Low frequency liners for turbofan engines

Sugimoto, R., Astley, R. Jeremy and Murray, Paul B. (2010) Low frequency liners for turbofan engines. In Proceedings of 20th International Congress on Acoustics, ICA 2010: Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society. International Commission for Acoustics. 4 pp .

Abstract

Reducing aircraft noise is critical to the growth of air transport and for quality of people’s life. The aircraft noise is composed of contributions from various source mechanisms and fan noise is one of the dominant components at take-off and landing for aircraft with modern high bypass ratio turbofan engines. Fan noise is generated at the fan, propagates through engine intake and bypass duct and is radiated to the outside. Acoustic liners are applied on the internal walls to attenuate the fan noise while it propagates through the engine ducts. Typical engine duct liners are either so-called single degree of freedom (SDOF) or double degree of freedom (DDOF) liners. SDOF liners consists of a porous facing sheet backed by a single layer of cellular separator such as honeycomb cells with solid backing plate, and in case of DDOF liners two cellular layers are separated by porous septum sheet. The acoustic performance of such liners is strongly dependent on the depth of the cell(s). Generally these liners are selected to be most effective to reduce community noise measured with EPNL(dB) and typical liner cell depth is around 1 to 2 inches. In order to increase the attenuation by the liners at lower frequencies the cell depth must be made larger, which is often prohibited by the mechanical design constraints. One remedy can be the acoustic liners having L-shaped geometry so that it can fit in a shallower space.
In this study a potential of folded cavity liners is investigated. Such liners have the potential to behave like a mixture of deep and shallow liners. They have more complex frequency characteristics due to the fold compared to conventional liners and can be used to reduce noise over wider frequency range. Finite element models are used to assess the acoustic
performance of liners.

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Contributors

Author: R. Sugimoto

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