The present invention relates to radial compressors, and in particular, to radial compressors with blades tuned according to natural frequency.
Gas turbine engines typically include several sections such as a compressor section, a combustor chamber, and a turbine section. In some gas turbine engines, the compressor section includes a radial compressor with a series of main blades and splitter blades connected by a disc. During operation of the gas turbine engine, the main blades and splitter blades can be subject to vibratory excitation at frequencies which coincide with integer multiples, referred to as harmonics, of the radial compressor's rotational frequency. As a result of the vibratory excitation, the main blades and/or the splitter blades can undergo vibratory deflections that create vibratory stress on the blades. If the vibratory excitation occurs in an expected operating speed range of the radial compressor, the vibratory stresses can create high cycle fatigue and cracks over time.
According to the present invention, a gas turbine engine includes a radial compressor having first and second blades. The first blade has a tuned leading edge that prevents either blade from exciting at a natural frequency at speeds within an expected operating speed range.
Another embodiment includes a method for tuning a radial compressor. The method includes designing the radial compressor to have a first blade connected to a second blade having a substantially different shape from the first blade by a disc, wherein the first and second blades have first and second blade resonant modes that excite in an expected operating speed range of the radial compressor, tuning both the first and second blades by modifying mass quantity on the first blade at a primary anti-node of the first blade resonant mode, and fabricating the radial compressor as tuned.
Hub 16 can be attached to a compressor shaft of a gas turbine engine (not shown). In operation, air from a turbine inlet (not shown) can pass over leading edge 28, is compressed by blades 12 as radial compressor 10 rotates, and passes over trailing edge 30 on its way to a combustion chamber (not shown). Because operation of gas turbine engines is well known in the art, it will not be described in detail herein. However, during engine operation, various aero-excitation source frequencies can be created as air passes over components of the gas turbine engine, such as inducer or exducer vanes. Different source frequencies can be created at different operating speeds. These source frequencies are transmitted to the air, causing unsteady fluid pressure, and can then be transmitted to radial compressor 10. Radial compressor 10 can have one or more natural frequencies (also called resonance frequencies) in which one or more blades 12 and/or disc 14 will vibrate. If a natural frequency coincides with an aero-excitation source frequency, an interference can occur, causing undesired harmonic vibration. A variety of possible blade anti-nodes 34 are illustrated on free edges 26 of blades 12. Primary anti-node 35 is that with the greatest deflection of all blade anti-nodes 34 on a particular blade 12. If a particular blade 12 has two anti-nodes 34 with almost the same deflection, both can be referred to as primary anti-nodes 35, and any other anti-nodes 34 can be referred to as secondary anti-nodes 34.
For example, radial compressor 10 has a variety of natural frequencies associated with nodal diameter n that are potentially excitable at different operating speeds. However, radial compressor 10 only has two natural frequencies 56 and 58 associated with nodal diameter n that occur in the expected operating speed range. As illustrated, natural frequency 56 corresponds to splitter blade 20 and natural frequency 58 corresponds to main blade 22. It can be desirable to tune radial compressor 10 such that natural frequencies 56 and 58 excite outside of the expected operating speed range. For example, radial compressor 10 could be tuned such that natural frequencies 56′ and 58′ occur below lower bound line 54. In that case, natural frequencies 56′ and 58′ will not be excited in the expected operating speed range. Natural frequencies 56′ and 58′ could, however, be excited for a period of time as the gas turbine engine speeds up during initial startup and shutdown. Alternatively, radial compressor 10 could be tuned such that natural frequencies 56″ and 58″ occur above upper bound line 52. In that case, natural frequencies 56″ and 58″ will not be excited in the expected operating speed range nor during initial startup and shutdown.
After it is determined that splitter blade 20 has the slower blade resonant mode, location of one or more blade anti-nodes 34 of the blade resonant mode for splitter blade 20 is identified (step 110). Blade anti-nodes 34 typically occur along free edge 26, and in particular, along leading edge 28. If there is more than one blade anti-node 34 along free edge 26, one or more primary anti-nodes 35 have greater deflection than all other blade anti-nodes 34 of the blade resonant mode shape in question. In radial compressors such as radial compressor 10, one primary anti-node 35 is typically positioned along leading edge 28. Location of blade anti-nodes 34 can be determined through eigenvalue solutions, in a manner known in the art. Main blade 22 also has one or more blade anti-nodes 34, however, the present method does not involve direct tuning of these anti-nodes 34.
