Patent Description:
Centrifugal compressors include an impeller surrounded by a shroud and a diffuser downstream therefrom. They achieve a pressure rise by adding kinetic energy to a flow of fluid through the impeller. The combination of the rapid rise in pressure and the relatively high curvature of the flow path from an axial to a radial direction in the centrifugal compressors may cause a relatively high adverse pressure gradient to develop as the fluid flow negotiates the curved shroud surface. This phenomenon may generally be observed with compressible fluids. This may result in a build-up of the boundary layer at the curved shroud surface due to the change between axial momentum to radial momentum of the fluid flow. Flow blockage may occur in the centrifugal compressors, especially at or aft the bend area of the impeller. Such flow blockage may reduce the pressure gains achieved by the centrifugal compressor. Large flow blockage may impose high incidence on the diffuser downstream of the impeller.

<CIT> discloses a prior art centrifugal compressor comprising an anti-surge device.

<CIT> discloses a prior art turbo-machine with means for removing a boundary layer by suction.

<CIT> discloses a prior art diaphragm for a centrifugal compressor.

According to the present invention, there is provided a centrifugal compressor as set forth in claim <NUM>.

In an embodiment of the above, the impeller has a shroud side and an opposite hub side, the cavity located on the shroud side of the impeller.

In an embodiment of any of the above, the cavity is annular, the cavity extending circumferentially about the axis.

In an embodiment of any of the above, the shroud has a curved gas path surface having a bend portion extending between an inducer portion and the exducer portion, at least part of the cavity radially aligned with the bend portion.

In an embodiment of any of the above, the shroud has a curved gas path surface having a bend portion extending between an inducer portion and the exducer portion, at least part of the cavity extends radially along the exducer portion.

In an embodiment of any of the above, the at least one reinjection port is defined through a shroud wall extending along the impeller blades.

In an embodiment of any of the above, the at least one reinjection port defines a reinjection outlet in a gas path surface of the shroud, the gas path surface defining a bend portion extending between an inducer portion and the exducer portion, the reinjection outlet located within about one third of the chord of the impeller from the exducer end.

In an embodiment of any of the above, the at least one reinjection port has a convergent shape between the cavity and the main flow passage.

In an embodiment of any of the above, the at least one reinjection port is an annular slot defined through the shroud about the axis.

In an embodiment of any of the above, the shroud has a gas path surface facing the impeller, the annular slot having an outlet defined at the gas path surface, the outlet having a radial dimension w and the impeller having an axial width H at the exducer end of the impeller, a ratio w/H is <NUM> ≤ w/H ≤ <NUM>.

In an embodiment of any of the above, the centrifugal compressor includes a plurality of reinjection ports defining a series of circumferentially spaced apart holes extending through a gas path surface of the shroud and angled circumferentially in a direction of rotation of the impeller from the cavity towards the main flow passage.

In an embodiment of any of the above, the centrifugal compressor includes a plurality of reinjection ports defining a series of circumferentially spaced apart slots extending through a gas path surface of the shroud between the cavity and the main flow passage, the slots having a radially outward directional component from the cavity towards the main flow passage.

In an embodiment of any of the above, the at least one extraction port is located upstream of the exducer end of the impeller, a projection of a center line of the at least one extraction port intersecting with the impeller.

According to a further aspect of the present invention, there is provided a compressor section 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 aircraft engines in general, and other types of gas turbine engines in particularly, including but not limited to turboshaft and turboprop engines, auxiliary power units (APU), and the like.

The compressor section <NUM> of the engine <NUM> includes one or more compressor stages disposed in flow series. For instance, the compressor section <NUM> may comprise a number of serially interconnected axial compressor stages feeding into a radial compressor stage having a centrifugal compressor <NUM>. The centrifugal compressor <NUM> has a main flow passage FP defined therethrough and includes an impeller <NUM> having a disc <NUM> from which a plurality of circumferentially spaced-apart blades <NUM> extends. The impeller <NUM> is mounted for rotation within a shroud <NUM> about the central axis <NUM>. The disc <NUM> of the impeller <NUM> may be mounted to a shaft not shown) in the compressor section <NUM>, directly, or via a gearbox for instance.

