The Dangers of Thermal Fatigue in Gas Turbine Engines
Thermal fatigue is a failure risk that builds up over time due to the high temperature differentials during start up and shut down in gas turbine engines. Too much thermal fatigue will reduce the lifetime of turbine components, and especially rotating components like gas turbine blades, which are under additional stresses already.
What Creates Thermal Fatigue?
The alternating tensile and compressive stresses (the hysteresis loop) caused by hot and cold thermal cycling will stress the surface of turbine components and gradually worsen the root areas of small defects or cracks in the material structure. In this sense, it’s a crack growth mechanism with consequences not unlike another limiting factor: creep (deformation of the component under load at high temperatures).
What Components Are at Greatest Risk?
Thermal fatigue affects all components of gas turbine engines that reside in the hot gas path. Some of these would include:
- The combustor
- The transition piece
- Turbine nozzles
- Turbine blades
- Turbine discs
Sharp radius intersections on rotating components are typically the areas that experience the most localised stress. Each start/stop cycle creates a risk of thermal fatigue damage in those areas.
Dangers of Thermal Fatigue:
Life-Limiting For the Component
It’s common for thermal fatigue to be a major determining mechanism in the blade’s useful life. However, it is one of many such secondary failure mechanisms. Among other stresses, the gas turbine engine must cope with:
- Thermal fatigue from extreme thermal gradients differentials during start up and shut down
- Creep from load stresses at high temperatures
- Low-cycle fatigue from centrifugal forces during start-up
- High-cycle fatigue from vibration of the component
- Corrosion and oxidation from combustion gases
- Erosion and fouling due to contact with dust (and other particles) passing through the engine
These stresses each act as significant life-limiting mechanisms and must be addressed individually in the engine’s design, testing, monitoring, and maintenance. Thermal fatigue is mainly related to heat transfer. This will require attention to material properties such as thermal conductivity, modulus of elasticity, and coefficient of thermal expansion in the turbine blades and other components.
Temperature-Limiting For the Gas Turbine Engine
The highest temperatures in a gas turbine’s hot cycle occur at the end of the combustion chamber. The maximum temperature for the engine is therefore limited by the highest temperature that the turbine blades can withstand.
However, since this is one end of the extreme temperature differential in the engine, it’s also necessary to consider the secondary effects of thermal fatigue when cycling back and forth between the maximum and start-up temperatures. Too much potential for thermal fatigue with this temperature spread may further limit the engine’s ideal temperature capacity. Factor in the expected or intended number of starts per hour of operating time to determine the overall effect of thermal fatigue on blade life.
Dangerous Failures
Each gas turbine blade or other component can only experience a finite number of hot cycles at a particular level of thermal strain before it will begin to crack. Once the cracking starts, eventual failure and release of the blade (or a portion of the blade) due to continued thermal fatigue or creep ruptures becomes inevitable. This would send hot metal hurtling through the chamber and cause potentially catastrophic engine damage. It’s essential to track blade life, continuous temperature performance, and identify hot spots in gas turbine engines to prevent these failures from occurring.
Costly Maintenance & Downtime
Even if failure is avoided, thermal fatigue reduces blade life and requires more frequent downtime and component replacements for the engine in the field. Costly and time-consuming maintenance can be minimized with sufficient data on temperature performance and thermal fatigue. An optimized engine design and cooling mechanism will extend blade life.
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