Gas Turbine Combustion Cap Stiffness Design
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Solution Overview
Problem
The existing combustion chamber designs in gas turbine engines face issues with vibratory stresses, leading to potential breakage or cracks in the cantilevered caps due to resonance modes aligning with dynamic excitation frequencies, and previous solutions either compromise mechanical strength or increase production complexity and cost.
Innovation Solution
An annular wall forming a cap with an additional downstream portion for attachment to the chamber wall, which increases the stiffness and shifts resonance frequencies away from vibrational frequencies, while maintaining an optimal combustion flow passage section and aerodynamic separation, and is fixed using bolts for enhanced mechanical strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the cap is mounted cantilevered on the combustion chamber, then the aerodynamic separation function is achieved, but the cap is subjected to strong mechanical dynamic stresses and risk of breakage or cracks due to resonance modes
Solution Approach 1:
The cap is divided into two separate caps (inner cap and outer cap) that are mounted independently on the combustion chamber. This segmentation allows each cap to be optimized for its specific function while reducing the overall mechanical stress on the structure, as the load is distributed across multiple independent mounting points rather than a single cantilevered structure.
Solution Approach 2:
The mounting structure is extended in the axial dimension by adding downstream mounting portions to the caps. Instead of mounting only at the upstream end (cantilevered configuration), the caps are now mounted at both upstream and downstream locations, transforming the structural support from a single-point cantilever to a multi-point supported structure, thereby reducing bending moments and mechanical stresses.
2Reliability
If an annular damping ring is provided in the flap of the caps, then the resonance modes are shifted away from vibrational frequencies, but the mechanical strength is reduced and the ring may loosen or break
Solution Approach 1:
The damping function is extracted from a separate annular damping ring component and integrated directly into the cap structure itself. The damping material is applied as a coating or embedded layer on the inner surface of the cap, eliminating the need for a separate mechanical ring structure that would require additional fastening elements and reduce overall mechanical strength.
Solution Approach 2:
The cap structure combines the structural material (metal) with damping material in a composite configuration. The damping material is applied as a layer on the inner surface of the metal cap, creating a composite structure that provides both structural integrity and vibration damping properties without requiring separate mechanical components.
3Strength
If a one-piece fairing with connecting lugs is provided, then the mechanical strength is increased, but the passage section of combustion flow is reduced and production complexity increases
Solution Approach 1:
The fairing is segmented into separate inner and outer caps that are mounted independently on the combustion chamber. This segmentation simplifies manufacturing compared to a complex one-piece structure with laser-cut recesses, while the multiple mounting points provide sufficient mechanical strength. Each cap can be manufactured separately using simpler processes.
Solution Approach 2:
Instead of providing continuous connecting lugs throughout the fairing structure, local reinforcement is provided only at specific mounting locations where structural strength is needed. The caps are mounted at discrete upstream and downstream positions, providing local structural support without the complexity of continuous connecting elements.
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 provides a mechanically solid cap that reduces vibratory forces and overhang effects, maintaining aerodynamic function with increased resistance and simpler, cost-effective production, ensuring the cap's structural integrity and efficient combustion flow passage.
Implementation Method 1
The caps 10, 11 are therefore subject to these vibrations, in particular the outer cap 10. The caps 10, 11 are also subject to other dynamic excitation frequencies, in particular certain harmonic frequencies of the rotation speed of the rotating members of the turbojet engine. The caps 10, 11, mounted cantilevered, can have resonance modes close to the frequencies aroused and are therefore subjected to strong mechanical dynamic stresses.
Implementation Method 2
A first solution consists in providing an annular damping ring, housed in the flap 12, 13 of the caps 10, 11 (or only of the outer cap which is the most subject to the vibratory stresses); the flap 12, 13 is for this purpose wrapped around the rod to hold it. The friction generated by the presence of the rod provides a damping effect and therefore a displacement of the frequencies of the resonance modes of the caps 10, 11, which allows them to be removed from the vibrational frequencies to which the caps 10, 11 are subjected.
Data Source
Figure 1~2
Figure 3~5
Figure 6~7
AI summary
The combustion chamber, comprising outer (31) and inner (32) annular walls and a gas flow separating fairing with an annular wall (21) having rear (22) and forward (24) sections, has the forward section extended rearwards to provide an additional fixing to the combustion chamber's outer wall. The rear section (22) of the fairing wall is fixed to the combustion chamber wall outer surface and the rearward extension on the forward section is fixed to an inner surface flange (34).