Honeycomb Acoustic Panel for Fan Flutter Damping
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Solution Overview
Problem
Conventional damping technologies for flutter in turbomachines, such as Helmholtz resonators, face challenges in integrating high-volume cavities to effectively dampen pressure fluctuations within the frequency range of 50 to 300 Hz, leading to bulkiness and integration difficulties in turbomachine designs.
Innovation Solution
An acoustic treatment panel with a honeycomb structure and perforated sheets is designed to create resonant cavities within the existing fan casing volume, utilizing interconnected cells and strategically placed openings to form propagation channels, allowing for minimal mass and bulk modifications, and tuning Helmholtz resonators to lower frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Helmholtz resonators are used to dampen pressure fluctuations at 50-300 Hz, then flutter damping effectiveness is improved, but the volume of the resonator cavity becomes excessively large making integration difficult
Solution Approach 1:
The patent divides the large resonator cavity into multiple smaller cells arranged in a honeycomb structure. Each cell acts as an individual resonator, and collectively they provide the required damping effect while occupying significantly less volume than a single large cavity would require.
Solution Approach 2:
The honeycomb cells are nested within the existing fan casing structure, utilizing the available space efficiently. The cells are arranged in a compact hexagonal pattern that allows maximum packing density, effectively nesting multiple resonators within the constrained volume of the fan casing.
2Reliability
If voluminous chambers are added to form resonant cavities behind the acoustic air inlet lip or fan casing, then flutter damping is achieved, but the overall bulk of the turbomachine increases significantly
Solution Approach 1:
The resonant cavities are merged with the existing fan casing structure rather than being added as separate external components. The honeycomb cells are integrated into the casing wall, combining the structural function of the casing with the acoustic damping function of the resonators.
Solution Approach 2:
Instead of adding volume in the axial direction (lengthening the casing), the patent utilizes the radial and circumferential dimensions by implementing a honeycomb pattern on the casing surface. This transforms the problem from a one-dimensional volume addition to a two-dimensional surface utilization.
3Reliability
If the honeycomb core thickness is increased to 25-30 mm for decoupling mechanism, then fan-turbine shaft decoupling capability is improved, but the available space for acoustic treatment is reduced
Solution Approach 1:
The honeycomb structure serves dual functions: providing structural decoupling thickness and creating acoustic resonators. The local geometry of the honeycomb cells is optimized to provide both mechanical compliance for decoupling and acoustic resonance for damping, eliminating the need for separate acoustic treatment space.
Solution Approach 2:
The honeycomb core is designed to perform multiple functions simultaneously: structural decoupling, acoustic damping, and flow management. This multi-functionality eliminates the need for separate components and maximizes the utilization of the available thickness in the fan casing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables effective damping of fan flutter without major design modifications, utilizing existing volumes and minimizing mass penalties, while also providing acoustic attenuation for insufficient engine duct heights, effectively reducing vibrations and noise across targeted frequencies.
Implementation Method 1
Another way of disposing of this problem is the addition of a specific damper in the fan casing or the air inlet lip, the type of damper most currently used being the Helmholtz resonator.
Implementation Method 2
An acoustic treatment panel with a honeycomb structure and perforated sheets is designed to create resonant cavities within the existing fan casing volume
Data Source
AI summary
An acoustic treatment panel intended to be disposed on at least one wall of a turbojet in contact with a fluid flow, the panel comprising a first sheet, a second sheet parallel to the first sheet and having a first face intended to be in contact with a fluid flow and a second face facing the first sheet, and acoustic treatment cells extending between the first and second sheets and each including an enclosure and a cavity delimited by the enclosure. The enclosure of each cell comprises two openings facing one another, at least one of the openings being coincident with an opening of the enclosure of an adjacent cell.