Acoustically damped gas turbine engine
Disclosed is a gas turbine engine including a fan, a nacelle including a flutter damper forward of the fan, the flutter damper including an acoustic liner having a perforated radial inner face sheet and a radial outer back sheet, the acoustic liner configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter, a chamber secured to the radial outer back sheet, the chamber in fluid communication with the acoustic liner, and the chamber configured for peak acoustical energy absorption at a frequency range associated with fan flutter modes, and the engine includes (i) the nacelle and a core cowl forming a convergent-divergent fan exit nozzle; (ii) a variable area fan nozzle capable of being in an opened and closed, the opened position having a larger fan exit area than the closed position; and/or (iii) the fan being shrouded.
1. A gas turbine engine comprising:
a fan;
a nacelle including a flutter damper disposed forward of the fan, the flutter damper comprising:
an acoustic liner having a perforated radial inner face sheet and a radial outer back sheet, wherein a perforated section is defined by perforations in the radial outer back sheet,
the acoustic liner being configured for peak acoustical energy absorption at a frequency range that is greater than a frequency range associated with fan flutter;
a circumferential array of chambers secured around a circumference of a nacelle inlet, the chambers being circumferentially spaced from one another and cumulatively achieving a desired dampening volume,
each of the chambers being secured to the radial outer back sheet, and being in fluid communication with the acoustic liner, each of the chambers configured so that the perforated section is disposed thereunder and has a width W p , a length L p and a height corresponding to a height of the acoustic liner H Li , wherein L p is defined along an engine axial direction, and
each of the chambers defining a rectangular box with a front surface, a back surface, opposing side surfaces, and a bottom edge extending axially and circumferentially outwardly from each of the surfaces to define a mounting flange that geometrically conforms to an annular and axial profile of the nacelle inlet and that mounts each of the chambers to the nacelle inlet,
whereby a bottom surface of each of the chambers is formed by the radial outer back sheet, and wherein each of each of the chambers has a width W, a length L, and a height H, wherein L is defined along the engine axial direction,
wherein an acoustic volume V of the flutter damper is a sum of W p ×H Li ×L p and W×H×L, and wherein the perforated section and each of the chambers is configured so that L>L p within each of the chambers, and
each of the chambers is configured for peak acoustical energy absorption at a frequency range that is associated with one or more fan flutter modes; and
wherein:
(i) the nacelle and a core cowl form a bypass duct, the bypass duct forming a convergent-divergent fan exit nozzle; and/or
(ii) the gas turbine engine includes a variable area fan nozzle, the variable area fan nozzle is capable of being in an opened position and a closed position, wherein the opened position has a larger fan exit area than the closed position; and/or
(iii) the fan is a shrouded fan.
2. The gas turbine engine of claim 1 , wherein the fan operates within a flutter margin of between 2% and 10%.
3. The gas turbine engine of claim 2 , wherein blades of the fan have a mean roughness of less than about 28 Ra.
4. The gas turbine engine of claim 1 , including a variable area fan nozzle, and wherein the variable area fan nozzle is fixed in a semi-opened state.
5. The gas turbine engine of claim 4 , wherein the fan operates within a flutter margin of between 2% and 10%.
6. The gas turbine engine of claim 5 , wherein blades of the fan have a mean roughness of less than about 28 Ra.
7. The gas turbine engine of claim 4 , wherein:
the flutter damper has an impedance characteristic at one or more target frequencies defined as:
f target =f S,ND +Ω·ND
wherein
f S,ND is a resonance frequency corresponding to a structural mode of a rotating component;
ND is a nodal diameter count of the structural mode; and
Ω is a rotational speed of the rotating component; and
wherein the flutter damper has the following impedance characteristic at the one or more targeted frequencies:
R≥ 2ρ c
−3 ρc≤X≤− 0.6 ρc
wherein R is the real part of the impedance characteristic, X is the imaginary part of the impedance characteristic, ρ is air density, and c is speed of sound.
8. The gas turbine engine of claim 1 , wherein:
the flutter damper has an impedance characteristic at one or more target frequencies defined as:
f target =f S,ND +Ω·ND
wherein
f S,ND is a resonance frequency corresponding to a structural mode of a rotating component;
ND is a nodal diameter count of the structural mode; and
Ω is a rotational speed of the rotating component; and
wherein the flutter damper has the following impedance characteristic at the one or more targeted frequencies:
R≥ 2ρ c
−3 ρc≤X≤− 0.6 ρc
wherein R is the real part of the impedance characteristic, X is the imaginary part of the impedance characteristic, ρ is air density, and c is speed of sound.