Patent Description:
Diffuser pipes are provided in certain engines for diffusing a flow of high speed air received from an impeller of a centrifugal compressor and directing the flow to a downstream component, such as an annular chamber containing the combustor. The diffuser pipes are typically circumferentially arranged at a periphery of the impeller, and are designed to transform kinetic energy of the flow into pressure energy. Diffuser pipes seek to provide a uniform exit flow with minimal distortion, as it is preferable for flame stability, low combustor loss, reduced hot spots etc..

<CIT> and <CIT> disclose a diffuser construction.

The present invention provides a centrifugal compressor diffuser for an aircraft engine, as claimed in claim <NUM>.

The compressor as defined above and described herein may also include one or more of the following features, in whole or in part, and in any combination.

In certain aspects, one or both of the radially outer wall and the radially inner wall has a compound curvature.

In certain aspects, the radially inner wall has a compound curvature and the radius of curvature of the radially inner wall varies between the side walls, the radius of curvature of the radially outer wall being substantially constant between the side walls.

In certain aspects, the radially inner wall has a first curved portion joined to one of the side walls and having a first radius of curvature, a second curved portion joined to the other of the side walls and having a second radius of curvature equal to the first radius of curvature, and a third curved portion between the first and second curved portions having a third radius of curvature different from the first and second radii of curvature.

In certain aspects, the radially inner wall, the radially outer wall, and the side walls are free of planar portions.

In certain aspects, the outlet is peanut or kidney shaped.

In certain aspects, the outlet is kidney shaped and the radially inner wall has an indented portion, a height of the outlet defined between the radially outer wall and the indented portion, a ratio of a height of the indented portion over the height of the outlet over is between <NUM> and <NUM>.

In certain aspects, the outlet is symmetric about a plane passing through the outlet, the plane containing a line defining a height of the outlet between the radially inner and outer walls.

In certain aspects, a reference cross-sectional profile is defined in a plane normal to a pipe center axis of that diffuser pipe, the reference cross-sectional profile shaped as an ellipse and defining a reference width along a major axis and a reference height along a minor axis, the outlet having a shape different than the ellipse and having a width being equal to the reference width.

In certain aspects, an area of the outlet is equal to a reference area of the reference cross-sectional profile, and a center of area of the outlet is closer to the radially inner wall than a center of the reference area of the reference cross-sectional profile.

In certain aspects, the outlet includes a width line extending between points of the side walls disposed furthest from one another, the width line intersecting a line defining a height of the outlet and dividing the line defining the height into a first height portion extending between the radially outer wall and the width line, and a second height portion extending between the radially inner wall and the width line, a ratio of the first height portion over the second height portion being between about <NUM> and <NUM>.

In certain aspects, the line defining the height intersects the width line and divides the width line into a first width portion extending between one of the side walls and the line defining the height, and a second width portion extending between the other of the side walls and the line defining the height, a ratio of the first width portion over the second width portion being about <NUM>.

In certain aspects, the radius of curvature of the radially outer wall is different from the radius of curvature of the radially inner wall along a length of the diffuser pipe extending from the outlet to a bend in the diffuser pipe.

In certain aspects, the radially inner wall has an indented portion extending toward the radially-outer wall.

In certain aspects, the indented portion is symmetrical about a line defining a height of the outlet.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication along an engine center axis <NUM> 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 compressor section <NUM> may include a plurality of stators <NUM> and rotors <NUM> (only one stator <NUM> and rotor <NUM> being shown in <FIG>), and it includes a centrifugal compressor <NUM>.

The centrifugal compressor <NUM> of the compressor section <NUM> includes an impeller <NUM> and a plurality of diffuser pipes <NUM>, which are located downstream of the impeller <NUM> and circumferentially disposed about a periphery of a radial outlet 17A of the impeller <NUM>. The diffuser pipes <NUM> convert high kinetic energy at the impeller <NUM> exit to static pressure by slowing down fluid flow exiting the impeller. The diffuser pipes <NUM> may also redirect the air flow from a radial orientation to an axial orientation (i.e. aligned with the engine axis <NUM>). In most cases, the Mach number of the flow entering the diffuser pipe <NUM> may be at or near sonic, while the Mach number exiting the diffuser pipe <NUM> may be less than <NUM> to enable stable air/fuel mixing, and light/re-light in the combustor <NUM>.

