Patent Description:
Impellers of compressors of aircraft engines are subjected to temperature gradients, which may limit the choice of materials used to manufacture them. Cold portions of the impellers may be prone to cold dwell whereas hot portions may be prone to creep. Materials that are resistant to cold dwell may not be able to withstand hot temperatures and may limit outlet air temperatures of the compressor. Improvements are sought.

<CIT> discloses a method for fabricating a dual titanium alloy impeller, wherein a bore sub-element is fabricated from a first titanium alloy that displays characteristics such as high strength, and a body sub-element is fabricated from a second titanium alloy that displays characteristics such as high temperature creep resistance.

In one aspect, there is provided an impeller for a centrifugal compressor as claimed in claim <NUM>. Embodiments of the invention are as claimed in the dependent claims thereof.

In another aspect, there is provided an aircraft engine as claimed in claim <NUM>.

<FIG> illustrates an aircraft engine depicted as a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases. The fan <NUM>, the compressor section <NUM>, and the turbine section <NUM> rotate about a central axis <NUM> of the gas turbine engine <NUM>.

The compressor section <NUM> includes an axial compressor <NUM>, which may include one or more stage, each including stator vanes and rotor blades. The compressor section <NUM> further includes a centrifugal compressor <NUM> located downstream of the axial compressor <NUM> relative to a flow in a core flow path <NUM> of the gas turbine engine <NUM>. The centrifugal compressor <NUM> includes an impeller <NUM> and a diffuser <NUM> located downstream of the impeller <NUM>. The axial compressor <NUM> may be referred to as a low-pressure compressor whereas the centrifugal compressor <NUM> may be referred to as a high-pressure compressor.

Typically, impellers are made of a monolithic body defining a hub and blades protruding from the hub. In use, air flows from an inlet of the impeller to an outlet and between the blades. As the air is being compressed, its temperature increases. Therefore, a front face of the impeller is exposed to colder air than a rear face of the impeller. This may create temperature gradients that may lead to low cycle fatigue issues. Moreover, portions of the impeller may operate at temperatures that is more prone to cold dwell. This may happen in regions of the impeller being below a temperature threshold, which may be about <NUM> Fahrenheit (<NUM>). Therefore, in an embodiment not according to the claimed invention, the impeller may be made entirely of a material that is resistant to cold dwell. This material may be, for instance, Ti-<NUM>.

Some titanium alloys may be susceptible to cold dwell because of subsurface micro texture region (MTR). Failure of the impeller may be exasperated when a critical component undergoes long periods of dwell below of certain temperature threshold (e.g., <NUM> Fahrenheit (<NUM>)). Within titanium alloys, Ti-<NUM> may be the least susceptible to cold dwell and Ti-<NUM> may be the most prone to cold dwell. Hence, to avoid cold dwell, impellers could be manufactured out of titanium alloys, such as Ti-<NUM>.

However, this material may be susceptible to creep above a given temperature threshold, which may be about <NUM> Fahrenheit (<NUM>). Hence, Ti-<NUM> may limit a temperature of the air that exits the impeller. In some situations, it may be desired to inject air at a higher temperature in the combustor <NUM> since this may lead to efficiency and performance gains. A solution may be to use another material, such as Ti-<NUM> that is able to withstand higher temperatures without creeping. However, as explained above, Ti-<NUM> may be susceptible to cold dwell. Thus, colder regions of the impeller may present cold dwell issues during use if such colder regions are manufacture out of Ti-<NUM>.

In the context of the present disclosure, "creep" corresponds to a tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It may occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep may be more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point. The rate of deformation is a function of the material's properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function. Creep is usually of concern when evaluating components that operate under high stresses and/or high temperatures. Unlike brittle fracture, creep deformation does not occur suddenly upon the application of stress. Instead, strain accumulates as a result of long-term stress. Therefore, creep is a "time-dependent" deformation.

