in the aviation industry. Due to successful reductions of other turbofan jet engine noise sources, combustion noise is becoming a more prominent noise source. The measurement of combustion noise in the far field of an engine is complicated by the presence of other noise sources. An established method called 3S Array was developed to measure combustion noise but its use is limited by the assumptions required to implement this method. Conditions that the measured data needs to satisfy in order for the estimate to be valid have been identified in this thesis. An empirical method for calculating confidence intervals of this method has been developed and applied to measurements of a short cowl turbofan jet engine. Three new methods have been developed during this project to extend and enhance the 3S Array method for a wider range of scenarios. The first method combines a modal isolation method with the 3S Array method to extend its upper frequency limit in the engine duct. This method was tested in no-flow laboratory experiments and was found to be effective. The second uses an inverse method to separate the two components of combustion noise (the direct noise and the indirect noise) in the combustor combined with the 3S Array method to identify the contributions of the two components to the far field noise. This method was also tested in no-flow laboratory experiments which were partially successful due to errors in the estimated transfer functions. The third method is a new coherence-based measurement technique called 3S PCCSM which is a variation of the 3S Array method. 3S PCCSM makes it possible to use advanced source location/beamforming methods to isolate the combution noise from the other noise sources. The 3S PCCSM method was applied to acoustic measurements of a short cowl turbofan jet engine using delay-and-sum beamforming and the inverse beamforming methods. Each of the methods developed in this thesis are modular and so can be combined as needed for different scenarios. The methods developed in this thesis have applications outside the measurement of combustion noise.