<FIG> shows the impeller <NUM> and the plurality of diffuser pipes <NUM>, also referred to as "fishtail diffuser pipes", of the centrifugal compressor <NUM>. Each of the diffuser pipes <NUM> includes a diverging (in a downstream direction) tubular body <NUM>, formed, in one embodiment, of sheet metal. The enclosed tubular body <NUM> defines a flow passage <NUM> (see <FIG>) extending through the diffuser pipe <NUM> through which the compressed fluid flow is conveyed. The tubular body <NUM> includes a first portion <NUM> extending generally tangentially from the periphery and radial outlet 17A of the impeller <NUM>. An open end is provided at an upstream end of the tubular body <NUM> and forms an inlet <NUM> (see <FIG>) of the diffuser pipe <NUM>. The first portion <NUM> may be inclined at an angle θ1 relative to a radial axis R extending from the engine axis <NUM>. The angle θ1 may be at least partially tangential, or even substantially tangentially, and may further correspond to a direction of fluid flow at the exit of the blades of the impeller <NUM>, such as to facilitate transition of the flow from the impeller <NUM> to the diffuser pipes <NUM>. The first portion <NUM> of the tubular body <NUM> can alternatively extend more substantially along the radial axis R.

The tubular body <NUM> of the diffuser pipes <NUM> also includes a second portion <NUM>, which is disposed generally axially and is connected to the first portion <NUM> by an out-of-plane curved or bend portion <NUM>. An open end at the downstream end of the second portion <NUM> forms a pipe outlet <NUM> (see <FIG>) of the diffuser pipe <NUM>. Preferably, but not necessarily, the first portion <NUM> and the second portion <NUM> of the diffuser pipes <NUM> are integrally formed together and extend substantially uninterrupted between each other, via the curved, bend portion <NUM>.

The large radial velocity component of the flow exiting the impeller <NUM>, and therefore entering the first portion <NUM> of each of the diffuser pipes <NUM>, may be removed by shaping the diffuser pipe <NUM> with the bend portion <NUM>, such that the flow is redirected axially through the second portion <NUM> before exiting via the pipe outlet <NUM> to the combustor <NUM>. It will thus be appreciated that the flow exiting the impeller <NUM> enters the inlet <NUM> and the upstream first portion <NUM> and flows along a generally radial first direction. At the outlet of the first portion <NUM>, the flow enters the bend portion <NUM> which functions to turn the flow from a substantially radial direction to a substantially axial direction. The bend portion <NUM> may form a <NUM> degree bend. At the outlet of the bend portion <NUM>, the flow enters the downstream second portion <NUM> and flows along a substantially axial second direction different from the generally radial first direction. By "generally radial", it is understood that the flow may have axial, radial, and/or circumferential velocity components, but that the axial and circumferential velocity components are much smaller in magnitude than the radial velocity component. Similarly, by "generally axial", it is understood that the flow may have axial, radial, and/or circumferential velocity components, but that the radial and circumferential velocity components are much smaller in magnitude than the axial velocity component.

Referring to <FIG>, the tubular body <NUM> of each diffuser pipe <NUM> has a radially inner wall 22A and a radially outer wall 22B. The radially outer wall 22B is spaced further from the center axis <NUM> than the radially inner wall 22A. The tubular body <NUM> also has a first side wall 22C spaced circumferentially apart across the flow passage <NUM> from a second side wall 22D. The first and second side walls 22C,22D are curved. The first and second side walls 22C,22D have a non-zero curvature value. The first and second side walls 22C,22D are concave when viewed from the center axis <NUM> or from within the tubular body <NUM>, and are convex when viewed from outside the diffuser pipe <NUM>. The radially inner and outer walls 22A,22B and the first and second side walls 22C,22D meet and are connected to form the enclosed flow passage <NUM> extending through the tubular body <NUM>. The radially inner and outer walls 22A,22B and the first and second side walls 22C,22D meet and are connected to form a peripheral edge of the tubular body <NUM> which circumscribes the pipe outlet <NUM>. The radially inner wall 22A may correspond to the wall of the tubular body <NUM> that has the smallest turning radius at the bend portion <NUM>, and the radially outer wall 22B may correspond to the wall of the tubular body <NUM> that has the largest turning radius at the bend portion <NUM>.

