Patent Description:
Centrifugal compressors include an impeller and a diffuser downstream therefrom. Pressure at impeller exit may vary circumferentially due to pressure difference between pressure/suction sides of the impeller blades, and due to the turbulent nature of the flow as it travels downstream, especially after the bend area of the impeller. This may set up a circumferentially varying pattern of flow distortion that may degrade performance of both upstream impeller and downstream diffuser, which is undesirable.

<CIT>, <CIT> disclose prior art gas turbine engines. <CIT> discloses a prior art centrifugal compressor.

In one aspect and according to the claimed invention there is provided a gas turbine engine for an aircraft as set forth in claim <NUM>.

The gas turbine engine as defined herein may also include, in whole or in part and in any combination, one or more of the following features:.

In another aspect, there is provided a method for operating a centrifugal compressor of a gas turbine engine as set forth in claim <NUM>.

<FIG> illustrates an exemplary gas turbine engine <NUM> of a type preferably provided for use in subsonic flight. The exemplary gas turbine engine <NUM> as shown is a turbofan, 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. Also shown is a central longitudinal axis <NUM> of the engine <NUM>. Even though the following description and accompanying drawings specifically refer to a turbofan engine as an example, it is understood that aspects of the present disclosure may be equally applicable to other types of combustion engines in general, and other types of gas turbine engines in particularly, including but not limited to turboshaft and turboprop turbine engines, auxiliary power units (APU), and the like.

The compressor section <NUM> of the engine <NUM> includes one or more compressor stages, at least one of which includes a centrifugal compressor 14A. The centrifugal compressor 14A has a main flow passage defined therethrough and includes a rotating impeller <NUM> and a downstream diffuser <NUM>. The impeller <NUM> is mounted for rotation within an outer shroud <NUM> about the central longitudinal axis <NUM>. The impeller <NUM> may draw air axially, and rotation of the impeller <NUM> may increase the velocity and build pressure within a main gas flow as the main gas flow is directed though the diffuser <NUM>, to flow out in a radially outward direction under centrifugal forces.

Referring to <FIG>, the diffuser <NUM> is positioned immediately downstream of the exit of a rotating component of the compressor, which in the exemplary embodiment is the impeller <NUM>. The diffuser <NUM> may form a fluid connection between the impeller <NUM> and the combustor <NUM>, thereby allowing the impeller <NUM> to be in serial flow communication with the combustor <NUM>. The diffuser <NUM> may redirect the radial flow of the main gas flow exiting the impeller <NUM> to an annular axial flow for presentation to the combustor <NUM>. In some embodiments of the gas turbine engine <NUM>, the diffuser <NUM> may include vanes (not shown) downstream of the impeller exit by which the radial flow leaving the impeller <NUM> may exit the diffuser <NUM> and be led toward the next compressor stage or to the combustor <NUM>. In other embodiments of the gas turbine engine <NUM>, the diffuser may include one or more fishtail diffuser pipes directing the flow downstream of the impeller <NUM> to exit the diffuser <NUM>. The diffuser <NUM>, with or without vanes, may also reduce the velocity and increase the static pressure of the main gas flow when it is directed therethrough.

With continued reference to <FIG>, the diffuser <NUM> includes an annular diffuser body <NUM> mounted about the impeller <NUM>. The diffuser body <NUM> forms the corpus of the diffuser <NUM> and provides the structural support required to resist the loads generated during operation of the centrifugal compressor 14A. According to the claimed invention, the diffuser body <NUM> forms an annular diffuser ring extending circumferentially about the impeller exit 15B, and may have a vaned, vane-less, or semi-vaned space. The diffuser body <NUM> is mounted about a circumference of the compressor or impeller exit 15B so as to receive the main gas flow therefrom.

As shown, the impeller <NUM> includes impeller blades 15A extending from an impeller hub and having an axial bend extending radially outwardly with respect to the central longitudinal axis <NUM>, which allow the axial main gas flow upstream of the impeller <NUM> to be directed radially outwardly away from the central longitudinal axis <NUM>. The impeller <NUM> defines a shroud side, which corresponds to a region of the impeller <NUM> circumferentially surrounding the impeller blades 15A, and an opposed hub side, which is located downstream of the impeller <NUM>, at the impeller back plate side. For sake of clarity, the reader is referred to <FIG>, which shows a shroud side and an opposed hub side, respectively shown on the left side and right side of the illustrated example of centrifugal compressor 14A.