As shown in <FIG>, the impeller blades <NUM> extend from an axial inlet or inducer end <NUM> of the impeller <NUM> to a radial outlet or exducer end <NUM> at which the gas flow exits the impeller <NUM> substantially radially (<NUM> ± <NUM> degrees or between <NUM> and <NUM> degrees for instance) relative to the central longitudinal axis <NUM>. The impeller blades <NUM> define an intermediate bend 151A from axial to radial between the inducer end <NUM> and the exducer end <NUM>. The bend 151A generally defines a bend area of the impeller <NUM>. The impeller blades <NUM> 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 <NUM> versus the aft of the blades <NUM> caused by rotation of the impeller <NUM> and fluid interaction with the main gas flow. As will be seen herein after, this may set up a circumferentially varying pattern of flow distortion at an exit of the impeller downstream of the impeller blades <NUM>, in other words at the exducer end <NUM> or "tip" of the blades <NUM> of the impeller <NUM>.

In accordance with at least some embodiments, the shroud <NUM> encloses the impeller <NUM>, thereby forming a substantially closed system, whereby the compressible fluid enters axially the shroud <NUM>, flows through the main flow passage FP, and exits substantially radially outwardly relative to the engine axis <NUM>. The shroud <NUM> has a shroud body <NUM>, which makes up the corpus of the shroud <NUM> and provides it with its structure and its ability to resist the loads generated by the compressor <NUM> when in operation. The shroud body <NUM> has a gas path surface <NUM>, which is the face of the shroud <NUM> that is exposed to the fluid flow, and which defines a wall of the main flow passage FP of the shroud side of the impeller <NUM> as shown in <FIG>.

As shown in <FIG>, the gas path surface <NUM> of the shroud <NUM> has a curved profile, which may match the curvature of the impeller blades <NUM>, and which extends between an inducer portion IN and an exducer portion EX of the gas path surface <NUM>. The location and relative size of the inducer portion IN and the exducer portion EX on the gas path surface <NUM> of the shroud <NUM> may vary for different centrifugal compressors <NUM>. The locations of the inducer portion IN and the exducer portion EX may be given relative to a bend portion KN, or "knee", of the gas path surface <NUM>. Still referring to <FIG>, the bend portion KN can be defined by a bend length, which begins at a point where the substantially axial compressible fluid starts to curve or bend, and ends at a point where the compressible fluid first begins to flow in a substantially radial direction. The bend portion KN is demarcated in <FIG> by lines L, which extend in a direction normal to the gas path surface <NUM> at the location where the flow transitions from an axial direction, and where it transitions to a substantially radial direction. The inducer portion IN can be any part of the gas path surface <NUM> which is upstream of the bend portion KN, and the exducer portion EX can be any part of the gas path surface <NUM> which is downstream of the bend portion KN.

For the exemplary compressor <NUM> shown in <FIG> and <FIG>, the inducer portion IN corresponds to the part of the gas path surface <NUM> of the shroud <NUM> in proximity to the inducer end <NUM> of the impeller <NUM>. The inducer portion IN in the depicted embodiment is defined by a generally straight-line (or slightly curved) segment which is substantially parallel to the central axis <NUM>, and corresponds to the portion of the shroud <NUM> that receives the fluid flow. In the depicted embodiment, the inducer portion IN extends from the impeller inlet <NUM> to about one third (<NUM> ± <NUM>) of a chord A of the impeller <NUM> extending from the inducer end <NUM> to the exducer end <NUM>. Inducer portions IN having other configurations (or impeller relative chord) are contemplated.

Still for the compressor <NUM> shown in <FIG> and <FIG>, the exducer portion EX corresponds to the part of the gas path surface <NUM> of the shroud <NUM> in proximity to the exit or exducer end <NUM> of the impeller <NUM>. The exducer portion EX in the depicted embodiment is a substantially straight-line (or slightly curved) segment extending from the end of the bend portion KN of the gas path surface <NUM>. The exducer portion EX extends generally radially with respect to the central axis <NUM> at the exducer end <NUM>. In the depicted embodiment, the exducer portion EX extends from the exducer end <NUM> of the impeller <NUM> to about one third (<NUM> ± <NUM>) of the chord A of the impeller <NUM>. Exducer portions EX having other configurations (or impeller relative chord) are contemplated.