The tubular body <NUM> diverges in the direction of fluid flow F therethrough, in that the internal flow passage <NUM> defined within the tubular body <NUM> increases in cross-sectional area between the inlet <NUM> and the pipe outlet <NUM> of the tubular body <NUM>. This increase in cross-sectional area of the flow passage <NUM> through each diffuser pipe <NUM> may be continuous along the complete length of the tubular body <NUM>, or the cross-sectional area of the flow passage <NUM> may increase in gradual increments along the length of the tubular body <NUM>. In the depicted embodiment, the cross-sectional area of the flow passage <NUM> defined within the tubular body <NUM> increases gradually and continuously along its length, from the inlet <NUM> to the outlet <NUM>. The direction of fluid flow F is along a pipe center axis <NUM> of the tubular body <NUM>. The pipe center axis <NUM> extends through each of the first, second, and bend portions <NUM>,<NUM>,<NUM> and has the same orientation as these portions. The pipe center axis <NUM> is thus curved. In the depicted embodiment, the pipe center axis <NUM> is equidistantly spaced from the radially inner and outer walls 22A,22B of the tubular body <NUM>, and from the first and second side walls 22C,22D, through the tubular body <NUM>.

Referring to <FIG>, the tubular body <NUM> has a length L defined from the inlet <NUM> to the pipe outlet <NUM>. The length L of the tubular body <NUM> may be measured as desired. For example, in <FIG>, the length L is the length of the pipe center axis <NUM> from the inlet <NUM> to the pipe outlet <NUM>. In an alternate embodiment, the length L is measured along one of the walls 22A,22B,22C,22D of the tubular body <NUM>, from the inlet <NUM> to the pipe outlet <NUM>. Reference may be made herein to positions on the tubular body <NUM> along its length L. For example, a position on the tubular body <NUM> that is along a last <NUM>% of the length L is anywhere in the segment of the tubular body <NUM> that is upstream of the pipe outlet <NUM> a distance equal to <NUM>% of the length L. This same segment is also downstream of the inlet <NUM> a distance equal to <NUM>% of the length L. Similarly, a position on the tubular body <NUM> that is along a first <NUM>% of the length L is anywhere in the segment of the tubular body <NUM> that is downstream of the inlet <NUM> a distance equal to <NUM>% of the length L. This same segment is also upstream of the pipe outlet <NUM> a distance equal to <NUM>% of the length L.

The tubular body <NUM> is composed of many cross-sectional profiles <NUM> which are arranged or stacked one against another along the length L of the tubular body <NUM>. Each cross-sectional profile <NUM> is a planar contour that lies in its own plane that is transverse or normal to the pipe center axis <NUM>. <FIG> shows multiple cross-sectional profiles <NUM> in every portion <NUM>,<NUM>,<NUM> of the tubular body <NUM>, and it will be appreciated that many more cross-sectional profiles <NUM> may be defined at other locations along the pipe center axis <NUM>. In the depicted embodiment, the orientation of the cross-sectional profiles <NUM> in the frame of reference of the diffuser pipe <NUM> may vary over the length L of the tubular body <NUM>, depending on where the cross-sectional profiles <NUM> are located along the pipe center axis <NUM>. Each cross-sectional profile <NUM> defines the shape, contour, or outline of the tubular body <NUM> at a specific location along the pipe center axis <NUM>. Each cross-sectional profile <NUM> shows the shape, contour, or outline of the tubular body <NUM>, as defined by its interconnected walls 22A,22B,22C,22D, at a specific location along the pipe center axis <NUM>.

