AIRFOIL AIR SEAL ASSEMBLY
An air seal assembly for a gas turbine engine the air seal comprises a first assembly and a second assembly. One of the first assembly and the second assembly is rotatable relative to the other of the first assembly and the second assembly. The second assembly is aligned annularly with the first assembly and includes a circumferential surface with an abradable coating disposed annularly adjacent to the first and second airfoil tips. The first assembly includes at least one first airfoil with a first tip having an abrasive coating, and at least one second airfoil with a second tip absent the abrasive coating, the at least one first airfoil co-aligned axially and intermingled with a respective at least one second airfoil around a periphery of the first assembly.
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The invention relates generally to air seal assemblies for a gas turbine engine, and more specifically to rotor assemblies for air seal assemblies.
To maximize efficiency and minimize clearances, each operative section of a gas turbine engine (fan, compressor, and turbine) includes a variety of seals and coatings. Maintaining appropriate clearances between moving parts and adjacent stationary parts is critical to balancing efficiency and improving stability to limit damage to the engine. Too small of a clearance results in increased contact severity and frequency between components, particularly due to maneuver loads, rapid temperature changes, and other sudden changes in engine operation. Excessive clearance can cause efficiency losses from lost work embodied in the compressed gases escaping through gaps between respective rotor and stator elements. Large clearances also increase the risk and severity of operational instability such as compressor surge.
In one relatively simple example, every blade tip on a rotor is abrasively coated to run a groove into an abradable coating on the casing to form a seal. In another simple example, stator vanes include an abradable seal coating on all of the vane tips or shrouds that are rubbed by an abrasive coating on the rotor land. However, coating every blade tip or every vane is quite expensive and time-consuming. The coating process itself is subject to error which can also result in additional repair or scrapping of the entire component or assembly.
SUMMARYAn air seal assembly for a gas turbine engine the air seal comprises a first assembly and a second assembly. One of the first assembly and the second assembly is rotatable relative to the other of the first assembly and the second assembly. The second assembly is aligned annularly with the first assembly and includes a circumferential surface with an abradable coating disposed annularly adjacent to the first and second airfoil tips. The first assembly includes at least one first airfoil with a first tip having an abrasive coating, and at least one second airfoil with a second tip absent the abrasive coating, the at least one first airfoil co-aligned axially and intermingled with a respective at least one second airfoil around a periphery of the first assembly.
A rotor assembly comprises a rotor disc, at least one first rotor blade and at least one second rotor blade. The at least one first rotor blade includes a first airfoil tip with an abrasive coating. The at least one second rotor blade includes a second airfoil tip absent an abrasive coating. The at least one first rotor blade is intermingled and co-aligned with a respective at least one second rotor blade axially around a periphery of the rotor disc.
Engine 10 is shown as a dual-spool turbofan engine. However, this is merely one application of the inventive concepts described herein. In the example dual-spool engine, the blade, vane, and seal arrangements described below can be readily adapted to other compressor sections as well as to fan section 12, low-pressure compressor 14, and turbine sections 20, 22. The concepts can also be adapted to other turbofan engines with a single spool or with multiple spools, as well as to other types of gas turbine engines including industrial gas turbines (IGTs), turboprops, and turboshafts.
Here, rotor assemblies 30 are integrally bladed rotors (IBR's), which are also known in the art as blisks. IBR assemblies 30 include both first blades 40 and second blades 46 integrally secured thereto. Each IBR 30 has at least one first blade 40 with abrasively coated airfoil tip 42 and at least one second blade 46 with uncoated airfoil tip 48. Coated first airfoil tips 42 interact with seal rings 36 to form a groove for a blade outer air seal (BOAS) in outer abradable coating 38A. Since not every blade 40, 46 is coated on each rotor 30, the example depicted in
At lower pressure stages, shrouded vanes 31 are mounted conventionally to casing 34. Shrouded vanes 31 include conventional labyrinth seal 50 to minimize airflow therepast. In some embodiments, labyrinth seals 50 are disposed in lower compressor stages while the upper compressor stages can include the inner air seal as described below. The particular number and distribution of inner air seals and labyrinth seals can be determined via simulation.
Similar to the BOAS, cantilevered vanes 32, 33 can be adapted to interact with inner abradable coatings 38B on adjacent rotor lands 39 to form an inner air seal. Casing 34 includes one seal ring 36 at each stator stage. At least one first cantilevered vane 32 is intermingled circumferentially with at least one second cantilevered vane 33 around each seal ring 36. First vane 32 has abrasively coated first airfoil tip 35 and second vane has second airfoil tip 37 absent an abrasive coating.