A diffuser <NUM> is disposed immediately downstream of the impeller <NUM> for converting kinetic energy to an increased potential energy/static pressure by slowing down the airflow through the diffuser <NUM>. Referring jointly to <FIG> and <FIG>, it can be seen that the diffuser <NUM> forms a fluid connection between the impeller <NUM> and the combustor <NUM> (see <FIG>), thereby allowing the impeller <NUM> to be in serial flow communication with the combustor <NUM>. The exemplified diffuser <NUM> is configured to 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 <NUM> 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 <NUM> 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, is configured to reduce the velocity and increase the static pressure of the main gas flow as it flows therethrough. The exemplified 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 <NUM>. The diffuser body <NUM> is mounted about a circumference of the compressor or impeller exit so as to receive the main gas flow therefrom. In some embodiments, such as the depicted one, the diffuser body <NUM> forms an annular diffuser ring 171A extending circumferentially about the impeller exducer end <NUM>. In the depicted embodiment, the annular diffuser ring 171A defines a vaneless space 171B downstream of the impeller <NUM>. As shown, the vaneless space 171B defines a wall facing radially inwardly towards the exducer end <NUM> of the impeller <NUM>. The flow exiting the impeller <NUM> is directed to the vaneless space 171B radially outwardly before being redirected in other directions via other parts of the annular diffuser ring, for instance towards the combustor <NUM>.

Flow blockage is a phenomenon observed in many centrifugal compressors, in particular with compressible fluids. The flow of a compressible fluid at the exit of the impeller <NUM> may be highly turbulent. The pressure of such compressible fluid may be raised rapidly after the impeller inducer end <NUM>, starting at the intermediate bend 151A. The combination of the rapid rise in pressure and the relatively high curvature of the shroud gas path surface <NUM> may cause a relatively high adverse pressure gradient to develop as the compressible fluid negotiates the curved shroud gas path surface <NUM> from axial to radial. This may result in a build-up of the boundary layer at the curved shroud gas path surface <NUM> due to the change between axial momentum to radial momentum of the compressible fluid. Part of the flow may "stagnate" in the boundary layer or have a lower velocity than away from shroud gas path surface <NUM> (positive gradient projecting out from the curved gas path surface <NUM>), with such boundary layer tending to reduce the velocity of the flow in the vicinity therewith. In other words, aft of the bend area of the impeller <NUM>, the boundary layer bordering the curved shroud gas path surface <NUM> may thicken and may be characterized as a low momentum flow layer, which may lead to increased flow blockage. Such flow blockage may reduce the pressure gains achieved by the centrifugal compressor <NUM> and/or weaken/deteriorate the main flow exiting the bend area of the impeller <NUM>, which may thus fail to negotiate the curved shroud gas path surface <NUM> and cause even more flow blockage as the flow follows its path to the impeller exit. Flow blockage may impose high incidence on the diffuser <NUM> downstream of the impeller <NUM>.

Referring to <FIG>, the centrifugal compressor <NUM> includes a cavity <NUM> fluidly communicating with the main flow passage FP via at least one extraction port <NUM> and at least one reinjection port <NUM> extending between the cavity <NUM> and the main flow passage FP. The reinjection port <NUM> is fluidly connected to the main flow passage FP upstream of the extraction port <NUM> relative to a flow direction (see direction of dashed line arrow in <FIG>) through the main flow passage FP of the impeller <NUM>. As will be further described below in connection with other features of the centrifugal compressor <NUM> referred to herein, re-energizing the fluid flow upstream of the impeller exit (i.e. the exducer end <NUM> of the impeller <NUM>) by recirculating part of the fluid flow extracted close to the impeller exit in the exducer portion EX of the shroud <NUM> may improve the conditions of the flow exiting the impeller <NUM>, whereby the function of the diffuser <NUM> downstream therefrom may be facilitated. Recirculation may allow low momentum flow at the exducer end <NUM> of the impeller <NUM> to be reduced/removed and returned upstream, near the bend area of the impeller <NUM>, with higher momentum. The introduction of higher momentum near the bend area of the impeller <NUM> may allow re-energizing the boundary layer, which may become more tolerant to flow separation. Such improved conditions of the flow exiting the impeller <NUM> may favorably affect the performance of the diffuser <NUM> downstream thereof. In some cases, improving impeller exit conditions may lead to improve diffuser performance especially at high speeds where diffuser controls may be more likely to stall.

