Acoustic liners have desirable acoustic attenuation properties and thus are commonly used to reduce noise in jet engines. The liners typically are placed upstream and downstream of the rotors (fans) to absorb sound before it propagates out of the inlet and exhaust ducts. Noise attenuation could be dramatically improved by increasing the area over which a noise-reducing material is applied and by placing the material closer to the noise source.

Researchers at NASA Glenn and NASA Langley have discovered that the Haynes® 25 metallic foam is compatible with the environment in the region close to the rotor blades. The Haynes 25 metallic foam achieves the twin goals of increasing the area covered by the noise-attenuating material and bringing it closer to the noise source. NASA also is working on alternative foam materials that are lighter and provide acceptable acoustic performance. Although the current work is applied to a fan, the high temperature capability of the metal foams enables applications to engine core components such as the turbine.

For many years, engineers have known that bulk acoustic liners provide better noise reduction over a range of frequencies. However, materials currently available are not well-suited for use in the harsh environment within a jet engine. Previous attempts at providing acoustic liners that could withstand these harsh conditions resulted in materials that reduced aerodynamic performance and could not hold up against rotor-tip rub, making them unsuitable for use in the region over the fan rotor.

Researchers at NASA Glenn and NASA Langley have acoustically tested the Haynes 25 metallic foam, which significantly reduces noice over a range of frequencies and fan speeds. Besides having these favorable bulk acoustic liner properties, the material can withstand the harsh engine environment and can be designed to minimize aero dynamic losses. The new metallic foam acoustic liner material has been shown to have a long life in an oxidizing environment, withstanding a temperature of 1000°C in a burner rig for 30 minutes. The foam does not readily absorb fluids, such as hydraulic fluid, that reduce the material’s efficacy. Nor have the freezethaw cycles, inherent in moving between high altitudes and ground level, resulted in the structural concerns associated with bulk acoustic materials currently on the market. Stress tests revealed that the metallic foam can withstand expected mechanical loads, having held up well under compression, bending, and tensile stress.

The Haynes 25 metalic foam can double as a rotor-tip rub strip. Replacing the rub-strip layer in the containment system surrounding the fan rotors with this acoustic metallic foam brings a noise-reduction material into this region. The material properties of the foam, such as temperature, density, porosity, and weight can also be tailored to suit a specific application. While the current work is intended to reduce turbofan noise, the method is applicable to other applications such as ground power systems and ventilating fans.
 

Haynes is a registered trademark of Haynes International, Inc..