Table of Contents
7.1.4 Remedies for Blade Tip Rubbing Damage
Measures for preventing rubbing damage:
- optimized constructive layout of the rubbing zone of the affected part group and engine structure
- active clearance control
- suitable material selection for the tribo-system, i.e. blade tip and rub coating
- tuned engine operation
- observation and analysis of the rubbing process in technical trials
- computer-aided analysis of clearance gap behavior
Constructive layout (passive clearance control)
Good gap behavior over the entire operating time of an engine is determined as early as the design concept stage. This includes the number of shafts (two- or three-shaft engine, Fig. "Sealing advantages of 3-shaft engines"), the position of the main bearings, especially the fixed bearings, the positioning and adjustment of the engine suspension, and the design of the housing (double-walled, axially split, rings).
Engine stiffness is determined by housing stiffness, which determines bending of the engine core (“backbone bending”) due to internal and external forces. If stiffness is insufficient, supports can also be used (Fig. "Clearance changes by flight loads"), but this method should not be a first choice.
The design of the housing flange contributes to the gap distribution around the circumference. Axial flange (Fig. "Pressure casing cracks due to stiffness jumps") stiffens the housing locally around the circumference and prevents this zone from expanding due to internal pressure. The local changes of thermal capacity and heat dissipation combined with the increased stiffness at the flange result in a noticeably different expansion behavior than in than in the thin housing walls (passive gap control). This promotes self-increasing rubbing in the flange area (see Fig. "Rubbing traces indicate influences").
The components participating in rubbing must be suitably designed. Proper application of reinforcements and/or heat balance-influencing masses can control strain caused by temperature and pressure changes (Figs. "Tip clearance control types" and "Thermal adjusting housing expansion").
The rubbing process and the consequences of enlarged gaps can be better controlled by adjusting the coating geometry in the housing (Fig. "Stiffening rings preventing deformations by G-forces") and on the rotors (7.1.4-13). Patterning the coating surface is a suitable method (“casing treatment”).
Active clearance control:
Active clearance control (ACC) is a controlled system that adjusts the gap to optimal size even during unsteady operation (Ref. 7.1.4-5). Appropriate cooling or heating with the fan air or compressor air enables active clearance control in the compressor and turbine (Figs. "Active clearance control ACC" and "Clearance behaviour by ACC"). Rotors can be brought nearer to the housing with the aid of internal ventilation which adjusts their strain. This avoids unwanted rubbing or overly large clearances (Figs. "Tip clearance control types", "Thermal adjusting housing expansion", Active clearance control ACC" and "Clearance behaviour by ACC").
There have been many attempts to develop regulated ACCs for serial implementation (intelligent clearance control = ICC, Ref. 7.1.4-4). These systems measure the tip clearance at several points around the circumference and adjust the inner housing diameter accordingly. Uneven gaps around the circumference could also be compensated for with segmented inner housing rings (Fig. "Backbone bending' by aerodynamic forces"). It would be especially advantageous if these systems could prevent or compensate for wear of the housing coatings (rubbing, corrosion, erosion). A significant problem with these systems is the creation of sensors that guarantee sufficiently exact clearance measurements over long operating times under the engine-specific conditions in the sensor area (Refs. 7.1.4-6 and 7.1.4-7).
ICCs have potential drawbacks that must be taken into account by the designer. If the regulators fail, the housing segments must not contract like a shoe brake. If individual segments are drawn back too far, the uneven clearance around the circumference can cause the blades and rotor to oscillate. Foreign objects/fragments can break off of adjustment systems in the housing and enter into the engine`s gas flow. The sensors and the adjuster must not increase engine weight.
Material selection for optimizing clearance maintenance and rubbing begins with the components involved. Stiff housing materials (Fig. "Backbone bending' by aerodynamic forces"), such as steel and Ti alloys, can noticeably reduce elastic deformations. Thermal strain can be minimized through use of materials with especially small expansion coefficients. These include steels and Ni alloys rather than light metals based on Mg, Al, or Ti. 13%-Cr steels are used in certain areas of rotors and housings because of their especially small heat strain.
Material selection for the tribo-system of the rubbing process demands special experience (see Chapter 7.1.1). Blade tips, as well as the rub coatings on housings or rotor spacer rings, must be precisely tuned to one another in their rubbing behavior and additionally be guaranteed to last throughout the required long life spans. The main objective is ensuring the lowest possible mechanical and thermal stressing of the blade tips and blade leaf with the least possible amount of wear.
