Acoustic liner and gas turbine engine with such acoustic liner

1. An acoustic liner which comprises:

a facing sheet that comprises holes, the facing sheet having a porosity and a facing sheet thickness,

a backing sheet,

a plurality of cells arranged between the facing sheet and the backing sheet, the cells each having a cell depth and each cell defining a cavity,

a plurality of internal necks beginning at the facing sheet and extending from an inner side of the facing sheet towards the backing sheet but ending prior to reaching the backing sheet so that lengths of the internal necks are less than the cell depth and the internal necks are spaced a distance from the backing sheet, each of the internal necks being located around a respective one of the holes of the facing sheet, thereby extending a longitudinal length of the hole,

wherein the acoustic liner is adapted to have a peak attenuation at a frequency in a range between 4,000 Hz and 8,000 Hz in a temperature condition in which a sound velocity is 600 m/s,

wherein the cell depth of the cells is in a range between 2 mm and 5 mm,

wherein each of the internal necks has a length in a range between 0.1 mm and 2 mm,

wherein the facing sheet including the internal necks, walls of the cells and the backing sheet are formed as one part, manufactured by 3D printing;

wherein each of the holes has a closed perimeter;

wherein each of the holes in the facing sheet includes a respective one of the internal necks;

wherein each cell of the acoustic liner is connected with a defined plurality of the holes in the facing sheet and the defined plurality of internal necks respectively surrounding the defined plurality of holes;

wherein the facing sheet is an outermost element of the acoustic liner facing a volume to be attenuated;

wherein the facing sheet defines an outermost boundary of the cells;

wherein the facing sheet forms an outer portion of each of the internal necks;

wherein the facing sheet is monolithic with each of the internal necks.

2. The acoustic liner of claim 1 , wherein the facing sheet has a porosity in a range between 1% and 4%.

3. The acoustic liner of claim 1 , wherein the facing sheet thickness is in a range between 0.3 and 2 mm.

4. The acoustic liner of claim 1 , wherein the holes each have a diameter in a range between 0.1 mm and 1.6 mm.

5. The acoustic liner of claim 4 , wherein the holes each have a diameter equal to or smaller than 1.0 mm.

6. The acoustic liner of claim 1 , wherein each of the internal necks has a hollow cylindrical form.

7. The acoustic liner of claim 1 , wherein a diameter of each of the cells is in a range between 5 mm and 20 mm.

8. The acoustic liner of claim 1 , wherein an overall thickness of the acoustic liner, including the facing sheet thickness and a thickness of the backing sheet, is in a range between 2 mm and 6 mm.

9. The acoustic liner of claim 1 , wherein the cells are arranged in a honeycomb structure, wherein the walls of the cells of the honeycomb structure have a wall thickness in a range between 0.05 and 0.5 mm.

10. The acoustic liner of claim 9 , wherein the backing sheet comprises holes each having a size which is adapted to remove powder from the cells produced during the 3D printing.

11. The acoustic liner of claim 1 , wherein the defined plurality of holes in the facing sheet includes three or more holes per each cell.

12. The acoustic liner of claim 1 , wherein the facing sheet is a single layer facing sheet.

13. The liner of claim 1 , wherein the range of the cell depth of the cells is between 2.5 and 3.5 mm and the length of the each of the internal necks is between 1 mm and 1.4 mm.

14. The liner of claim 1 , wherein the range of peak attenuation is between 6,001 Hz and 8,000 Hz in the temperature condition in which the sound velocity is 600 m/s.

15. A gas turbine engine for an aircraft comprising:

an engine core comprising a compressor, a combustion equipment, and a turbine, and

an acoustic liner attached to at least one surface of the turbine, the acoustic liner comprising:

a facing sheet that comprises holes, the facing sheet having a porosity and a facing sheet thickness,

a backing sheet,

a plurality of cells arranged between the facing sheet and the backing sheet, the cells each having a cell depth and each cell defining a cavity,

a plurality of internal necks beginning at the facing sheet and extending from an inner side of the facing sheet towards the backing sheet but ending prior to reaching the backing sheet so that lengths of the internal necks are less than the cell depth and the internal necks are spaced a distance from the backing sheet, each of the internal necks being located around a respective one of the holes of the facing sheet, thereby extending a longitudinal length of the hole,

wherein the cell depth of the cells is in a range between 2 mm and 5 mm,

wherein each of the internal necks has a length in a range between 0.1 mm and 2 mm,

wherein the acoustic liner is adapted to have a peak attenuation at a frequency in a range between 4,000 Hz and 8,000 Hz in a temperature condition in which a sound velocity is 600 m/s,

wherein the facing sheet including the internal necks, the walls of the cells and the backing sheet are formed as one part, manufactured by 3D printing;

wherein each of the holes has a closed perimeter;

wherein each of the holes in the facing sheet includes a respective one of the internal necks;

wherein each cell of the acoustic liner is connected with a defined plurality of the holes in the facing sheet and the defined plurality of internal necks respectively surrounding the defined plurality of holes; and

wherein the facing sheet is an outermost element of the acoustic liner facing a volume to be attenuated;

wherein the facing sheet defines an outermost boundary of the cells;

wherein the facing sheet forms an outer portion of each of the internal necks;

wherein the facing sheet is monolithic with each of the internal necks.

16. The gas turbine engine of claim 15 , wherein the acoustic liner is attached to guide vanes or struts of the turbine.

17. The gas turbine engine of claim 15 , wherein the acoustic liner is attached to an annular structure surrounding a main gas path through the turbine.