Technology for quieter aircraft engines

Conventional perforated acoustic liners are currently used in hot areas of aeroengines 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.

In the EU-funded project 'Integration of a hot stream liner into the turbine exit casing (TEC)' (HOSTEL), the objective was to develop an acoustic liner concept that is suitable for the turbine exit casing.

The HOSTEL liner concept relied on a liner top sheet assembly consisting of a thin metal foam layer in combination with a perforated sheet metal. Previous research had demonstrated that such a structure shows good acoustic performance almost independent of flows and temperature. The metal foam layer is brazed to a classical honeycomb sandwich structure and is compressed to a pre-determined degree. This helps increase the acoustic resistance and lower liner non-linearity with respect to both flow and temperature variations.

Scientists performed a number of simulations on one cell of the reference liner with a single hole. Several flow cases with different flow mean speeds, temperatures and foam material parameters were calculated to study the liner cell performance variations. Focus was placed on investigating the acoustic power balance — the relation of the acoustic power scattered by the cell compared to the input power.

Project partners also performed vacuum brazing of the test samples. Intensive efforts were devoted to solving issues regarding the interaction of the brazing material with the nickel foam.

HOSTEL findings should help project partners tune a hybrid acoustic liner to the blade passing frequencies of the Sustainable and Green Engine (SAGE) 4 Geared Turbofan demonstrator that has a low-pressure turbine.

HOSTEL research should help the development of acoustic liners that are satisfying thermal and other functional requirements, as they are fundamental in ensuring quiet aircraft.