The manufacturing process of rub coatings determines their operating behavior. The bond strength of adhesively bonded or poured elastomers is to a large degree dependent on adherence to the given processing parameters (e.g. humidity, temperature, air purity, wet life, processing date). During the manufacture of thermal spray coatings, for example, adhering to the processing parameters for preparation of the bond surface and the temperature control during the spraying process determines the residual stresses in the coating and therefore its bond strength.
Clearance behavior is strongly influenced by engine operation. Operating influences such as quick starts, special starting intervals (Figs. "Engine position influences damages" and "Compressor blade fracture by thermal bending"), erosive environments, and corrosion (see Chapter 5.3 and Chapter 5.4) can, in identical engines, lead to very different damage mechanisms which affect the life span of the engine.
The damaging effects auxiliary materials such as cleaners and solvents, greases, and fuel can have on rub coatings, especially elastomers (Fig. "Elastomer rubbing coatings"), must not be underestimated. They can induce or accelerate aging through thermal loads (e.g. embrittlement, shrinking, loss of bond strength). Therefore, only approved auxiliary materials may be used. Elastomer coatings should be additionally secured against delamination by form fitting (Fig. "Elastomer rubbing coatings").
The availability of constantly improving calculation programs allows very detailed analyses of clearance behavior as early as the construction phase. However, as has been shown time and again, only serial operation can prove the successfulness of a design.
Analyses of technical trials:
In order to conceptualize and implement specific remedies, a complete understanding of the clearance gap-affected components, operating influences, and effects is necessary. Despite the available tools in the form of hardware and software, engines may exhibit unexpected anomalies. In these cases, observing the clearance changes in a running engine can be an important aid. Static (images of certain phases of operation) and dynamic observations (film) of the clearance gap can be achieved in technical trials through use of X-ray inspections (Fig. "X-ray clearance control in engines", Ref. 7.1.4-3).
Chemical and physical analysis of the particles in the waste gas gives important clues as to the time at which rubbing occurred, as well as the components involved.
With the aid of acceleration sensors and vibration analysis, conclusions can be drawn as to the rubbing components and anomalies of the specific rubbing process (Ref. 7.1.4-2).
Figure "Sealing advantages of 3-shaft engines": The sensitivity to long-term efficiency deterioration is determined by the engine design. For this reason, an OEM propagates the triple-shaft principle (depiction corresponds to available prospectus material) in large turbofan engines as being especially resistant to housing deformation and shaft deflection.
One advantage is short shafts, i.e. short distances between main bearings. This ensures bending resistant rotors and minimizes their deflection. The housings are made of two shells, where the inner shell is protected from deflection of the outer shell by a soft suspension (bottom diagram). The inner housing therefore remains round despite ovalization of the outer housing (compare with Fig. "Clearance behaviour by ACC").
Figure "Long term performance of 3-shaft engines": These diagrams are based on information from a manufacturer of large turbofan engines with a triple-shaft design. The benefits of lower fuel consumption in triple-shaft engines are especially obvious after long operating times with many starts. After about 4000 start-up/shut-down cycles, the SFC of a double-shaft engine will have increased up to 2% more than that of a triple-shaft engine. This corresponds roughly to the profit margin of a “good” airline, which shows the significance of this kind of engine behavior.
The difference in turbine intake temperatures compared with the double-shaft engine is especially noteworthy. If the temperature increase of 40 °C or more (in double-shaft engines) were not compensated for by cooling measures or regulator settings and was instead transmitted to the parts, it would shorten the life of the engine by a factor of four. When added to the increased fuel costs due to the higher SFC, the high cost of replacement parts would make this engine behavior extremely expensive.
Figure "G-forces influencing tip clearances": The position and configuration of the engine suspension strongly influence the housing deformations. If the designer takes this into account, he can influence the clearance gap behavior as early as the design stage. In tactical aircraft engines, curving flight and landing (aircraft carrier) can create G-forces so large that the clearance gap is bridged near the suspension (Ref. 7.1.4-5). The bottom diagrams show the typical ovalisation of a housing with the resulting local rubbing zones. The stiffness of the housing section that accepts the bearing- and suspension forces is a determining factor (Ref. 7.1.2-18).
