Improved turbofan intake liner design and optimization

Abstract

In modern turbofan engines, fan noise is one of the principle noise sources due to increased bypass ratio of the engines. Acoustic liners in the intake and bypass sections of the engine are effective in mitigating noise generated by the fan. They also play an important role in reducing fan blade instabilities by minimizing low-frequency acoustic reflections within the intake. Any damage on the lined surface has the potential to compromise the effectiveness of the liner; especially, its noise suppressing capabilities. The research presented in this thesis explores these areas on a much wider scale and aims to obtain an improved design of turbofan liners. The work performed ranges from developing an efficient strategy to optimize intake liners automatically to reduce both community noise and low-frequency acoustic reflections, to investigating the effects of liner damage and repair on the performance of zero-splice intake liners. Computational Aero-Acoustic (CAA) models have been used to predict radiated noise from a turbofan intake and the results have been validated against Rolls-Royce rig and engine test data. Adjustments have been made to the linear predictions to account for non-linear propagation effects which are significant at high fan speeds. Intake liners have been optimised to mitigate radiated noise in the far-field by using the CAA code within automated optimisation routines. The cumulative process time of these automated techniques seems to be within the acceptable limit by the industry. The acoustic effects of liner damage and repair on liner performance have been assessed by using analytical and computational prediction models. The effects of the extent and the location of the damage or the repaired surface on the overall performance of the liner is assessed. Some preliminary rules and guidelines have also been proposed in order to quantify the acoustic effects of the damage. The acoustic impact of different intake liners on low-frequency reflections have been investigated by using computational models. The results show that high resistance liners are more effective in minimizing acoustic reflections within the intake.

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