Table of Contents
11.2.3 The Turbine
Engine types can be categorized according to their number of shafts, i.e. the number of turbines. Compressors of single-shaft engines (typical for older models) are usually powered by one multi-stage turbine. Dual-shaft engines have an HPT - high pressure turbine and a LPT - low pressure turbine, while triple-shaft engines also have an IPT -intermediate pressure turbine (see Fig. "Triple shaft engine configuration").
The high-pressure turbine is usually subject to the greatest thermal (highest gas temperature) and mechanical (highest RPM) loads. Its blades have the shortest lives. The first turbine stage after the combustion chamber not only has the highest temperature levels, but it is also exposed to an especially uneven temperature distribution in the gas flow (Fig. "Temperature variation at the combustion chamber outlet"). Temporal and localized temperature peaks stress the first turbine stator vanes (turbine nozzle). For this reason, these are cooled especially intensively (Fig. "Cooling systems of turbine blades and vanes").
The following is a description of typical specific problems of the various turbines.
High pressure turbine: In accordance with the high RPM, the high pressure rotor is subject to centrifugal forces. The blades are directly exposed to the hot gas flow as it leaves the combustion chamber. The gas temperatures are usually above the softening/melting temperature of the blade materials. In this area, the temperature distribution is not yet even, nor has the infeed of cooling air effectively caused the temperatures to decrease considerably. This is especially true for the first turbine guide vanes. Traditionally, the newest and most elaborate technologies are first used in the turbine in order to ensure an acceptable life span. Single crystal materials, extremely complex cooling air configurations, and thermal barrier coatings are all used. This technological pioneering also increases risks. In addition, damaging effects such as increased temperatures during compressor surges, erosion and OOD due to coking, or vibrations caused by unstable combustion can be expected (Chapter 11.2.2). The clearance gap at the tips of the rotor blades is especially important, and has an especially great effect in connection with these relatively short blades (Volume 2, Chapter 7.1.3). Rotor blades of older engine types were outfitted with shrouds to improve the seal effect. However, the blades of modern engines are usually shroudless. The clearance gap is created with seal segments that are affixed to the housing. Minimizing this tip clearance gap is one of the greatest technological challenges, and therefore also a problem zone (Volume 2, Chapter 7.1.3). Wear due to rubbing, erosion, and oxidation all increase the tip clearance during the operating life. The high pressure turbine receives its cooling air from the area of the compressor exit, so that the pressure level is sufficient to form a cooling air film around the blades. In modern engines, the pressure of the cooling air for the rotor blades (of the first stage) is increased with the aid of a small cover plate or a similarly functioning device (e.g. conical hollow shaft). The temperature of the cooling air can only be lowered to a certain degree, since there is a danger of unallowably high thermal strain occurring in the blades, which could unallowably reduce the life span of the latter due to the high thermal fatigue stress. The cyclical and static loads on the HPT are responsible for the limited life of this family of parts, especially the rotor blades. They must usually be replaced first. Acquiring new parts and the expense of maintenance for these high-technology components are responsible for a significant portion of operating costs.
Intermediate pressure turbine: The IPT is an additional component in triple-shaft engines. The loads on IPTs are often underestimated. Their RPM rate is lower than that of the high pressure turbine, but it is high nonetheless. The cooling air film in the high pressure turbine has lowered the gas temperature in addition to the expansion, and the temperature peaks have evened out. Therefore, the cooling air is reduced in comparison to the high pressure turbine and/or it has already been used for cooling. This means that the blade temperatures are also near the limit values of their materials. Experience shows that the relatively long and slender blades have a tendency to vibrate. For this reason, they are outfitted with stiffening shrouds that may be braced against one another.
Low pressure turbine: The LPT usually has several stages and powers the low pressure rotor, which is the low pressure compressor (the booster in fan engines) and/or the fan. Power can also be taken off by an external source (e.g. helicopter rotor). RPM and temperatures are relatively low, so the blading remained uncooled in older engines. However, in modern engines the temperatures at the turbine inlet are high enough to require at least simple cooling, and in some cases, single crystal materials. The LPT tends to specific damages such as sulfidation, which are promoted by specific factors such as the operating temperatures of the turbine parts (Volume 1, Chapter 188.8.131.52). A unique problem occurs during a shaft failure (Volume 1, Chapter 4.5), when the core engine continues to supply the powering gas and causes the turbine to accelerate to overspeed within seconds. In this case, blades and disk fragments can fly off and escape from the engine (Volume 2, Chapter 8.1). High and intermediate pressure turbines are less likely to “run away”, since a shaft failure in these turbines causes a compressor in the core engine to lose power. This means that there is less hot gas available for accelerating the rotor.
Power turbine: Engines that export power via a shaft (helicopter engines, etc.) are outfitted with a power turbine. This usually consists of the last turbine stages. In special cases, the PT is outfitted with a variable stator. This allows it to accelerate from standing with high torque. The problems in PTs are largely the same as in LPTs. If a variable stator is used, specific weak points of the adjustment mechanism, such as the bearings of the vanes and the play-free actuation, must be controlled. Because thermal strain and insufficient gliding conditions prevent the guide vane bearings from being located at the tips, there is an increased danger of LCF and HCF fractures. Experience has shown that axial thermal strain, combined with the tight tip gaps to the hub, can easily make the vanes come into sporadic contact with the hub, putting dangerous flexural stress on the vanes.