According to the embodiment illustrated in <FIG>, the 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 FP from the cavity <NUM>, where the main flow passage wall is located adjacent the exducer end <NUM> of the impeller <NUM>. In the depicted embodiment, where the diffuser body <NUM> forms an annular ring 171A, the cavity <NUM> may be circumscribed by the annular ring 171A and an adjacent portion of the shroud <NUM>. As shown, the diffuser body <NUM> defines a radially outward peripheral wall of the cavity <NUM>, and the shroud <NUM> defines a radially inward peripheral wall of the cavity <NUM>. In other embodiments, the cavity <NUM> may be an internal cavity defined solely in the diffuser body <NUM> or solely in the shroud <NUM>. The cavity <NUM> may be located on the hub side in other embodiments, though there may be structural restriction (limited available space) rendering such placement less desirable depending on the engines.

In the depicted embodiment, the cavity <NUM> is an annular chamber extending circumferentially about the axis <NUM> (see <FIG>). As shown, the cavity <NUM> defines a chamber or internal volume with a volumetric footprint larger than that of the ports <NUM>, <NUM>.

In other embodiments, such as shown in <FIG>, the cavity <NUM> may be discontinuous, such that the cavity <NUM> may define a series of spaced apart (or segmented, fully or partially) sub-chambers distributed about the impeller <NUM> (impeller not shown on <FIG>), though having the cavity <NUM> in the form of an annular chamber extending over <NUM> degrees about the impeller <NUM> may allow flow communication between the main flow passage FP and the cavity <NUM>, via the ports <NUM>, <NUM> with more freedom than in embodiments with spaced apart sub-chambers. In some embodiments, such as shown in <FIG>, it may be desirable, on a structural standpoint for instance, to have partition walls PW segmenting the cavity into a series of spaced apart sub-chambers 180A within the shroud <NUM>, about the impeller <NUM>. Depending on the embodiments, such partition walls PW may partially or fully partition the cavity <NUM>, such that sub-chambers 180A may or may not be fluidly connected to each other otherwise than via flow communication with the main flow passage FP. Such partition walls PW may contribute to the structural integrity of the shroud <NUM>.

In at least some embodiments, the cavity <NUM> is configured to decelerate the flow entering the cavity <NUM>. The flow entering the cavity <NUM> may slow down because of the size/volume of the cavity <NUM>. In at least some embodiments, the cavity <NUM> may be sized and/or shaped to maximize the flow deceleration, within the limited available space in the engine <NUM>. For instance, in a particular embodiment, the size of the cavity is maximized within the limited dedicated space within the engine <NUM>. Slowing down the flow may reduce skin friction loss as the flow is redirected to be reinjected through the reinjection port <NUM>. Reducing a velocity of the flow via the cavity <NUM> before it gets reinjected in the main flow passage FP via the reinjection port <NUM> may facilitate redirecting the flow to turn more easily, in particular with high pressure ratio systems, such as aircraft engines.

Returning to <FIG>, the cavity <NUM> is located radially outward relative to the inducer portion IN and the bend portion KN, on the shroud side of the impeller <NUM>. The cavity <NUM> is at least in part radially aligned with the bend portion KN (in some cases, the entire footprint of the cavity <NUM> is axially aligned about the bend portion KN). As shown, at least part of the cavity <NUM> extends radially along the exducer portion EX. The cavity <NUM> may be located somewhere else in other embodiments, though fluid flow communication at the impeller exit with the main flow passage FP could require more plumbing/conduits to channel the flow at the impeller exit.

As mentioned above, the cavity <NUM> is in fluid communication with the main flow passage FP at the impeller exit via at least one extraction port <NUM>. Referring to <FIG>, in the embodiment of the impeller <NUM> depicted in dashed lines, the extraction port <NUM> is located downstream of the impeller <NUM>, adjacent the exducer end <NUM> of the impeller <NUM>. In other embodiments, the extraction port <NUM> may be slightly upstream of the exducer end <NUM> such that a projection of a central line X of the extraction port <NUM> may intersect with the impeller (see this scenario in <FIG>, where impeller outlet <NUM> may be at the dotted lines (identified as <NUM>') instead of dashed lines, in an alternate configuration of the centrifugal compressor), or the projection of the central line X may be generally at a same distance from the central axis <NUM> as the exducer end <NUM>, among other possibilities.

