Impeller containment system

[0001] The application relates generally to rotor containment structures and, more particularly, to a compressor impeller containment system.

[0002] Aviation regulations require engine manufacturers to demonstrate fragments containment following a compressor impeller tri-hub burst event. While known impeller containment structures have various advantages, there is still room in the art for improvement.

[0003] In one aspect, there is provided a compressor impeller containment system comprising: an impeller mounted for rotation about an axis, the impeller having an impeller hub and impeller blades projecting radially outwardly from the impeller hub; and a catcher ring surrounding the impeller hub at an axial location spaced from the impeller blades, the catcher ring having a radial thickness R1 defined between a radially inner diameter surface and a radially outer diameter surface, the catcher ring further having a radially inner hollow portion extending from the radially inner diameter surface to an intermediate radial location between the radially inner diameter surface and the radially outer diameter surface, the radially inner hollow portion defining an internal cavity extending circumferentially around the axis, and a radially outer solid portion extending radially outwardly along a radial thickness R2 from the intermediate radial location to the radially outer diameter surface, wherein ¾ R1≥R2≥½ R1.

[0004] In another aspect, there is provided an auxiliary power unit comprising: a compressor including an impeller mounted for rotation about an axis, the impeller having an impeller hub and impeller blades projecting radially outwardly from the impeller hub; and a catcher ring disposed on a backside of the impeller blades directly around the impeller hub, the catcher ring having a radially outer diameter surface and a radially inner diameter surface, the radially inner diameter surface disposed radially adjacent to a radially outer surface of the impeller hub, the radially inner diameter surface and the radially outer diameter surface defining therebetween a radial thickness R1 of the catcher ring, and wherein the catcher ring has a hollow body portion extending along at most one third of the radial thickness R1 of the catcher ring as measured from the radially inner diameter surface, and a solid body portion extending radially outwardly from the hollow body portion to the radially outer diameter surface.

[0005] In a further aspect, there is provided a compressor impeller hub containment ring comprising: a sheet metal inner ring component concentrically mounted inside a forged outer ring component, the sheet metal inner ring component having an open cross-section, the forged outer ring component having a solid cross-section, the forged outer ring component closing the open cross-section of the sheet metal inner ring and cooperating therewith to form an internal cavity around an inner circumference of the compressor impeller hub containment ring.

[0010] FIG. 1 illustrates a turboshaft engine 10 suitable for use as an auxiliary power unit (APU) of an aircraft. The engine 10 generally comprises in serial flow communication, a compressor section 12 for pressurizing the air, a combustor 14 in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section 16 for extracting energy from the combustion gases.

[0011] The engine 10 in this example can be seen to include a high pressure spool 18, including a compressor impeller 19 and a high-pressure turbine 20, and a low pressure spool 22, including a low-pressure turbine 24. The low pressure spool 22 leads to a power shaft via a gear arrangement. The high pressure spool 18 can be refer to herein as a compressor spool and the low pressure spool 22 can be referred to herein as the power spool.

[0012] The exemplified compressor impeller 19 is provided in the form of a centrifugal impeller mounted for rotation about the engine axis A. The impeller 19 comprises an annular hub 19a and a set of circumferentially distributed impeller blades 19b integrally projecting from the hub 19a for compressing incoming air as the blades 19b rotate with the hub 19a. An impeller shroud 25 has a curved portion that closely contours a shape of the blades 19b. The shroud 25 and the impeller blades 19b are configured for directing the compressed air radially outwardly into a diffuser 27 surrounding the tip of the impeller blades 19b.

[0013] Generally, engine rotors, such as the compressor impeller 19, are limited by fatigue strength. Consequently, their burst speeds can be considerably higher than operating speeds. When a rotor bursts, the fragments retain virtually all the rotor's original rotational energy. Each fragment now has two components of energy: a rotational component and a translational component. In practice, a rotor will break from a single failure origin, often from a fault in the bore of the annular hub where the stress is often the maximum. The exact fracture mode is unpredictable and can result in fragments of various sizes and shapes. The theoretical configuration which produces the maximum proportion of energy, and therefore the most dangerous configuration, is a failure which produces three equal weight pieces. Therefore, this is the mode usually prescribed for testing, and it is known as a “tri-hub failure”. Testing are usually achieved by deliberately cutting equally spaced slots in the hub to thereby weaken it to the point where it bursts at, or marginally above, the maximum operating speed. The tri-hub failure mode has become a standard for testing. Engine manufacturers have to demonstrate by testing that the containment structure around the compressor impeller is strong enough to absorb the energy of the three parts when the impeller breaks apart during such a test.

[0014] Heretofore, rotor containment structures have been axially long and radially thick such that their cross-sections have been massive relative to adjacent normal engine structure. A common assumption among designers is that the containment rings circumscribing engine rotors must have as much weight as possible to absorb the energy of the burst fragments by deflecting and expanding. However, as will be seen hereafter, the inventors have found that it is possible to minimize the weight of an impeller containment structure without negatively affecting its energy absorption capacity by introducing a compliant hollow portion in the radially inner half portion of a catcher ring surrounding the hub of the impeller.