In the context of the present disclosure, "cold dwell" refers to the reduction in the fatigue life-time of a component as a result of exposing the component to a constant high mean stress during cruising, between the ramping up of the load, during take-off for instance, and the ramping down of the load, on landing for instance. The "cold" of cold dwell fatigue refers to the fact that this phenomenon may happen at temperatures of around <NUM> degrees Celsius or less, in a relatively cold part of the impeller. Cold dwell fatigue complexity raises a host of fundamental questions about plasticity, creep and fracture in titanium and its alloys. Many metallurgical factors may affect cold dwell fatigue: alloy composition: the most susceptible Ti alloys are those containing high volume fractions of the alpha (HCP) phase and low volume fractions of the beta (BCC) phase); microstructure: the most susceptible alloys contain clusters of alpha grains that have small misorientations between them (microtexture); duration of the dwell and the loading during the dwell; creep; fracture morphology: the crack initiates below the surface of the specimen and consists of facets almost parallel to the basal plane of the alpha phase.

Referring now to <FIG>, the impeller <NUM> is shown in greater detail. The impeller <NUM> may at least partially alleviate the aforementioned drawbacks. The impeller <NUM> may be made of two bodies <NUM>, <NUM> each made of a respective material. A first one of these two materials is selected for its creep resistance and a second one of these two materials is selected for its cold dwell resistance.

The impeller <NUM> has a front face 30A and a rear face 30B. The impeller <NUM> has an inlet 30I and an outlet 30O. The front face 30A is proximate the inlet 30I and the rear face 30B is proximate the outlet 30O. In use, the front face 30A is facing a front cavity 31A and the rear face 30B is facing a rear cavity 31B. A temperature of the air in the front cavity 31A is less than that in the rear cavity 31B. Typically, the air in the rear cavity 31B and being in contact with the rear face 30B of the impeller <NUM> may be at about <NUM> Fahrenheit (<NUM>) and above. Hence, thermal gradients may be present within the impeller between the front face 30A and the rear face 30B.

The impeller <NUM> includes an outer hub body <NUM> including a first material. In some embodiments, the outer hub body <NUM> may be made entirely of the first material. The outer hub body <NUM> extends around the central axis <NUM>. The outer hub body <NUM> may extend annularly around a full circumference around the central axis <NUM>. The outer hub body <NUM> defines a gaspath face 32A that extends from the inlet 30I to the outlet 30O of the impeller <NUM>. The gaspath face 32A extends radially away from the central axis <NUM> from the inlet 30I to the outlet 30O such that the flow deviates away from the central axis <NUM> as it is being compressed. The outer hub body <NUM> may define a bore 32B for receiving a shaft of the gas turbine engine <NUM>. The outer hub body <NUM> may define a recess 32C that extends axially between the front face 30A and the rear face 30B.

The impeller <NUM> includes blades <NUM> protruding from the gaspath face 32A and circumferentially distributed around the central axis <NUM>. The blades <NUM> extends from roots 33A at the gaspath face 32A of the outer hub body <NUM> to tips 33B and from leading edges 33C at the inlet 30I to trailing edges 33D at the outlet 30O. The blades <NUM> have opposed pressure and suction sides. Flow passages are defined between the blades <NUM>. The flow passages may curve from extending substantially axially at the inlet 30I to extending substantially radially at the outlet 30O.

The impeller <NUM> further includes an inner hub body <NUM> including a second material. In some embodiments, the inner hub body <NUM> may be made entirely of the second material. The inner hub body <NUM> extends around the central axis <NUM>. The inner hub body <NUM> may extend annularly a full circumference around the central axis <NUM>. The inner hub body <NUM> is received within the recess 32C such that the outer hub body <NUM> axially overlaps the inner hub body <NUM> and that the outer hub body <NUM> extends around the inner hub body <NUM>. The inner hub body <NUM> is secured to the outer hub body <NUM>. Thermal gradients in the impeller <NUM> may be lower than one-piece impellers. Therefore, the outer hub body <NUM> may be operating above cold dwell threshold and inner hub body <NUM> may be operating below cold dwell threshold.