In accordance with at least some embodiments, the extraction port <NUM> is defined by a gap extending radially between the diffuser body <NUM>, or diffuser ring 171B if present, and the shroud <NUM>. The gap may be an annular gap that extends circumferentially about the central axis <NUM> of the impeller <NUM>, as shown in <FIG>). In accordance with such an embodiment, there is a single extraction port <NUM> extending between the cavity <NUM> and the main flow passage FP, with such extraction port <NUM> extending annularly about the impeller <NUM>. The presence of such gap may also allow thermal expansion of the shroud <NUM> and/or diffuser <NUM>, without interference of the diffuser ring 171B with the shroud <NUM> at such location. In other embodiments (not shown), the extraction port <NUM> may be defined through a portion of the shroud <NUM>. In such case, the main flow passage wall through which the extraction port <NUM> is defined is part of the shroud <NUM>. Having the extraction port <NUM> defined through the shroud <NUM> instead of at a gap between the shroud <NUM> and the diffuser <NUM> may increase vibration and/or weaken the shroud <NUM>, though this could be contemplated. In other embodiments, the extraction port <NUM> may be defined through a portion of the diffuser body <NUM>. In such case, the main flow passage wall through which the extraction port <NUM> is defined is part of the diffuser body <NUM>. This may depend on the location of the cavity <NUM> (within the shroud <NUM> or within the diffuser body <NUM>).

In the depicted embodiment, the extraction port <NUM> in the form of the annular gap between the shroud <NUM> and the diffuser body <NUM> extends axially, parallel to the central axis <NUM>. The extraction port <NUM> may extend angularly, radially inwardly or outwardly, from the inlet <NUM> in other embodiments. In the depicted embodiment, the extraction port <NUM> has a constant cross-section from the inlet <NUM> to the cavity <NUM>, though the cross-section may vary in size and/or shape (e.g. convergent, divergent or both) in other embodiments.

In other embodiments, the gap may be discontinuous, i.e. not extending continuously over the entire circumference of the impeller <NUM>. For instance, in some embodiments where the gap is discontinuous, such as shown in the example of <FIG>, the gap may define a series of spaced apart inlets <NUM> defined through the main flow passage wall and that extend between the cavity <NUM> and the main flow passage FP. For instance, the inlets <NUM> may be circumferentially equally spaced apart about the impeller <NUM>. The inlets <NUM> may be unevenly distributed along the circumference of the impeller <NUM> in other cases.

In some embodiments, such as shown in <FIG>, the extraction ports <NUM> may be defined at an interface between the shroud <NUM> and the diffuser body <NUM>. In other words, the shroud <NUM> and the diffuser body <NUM> may mate at a common edge, where they contact each other between circumferentially adjacent extraction ports <NUM>. At such interface between the diffuser body <NUM> and the shroud <NUM>, the common edge of the shroud <NUM> and the diffuser body <NUM> may form respective radially inward and radially outward wall of the extraction ports <NUM>.

Referring back to the embodiment of <FIG> and <FIG>, features of the reinjection port <NUM> will now be discussed. As mentioned above, the cavity <NUM> is in fluid communication with the main flow passage FP via at least one reinjection port <NUM> upstream of the extraction port <NUM>.

The reinjection port <NUM> defines an outlet <NUM> in the gas path surface <NUM> of the shroud <NUM>. The outlet <NUM> is located in the exducer portion EX. The outlet <NUM> is closer to the impeller outlet <NUM> than from the impeller inlet <NUM>. The outlet <NUM> is located past the bend portion KN, in the exducer portion EX. The outlet <NUM> may be located within about one third (<NUM> ± <NUM>) of the chord A of the impeller <NUM> from the exducer end <NUM>. In some cases, the location of the outlet <NUM> may be in the last one third (<NUM> ± <NUM>) of the chord A of the impeller <NUM>. The outlet <NUM> may be located where the bend portion KN transitions to the exducer portion EX. Such location may be further than about one third (<NUM> ± <NUM>) of the chord A from the exducer end <NUM>, depending on the compressors <NUM> and/or profile of the impeller <NUM>.

In the depicted embodiment, there is a single reinjection port <NUM> extending annularly about the central axis <NUM>. The reinjection port <NUM> is in the form of a circumferential slot defined in the gas path surface <NUM> (see <FIG>). As shown, the outlet <NUM> in the shroud gas path surface <NUM> having a radial width w. A ratio between the width w and an axial width H of the impeller <NUM> at the impeller outlet <NUM> may be <NUM> ≤ w/H ≤ <NUM> in some embodiments. In some embodiments, such ratio w/H may allow obtaining a maximum flow and a maximum flow velocity at the reinjection port <NUM>. Other ratios may be contemplated in other embodiments.