The impeller blades 15A each have a pressure side and a suction side, named as such with reference to the pressure differential between the gas flow pressure to the fore of the blades 15A versus the aft of the blades 15A caused by rotation of the impeller <NUM> and fluid interaction with the main gas flow. This may set up a circumferentially varying pattern of flow distortion at the impeller exit 15B, which is defined downstream of the impeller blades 15A, in other words at the exducer of the impeller <NUM> adjacent the tip of the impeller <NUM>, or more specifically the tip of the blades 15A of the impeller <NUM>. A pressure differential may occur between the pressure side and the suction side of the impeller blades 15A. As such, the pressure at impeller exit 15B may vary circumferentially due to a pressure difference between the pressure side and the suction side of the impeller blades 15A during rotation. This may create flow turbulence of the main gas flow travelling through the impeller <NUM>, which may build along the impeller blades 15A, and more particularly after the bend area 15C of the blades 15A. This flow distortion may degrade performance of the gas turbine engine <NUM> as a whole and/or more specifically upstream of the impeller <NUM> and downstream of the diffuser <NUM>.

According to the claimed invention, such as shown in <FIG> and <FIG> for example, a cavity <NUM> is disposed on the shroud side of the impeller <NUM>, on one side of a main flow passage wall separating the main flow passage from the cavity <NUM>, where the main flow passage wall is located adjacent the impeller exit 15B. According to the claimed invention the diffuser body <NUM> forms an annular ring and the cavity <NUM> is circumscribed by the annular ring and an adjacent portion of the outer shroud <NUM>. In an embodiment, such as shown at least in <FIG>, the diffuser body <NUM> defines a radially outward peripheral wall of the cavity <NUM>, and the outer shroud <NUM> defines a radially inward peripheral wall of the cavity <NUM>. In other embodiments not covered by the claimed invention, the cavity <NUM> may be an internal cavity defined solely in the diffuser body <NUM> and/or in the outer shroud <NUM>. The cavity <NUM> may have any suitable internal volume and/or shape.

The cavity <NUM> is in fluid communication with the main flow passage exiting the impeller <NUM>, in other words at the impeller exit 15B, via at least one (i.e. one or more) apertures <NUM>, as will be seen. Flow enters and exits the cavity <NUM> via the same flow passage(s) defined through the one or more apertures <NUM>, which in the present embodiment comprise a series of apertures <NUM> defined through the main flow passage wall and extending between the cavity and the main flow passage. The cavity <NUM> is an annular chamber that is closed but for the apertures <NUM>, such that the apertures <NUM> provide the sole fluid connection to the closed chamber.

In an embodiment, the apertures <NUM> are circumferentially equally spaced apart about the impeller <NUM>. This may be different in other embodiments, where, for instance, the apertures <NUM> may be unevenly distributed along the circumference of the impeller <NUM>. In an embodiment, the centrifugal compressor 14A includes a number of apertures <NUM> at least more than half the number of impeller blades 15A (rounded up), including the blades 15A within an exducer portion and extending to the impeller exit 15B. This may balance the flow exchange between the cavity <NUM> and the main flow passage between the suction side of a blade 15A and the opposite pressure side of an adjacent one of the blades 15A over the circumference of the impeller <NUM>.

In an embodiment, such as shown in <FIG>, the series of apertures <NUM> are defined at an interface between the outer shroud <NUM> and the diffuser body <NUM>. In other words, the outer shroud <NUM> and the diffuser body <NUM> mate at a common edge, where they contact each other between circumferentially adjacent apertures <NUM>. At such interface between the diffuser body <NUM> and the outer shroud <NUM>, the common edge of the outer shroud <NUM> and the diffuser body <NUM> form respective radially inward and radially outward wall of the apertures <NUM>. In other embodiments, such as shown in <FIG>, the series of apertures <NUM> are defined through a portion of the diffuser body <NUM>, and provide fluid communication between the main flow passage at the impeller exit 15B and the cavity <NUM>. In such case, the main flow passage wall through which the apertures <NUM> are defined is part of the diffuser body <NUM>. In other embodiments, such as shown in <FIG>, the series of apertures <NUM> are defined through a portion of the outer shroud <NUM>. In such case, the main flow passage wall through which the apertures <NUM> are defined is part of the outer shroud <NUM>. This may depend on the location of the cavity <NUM> (within the shroud <NUM> or within the diffuser body <NUM>).