In the embodiment shown, the second material of the inner hub body <NUM> is more cold dwell resistant than the first material of the outer hub body <NUM>. The first material of the outer hub body <NUM> may be more creep resistant than the second material of the inner hub body <NUM> at an operational temperature of the impeller <NUM>. The first material may be, for instance, Ti-<NUM> and the second material may be, for instance, Ti-<NUM>. Therefore, the impeller <NUM> uses the first material, which may be Ti-<NUM>, for the outer hub body <NUM> thereby allowing higher outlet air temperatures and uses the second material, which may be Ti-<NUM>, for the inner hub body <NUM>, which may be less prone to cold dwell than the first material. Thus, each of the hot and cold portions of the impeller <NUM>, corresponding respectively to the outer hub body <NUM> and the inner hub body <NUM>, is made of a material able to resist a specific phenomenon associated with these temperatures, that is, the creep and the cold dwell.

Cold dwell is a phenomenon that, as explained above, arises with some engine parts exposed to colder engine temperatures, typically below about <NUM> Fahrenheit (<NUM>). Hence, even if a given material is susceptible to cold dwell, it may not be a problem if said material is being used in a location of the gas turbine engine where its temperature is expected to be more than <NUM> Fahrenheit (<NUM>). Creep, as explained above, is exacerbate with an increase in temperature. Therefore, the first material of the outer hub body <NUM> would be susceptible to cold dwell if it were used for the inner hub body <NUM>, but, since it is used for the outer hub body <NUM> being, in operation, at a temperature higher than about <NUM> Fahrenheit (<NUM>), cold dwell may not be an issue.

Still referring to <FIG>, the inner hub body <NUM> defines a major portion, that is at least half, of the front face 30A of the impeller <NUM>. According to the claimed invention, an entirety of the rear face 30B is defined by the outer hub body <NUM>. Hence, the inner hub body <NUM> of the impeller <NUM> may be free of exposure to the air flowing within flow passages defined between the blades <NUM> and along the gaspath face 32A of the outer hub body <NUM>. The inner hub body <NUM> may be further free of exposure to the air in the rear cavity 31B located rearward of the impeller <NUM>. Therefore, in use, the temperature of the inner hub body <NUM> may remain below <NUM> Fahrenheit (<NUM>), where it may not be susceptible to creep, and a temperature of the outer hub body <NUM> may remain above <NUM> Fahrenheit (<NUM>), where it may not be susceptible to cold dwell.

It will be appreciated that, in some embodiments, all parts of the impeller <NUM> located radially inwardly of a radial position of the gaspath face 32A at the inlet 30I of the impeller <NUM> may be defined by the inner hub body <NUM>.

An annular gap <NUM> is defined between an inner face 32D of the outer hub body <NUM> and an outer face 34A of the inner hub body <NUM>. The inner face 32D oriented toward the inner hub body <NUM> and the outer face 34A being oriented toward the outer hub body <NUM>. This annular gap <NUM> may contain air and may help in limiting heat from being transferred from the outer hub body <NUM> to the inner hub body <NUM>. As shown, the annular gap <NUM> extends radially towards the central axis <NUM> as the annular gap <NUM> extends axially toward the rear face 30B and axially toward the outlet 30O of the impeller <NUM>. This may have the effect of maximising the area of the front face 30A of the impeller <NUM> that is defined by the inner hub body <NUM> and to maximize the area of the rear face 30B of the impeller <NUM> that is defined by the outer hub body <NUM>.

The inner hub body <NUM> is secured to the outer hub body <NUM> at two attachment locations, which include a fore attachment location 36A and a rear attachment location 36B, which may be axially offset from the fore attachment location 36A relative to the central axis <NUM>. In some cases, more or less than two attachment locations may be used. As shown, the inner hub body <NUM> is free of contact with the outer hub body <NUM> between the two attachment locations 36A, 36B. The two attachment locations 36A, 36B are herein axially and radially offset from one another. The fore attachment location 36A is located proximate the front face 30A of the impeller <NUM> and located radially outwardly of the rear attachment location 36B, which is located proximate the rear face 30B of the impeller <NUM>.

