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Stress systems Experienced: Fan vs HP Turbine Rotor Assemblies

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Introduction

The gas turbine is an extremely complex piece of machinery, one that requires various components to behave in different environments without failure (or rather an extremely low risk of failure: less than 1×10^-8 per flying hour). This makes it particularly difficult to provide an accurate lifetime prediction as many of the components are not subjected to a constant stress/temperature, have differing damage mechanisms and loading conditions. For instance, components within the fan system at the front of the compressor require very different approaches to lifing compared with components in the cooled first stage High Pressure (HP) rotor assembly. These particular components will be investigated within this essay. In addition the types of defects that arise in service vary between components due to manufacturing processes, materials used and other factors, which in turn impacts on their effective service life which needs to be taken into consideration when estimating fatigue lifetimes.

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Regardless of the component, in order to provide an accurate fatigue lifing estimation the operating stress-temperature behaviour and both in service and machinery defects need to be thoroughly understood. It is not only sufficient to understand how the damage caused by machinery defects affects the initial life but also to understand how the defect damage will progress in service. However the impact of damage caused in service and how it affects the initial lifing calculations also needs to be grasped (whether it promotes further damage or actually extends the lifetime (for example oxide promoting crack tip closure). A crucial aspect of the lifing is being able to understand the critical features of each component and how they can fail in service. The potential accumulation of in service damage is also necessary to the provide accurate lifing estimations to avoid unexpected failure of components (in particular critical components such as disks).

Critical components within aero gas turbines (such as disks) are subject to design specifications which dictate the adherence of an Engineering Plan. [1] The Engineering Plan “means a compilation of the assumptions, technical data and actions required to establish and to maintain the life capability of an Engine Critical Part. The Engineering Plan is established and executed as part of the pre- and post-certification activities.” In accordance with “CS-E 515 Engine Critical Parts” the integrity of the critical components specifies that a suitable lifetime estimation includes consideration of the following: combinations of loads, material properties, environmental influences and operating conditions, including effects of parts influencing these parameters, are sufficiently well known or predictable, by validated analysis, test or service experience, to allow each Engine Critical Part to be withdrawn from service at an Approved Life before Hazardous Engine Effects can occur. Appropriate Damage Tolerance assessments must be performed to address the potential for Failure from material, manufacturing and service-induced anomalies within the Approved Life of the part. The Approved Life must be published as required in CS-E 25 (b). [1]

In order to satisfy these criteria during disk design, a specified overspeed criterion is established (in case a shaft fails for example) and the disk must resist bursting at this overspeed. In addition the Low Cycle Fatigue (LCF) and Creep life must be sufficient as stated in the standards.

HP Turbine disks necessitate extremely cautious design because if a disk were to fail through bursting there is too much energy contained by the disk to contain it within the engine carcass. At best the engine will no longer function, at worst the airframe security and safety of passengers/general public is greatly threatened. The acceptable burst criterion for turbine disks is 125% overspeed with no burst of a disk with the minimum strength. Bursting of a disk occurs due to gross plasticity when the local peak stress is equal to the fracture stress of the material. Material selection also plays a key role, Nickel is used as disk material within HP Turbine disks and due to its strain hardening characteristics cannot allow stress redistribution at the bore through plastic deformation. This is in direct comparison to Titanium disks in the Fan which are much less subject to strain hardening and thus are able to redistribute stresses. The thicker material at the bore helps to achieve the burst criterion by providing more area to the most highly hoop stressed portion of the disk. When bursting occurs the hoop stress is constant from the bore to the rim and also equal to the Area Weighted Mean Hoop Stress (AWMHS).

With HP turbine disks the centrifugal loading arises due to the rotation of the disk material itself as well as a contribution from the blades attached at the rim. There are also thermal gradients between the rim and the bore of the disk. These are the major loads experienced by the disk however it is important to note (for a holistic approach) that there will be additional loads from the bolts connecting the shafts/spacers to the disks. These are the three main sources of stress generation in a disk. The thermal gradients can cause both compressive and tensile stresses within the disk; as the rim of the disk is hotter than the bore there is an element of thermal expansion generating tensile stresses within the rim and the cooler bore generates compressive stresses acting closer to the bore. These temperature gradients also affect the hoop and radial components of the centrifugal forces. The hoop stresses are induced in tension at the bore and drop to compressive at the rim. The radial stresses at both the bore and the rim are zero but rise to their peak value within the diaphragm. Finally there are nip loads on the rim due to clamping the spacers to the rims.