Next splitter blade 20 is tuned at blade anti-nodes 34 (step 112). Tuning is performed by modifying mass localized at one or more blade anti-nodes 34 on splitter blade 20. Increasing mass at blade anti-nodes 34 decreases natural frequency, and decreasing mass at blade anti-nodes 34 increases natural frequency. When mass at blade anti-nodes 34 on splitter blade 20 is reduced, its natural frequency can be increased from natural frequency 56 (shown on
Step 112 can be repeated to tune all of splitter blades 20. It can be relatively effective and efficient to modify mass only at primary anti-node 35 on leading edge 28 of each of splitter blades 20. If further tuning is desired, mass quantity can be modified at additional blade anti-nodes 34 of splitter blades 20. After tuning is complete, radial compressor 10 can have no natural frequencies that excite in the expected operating speed range. Leading edge 28 on splutter blade 20 is tuned to prevent either blade from exciting at a natural frequency at speeds within an expected operating speed range.
Some or all of steps 100-112 can be performed physically, electronically, or both. If steps 100-112 are performed electronically, radial compressor 10 can then be fabricated as electronically tuned. Radial compressor 10 can be fabricated using techniques such as forging and machining.
Splitter blade 20″ can also be modified by adding mass at tuned portion 204″. For example, mass addition can be achieved by smoothly and continuously increasing thickness of splitter blade 20″ at tuned portion 204″ from non-tuned thickness 206 to increased mass tuned thickness 208. Smooth mass modification allows for reduced aerodynamic impact and flow separation. Such a mass increase on splitter blade 20′ would reduce its natural frequency. This example corresponds to ND interference map 50 on
After splitter blade 20″ is tuned, its contour profile geometry can be optimized to reduce stress concentration while maintaining a desirable aero-constraint on an incident angle of leading edge 28″ within about 2 degrees. All of radial compressor 10 can be tuned similarly to cyclic sector 200″ such that splitter blade 20″ is one of a plurality of substantially similar tuned splitter blades. In the illustrated embodiment, thickness of leading edge 28 of main blade 22 is neither increased nor decreased. Main blade 22 need not be modified because modification to splitter blade 20″ tunes both splitter blade 20 and main blade 22. In an alternative embodiment, thickness of leading edge 28 of main blade 22 can be modified, while splitter blade 20″ remains unmodified.
It will be recognized that the present invention provides numerous benefits and advantages. For example, tuning radial compressor 10 moves natural frequencies out of an expected operating speed range and, therefore, reduces vibratory stresses and cracks in radial compressor 10. By modifying mass at primary anti-node 35, tuning can be more efficient and more effective than by modifying mass at other locations on blades 12, disc 14, or elsewhere in the gas turbine engine. Additionally, by modifying mass at leading edge 28 instead of at trailing edge 30, problems associated with mass modification at trailing edge 30 can be reduced (such as weakening the blades due to elastic deformation if trailing edge 30 is made thinner or increasing steady state stress if trailing edge 30 is made thicker). This invention can be particularly useful in applications where it is undesirable to modify mass of one of splitter blade 20 or main blade 22, since mass can be modified on the other blade to tune natural frequency of both blades.
While the invention has been described with reference to exemplary embodiments, 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 embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. For example, blades 12 and disc 14 need not be configured as specifically illustrated so long as they are part of a radial compressor that benefits from tuning as described.
Reference is made to application Ser. No. _______ entitled “RADIAL COMPRESSOR WITH BLADES DECOUPLED AND TUNED AT ANTI-NODES”, which is filed on even date and is assigned to the same assignee as this application. Reference is also made to application Ser. No. 11/958,585 entitled “METHOD TO MAXIMIZE RESONANCE-FREE RUNNING RANGE FOR A TURBINE BLADE”, filed on Dec. 18, 2007 by Loc Q. Duong, Ralph E. Gordon, and Oliver J. Lamicq and is assigned to the same assignee as this application.