The fore and rear attachment locations 36A, 36B include each a tongue-and-groove connection between the outer hub body <NUM> and the inner hub body <NUM>. More specifically, the outer hub body <NUM> defines a fore groove 32E and a rear groove 32F whereas the inner hub body <NUM> defines a fore tab 34B and a rear tab 34C. The fore tab 34B is received within the fore groove 32E and the rear tab 34C is received within the rear groove 32F. Tight fit engagements may be defined between the fore and rear tabs 34B, 34C and the outer hub body <NUM>. More specifically, the fore and rear tabs 34B, 34C may be biased radially inwardly against the outer hub body <NUM> to limit the outer hub body <NUM> from moving radially outwardly relative to the inner hub body <NUM>. In other words, radially inner faces of the fore and rear tabs 34B, 34C, which face a radially inward direction, may be in a tight fit engagement against radially outer faces of the outer hub body <NUM>; the radially outer faces facing a radially outward direction toward the fore and rear grooves 32E, 32F. It will be appreciated that, alternatively, the tabs may be defined by the outer hub body <NUM> and the grooves may be defined by the inner hub body <NUM>. Any other suitable means may be used to secure the outer hub body <NUM> to the inner hub body <NUM> such as, for instance, keyway engagement, dog and slot, fasteners, threaded connections, and so on.

The disclosed impeller <NUM> may not exhibit a trihub failure mode as those of one-piece impellers. The reason is that any cracks emanating from the outer hub body <NUM>, typically from the rear face 30B and towards a bore 34D of the inner hub body <NUM> or from the bore 34D of the inner hub body <NUM> toward the rear face 30B may be stopped at the interface (e.g., annular gap <NUM>) of the two pieces. If failure of the outer hub body <NUM> at the rear face 30B were to occur, it would lead to fragments smaller than one-piece trihub. If failure of the inner hub body <NUM> were to occur at the bore 34D, unlike the one-piece impeller, it would be contained by the outer hub body <NUM> since the outer hub body <NUM> extends around and contains the inner hub body <NUM>. Moreover, due to superior hot creep properties of the first material of the outer hub body <NUM>, the chance of creep-fatigue interaction (CFI) may be reduced for a given compressor outlet temperature compared to a one-piece impeller. Therefore, the impeller <NUM> may address both the cold dwell and CFI. This may allow to open design space for aircraft engines with higher compressor outlet temperatures.

Claim 1:
An impeller (<NUM>) for a centrifugal compressor (<NUM>), the impeller (<NUM>) being rotatable about a central axis (<NUM>), comprising:
a front face (30A) and a rear face (30B) axially rearward of the front face (30A), the rear face (30B) extending radially outwardly relative to the central axis (<NUM>) from a bore (32B) of the impeller (<NUM>);
an outer hub body (<NUM>) including a first material and extending around the central axis (<NUM>), the outer hub body (<NUM>) defining a gaspath face (32A) extending from an inlet (30I) to an outlet (<NUM>), the gaspath face (32A) extending radially away from the central axis (<NUM>) from the inlet (30I) to the outlet (<NUM>);
blades (<NUM>) protruding from the gaspath face (32A) and circumferentially distributed around the central axis (<NUM>); and
an inner hub body (<NUM>) extending around the central axis (<NUM>), the inner hub body (<NUM>) being secured to the outer hub body (<NUM>), the outer hub body (<NUM>) axially overlapping and extending around the inner hub body (<NUM>), the inner hub body (<NUM>) made of a second material being more cold dwell resistant than the first material,
wherein the inner hub body (<NUM>) has an outer face (34A) facing an inner face (32D) of the outer hub body (<NUM>); and
the impeller (<NUM>) comprises an annular gap (<NUM>) between the outer face (34A) and the inner face (32D),
characterised in that:
an entirety of the rear face (30B) is defined by the outer hub body (<NUM>); and
the annular gap (<NUM>) extends radially towards the central axis (<NUM>) as the annular gap (<NUM>) extends axially toward the outlet (<NUM>).