The scetch at the bottom right shows schematic the deformation of the struts, rings and of the bearing suspension area of a turbine casing by a vertical force at the rotor (Ref. 7.1.4-16)
Figure "Pressure casing cracks due to stiffness jumps": Housings with or without axial flange behave very differently under strain (Ref. 7.1.4-11). The changes in heat capacity, heat dissipation, and stiffness in the region of the axial flange create unevenness around the circumference. Much experience is necessary in order to change from one construction principle to another. The stiffness jump in the area of the axial flange leads the housing to “cave in” in this area (compare Fig. "Rubbing traces indicate influences"). This characteristic must be taken into account by the designer, since it may cause clearances to decrease further due to heat strain during rubbing.
Housings with axial flange have considerable advantages for inspection and repairs of the blading on the engine. However, engines with housing rings exhibit very little long-term efficiency deterioration.
Figure "Tip clearance control types": The top diagrams depict various options for clearance control. These are based on the following effects:
“A”: Materials with suitable thermal expansion and tuned wall thickness.
“B”: Increasing the thermal inertia of the housing through use of an inner thermal insulation layer.
“C”: Local stiffness increases to minimize elastic expansion. The mass increase also locally increases thermal capacity and heat dissipation of the housing.
“D”: Cooling and/or heating the housing with air blown onto its surface from specially shaped pipes (right-angle cross-section).
“E”: Using insulating padding to increase thermal inertia (especially in the LPT, Fig. "Active clearance control ACC" and HPC, bottom left diagram).
“F”: Internal housing structuring struck by the air flow (e.g. in the HPC and the HPT, bottom right diagram).
“G”: Intelligent clearance control systems with gap measurement and segment adjustment.
Figure "Thermal adjusting housing expansion": Combining options “B” and “F” from Fig. "Tip clearance control types" allows especially good clearance control (Refs. 7.1.4-8 and 7.1.4-9). Also, the inner housing should have limited radial movement against the outer housing (bottom diagram).
Figure "Active clearance control ACC" (Ref. 7.1.4-5): Active Clearance Control (ACC) systems usually find limited use compared with the original intentions in the development project E3E. They find use in modern engines in high-pressure compressors (HPC), high-pressure turbines (HPT), and low-pressure turbines (LPT, bottom diagram). The air extraction must be adjusted to minimize energy loss in the temperature and pressure as well as the demands of the housing, i.e. seal rings and seal segments. The bottom left diagram is an example in which the air (about 0.7% of the engine`s air flow-rate) necessary for the expansion of a double-shell HPC housing (see Fig. "Clearance behaviour by ACC") is extracted by a valve (VI) and then redirected into the LPT in order to recover its energy. In single-walled configurations, the HPC housing has air blown onto it from the outside, similar to a turbine housing (see Fig. "Tip clearance control types" “D”).
The bottom right diagram depicts, as does the top diagram, air taken from the fan region (corresponding to about 0.3% of the core engine`s air flow-rate) and directed by valves (V2 and V3) onto turbine housings to cool it. This air is then directed into the exhaust gas flow in order to recover its energy.
If an ACC is used to heat or cool depends on the adjusted exit clearance gap. If the cold gap is very small, the housing must be heated with air from the rear compressor during start-up. If the cold gap is adjusted to a larger size, the housing must be cooled so that the lengthening of the blades minimizes the clearance gap.
Figure "Clearance behaviour by ACC" (Ref. 7.1.4-1): The top diagram depicts the typical expansion characteristics, i.e. the temporal clearance changes, of a high-pressure compressor with a double-walled housing (compare with Fig. "Sealing advantages of 3-shaft engines"). Due to the high pressure in this area, passive clearance control is used. Fig. "Tip clearance control types" shows that active clearance control can be used even in HPCs with double-walled housings. The inner housing is elastically suspended in the outer housing and can expand relatively freely. The internal pressure and the expansion it induces are absorbed by the outer housing.
In the turbine area, air from the high-pressure compressor is fed onto internal structures, while air from the fan is fed onto the outer housing walls (bottom right diagram). The low-pressure turbine is usually struck by fan air from the outside, and additional insulating pads can be installed between the blading and housing (second diagram from bottom). This minimizes blade tip clearance, especially during cruising flight (bottom diagram).