Referring to <FIG>, and as described in greater detail below, the cross-sectional profiles <NUM> may vary over the length L of the tubular body <NUM>. The cross-sectional profiles <NUM> are different over the length L of the tubular body <NUM>. Each cross-sectional profile <NUM> may be unique, and thus different from the other cross-sectional profiles <NUM>. An area of the cross-sectional profiles <NUM> varies along the length L of the tubular body <NUM>. The area of a given cross-sectional profile <NUM> is defined between the inner, outer, first side, and second side walls 22A,22B,22C,22D in the cross-sectional profile <NUM>. The area of the cross-sectional profiles <NUM> increases over the length L of the tubular body <NUM> in the direction of the pipe outlet <NUM>. This is consistent with the diverging flow passage <NUM> defined by the tubular body <NUM>.

<FIG> shows one such cross-sectional profile <NUM> taken along the line IIIB-IIIB in <FIG>, which is at the pipe outlet <NUM>. Referring to <FIG>, both the radially outer wall 22B and the radially inner wall 22A are curved at the pipe outlet <NUM>. The radially inner and outer walls 22A,22B have a curvature greater than zero. The radially inner and outer walls 22A,22B have a radius of curvature that is less than infinity. The radially outer wall 22B curves in a direction toward the center axis <NUM>, such that it is concave when viewed from the center axis <NUM> and convex when viewed from outside the diffuser pipe <NUM>. The pipe outlet <NUM> in <FIG> is thus free of planar portions. The radially inner and outer walls 22A,22B and the side walls 22D are curved along all or substantially all of their lengths. The radially inner and outer walls 22A,22B and the side walls 22D have curvatures greater than zero along all or substantially all of their lengths. The radially inner and outer walls 22A,22B and the side walls 22D are free of straight lines along all or substantially all of their lengths.

Referring to <FIG>, the pipe outlet <NUM> is symmetric about a line H defining a height of the tubular body <NUM>. The pipe outlet <NUM> thus has the same shape or contour on each side of the line H. When one half of the pipe outlet <NUM> is folded about the line H, it will have the same shape as the other half of the pipe outlet <NUM>. Referring to <FIG>, the line H extends between the radially inner and outer walls 22A,22B. The line H extends generally radially to the center axis <NUM>, where "generally radially" is understood to mean that the line H may have axial, radial, and/or circumferential directional components, but that the axial and circumferential directional components are much smaller in magnitude than the radial directional component. Referring to <FIG>, the line H defines the maximum height of the tubular body <NUM> between an apex point AP on the radially outer wall 22B and the furthest point from the apex point AP on the radially inner wall 22A, defined along a generally radial direction. Referring to <FIG>, the line H is defined between radially spaced-apart maxima and minima on the radially outer wall 22B and the radially inner wall 22A, respectively. Referring to <FIG>, the line H is a generally radial line relative to the center axis <NUM> that extends from an inflection point on the curved, radially outer wall 22B to a point on the radially inner wall 22A. Referring to <FIG>, the line H extends through the pipe center axis <NUM>.

The diffuser pipe <NUM> disclosed herein therefore has, for one or more locations along its length L, a cross-sectional profile <NUM> that is curved along both of its radially inner and outer walls 22A,22B, and which is symmetrical about a generally radial line through the cross-sectional profile <NUM>. This shape for the cross-sectional profile <NUM> may be referred to as an elliptical polygon (or "elliptogon"). As described in greater detail below, other similar shapes for the cross-sectional profiles <NUM> of the diffuser pipe <NUM> are also possible, such that the present disclosure presents different diffuser pipe <NUM> cross sectional shapes that may improve the stiffness of the diffuser pipe <NUM> stiffness by increasing its moment of inertia while maintaining its performance.

