CMC Seal Segment Pin Bore Design for Gas Turbine Shroud
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Solution Overview
Problem
Ceramic matrix composite (CMC) materials used in gas turbine engines face limitations due to low allowable stress, high stiffness, and lower thermal expansion rates compared to metal alloys, leading to poor load distribution and limited use in high-pressure applications, especially in turbine shroud segments where machining is undesirable and results in short lifespan.
Innovation Solution
A segmented turbine shroud design featuring a CMC seal segment with an arcuate flange and elongated pins, where the pin-receiving bore is at least 70% of the pin's length, and radial members with pin-receiving bores spaced from the arcuate flange, allowing for improved load distribution and reduced stress on the CMC material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If CMC materials are used in turbine shroud segments, then resistance to high temperature degradation is improved, but allowable stress is reduced to an order of magnitude lower than metal alloys
Solution Approach 1:
The shroud is divided into multiple segments with seal elements positioned between them. This segmentation allows the CMC material to be used in high-temperature zones while the seal elements handle the sealing function, accommodating the material's lower allowable stress through distributed load management across multiple components rather than requiring the entire shroud to withstand high stresses.
Solution Approach 2:
Seal elements are introduced as intermediary components between the CMC shroud segments and the turbine blades. These seal elements serve as mediators that handle the sealing function and accommodate thermal expansion differences, allowing the CMC material to operate in its optimal high-temperature range without being constrained by its lower stress tolerance.
2Stability of the object's composition
If CMC material components are used, then thermal expansion rate is reduced, but load distribution at transfer points deteriorates
Solution Approach 1:
Seal elements with compliant materials are positioned at the interfaces between CMC shroud segments and metal support structures. These intermediary seal elements accommodate the thermal expansion mismatch between CMC and metal materials, distributing loads more evenly across the interface rather than concentrating stresses at discrete transfer points.
Solution Approach 2:
The design incorporates seal elements that can change their physical parameters (such as compression and deformation) to accommodate thermal expansion differences. By allowing the seal material to compress and deform, the system adapts to thermal cycling and expansion, distributing loads more uniformly across the shroud-segment interfaces.
3Ease of manufacture
If u-shaped CMC sealing segments are machined with holes and slots, then mounting pin attachment is enabled, but lifespan is reduced due to recession in hot, humid environment
Solution Approach 1:
The mounting holes and slots are extracted from the CMC seal segment itself and relocated to the metal support structures. This allows the CMC material to remain solid and intact, avoiding the recession and degradation that occurs when CMC is machined. The metal support structures, which are more tolerant of machining operations, now contain the mounting features.
Solution Approach 2:
Instead of machining the CMC segment to create mounting features, the design inverts the approach by having the metal support structures provide the mounting holes and slots. The CMC seal segment is then attached to these pre-formed metal mounting features, reversing which material undergoes machining operations.
4Weight of moving object
If thin walls are used in sealing segment, then contact area with mounting pin is reduced, but edge loading stresses increase severely
Solution Approach 1:
Metal support structures with thicker walls are introduced as intermediary components between the mounting pins and the CMC seal segment. These metal intermediaries bear the edge loading stresses from the mounting pins, allowing the CMC seal segment to maintain thin walls for weight reduction without directly承受 the high contact stresses.
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 design enhances the use of CMC materials in high-pressure, high-temperature applications by reducing stress and extending the lifespan of components, enabling efficient gas turbine operation with improved sealing and reduced machining requirements.
Implementation Method 1
the CMC seal segment portion defining a pin-receiving bore is radially spaced from the arcuate flange by a spacing flange... the CMC seal segment portion defines a bore having a length that is at least 70% of the length of the elongated pin received therein
Data Source
AI summary
A ceramic matrix composite (CMC) seal segment for use in a segmented turbine shroud for radially encasing a turbine in a gas turbine engine. The CMC seal segment comprises an arcuate flange having a surface facing the turbine and a portion defining a bore for receiving an elongated pin, with the bore having a length that is at least 70% of the length of the elongated pin received therein. The CMC seal segment is carried by the carrier by at least one of the elongated pins being received within the bore. The CMC seal segment portion defining a pin-receiving bore is radially spaced from the arcuate flange by a spacing flange extending radially outward from the arcuate flange.