Figure "ACC cooling air pipe optimizing": The configuration of the profile of the ACC's air pipes can considerably improve the thermal conductivity, i.e. the cooling from directed air and convection (Ref. 7.1.4-9). Pipes with an angular cross-section that has been fitted to the housing contour shorten the distance between the air jet exit and the surface it is directed at. They are additionally better at directing the air flow along the housing surface for a long distance and thus increase the amount of the housing surface that is intensively cooled. The smaller gap between the pipe and housing makes higher air flow speeds possible (compared with round pipes). Changing from round pipe cross-sections to angular ones in the high-pressure turbine region of a large turbofan engine resulted in fuel consumption being reduced by 0.65 %.
Figure "Stiffening rings preventing deformations by G-forces": Clearance control in large fans is especially challenging (Ref. 7.1.4-1). The housings, which are as light as possible due to their large diameter, must accept and transfer powerful aerodynamic force (Fig. "Engine affecting by inertia forces / acceleration") and inertial force (Fig. "Compressor blade fracture by thermal bending") into the engine suspension. The relatively massive cross-section of the pylon creates a local flow disruption in the fan and affects the surge limit. This makes it especially important that the surge limit is not brought closer by uneven and/or large blade tip clearance in the fan. Further improvements in the surge limit can be achieved through casing treatment. This is commonly circumferential grooving, the geometry and axial positioning of which must be optimized for the specific situation.
Figure "Behaviour of rub-tolerant systems": In rub systems, the multitude of requirements mean that there must always be compromises. The diagram lists the specific requirements for abradable and rub-tolerant systems, as well as their advantages and disadvantages. The influences on rub systems (Fig. "Rub tolerant blade tip systems") and their properties are described in depth in Chapter 7.1.1.
Figure "Minimizing blade stresses during rubbing": In order to increase the distance to the surge limit (see Fig. "Coatings at tip seals") the coating in the housing above the blade tip can be patterned (“casing treatment”). In fan stages, this is done with circumferential grooves. If the intake flow is disturbed, this casing treatment increases the surge limit by 3-5% (Ref. 7.1.4-1) without decreasing efficiency. Axial grooves that spiral slightly in a circumferential direction increase the surge limit even further, but decrease efficiency. New casing treatment designs can increase the surge limit by about 20% with disturbed intake flows. Sufficiently large surge limits allow the chord length of the rotor blades to be shortened, reducing the weight of the fan (rotor, containment).
Treating the casing coating can also be done to optimize the rubbing process. For example, it can limit the friction forces, and therefore the bending stress on the blades.
Figure "Technologies at blade tip": Armoring the blade tip should improve its cutting effect and thus lower friction force and heat creation considerably. However, the dynamic strength of the armored blade tip edge is problematic (Fig. "Compressor blade cracks by rubbing"), since this area tends to dynamic cracks (top diagram).
Appropriate constructive design of the blade tip is especially important with armorings (galvanically, thermally sprayed, soldered). Armorings usually have a considerably lower dynamic strength than the undamaged base blade material. It would be more realistic to compare the dynamic strength of the armored blade with that of an unarmored rubbed blade. Additionally, due to the greater distance to the outer fibers, bending loads stress overhanging coatings (bottom left diagram) more highly than those whose sides are flush with the blade edge (bottom right diagram). Reworking the blade profile after tip armoring is recommended, but costly.
Figure "Optimizing blade tip coatings" (Refs. 7.1.4-10 and 7.1.4-12):
The extreme operation conditions (erosion, oxidation) in high-pressure turbines demand relatively hard ceramic coatings on the seal segments that form the gap with the turbine rotor blade tips in the housing. Therefore, these blade tips must be armored to protect them during rubbing. This is done with hard particles embedded in a bonding layer. Both the hard particles as well as the bond layer have serious specific problems. This is why, in certain cases, a switch is made back to unarmored blade tips.
For the rubbing behavior, optimal bracing of the particles in the bonding layer and sufficient “cutting volume” to absorb removed coating material are important (top and bottom diagrams). Optimal distribution of the particles (especially if they are relatively large) on the rub surface is vital (Ref. 7.1.2-14). Small particles do not allow as much cutting volume and tend to smear.
There are two primary materials that are used as hard particles:
Cubic boron nitride (CBN): These particles are extremely hard and unreactive. This makes them suitable for long-term use without damaging the bond layer. However, the oxidation resistance of these particles is so low, that they are completely destroyed by oxidation within one hour of operation in the typical environment of modern high-pressure turbine blade tips (middle diagrams). Therefore, these armorings are used in places where the maximum rubbing and abrasion occurs during the testing/rub-in runs, after which blade tips without armoring are acceptable.
