High Pressure Turbine: Stress Sources And Cooling

The stress sources presented here will focus on two crucial components of high-pressure turbine (HPT), the blades and the disc. These stresses are responsible for the development of failure mechanisms like creep and low cycle fatigue.

Blade Stress Sources

The main stress sources in the HPT blades are the following:

• The centrifugal body force of disc, which is caused due to the rotary inertia field.
• The radial centrifugal load, which is produced by the “dead” mass of the blades, shrouds, etc and acts on the circumference of the disc like a “rim stress”.
• The temperature gradient between the bore and the rim of the disc, in combination with the coefficient of thermal expansion, produces a thermal stress.
• The torque load, which produces shear stresses in the body of the disc. This occurs either by steady-state torque transmission from the turbine to compressor or by the inertia loading creation as the machine speeds up or slows down.
• The bending loads, which act on the disc due to the pressure difference across the stage or due to the gas bending loads on the blades.

The HPT is designed to work at temperatures that are higher than the melting point of the components materials that it is composed. Consequently, apart from improvements in the materials properties that are used, an effective cooling system is required in order to reduce the temperature at acceptable levels. Turbine blades must be made from a material that meets the following requirements:

• It must have a high melting point.
• It should be resistant to oxidation.
• It must have high-temperature strength and microstructure stability.
• It should have low density and high stiffness.
• It must be easily fabricated and with low cost.
• It should have reproducible performances.

The most common materials that meet the above requirements are nickel-based alloys.

Cooling systems for gas turbine engines are constituted of a number of air flow paths, which are parallel to the main gas path. The air that flows within these paths is extracted from the compressor either by slots in the outer casing or by gaps and holes in the drum. The air then is transferred either internally through orifices and labyrinths, or externally by pipes outside of the engine casing. If the air is extracted earlier from the compressor, the performance loss will be lower, due to the less work that has been given to air by the compressor.

A blade cooling technique is the thermal barrier coating (TBC), which is used in combination with other cooling techniques. By adding a thin layer of coating (i.e. zirconia-based ceramic) in the material of the blade the temperature inside the blade falls and the component’s life increases.

The barrier coatings have low thermal conductivity, which can provide a temperature reduction of almost 150°C across a layer of 200 μm thickness. This means that the main blade material will experience a temperature of 150°C lower than before being coated. This can result in the following:

• If the coated blades continue to work at the same external temperature, either their lifetime will increase (because the temperature of the blades will be reduced), or the cooling air flow will be reduced.
• If the coated blades work at higher external temperatures the turbine efficiency will increase. 

The advantages that can be achieved with the use of thermal barrier coatings are the following:

1. Lower blade metal temperature.
2. Lower transient thermal stress.
3. Higher operating temperature and particularly, higher turbine entry temperature (TET), which leads to a higher efficiency.
4. Reduction in the air flow that is required for cooling.
5. Improved resistance to corrosion.

References

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