The centrifugal force experienced by a rotating component is proportional to the mass of the rotating component, the radius of the disk and the angular velocity:

Equation 1 F=Mrω^2

Due to the required use of Nickel based superalloys (in order to cope with the high gas flow temperatures in the HP turbine) and its high density (in the region of 8.3g/cm^3 [2]), it is clear from Equation 1 that in order to minimise the stresses the best approach is to reduce the mass and radii of the disks. This can be achieved by going through a number of design iterations and changing the disk design wherever possible. As the stresses in the disk bore are normally higher than in the rim, less material is required towards the rim in order to sustain these lower stresses. As such disk designs often follow a “milk bottle” design with a taper towards the rim.

The stresses generated within HP turbine disks are characterised by the higher rim loads due to the blade assemblies mounted on the edge of the rim and the loads generated by the gas flow. A modern day hollow Trent 1000 fan blade can be seen in Figure 2 with a comparative HP turbine blade in Figure 2 (The multi-pass cooling channels with a thermal barrier coating (TBC)). There is also an angular velocity gradient along the length of the disk as the radius increases in addition to an extreme thermal gradient at the rim/ blades due to the very high temperature of the gas flow. This thermal gradient will always be rather extreme due to the greater amount of material in the bore being able to provide good heat redistribution around the disc and the always present high temperature gas flow. In the hotter regions tensile radial stresses are induced because of these thermal gradients. However with respect to hoop stresses the thermal gradients provide a tensile hoop stress region within the bore and a compressive region within the rim. As a result the disc is very highly radially stressed in the diaphragm section and zero radial stress at the rim and bore. Thus with a high stress in the thinnest region of the disc the creep of this region is very important when considering damage tolerant design. In order to combat this the microstructure at the bore will be a fine grain structure in order to provide the strength whilst at the rim a coarse grain microstructure is used in order to reduce the amount of creep occurring.

Whilst the magnitudes of stress in both fan and HP turbine blades are different, the stress sytems themselves are remarkably similar. Both will experience stress and torsion due to the high level of centrifugal loading, however they will also be subjected to bending moments in both tangential and axial directions due to an off- centre centre of gravity and the forces imparted by the high speed gas flow.

It is quite clear that the magnitude of stress of Nickel based HP turbine is higher than that of Titanium fan blades due to the higher density of Nickel versus Titanium. There are also the thermal gradients present in HP turbine discs which are practically nil in the fan system due to the entire assembly being “submersed” in the oncoming ambient flow. The HP turbine disks and blades have multi pass cooling channels within them which can act as stress raising features and also act as a means for generating more thermal gradients within the components as bulky material struggles to conduct the cooling efficiently. The method of mounting the blades to the disk also provides its own considerations; at the bottom of a Titanium fan blade is a dovetail joint which allows it to be slotted into the disk, whilst on a Nickel HP Turbine blade there is a fir tree joint to connect it to the disk (The fir tree joint is used in turbines due to the greater stresses present, at the bottom of the fir tree recess (within the disk) is a greater area of material which allows the management of the greater stresses and both decrease towards the turbine blade). The fir tree root compared to the dovetail also offers a far more complex geometry and as such is a much higher stress raising feature. As such the ability of the components to resist creep deformation is crucial due to the local stresses being so much greater. Obviously the effects of creep deformation will be much greater in the HP turbine section given the greater temperatures, however Nickel based superalloys have a good ability to retain their strength at elevated temperatures even up to their melting temperature. In terms of fatigue cycles the vibration of the blades during operation can be considered as High Cycle Fatigue (HCF) and the differing stresses with engine speeds as LCF.

With HP Turbine blades an issue of particular interest is Thermo -Mechanical Fatigue (TMF) which deals with the interaction between varying stresses and temperatures on fatigue lifetimes. Environmental attack also occurs within the turbine sections (High Pressure and Low Pressure (LP)) namely oxidation and hot corrosion. Within the HP Turbine Type I Hot Corrosion (High Temperature Hot Corrosion, HTHC) and the effects of oxidation are prevalent due to the elevated temperatures, whilst within the LP Turbine the effect of Type II Hot Corrosion (Low Temperature Hot Corrosion, LTHC) is most prevalent. Essentially Hot Corrosion requires the presence of salt and SO2 (either from the gas or the salt melt) and temperature to form pitting damage in the Nickel base material. This is an important consideration for in service damage accumulation

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