[0015] Referring to FIG. 1, it can be seen that the containment system of the impeller 19 comprises a catcher ring 30 for protecting engine components from tri-hub burst. The catcher ring 30 is made of strong material such as Inconel® 625 steel, titanium or the like. The catcher ring 30 surrounds the impeller hub 19a at an axial location spaced from the impeller blades 19b. For instance, the catcher ring 30 may be axially positioned at the downstream end of the impeller hub 19a on the back side of the impeller blades 19b and in closed radial proximity to a cylindrical neck portion 19a′ of the impeller hub 19a (i.e., the radially inner diameter surface of the catcher ring 30 is directly radially next to the radially outer surface of the neck portion 19a′ of the impeller hub 19a). Such close proximity is intended to restrict the motion of any burst fragments before they gain too much kinetic energy. In so doing, the entire volume of the containment structure is utilized in the containment process, wherein in applications where the containment structure is remote from the respective hub, the translational impacts are on localized regions of the structure so that the structure is unevenly loaded and the material is less efficiently utilized. The catcher ring 30 is coaxially supported around the impeller hub 19a by any appropriate structural components of the engine 10. For instance, the catcher ring 30 may be welded or otherwise suitably mounted to a hairpin connector (not shown) which is, in turn, bolted or otherwise suitably mounted to a structural case of the engine 10.

[0016] FIG. 2 illustrates a cross-section of the catcher ring 30. It can be appreciated that the catcher ring 30 has a total radial thickness R1 extending from a radially inner diameter surface 30a to a radially outer diameter surface 30b. The catcher ring 30 is characterized by a radially inner hollow portion 30c circumscribed by a radially outer solid portion 30d. The radially inner hollow portion 30c extends radially from the radially inner diameter surface 30a to an intermediate location 30e between the radially inner diameter surface 30a and the radially outer diameter surface 30b. The radially inner hollow portion 30c defines an internal cavity 30f extending circumferentially continuously around the central axis of the catcher ring 30. According to some embodiments, the internal cavity 30f has a constant section along its circumference. However, it is understood that the cross-section of the cavity 30f may vary along its circumference. The radially outer solid portion 30d extends radially from the intermediate location 30e to the radially outer diameter surface 30b. Analytical results have shown that with such a hollow ring configuration, it is possible to reduce the mass of the catcher ring 30 by about 25% while virtually preserving the same energy absorption capacity as that of a solid ring having the same outline dimensions. Indeed, the radially inner hollow portion 30c may be configured to plastically deform under the impact of the tri-hub burst fragments, thereby absorbing energy. The burst fragments hitting the radially inner diameter surface 30a of the catcher ring 30, introduce a high hoop or circumferential stress on the radially inner hollow portion 30c of the catcher ring 30, thereby causing the radially inner hollow portion 30c to deform and, thus, absorb energy. In contrast, the solid radially inner portion of conventional solid rings tend to fracture under the impact of the burst fragments, thereby preventing the radially inner portion of solid rings to fully contribute to the energy absorption process. By removing some of the mass from the radially inner half of the catcher ring 30 to form a hollow compliant portion, it may be possible to initially absorb more energy via radial deflection than it would be feasible via fracture. Once fractured, the radially inner hollow portion 30c looses its capacity to resist against the hoop energy of the burst fragments. However, the radially outer solid portion 30d is configured to resist against the remaining hoop energy of the burst fragments amortized by the radially inner hollow portion 30c.

[0017] As can be appreciated from FIG. 2, the radially outer solid portion 30d has a radial thickness R2. According to some embodiments, the radial thickness R2 is more than half the total radial thickness R1 of the catcher ring 30 and equal or less than three quarter of the total radial thickness R1. It has been found that this range of radial thickness ratios allows to reduce the weight of the catcher ring 30 without negatively affecting the energy absorption capacity thereof. According to some engine applications, satisfactory results have been obtained with a radial thickness of the radially inner hollow portion 30c equal to or less than about one third of the total radial thickness R1 (that is with R2≥⅔ R1). Still according to some embodiments, the thickness T1 of the wall of the radially inner hollow portion 30c is less than about ⅛ of the axial thickness T2 of the radially outer solid portion 30d as defined between opposed front and back faces of the catcher ring 30. Still referring to FIG. 2, it can be appreciated that the cross-section area A1 of the internal cavity 30f is less than the cross-section area A2 of the radially outer solid portion 30d of the exemplified catcher ring 30. According to some embodiments, the cross-section area A1 of the internal cavity 30f is less than half the cross-section area A2 of the radially outer solid portion 30d. According to some embodiments, some or all of the above relative parameters between the radially inner hollow portion 30c and the radially outer solid portion 30d of the catcher ring 30 may be combined to optimize weight savings without negatively affecting the containment functionality of the catcher ring 30.

