Suppressing noise of turbofan engines
Conventional perforated acoustic liners are currently used in hot areas of aero engines to abate noise. However, the present liner technology for engine intake does not meet thermal and other functional requirements regarding the hot exhaust gases in turbines. A new liner enables the use of acoustic absorbers also in hot regions of the engine, thereby strongly reducing noise from the core duct (hot streams). In the project HOSTEL (Integration of a hot stream liner into the turbine exit casing (TEC)), researchers successfully integrated an acoustic absorber into the turbine exit casing structure of an aircraft engine. The HOSTEL liner concept relied on a liner top sheet assembly consisting of a thin metal foam layer in combination with a perforate sheet metal. Previous research had demonstrated that such a structure shows good acoustic performance almost independent of flows and temperature. The metallic foam layer is brazed to a classical honeycomb sandwich structure and is compressed to a predetermined degree. This helps increase the acoustic resistance and lower the liner's non-linearity with respect to both flow and temperature variations. Researchers used suitable alloys based on nickel for the liner for a typical temperature of around 700 °C. These include Inconel 625 that is used for the solid and perforated sheets as well as the honeycomb core. They also successfully applied a simulation technique – a frequency-domain, linearised formulation of the Navier-Stokes equations – to validate duct aeroacoustics of orifice plates and area expansions. The methodology proved to be extremely time efficient and to accurately simulate acoustic propagation in ducts with flows and fine geometry. Research findings should help project partners tune a hybrid acoustic liner to the blade, passing frequencies of the Sustainable and Green Engine 4 (SAGE4) Geared Turbofan demonstrator that has a low-pressure turbine. Research should also help develop acoustic liners that are satisfying thermal and other functional requirements, as they are fundamental in ensuring quiet aircraft.
Keywords
Noise, turbofan engines, acoustic liner, turbines, hot streams