Airfoils, which can include one or both of rotor blades and stator vanes, previously included abrasive coatings on the tip of each and every airfoil tip for a given stage to facilitate formation of inner and outer air seals. However, as will be seen in detail below, not every airfoil includes an abrasively coated tip, which saves processing time and effort. It should be noted that coated airfoil tips and numerous other elements have been exaggerated for clarity. Other useful elements in the various stator and rotor stages, such as dampers and anti-rotation devices, have also been omitted for clarity.
Casing 34 includes outer seal ring 36 secured annularly around rotor 30 with outer abradable coating 38 disposed radially adjacent to coated blade tips 42 and uncoated blade tips 48. These combine to form a BOAS according to the details below. Depending on exact tolerances, abrasively coated first airfoil tips 42 closely approach or actually contact the annularly adjacent abradable coating 38A. In this example, prior to operating in run-in mode for the first time, there is a larger gap between the surfaces of abradable coating 38A and uncoated blade tips 48, as compared to the gap between abradable coating 38A and abrasively coated tips 42. This shorter length helps ensure the abrasive coated tips 42 contact abradable coating 38A rather than uncoated tips 48 both during run-in mode and normal operational mode.
When the engine is cold, newly assembled or refurbished, and has not yet been operated in a run-in mode, coated blade tips 42 will nearly or actually touch abradable coating 38, while uncoated tips 48 generally avoid contact. As the engine is run in for the first time with new or refurbished seal components, the speed is quickly ramped up and down, on the order of only a few seconds per cycle, to take advantage of the centrifugal expansion of IBR assembly 30. This simulates the effects of a quick excursion to full engine throttle under normal operation, which can occur for example during takeoff and emergency maneuvers. Centrifugal expansion caused by quick acceleration of IBR assembly 30 results in first blades 40 extending spanwise, where abrasively coated first airfoil tips 42 wear into abradable coating 38A.
During run-in operation, second blades 46 generally avoid contact with abradable coating 38. However, when the engine is operating normally and both rotor 30 and seal ring 36 have centrifugally and thermally expanded, both coated tips 44 and uncoated tips 48 form a seal with the groove. This minimizes surge (backward airflow) through the compressor and eccentricity of rotor 30.
As was seen in
Minimizing tip clearances between stages optimizes the seal to improve efficiency and operational stability of the engine. As IBR 30 (shown in
It should be noted that blades 40, 46 including respective blade tips 44, 48 may have additional features and/or more complex geometries than shown in
Abradable coating 38A (as well as inner coating 38B shown in
As seen here, first blades 40 can remain slightly longer than second blades 46 both before and after run-in. In this way, first blades 40 will continue to preferentially contact abradable coating 38A as opposed to second blades 46. In situations such as during high maneuver loads, rotor 30 is displaced relative to casing 34. This could cause both types of blades 40, 46 to penetrate beyond abradable coating 38, and potentially an underlying thermal coating strike or seal ring 36. In such a case, it is preferable that coated first airfoil tips 42 rather than uncoated tips 48 absorb the majority of the contact forces. When blades 40, 46 comprise titanium alloys, they are less abradable than seal ring 36, which is itself less abradable than coating 38. Continued friction between the components can produce excessive heat and cause a titanium fire. In the case of nickel or other superalloy blades, significant wearing of both second blade 46 and seal ring 36 are likely in the event of inadvertent contact.
To form a conventional BOAS, each and every blade tip is coated with abrasive material. This is done because varying the blade weight increased the risk of rotor imbalance either during or after run-in. Increased stresses on blade tips was also thought to increase risk of tip or blade damage caused by fewer tips performing more of the work in forming the seal groove. Similarly, it was also required that all vanes in an individual stage have an abradable coating to form inner seal grooves because leaving some vane tips uncoated could cause metal buildup around the vane tips when the abrasively coated rotor lands strike the uncoated metal. This eventually led to larger clearances and greater efficiency losses. However, coating each and every airfoil tip with abrasive material introduces several complications in addition to increased costs. The coating process weakens the airfoil itself, making it more susceptible to bending stresses particularly around the tip. In addition, coating every airfoil tip increases processing time, effort, and opportunity for blade damage and scrapping.
IBR assembly 30 can be balanced to help minimize damage due to uneven rotational moments and wearing in of groove 52A. Particular intermingling of first blades 40 with second blades 46 is done with the goal of balancing the center of gravity of IBR assembly 30. The minimum number and distribution of coated blade tips 42 relative to uncoated tips 48, will depend on a confluence of factors, including the CTE of airfoil, rotor, coating, and casing materials, centrifugal expansion of the rotor, operating temperatures and pressures, rotational speed and harmonics based on blade and rotor shape, desired and minimum tip clearances, removal of airfoil material to facilitate coating, among others. Modern analytic and predictive software tools, such as SIMULIA®, available from Dassault Systèmes of Paris, France, can be used model and analyze the combined effects of these and other blade and operational characteristics in an effort to balance IBR assembly 30 during both run-in mode and normal operational mode.