The shapes for the cross-sectional profile <NUM> may help to increase the dynamic stiffness of the diffuser pipe <NUM> while retaining its aerodynamic performance. The diffuser pipe <NUM> is joined to the casing of the impeller <NUM> such that the tubular body <NUM> is cantilevered from the point of attachment and is subjected to bending or flexion. The tubular body <NUM> at the inlet <NUM> may have a flange or other mounting member that may be fastened to the casing of the impeller <NUM> to fixedly attach the diffuser pipe <NUM> to the casing. The unattached remainder of the diffuser pipe <NUM>, and the pipe outlet <NUM>, "overhangs" and is free of other structural support, such that they are cantilevered from the casing. This may cause a movement of the pipe outlet <NUM> toward and away from the center axis <NUM>, a movement sometimes referred to as "flapping", during operation of the engine <NUM>. Additionally, the shapes of the cross-sectional profile <NUM> as described herein (including the elliptogon, kidney and peanut shapes) may provide an increase in the area moment of inertia of the diffuser pipes <NUM> relative to typical, elliptically shaped, pipe such as that of the reference cross-sectional profile 27R. The area moment of inertia of the cross-sectional profile <NUM> is a property which can be used to predict deflection and/or bending stress of the pipe having such a cross-sectional profile. Because the proposed shapes of the cross-sectional profile <NUM> have a greater area moment of inertia relative to a corresponding elliptical profile 27R, the diffuser pipe <NUM> having such a cross-sectional profile <NUM> may be stiffer and thereby increasing its natural frequency.

By providing the cross-sectional profile <NUM> with the shapes, it may be more difficult for the pipe outlet <NUM> to displace radially during operation of the engine <NUM> such that the diffuser pipe <NUM> is stiffened. By providing the cross-sectional profile <NUM> with the shapes, it may be possible to increase an area moment of inertia (i.e. a property of a shape used to predict deflection and bending stress). Since the cross-sectional profile <NUM> with the shapes may increase the moment of inertia, the diffuser pipe <NUM> may be stiffer, thereby increasing a natural frequency of the diffuser pipe <NUM>. By providing the cross-sectional profile <NUM> with the shapes, it may thus be possible to change the natural frequency of the diffuser pipe <NUM>, such that the natural frequency is outside the range of certain vibratory frequencies which can exist within the engine operating envelope and can cause cracking and fatigue of the diffuser pipe <NUM>. By providing the cross-sectional profile <NUM> with the shapes, it may be possible to tune or select the natural frequency of the diffuser pipe <NUM> such that the natural frequency does not coincide with engine dynamics excitation frequencies over the entire engine operating range. Providing the cross-sectional profile <NUM> with the shapes may allow the length L of the diffuser pipe <NUM> to be increased without negatively impacting its vibrational response or its aerodynamic response, and thus make such a longer diffuser pipe <NUM> suitable for use in an engine <NUM> with increased power or size. Furthermore, providing the diffuser pipe <NUM> with the shapes of the cross-sectional profile <NUM> may not require expensive manufacturing techniques or retooling. By making the cross-sectional profile <NUM> "taller" by curving radially outwardly the radially outer wall 22B, it may be possible to stiffen the diffuser pipe <NUM> against flexion or bending motions.

There are many possible shapes for the cross-sectional profiles <NUM> within the scope of the present disclosure. For example, and referring to <FIG>, the shape of the cross-sectional profile <NUM> at the pipe outlet <NUM> is an elliptical polygon (sometimes referred to as an elliptogon). The shape of the cross-sectional profile <NUM> at the pipe outlet <NUM> is not oblong, where an oblong shape is an elongated rectangle or oval with parallel sides. The shape of the pipe outlet <NUM> is not oval. The shape of the pipe outlet <NUM> is different from a shape defined by two semi-circles with the same radius spaced apart and interconnected by parallel lines. The shape of the pipe outlet <NUM> has all curved lines represented by the radially inner and/or outer walls 22A,22B and side walls 22D. The shape of the pipe outlet <NUM> is free of parallel lines. Some conventional pipes, in contrast, have oblong, elliptical and symmetrical cross-sectional shapes along the downstream region of the diffuser pipe. Some non-limiting examples of specific shapes for the cross-sectional profiles <NUM> are now described in greater detail.