The reinjection port <NUM> is angled radially outwardly from the cavity <NUM> to the outlet <NUM>. The reinjected flow may thus have a direction component that is tangential to the shroud gas path surface <NUM> and/or a radial direction component such as the flow in the main flow passage FP. Such orientation tangential orientation of the reinjected flow relative to shroud gas path surface <NUM> may minimize mixing loss and further improve the performance of the centrifugal compressor <NUM> and/or diffuser <NUM> downstream thereof.

A radial angle θ of a central line Y of the reinjection port <NUM> at the outlet <NUM> with respect to the central longitudinal axis <NUM> is <NUM>° ≤ θ < <NUM>° or in some cases <NUM>°≤ θ < <NUM>°. Maximizing the tangential direction component of the reinjected flow may be desirable to minimize mixing loss at the reinjection point.

In the depicted embodiment, the reinjection port <NUM> is tapered in a direction extending from the cavity <NUM> toward the main flow passage FP (i.e. it forms a converging exit passage). As shown, the reinjection port <NUM> has an outlet <NUM> defined in the shroud gas path surface <NUM> that has a cross-section smaller than a remainder of the reinjection port <NUM>. The reinjection port <NUM> is a converging (progressively or constantly) channel towards the main flow passage FP. Fluid flow reinjected into the main flow passage FP via the reinjection port <NUM> may thus be accelerated via the converging reinjection port <NUM>. As the flow in the cavity <NUM> has a lower velocity, having the converging reinjection port <NUM> may reduce flow distortion at the reinjection point, with a reinjection flow at a velocity closer to the velocity of the flow in the main flow passage FP. In some cases, the converging reinjection port <NUM> has a cross-section differential of <NUM>:<NUM> from the cavity <NUM> to the outlet <NUM>, in some other cases, <NUM>:<NUM>, in some other cases more than <NUM>:<NUM> or less than <NUM>:<NUM>. Having a ratio of <NUM>:<NUM> or higher may provide more velocity hence more convergence, in some embodiments. In a particular embodiment, where the reinjection port <NUM> is in the form of a circumferential slot having a radial width w, the reinjection port <NUM> has a cross-sectional differential greater than <NUM>:<NUM> and a length taken between the cavity <NUM> and the outlet <NUM> along line Y ≥ <NUM> times the radial width w (or between about <NUM> and <NUM> times the radial width w). A cross-sectional differential of <NUM>:<NUM> or higher (e.g. between <NUM>:<NUM> and <NUM>:<NUM>).

The reinjection port <NUM> may have other suitable shapes in other embodiments. For instance, the reinjection port <NUM> may have a convergent-divergent shape, such that the reinjection port <NUM> may have a choked cross-section, i.e. a cross-sectional area that reduces before enlarging toward the outlet <NUM>. The reinjection port <NUM> may have a constant cross-section in other embodiments.

In other embodiments, there may be a plurality of reinjection ports <NUM>, in the form of circumferentially spaced apart holes about the central axis <NUM>. In such cases, the reinjection ports <NUM> may have many suitable cross-section shapes. In embodiments where the reinjection ports <NUM> have a round shape (e.g. circular shape), the round shape may be elongated, such as in an oval or elliptical shape. This is shown in the example of <FIG>. In some other embodiments, the apertures <NUM> may have other shapes, such as a rectangular cross-sectional shape. In some embodiments, the reinjection ports <NUM> have a constant cross-section shape, though the cross-section shape may vary from the cavity <NUM> to the outlet <NUM>. Also, while all the reinjection ports <NUM> may have a uniform cross-section shape in an embodiment, one or more reinjection ports <NUM> may have different cross-section shapes than one or more other reinjection ports <NUM>, in some embodiments.

In addition to or instead of being tapered and/or radially angled, the reinjection ports <NUM> may be circumferentially angled relative to a plane normal to the central longitudinal axis <NUM> (see <FIG>). In other words, in some embodiments, the outlets <NUM> of the reinjection ports <NUM> may be circumferentially offset relative to a remainder of their respective reinjection ports <NUM>. The reinjection ports <NUM> may be angled circumferentially in a direction of rotation of the impeller <NUM>, from the cavity <NUM> towards the main flow passage FP, which may minimize mixing loss at the reinjection point in the main flow passage FP.