The apertures <NUM> are located downstream of the impeller <NUM>, adjacent the impeller exit 15B. The apertures <NUM> allow bidirectional flow communication between the cavity <NUM> and the main flow passage at the tip of the impeller blades 15A. In other words, flow may enter and exit the cavity via the apertures <NUM> in an alternating sequence as the impeller <NUM> rotates. More particularly, as the impeller <NUM> rotates, a portion of the fluid of the main flow passage at the impeller exit 15B enters the cavity <NUM> and reduces in velocity. The pressure field inside the cavity <NUM> may thus be different than that of the flow just outside the cavity <NUM>. This pressure difference drives the flow in and out the cavity <NUM>. During rotation of the impeller <NUM>, there are flow separation regions at the impeller exit 15B that can occur, which may be caused by the pressure differential from the suction side to the pressure side of the blades 15A. These flow separation regions have low pressure and rotate around with the impeller <NUM>. When they encounter higher pressure from inside the cavity <NUM>, fluid inside the cavity <NUM> flows out from the cavity <NUM>, which helps to rebalance the pressure in and/or "reenergize" these low pressure flow regions. Then, as fluid leaves the cavity <NUM> through the apertures <NUM>, the flow pressure inside the cavity <NUM> will drop, which will result in having a higher pressure flow in the main flow passage from outside the cavity <NUM> to re-enter the cavity <NUM>. This bidirectional flow cycle entering and leaving the cavity <NUM> may thus continue so long as the impeller <NUM> rotates and generate sufficient pressure differential between the pressure side and suction side of the blades 15A to provide such flow distortion. Thus, a constant momentum exchange between the fluid inside the cavity <NUM> and the main flow passage occurs during operation. In some embodiments, as these regions of flow separation reenergized via the fluid from the cavity <NUM> are sucked out from the cavity <NUM> through the apertures <NUM>, the flow condition into the diffuser <NUM> downstream the impeller exit 15B has a more uniform flow stream. Circumferential/axial flow distortions originating from the impeller <NUM> are therefore damped before flowing through the diffuser <NUM>.

In operation, the centrifugal compressor 14A of the gas turbine engine <NUM>, which has the impeller <NUM> that rotates within the outer shroud <NUM> about the longitudinal axis <NUM>, has a bidirectional flow communication provided between the cavity <NUM> located on a shroud side of the impeller <NUM> and a main flow passage at the impeller exit 15B via the series of apertures <NUM> extending between the cavity <NUM> and the main flow passage at the impeller exit 15B. Referring to <FIG>, in an embodiment, the apertures <NUM> have their aperture axis 32A at a radial angle when viewed in a meridional plane of the centrifugal compressor 14A relative to the central longitudinal axis <NUM>. As such, the fluid flowing out from the cavity <NUM> via the apertures <NUM> may have a radial component relative to the main flow passage, which may reduce flow mixing losses and help reducing the flow distortion downstream of the impeller exit 15B, as discussed above. More particularly, in an embodiment, the apertures <NUM> have a cavity-side opening <NUM> and a main flow passage side opening <NUM> (also referred to herein as an "impeller-side opening" <NUM>), and the aperture axes 32A are radially inwardly angled with respect to the central longitudinal axis <NUM> in a direction extending from the main flow passage side opening <NUM> to the cavity side opening <NUM>. According to the claimed invention, when viewed in a meridional plane, such as shown in <FIG>, the cavity side opening <NUM> is disposed radially inward relative to the main flow passage side opening <NUM>. Additionally, the impeller-side opening <NUM> is located radially outward from the impeller exit 15B, relative to the longitudinal central axis <NUM>. The cavity-side opening <NUM>, which is disposed radially inward from the impeller-side opening <NUM>, is therefore located closer to the central axis <NUM> than the impeller-side opening <NUM>.

