Thermal gradient is a physical quantity that displays which direction and at what rate temperature changes most rapidly surrounding a specific location. In rotating components, such as disks for turbine engines, manufacturers measure thermal gradient to obtain data about how much thermal stress a component is placed under during operating conditions.
About Thermal Gradient
Gas turbines and engines continue to improve efficiency, which leads to the creation of hotter operating conditions. These extreme conditions put machinery components under intense stress. Temperature changes from flowing air and gas continuously expose the equipment as it performs work during compression; this is due to energy being added during the combustion and expansion of gases throughout the stages of a turbine.
Air and gas temperatures influence the metal temperatures of each disk rim, causing the center (or bore) of the disk’s temperature to stay lower. This is from natural or active cooling, producing a radial temperature gradient spread across the disk’s radius.
During operation, thermal gradients display rim to bore on a disk. Temperature differences between a disk’s bore and rim can differ by hundreds of degrees, despite only a few inches of radial space.
According to Fourier’s Law of Heat Conduction, heat flows from the hotter parts to the cooler parts of a solid—in this case, heat flows from the rim to the bore, causing expansion in the rim and compressive thermal stresses in the bore.
The degree of thermal gradient represents stress to the disk that is in addition to the centrifugal stress resulting from operation. When temperature gradients are severe, the disk is placed under significant thermal stress, compromising its operating safety.
How Thermal Gradient Works in Spin Testing Services
Spin testing services are used to observe thermal gradients in manufactured parts. This includes turbomachinery and other rotating machinery, such as turbine engines, high-speed electric motors, and systems for energy storage.
Spin testing measures thermal gradient under conditions that closely represent the operating environments of engines and turbines. It applies a thermal gradient to the disk that is tested at uniform temperatures to best replicate the operating conditions of the engine.
Using multi-zoned ovens with zoned heating and cooling capabilities, engineers apply radial and axial temperature gradients to the component to observe thermal gradient conditions as if under operation.
Spin testing for thermal mechanical failures synchronizes the thermal gradient with the engine’s speed, observing the significant radial thermal gradients between a disk’s rim and bore, causing disk stress. Thermal gradients cause thermal hoop stress to the disk, greatly contributing to its overall stress.
Experts measure local differences in thermal expansion amounts, as gradients can vastly alter a disk’s three-dimensional stress state. This is especially important since most disk designs feature complex cross-sectional geometries.
An Essential Tool in Spin Testing
Thermal gradient in spin testing services helps manufacturers validate component designs, selecting the right materials and coatings for best performance in high-temperature environments under complex stress conditions. The results provide valuable data that is used to improve disk performance and service life.
Resources:
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Effects of temperature distribution and elastic properties of materials on gas-turbine-disk stresses
- Thermal Gradient
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