Silicon carbide (SiC): This material is sufficiently hard and extremely resistant to oxidation. However, the problem with this material is its aggressive reaction with metallic bond layers such as MCrAlY materials or high-temperature solders. To solve this problem, these particles are coated with an intermediate layer (Ref. 7.1.4-13) that is intended to serve as a diffusion barrier. However, this has failed to work satisfactorily. The SiC can come into contact with the bond coating through weak points at the particle edges and dissolve in a short time. This can result in the creation of graphite nests such as in spherulitic cast iron or malleable cast iron (middle diagrams). The reactions with the matrix cause it to lose strength and embrittle, which worsens the operating behavior of the blade tip base material.
Figure "Blade stresses by grooved rub coatings" (Ref. 7.1.4-11): Protecting the rotor drum against wear and overheating (see Figs. "Compressor rotor damages by rubbing" and "Compressor rotor perforated by rubbing") requires wear-resistant, hard, rub-tolerant coatings with thermal insulation properties. In order to keep thermal and mechanical loads on the blades during rubbing as low as possible, as well as minimizing smearing (depositing), coatings are grooved in special cases (if the increased roughness is acceptable). Circumferential grooves provide a better seal, but their interlocking can put large bending stress on the blade leaf (left diagram) in case of simultaneous combined axial and radial motion of the contact surfaces (right diagram). In order to avoid this, the grooves can be spiraled (middle diagram). The grooves then make a continuous, machining feed motion due to their rotation. This creates only a small axial cutting force.
Figure "Resin rub-coatings technology": Housings in the compressor region with sufficiently low operating temperatures can be coated with a filled synthetic resin layer by a centrifugal casting process. This coating has not only good rubbing behavior, but also dampens vibrations of the housing and/or stator vanes. Longer operating times can cause aging problems in the coating (see also Fig. "Elastomer rubbing coatings"). The diagram depicts a small helicopter engine in which these coatings are used.
Figure "X-ray clearance control in engines" (Ref. 7.1.4-3): Measuring the actual clearance during engine operation is necessary in order to implement targeted measures. Inspections can be made in testing rigs, if one does not consider G-forces and gyroscopic forces. With the aid of a powerful X-ray source, the clearance behavior of the engine in real time can be analyzed and/or examined in saved images. Pulsing the X-ray at 50-500 1/s makes a stroboscopic effect possible. Larger engines can be examined in an overview or in enlarged details.
The results should be evaluated by combining them with findings from both conventional instruments and disassembly.
This procedure is especially suitable for measuring axial clearance/axial motion of the rotor relative to the housing. Of example, in case of changes in the axial bearing loads or an especially flexible bearing suspension.
Recommendations for Preventing Damage from Rubbing
- Before specific measures can be taken, the influences on the rubbing process must be precisely analyzed (clearance behavior, rubbing areas, rubbing anomalies, material properties of the tribo-system, etc.).
- Coating development is extremely time-consuming and expensive. Therefore, sufficient time and money must be available for the manufacturing technology and quality control. It is better to conduct continuous development, which can draw on insights gained from during serial application
- If possible, only use proven rub systems.
- With new rub systems, sufficiently realistic trials must be conducted with original parts. The following factors must be closely observed: heat development, mechanical loads (bending, vibrations), and burring.
Coatings intended for use at high temperatures must be tested for bond weakening due to oxidation from beneath.
- Testing whether or not the rubbing products can damage other parts (erosion, corrosion, diffusion, blocking of the hot parts, etc.).
- Coatings on the rotor hub (spacer rings) must have their LCF behavior (bond strength) tested in cyclical centrifugal tests (if possible combined with temperature cycles).
- Blade tip wear should be as little possible (reparability!).
- Blade tip armoring must not lower dynamic strength significantly. Dynamic trials should be conducted to ensure this.
- Rub coatings must be inspected for aging during operation. This is closely tied to the predicted life span, temperature, corrosion, oxidation, erosion, and dynamic fatigue.
- As much as possible, rub systems must be designed in such a way that rubbing is not self-increasing.
- Optimizing clearance behavior through use of active (ACC, ICC) and/or passive clearance controls.
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