These tools operate by using existing CAD definitions of the various components and modifying variable characteristics of the simulated components within the required parameters. Simulation occurs under different operating conditions to identify suitable and optimal blade distributions. For example, Monte Carlo simulations can be performed with these or other software tools in order to identify and analyze suitable blade distributions as well as to calculate and minimize failure risks given the randomness of possible operating conditions.
Using this or similar software, in combination with empirical testing, suitable and/or optimal intermingling of first rotor blades 40 with second blades 46 can be determined and tested over a wide range of normal and abnormal conditions, including bird strike and blade-off. These analytic tools can also be used to identify optimal conditions for run-in, including slower rotation, preheating or cooling of inlet air to expand blades relative to the casing, etc. In certain embodiments, the ratio of second rotor blades 46 with uncoated tips 48 to first rotor blades 40 with coated tips 42 ranges from about 12:1 to about 1:1. In certain of those embodiments, the ratio of second blades 46 to first blades 40 ranges from about 9:1 to about 3:1. In yet certain of those embodiments, the ratio of second blades 46 to first blades 40 is about 6:1. These blades will generally but are not required to be evenly distributed. The ratio of second blades 46 to first blades 40 tends to decrease with the overall swept diameter of IBR 30 because more energy is absorbed by blade tips to form a larger groove 52A in abradable coating 38.
In the example shown in
Blades 40, 46 can be welded around the periphery of a rotor disc or alternatively, the entire IBR assembly 30 can be machined out of a single block of metal like a titanium or nickel alloy. In some examples, such as for cold-side applications in fan section 12 and compressor sections 14, 16 (shown in
In certain embodiments, the clearance between coated tip 42 and the deepest point of seal groove 52A can range from about 1.0 mil (about 0.25 mm) to about 5.0 mils (about 1.3 mm) while the corresponding clearance for uncoated tips 48 can range from about 2.0 mils (about 0.50 mm) to about 10.0 mils (about 2.5 mm). It will be noted that first vanes 32 (shown in
Here, tip coating 54 is an abrasive coating, such as a cubic boron nitride (CBN) based material, while the blade material is a titanium alloy such as Ti-6Al-4V. Blades in higher compression stages or in the turbine may require a more temperature resistant blade or abrasive coating. Coating 54 can be added any time after the airfoil tips 42 are formed, such as is shown in
Alternatively, depending on the results of simulation and real-life testing, second blades 46 can have substantially the same underlying spanwise length and other dimensions (within an acceptable tolerance) as blade 40′. It should be noted that in the previous example where L1<L2, seal ring 36 has a lesser CTE than the rotor assembly 30 to limit the depth of groove 50 formed by first blades 40, as well as reducing the chances of second blades 46 striking the interior of seal groove 50 (shown in
The strength of blade material around first airfoil tip 42″ can be weakened by the heat and pressure of the coating process, making them more susceptible to bending stresses. Thus with all airfoils on a rotor or stator coated with an abrasive material, they are susceptible to damage during the run-in mode and during high maneuver loads. To partially alleviate this risk, titanium alloy blades sometimes included two 45° chamfer cuts chordwise along one or both the suction and pressure surfaces between the leading and trailing edges.
Replacing chamfers with rounded squealer tip cuts 70 moves the stress peaks, particularly those resulting from second- and third-order bending resonances, into the thicker uncoated part of finished first blade 40. With the stress peaks pushed past the runout of tip cuts 70 to points adjacent leading edge 62 and trailing edge 64, first blade 40 can better absorb contact with abradable coating 38A (shown in
Squealer tip cuts 70 have previously been used on many types of uncoated airfoils to reduce tip leakage. This is ordinarily accomplished by allowing for a smaller tip clearance while reducing the risk and magnitude of damage in the event that the blade strikes the casing or other outer stationary structure. Such tip cuts are thus most frequently made at the center of the tip between the suction and pressure surfaces rather than on the periphery as seen above.
Regarding second blades 48 (and second vanes 33 shown in
Squealer tip cuts 70 also can improve the balance of rotor 30 (shown in
First and second vanes 32, 33 are analogous to first and second rotor blades 40, 46 shown in previous figures. The first airfoils (vanes 32 and blades 40) include abrasive coatings on the respective first airfoil tips, which rub a seal groove (50A or 50B) into an adjacent abradable seal coating region (38A or 38B). In contrast, using an abrasively coated rotor land and uncoated vane tips led to larger clearances and greater efficiency losses caused by buildup of metal rubbed from the vane tips.