The reinjection ports <NUM> may have various suitable cross-section, such as a round or oval cross-section, whether or not constant over the whole length of the reinjection port <NUM>. As other possibilities, with or without the tapering, the reinjection ports <NUM> may also take the form of a series of elongated slots. For instance, the elongated slots may have an arcuate cross-section shape, though other cross-section shapes may be contemplated. The arcuate cross-section shaped slots may have their radius oriented toward the central longitudinal axis <NUM>, such as shown in <FIG>. The arcuate cross-section shape may also have their radius oriented differently, for instance away from the central longitudinal axis <NUM>, in other embodiments. The elongated slots may extend through the main flow passage wall defined by the shroud <NUM> (i.e. and surface <NUM>) with a radially outward directional component from the cavity <NUM> towards the main flow passage FP (see dashed line showing in-plane extension of the slots), such as to define an angle θ as discussed above with reference to <FIG>, to be as much tangentially as possible to the main flow passage FP.

Referring jointly to <FIG> and <FIG>, during operation of the centrifugal compressor <NUM>, the pressure inside the centrifugal compressor <NUM> increases from the inducer end <NUM> to the exducer end <NUM> of the impeller <NUM>. There is thus a pressure gradient between the inducer end <NUM> and the exducer end <NUM>. The fluid flow is pressure driven, such that the flow will move from a high pressure region to a low pressure region. The extraction port <NUM> is located at a higher pressure region than the reinjection port <NUM>. The pressure differential between the extraction port <NUM> and the reinjection port <NUM>, which are interconnected between the cavity <NUM> and the main flow passage FP, induces a recirculation loop in the recirculation direction illustrated by the arrows X and Y in <FIG>. Recirculation may be maximized by increasing the pressure differential between the extraction port <NUM> and the reinjection port <NUM>. This may be obtained by having the extraction port <NUM> and the reinjection port <NUM> at a greater radial distance (taken relative to axis <NUM>) from each other such as to have a greater pressure differential between them.

A method of re-energizing a flow in an exducer portion of a centrifugal compressor as discussed above is also disclosed, but not separately claimed. The method includes circulating part of the flow through the cavity <NUM> having at least one extraction port <NUM> fluidly connected to the main flow passage FP of the compressor <NUM> downstream of at least one reinjection port <NUM> fluidly connected to the main flow passage FP. In some cases, circulating part of the flow includes extracting said part of the flow downstream of the exducer portion EX. In some cases, circulating includes reinjecting at least a fraction of said part of the flow back to the main flow passage FP at a location radially inward relative to the extraction port <NUM>, in the exducer portion EX. In some cases, injecting at least said fraction of said part of the flow includes accelerating said fraction of said part of the flow through the injection port <NUM>.

Claim 1:
A centrifugal compressor (<NUM>) for an aircraft engine, comprising:
an impeller (<NUM>) mounted for rotation about an axis (<NUM>), the impeller (<NUM>) having impeller blades (<NUM>) extending from an inducer end (<NUM>) to an exducer end (<NUM>);
a shroud (<NUM>) extending over the impeller blades (<NUM>);
a main flow passage (FP) defined between the shroud (<NUM>) and the impeller (<NUM>);
a cavity (<NUM>) fluidly communicating with the main flow passage (FP) via at least one extraction port (<NUM>) and at least one reinjection port (<NUM>), the reinjection port (<NUM>) fluidly connected to the main flow passage (FP) upstream of the extraction port (<NUM>) relative to a flow direction through the main flow passage (FP), the reinjection port (<NUM>) disposed upstream of the exducer end (<NUM>) of the impeller blade, in an exducer portion (EX) of the shroud (<NUM>), characterized in that the at least one reinjection port (<NUM>) has a central line (X, Y) extending from the cavity (<NUM>) to a reinjection outlet (<NUM>) defined in a gas path surface (<NUM>) of the shroud (<NUM>), the central line (X, Y) radially angled such that a projection of the central line (X, Y) at the reinjection outlet (<NUM>) extends radially away at an angle θ of <NUM>° ≤ θ < <NUM>° relative to the axis (<NUM>), in the flow direction.