[0018] Referring now to FIG. 3, it can be appreciated that the catcher ring 30 may consist of an assembly of two separately manufactured components. For instance, according to some embodiments, the catcher ring 30 may comprise a sheet metal inner ring component 32 concentrically mounted inside a forged outer ring component 34, the sheet metal inner ring component 32 and the forged outer ring component 34 respectively forming the radially inner hollow portion 30c and the radially outer solid portion 30d of the catcher ring 30.

[0019] The sheet metal inner ring component 32 may be rolled formed into an open cross-section ring. According to the illustrated embodiment, the rolled sheet metal inner ring component 32 has a generally U-shaped cross-section and includes a curved bridging portion 32a between a front leg portion 32b and a back leg portion 32c. The curved bridging portion 32a provides for a convex surface at the radially inner diameter of the catcher ring 30 adjacent to the impeller hub 19a. Still referring to FIG. 3, it can be appreciated that the back leg portion 32c extends axially away from the front leg portion 32b as the back leg portion 32c extends radially outwardly from the bridging portion 32a. This provides a radially inner annular surface that flares radially outwardly as it extends axially in a rearward direction. Such a surface profile at the radially inner diameter of the catcher ring is designed to provide a uniform gap between the catcher inner diameter and the impeller neck 19a′.

[0020] The sheet metal inner ring component 32 is welded at its outer diameter to the inner diameter of the forged outer ring component 34. As schematically illustrated in FIG. 2, the radially inner and the radially outer components 32, 34 may be welded or brazed along a weld line provided at the intermediate location 30e. Once so joined to the outer diameter of the sheet metal inner ring component 32, the forged outer ring component 34 closes the open cross-section of the sheet metal inner ring component 32 and cooperate therewith to form the internal cavity 30f. As can be appreciated from FIG. 2, the resulting internal cavity 30f has a tapering profile in a radially inward direction. The inner and outer profiles of the radially inward tapering cavity are similar to provide a uniform thickness. It is understood that the thickness is adjusted based on detailed calculations/analyses for each specific engine.

[0021] Referring back to FIG. 3, it can be appreciated that the forged outer ring component 34 has a solid rectangular cross-section. However, it is understood that the forged outer ring component 34 could have other cross-section shapes.

[0022] The operation of the catcher ring 30 is as follows. In the unlikely event of an impeller hub failure, the impeller hub 19a will tend to burst away from its associated drive shaft in a rearward and radially outward direction. The burst impeller fragments will immediately hit the inner diameter surface 30a of the closely surrounding catcher ring 30. As a result, the radially inner hollow portion 30c of the catcher ring 30 will radially deform, thereby absorbing energy. The inner hollow portion 30c will deflect until fracture. After the failure of the radially inner hollow portion 30c, the radially outer solid portion 30d of the catcher ring 30 will continue to resist against hoop energy to contain the burst fragments and, thus, mitigate damages on the structural parts surrounding around the impeller 19.

[0023] As can be appreciated from the foregoing description, the material cross-section required to provide the necessary shear and hoop strength to contain the burst can be minimized to reduce weight, while still maintaining a sufficient factor of safety for protection of the engine and aircraft systems and structure. This can be generally achieved by absorbing the initial energy of the burst by plastic deformation of a compliant hollow portion in the radially inner half of the catcher ring 30.

[0024] According to at least some embodiments, the hollow catcher ring is designed to minimize added material and thus added weight to the engine while still effectively protecting engine components from tri-hub burst.

[0025] It is noted that various connections are set forth between elements in the preceding description and in the drawings. It is noted that these connections are general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. A coupling between two or more entities may refer to a direct connection or an indirect connection. An indirect connection may incorporate one or more intervening entities. The term “connected” or “coupled to” may therefore include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements).

[0026] While the description may present method and/or process steps as a particular sequence, it is understood that, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the description should not be construed as a limitation.

[0027] Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. As used herein, the terms “comprises”, “comprising”, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. As used herein, the term “about” is intended to allow for a 10% variation of the associated numerical values.

[0028] While various aspects of the present disclosure have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the present disclosure. For example, the present disclosure as described herein includes several aspects and embodiments that include particular features. Although these particular features may be described individually, it is within the scope of the present disclosure that some or all of these features may be combined with any one of the aspects and remain within the scope of the present disclosure. References to “various embodiments,” “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. The use of the indefinite article “a” as used herein with reference to a particular element is intended to encompass “one or more” such elements, and similarly the use of the definite article “the” in reference to a particular element is not intended to exclude the possibility that multiple of such elements may be present.

[0029] The embodiments described in this document provide non-limiting examples of possible implementations of the present technology. Upon review of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the present technology. For example, it is understood that while the containment system has been described in the context of an auxiliary power unit engine, it is understood that similar principles could be utilized in other types of engines, pump, fans, etc., that include a compressor impeller. Other applications of this impeller containment system, such as in power generators used on land vehicles or in motors utilized in non-aerospace applications, are considered to be within the scope of the present application. Other applications such as would be recognized by the person of ordinary skill in the art are considered to be within the scope of the present disclosure. Yet further modifications could be implemented by a person of ordinary skill in the art in view of the present disclosure, which modifications would be within the scope of the present technology.