In other embodiments, the aperture axes 32A may be radially outwardly angled with respect to the central longitudinal axis <NUM>, such that the main flow passage side opening <NUM> may be radially outward relative to the cavity side opening <NUM>. According to the claimed invention, the main flow passage side opening <NUM> is located radially outward relative to the impeller exit 15B. The cavity side opening <NUM> may also be located radially outward relative to the impeller exit 15B, such as in the depicted embodiments, or in alternate embodiments, the cavity side opening <NUM> may be located radially inward relative to the impeller exit 15B. In an embodiment, the radial angle Ψ of the aperture axes 32A with respect to the central longitudinal axis <NUM> is -<NUM>° ≤ Ψ < <NUM>° or <NUM>°< Ψ ≤ <NUM>°. More particularly, in one embodiment, the radial angle Ψ is -<NUM>° ≤ Ψ < -<NUM>° or <NUM>°< Ψ ≤ <NUM>°, and in another embodiment the radial angle Ψ is -<NUM>° ≤ Ψ < -<NUM>° or <NUM>°< Ψ ≤ <NUM>°. In a further particular embodiment, the radial angle Ψ is <NUM>°± <NUM>°. The radial angle Ψ may be different in other embodiments, but still excluding about <NUM>° (i.e. <NUM>° ± <NUM>°). While all the apertures <NUM> have their aperture axis 32A uniformly radially angled with respect to the central longitudinal axis <NUM> in an embodiment, one or more apertures <NUM> may be radially angled differently from one or more other apertures <NUM>, in some embodiments.

According to the claimed invention, the apertures <NUM> are tapered in a direction extending from the cavity <NUM> toward the main flow passage. In other words, a cross-sectional area of the cavity side opening <NUM> is larger than a cross-sectional area of the main flow passage opening <NUM>, whereby fluid exiting the main flow passage to enter the cavity <NUM> through the apertures <NUM> passes in a divergent passage defined by the apertures <NUM> to reach the cavity <NUM>. Conversely, fluid exiting the cavity <NUM> to re-enter the main flow passage passes in a convergent passage defined by the apertures <NUM>. This may provide optimal swirl to the downstream diffuser <NUM> and/or provide better vortical structure which may increase mixing of the fluid exiting the cavity <NUM> with the fluid of the main flow passage at the impeller exit 15B. In other embodiments not covered by the claimed invention, the apertures <NUM> may be tapered in the opposite direction, if desirable. In a particular embodiment not covered by the claimed invention, the apertures <NUM> may have a convergent-divergent shape, such that the apertures <NUM> may have a choked cross-section, i.e. a cross-sectional area, between the cavity side opening <NUM> and the main flow passage side opening <NUM>, that is smaller than the cavity side opening <NUM> and/or the main flow passage side opening <NUM>. Referring to <FIG>, the apertures <NUM> have a conical shape, which may diverge or converge toward the cavity side opening <NUM>, depending on the embodiment. For instance, in an embodiment, the apertures <NUM> have a conical angle θ that is -<NUM>° ≤ θ < <NUM>° or <NUM>°< θ ≤ <NUM>°. In a particular embodiment, the conical angle θ is <NUM>°± <NUM>°. The conical angle θ may be different in other embodiments. In some embodiments, the apertures <NUM> may thus be radially angled (i.e. radially inwardly or radially outwardly angled relative to the central longitudinal axis <NUM>) and tapered toward the cavity side opening <NUM> or the main flow passage side opening <NUM>. The apertures <NUM> may or may not be all equally tapered, depending on the embodiment.

The apertures may have many suitable cross-section shapes. In embodiments where the apertures <NUM> have a round shape (e.g. circular shape), a diameter D1 of the apertures <NUM> at the cavity side opening <NUM> may be greater than a diameter D2 of the apertures <NUM> at the main flow passage side opening. In embodiments where the round shape is elongated, such as in an oval shape, these diameters D1 and D2 may be the measure of the maximum transversal dimension of the openings <NUM>, <NUM>. In other embodiments, the apertures <NUM> may have other shapes, such as a rectangular cross-sectional shape. In some embodiments, the apertures <NUM> have a constant cross-section shape, though the cross-section shape may vary from the cavity side opening <NUM> and the main flow passage side opening <NUM> in other embodiments. Also, while all the apertures <NUM> have a uniform cross-section shape in an embodiment, one or more apertures <NUM> may have different cross-section shapes than one or more other apertures <NUM>, in some embodiments.