By slightly reducing the length of vanes 32 and/or providing a slightly thicker abrasive coating on vanes 32, some of the vanes 33 can remain uncoated similar to
Also similar to first and second blades 40, 46, significant time and money is saved by only coating first vanes 32 and leaving second vanes 33 without an abrasive. Generally, only one vane per cluster need be coated. However, the number of coated vanes and their relative coating thickness is determined using similar considerations as for identifying the number of abrasively coated rotor blades. Thus a greater or lesser number of coated tips 35 may be required relative to uncoated tips 37. The effects of both inner and outer seal grooves 50A, 50B (shown respectively in
In this alternative embodiment, first blades 140 and second blades 146 include respective coated blade tips 142 and uncoated tips 148 (like those shown in
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims
1. An air seal assembly for a gas turbine engine the air seal comprising:
- a first assembly including at least one first airfoil with a first tip having an abrasive coating, and at least one second airfoil with a second tip absent the abrasive coating, each of the at least one first airfoil co-aligned axially and intermingled with a respective at least one second airfoil around a periphery of the first assembly; and
- a second assembly aligned annularly with the first assembly, the second assembly including a circumferential surface with an abradable coating disposed annularly adjacent to the first and second airfoil tips, with one of the first assembly and the second assembly being rotatable relative to the other of the first assembly and the second assembly.
2. The air seal assembly of claim 1, wherein the engine is operable in at least a run-in mode and an operational mode, the run-in mode including rotating the rotatable assembly intermittently to centrifugally expand the rotatable assembly for abrasively forming a circumferential seal groove into the abradable coating with the at least one first airfoil tip, and the operational mode including rotating the rotatable assembly continuously to centrifugally and thermally expand the rotatable assembly forming an air seal.
3. The blade outer air seal assembly of claim 1, wherein the seal assembly is installed into a compressor section of the gas turbine engine.
4. The air seal assembly of claim 1, wherein the at least one airfoil tip includes at least one squealer tip cut under the abrasive coating.
5. The air seal assembly of claim 4, wherein the at least one airfoil tip includes a first squealer tip cut proximate the pressure surface and a second squealer tip cut proximate the suction surface.
6. The air seal assembly of claim 1, wherein the abrasive coating comprises cubic boron nitride (CBN) suspended in a matrix.
7. The air seal assembly of claim 1, wherein the abradable coating comprises a boro-nitride ceramic.
8. The air seal assembly of claim 1, wherein the at least one first airfoil and the at least one second airfoil each comprise a titanium alloy.
9. The air seal assembly of claim 1, wherein the first assembly is a rotor assembly and the second assembly is a stator assembly.
10. The air seal assembly of claim 9, wherein the at least one first airfoil is intermingled with the respective at least one second airfoil based at least in part on results of a Monte Carlo simulation.
11. The air seal assembly of claim 9, wherein the first and second airfoils are rotor blades integrally formed with a rotor disc.
12. The air seal assembly of claim 1, wherein the first assembly is a stator assembly, and the second assembly is a rotor assembly.
13. The air seal assembly of claim 12, wherein the first and second airfoils are cantilevered stator vanes.
14. A rotor assembly comprising:
- a rotor disc;
- at least one first rotor blade including a first airfoil tip with an abrasive coating; and
- at least one second rotor blade including a second airfoil tip absent an abrasive coating, the at least one first rotor blade intermingled and co-aligned with a respective at least one second rotor blade axially around a periphery of the rotor disc.
15. The rotor assembly of claim 14, wherein the at least one first rotor blade is intermingled with the respective at least one second rotor blade for rotational balance of the rotor assembly, the intermingling determined using a Monte Carlo simulation.
16. The rotor assembly of claim 14, wherein a ratio of the at least one first rotor blade and the at least one second rotor blade is between about 1:12 and about 1:1.
17. The rotor assembly of claim 16, wherein the ratio of the at least one first rotor blade and the at least one second rotor blade is between about 1:3 and about 1:9.
18. The rotor assembly of claim 17, wherein the ratio of the at least one first rotor blade and the at least one second rotor blade is about 1:6.
19. The rotor assembly of claim 14, wherein the at least one first rotor blade and the at least one second rotor blade are integrally formed with the rotor disc.
20. The rotor assembly of claim 14, wherein the first airfoil tip includes at least one squealer tip cut under the abrasive coating.
Type: Application
Filed: Sep 23, 2011
Publication Date: Mar 28, 2013
Applicant: UNITED TECHNOLOGIES CORPORATION (Hartford, CT)
Inventors: Charles P. Gendrich (Middletown, CT), Bradley L. Pike (Burlington, CT), Christopher R. McNeill (Plainville, CT), David J. Pitney (Moodus, CT)
Application Number: 13/242,297
International Classification: F01D 11/08 (20060101); F01D 5/14 (20060101);