Gas Turbine Exhaust Casing Heat Shield Gap Design
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Solution Overview
Problem
Exhaust ducts of gas turbine engines face high thermal stresses due to thermal gradients between the exhaust struts and the outer shroud, which reduces low-cycle fatigue life and necessitates increased metal thickness, thereby increasing the overall weight of the exhaust system.
Innovation Solution
A heat shield is radially positioned outwardly from the outer shroud with a gap between the shroud and the heat shield, forming a thermal air barrier to insulate the interface between the strut and the shroud, reducing thermal stresses and allowing for a thinner shroud without compromising durability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal thickness of the exhaust duct is increased to reduce thermal stresses and improve durability, then the durability and strength are improved, but the overall weight of the exhaust system increases
Solution Approach 1:
A heat shield is introduced as an intermediary component between the hot exhaust strut and the outer shroud. The heat shield creates a thermal air barrier that mediates heat transfer, protecting the shroud from direct thermal exposure and reducing thermal stresses without requiring increased metal thickness of the exhaust duct itself.
Solution Approach 2:
The thermal protection function is extracted from the structural shroud component and separated into a distinct heat shield component. This allows the shroud to be optimized for structural purposes with reduced thickness while the heat shield handles the thermal protection, resolving the contradiction between durability and weight.
2Strength
If metal thickness of the exhaust duct is increased to provide acceptable durability under thermal stresses, then the strength is improved, but the complexity of the exhaust system increases
Solution Approach 1:
The heat shield acts as a mediator that protects the exhaust duct from thermal stresses, allowing the duct to maintain adequate strength with reduced thickness. This modular approach actually reduces overall system complexity compared to a single thick-walled duct design.
Solution Approach 2:
The exhaust duct system is segmented into distinct functional components: the structural shroud, the heat shield, and the strut. This segmentation allows each component to be optimized independently for its specific function, simplifying the overall design and manufacturing process while maintaining required strength.
3Reliability
If a heat shield is added to create a thermal air barrier, then thermal stresses are reduced and durability is improved, but the device complexity increases
Solution Approach 1:
The heat shield serves as a relatively simple intermediary component that creates an effective thermal air barrier. Despite adding a component, the overall complexity increase is minimal because the heat shield is a straightforward structural element that integrates with existing exhaust duct architecture.
Solution Approach 2:
The heat shield utilizes the concept of thin protective barriers to create effective thermal insulation. This approach adds minimal structural complexity while providing significant thermal protection, as the heat shield can be designed as a thin-walled component that relies on trapped air for insulation.
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 effectively reduces thermal stresses and improves durability while minimizing weight by creating a thermal air barrier that insulates the shroud from bypass flows, allowing for a thinner shroud design.
Implementation Method 1
a gap defined radially between the shroud and the heat shield relative to a longitudinal axis and axially between a first end and a second end of the heat shield along the longitudinal axis, the gap configured to enclose at least a portion of an interface defined between the at least one strut and the shroud
Data Source
AI summary
An exhaust casing for a gas turbine engine comprises a shroud configured to surround an exhaust cone, and a heat shield attached to the shroud. The heat shield has a first end and a second end axially spaced apart from each other. A gap is defined radially between the shroud and the heat shield. The gap is configured to enclose at least a portion of an interface defined between at least one strut and the shroud. The exhaust casing is configured to surround the exhaust cone and configured to be connected to the exhaust cone via the at least one strut.