In addition to being tapered and/or radially angled, the apertures <NUM> may be circumferentially angled relative to a plane normal to the central longitudinal axis <NUM>. In other words, in some embodiments, the cavity side opening <NUM> and the main flow passage side opening <NUM> of the apertures <NUM> are circumferentially offset relative to each other. Such plane may correspond to the plane A-A shown in <FIG> taken transversally to the longitudinal axis <NUM>. The circumferential angle ζ may also be defined as the angle relative to a line tangent to the impeller exit 15B radius (perpendicular to the central longitudinal axis <NUM>). For instance, in an embodiment as shown in <FIG>, a circumferential angle ζ of the aperture axes 32A relative to the plane normal to the central longitudinal axis <NUM> is -<NUM>° ≤ ζ < <NUM>° or <NUM>°< ζ ≤ <NUM>°. More particularly, in an embodiment, the circumferential angle ζ is -<NUM>° ≤ ζ < -<NUM>° or <NUM>°< ζ ≤ <NUM>°, and in some cases the circumferential angle ζ is -<NUM>° ≤ ζ < -<NUM>° or <NUM>°< ζ ≤ <NUM>°. In a particular embodiment, the circumferential angle ζ is <NUM>°± <NUM>°. The circumferential angle ζ may be different in other embodiments. While all the apertures <NUM> have their aperture axis 32A uniformly circumferentially angled relative to the plane normal to the central longitudinal axis <NUM> in an embodiment, one or more apertures <NUM> may be circumferentially angled differently from one or more other apertures <NUM>, in some embodiments.

The apertures <NUM> have length L defined from the cavity side opening <NUM> and the main flow passage side opening <NUM> taken along the aperture axes 32A. In an embodiment, a ratio of the length L of the apertures <NUM> over the diameter D1 of the cavity side opening <NUM> is ≥ <NUM>. While all the apertures <NUM> have a uniform length L in an embodiment, one or more apertures <NUM> may have a greater length than one or more other apertures <NUM>, in some embodiments.

Although described above as being apertures <NUM> with a rounded cross-section shape in some embodiments, with or without the tapering of the apertures <NUM>, the series of apertures <NUM> may also take the form of a series of elongated slots. This is shown in <FIG>, for instance. In an embodiment, such as shown, the elongated slots have an arcuate cross-section shape, though other cross-section shapes may be contemplated. The arcuate cross-section shaped slots have their radius oriented toward the central longitudinal axis <NUM>, such as shown. The arcuate cross-section shape may also have their radius oriented differently, for instance away from the central longitudinal axis <NUM>, in other embodiments.

Whether the series of apertures <NUM> are in the form of a series of elongated slots, or with other cross-section shapes, as discussed above, both arrangements may provide better alignment of the flow exiting the cavity <NUM> with the flow downstream of the impeller exit 15B and upstream the diffuser passages leading toward the combustor <NUM> or another compressor stage when compared with impeller <NUM> and diffuser <NUM> assembly(ies) with a single, non-angled, circumferential slot between the cavity <NUM> and the main flow passage downstream of the impeller <NUM>, adjacent the tips of the impeller blades 15A, as may be provided in some cases, for instance. This may advantageously improve the cavity influence on downstream diffuser stall margin in some embodiments.

In an alternate embodiment, only a single aperture <NUM> is provided in the form of an annular slot extending circumferentially about the impeller exit 15B, instead of having the series of apertures <NUM> described above. In such embodiment, the annular slot would be radially angled and the above description in this regard would also be applicable to this embodiment. The annular slot may also have other characteristics described above with respect to other embodiments, including the characteristics described with respect to the tapered shape of the apertures <NUM> and location of the apertures <NUM> within the centrifugal compressor 14A.

Claim 1:
A gas turbine engine (<NUM>) for an aircraft, comprising:
a centrifugal compressor (14A) including an impeller (<NUM>) with impeller blades (15A) extending from a hub and a diffuser (<NUM>) downstream of the impeller (<NUM>), the impeller (<NUM>) mounted for rotation about a central longitudinal axis (<NUM>) within an outer shroud (<NUM>), a main flow passage extending between the hub and the outer shroud (<NUM>) to an impeller exit (15B) defined downstream of the impeller blades (15A), wherein a cavity (<NUM>) is disposed adjacent the exit (15B), the cavity (<NUM>) communicating with the main flow passage via at least one aperture (<NUM>) through a main flow passage wall from an impeller-side opening (<NUM>) to a cavity-side opening (<NUM>), the impeller-side opening (<NUM>) located radially outward from the impeller exit (15B) relative to the central longitudinal axis (<NUM>), and the cavity-side opening (<NUM>) located closer to the central longitudinal axis (<NUM>) than the impeller-side opening (<NUM>), wherein the cavity (<NUM>) defines a closed chamber, the at least one aperture (<NUM>) providing a sole fluid connection to the closed chamber, and the diffuser (<NUM>) includes an annular ring extending circumferentially about the impeller exit (15B), the cavity (<NUM>) circumscribed by the annular ring and a portion of the outer shroud (<NUM>), and the at least one aperture (<NUM>) is tapered in a direction extending from the cavity-side opening (<NUM>) toward the impeller-side opening (<NUM>).