Hybrid Dissipative/Reactive Silencer Predictions with Comparison to Measurement

Regular paper

Paul Williams

AAF Ltd.

Wednesday 3 june, 2015, 11:00 - 11:20

0.3 Copenhagen (49)

Gas Turbines predominantly generate a broadband noise spectrum, although strong tones are also present, often at relatively low frequencies. Dissipative silencers are commonly used to reduce noise within gas turbine systems, however they are inefficient at removing low-frequency noise and this leads to silencers which are overdesigned at higher frequencies. Therefore, by introducing new techniques designed specifically to target low-frequencies it is proposed that the overall cost of the silencer may be decreased. Reactive elements are known to be successful at targeting low frequencies but their non-acoustic limitations, including problems with minimising pressure drop, have traditionally prevented their use in gas turbine systems. This paper presents a new silencer design that combines dissipative and reactive silencer elements in order to target low frequency tonal noise, whilst at the same time minimising static pressure drop. This so-called hybrid silencer is designed using advanced finite element modelling techniques and new prototype designs are then tested in the laboratory. Comparison between prediction and experiment shows good agreement over a wide frequency range, which demonstrates the validity of the modelling approach. Results demonstrate that attaching a carefully designed reactive element to a dissipative element can deliver a large increase in performance at low frequencies. This improvement in performance is characterised by a resonance “peak”, which is designed to cover an octave band surrounding the target frequency in order to accommodate uncertainties in the noise output of a gas turbine. Furthermore, it is found that additional peaks occur at higher harmonics, which further increases the performance of the hybrid silencer beyond the plane wave region of the inlet duct, delivering additional and significant improvements in performance for the new hybrid silencer when compared to traditional dissipative designs and previous hybrid silencer concepts.

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