Referring to <FIG>, the shape for the pipe outlet <NUM> may be referred to as an elliptical polygon (or "elliptogon"). A radius of curvature ROW of the curved radially outer wall 22B is different from a radius of curvature RIW of the curved radially inner wall 22A. The radii of curvature ROW,RIW have different values, such that the curvatures of the radially inner and outer walls 22A,22B are different. In the shape of the pipe outlet <NUM> shown in <FIG>, the curvature of the radially inner wall 22A is much smaller than the curvature of the radially outer wall 22B. The radius of curvature RIW for the radially inner wall 22A is therefore much larger than the radius of curvature ROW of the radially outer wall 22B. For example, the radially inner wall 22A may have a curvature value of very small magnitude, particularly in comparison to the curvature value of the radially outer wall 22B. For example, and referring to <FIG>, a middle portion of the radially inner wall 22A around the line H is slightly curved, but the magnitude of its curvature is small and orders of magnitude less than the curvature of the radially outer wall 22B. In an embodiment, the radius of curvature RIW of the radially inner wall 22A tends toward infinity, and the radially inner wall 22A is represented in the cross-sectional profile <NUM> as an almost straight line being transverse to the line H. Referring to <FIG>, the line H is perpendicular to the almost straight radially inner wall 22A.

The apex point AP is a point on the line H. The apex point AP is the location on the radially outer wall 22B that is furthest from the center axis <NUM>. The apex point AP is the location on the radially outer wall 22B at which there is an inflection point in the curve of the radially outer wall 22B. The apex point AP is the location on the radially outer wall 22B at which the tangent to the curve of the radially outer wall 22B changes between a negative value for the slope of the tangent line and a positive value for the slope. From the apex point AP, the radially outer wall 22B curves in a direction toward the radially inner wall 22A (i.e. a radially inward direction) toward a radially outer end of each of the first and second side walls 22C,22D. The radially outer wall 22B has two curved segments on either side of the line H. From the apex point AP, each of the curved segments curves in a direction away from the apex point AP and toward the radially inner wall 22A, and joins to a radially outer end of one of the first and second side walls 22C,22D.

Referring to <FIG>, the pipe outlet <NUM> includes a width line WL extending between points of the first and second side walls 22C,22D that are disposed furthest from one another. The length of the width line WL corresponds to the largest width of the pipe outlet <NUM>, where the width is defined between the first and second side walls 22C,22D. The width line WL may or may not extend through the pipe center axis <NUM>. The width line WL intersects the line H defining the height of the tubular body <NUM> and divides the line H into a first height portion HP1 extending between the apex point AP on the radially outer wall 22B and the width line WL, and a second height portion HP2 extending between the radially inner wall 22A and the width line WL. The length of the first and second height portions HP1,HP2 are parameters which can be selected by a designer of the diffuser pipe <NUM> to obtain the desired shape for the pipe outlet <NUM>. For the pipe outlet <NUM> disclosed herein, a ratio of the first height portion HP1 over the second height portion HP2 (i.e. HP1/HP2) may be between about <NUM> and <NUM>. The first height portion HP1 is thus always greater than the second height portion HP2, by a factor ranging in value from <NUM> to <NUM>. This range of ratios helps to ensure that the radially outer wall 22B is curved, such that the shape of the pipe outlet <NUM> may be the desired elliptical polygon through the range of ratio values.

Referring to <FIG>, the line H defining the height intersects the width line WL and divides the width line into a first width portion WP1 extending between one of the first and second side walls 22C,22D and the line H, and a second width portion WP2 extending between the other of the first and second side walls 22C,22D and the line H. The length of the first and second width portions WP1,WP2 are parameters which can be selected by a designer of the diffuser pipe <NUM> to obtain the desired shape for the pipe outlet <NUM>. For the pipe outlet <NUM> disclosed herein, a ratio of the first width portion WP1 over the second width portion WP2 (i.e. WP1/WP2) may be approximately <NUM>. The first width portion WP1 is thus equal to the second width portion WP2. This helps to ensure that the pipe outlet <NUM> is symmetrical about the line H defining the height of the tubular body <NUM>, such that the shape of the pipe outlet <NUM> may be the desired elliptical polygon. Referring to <FIG>, the ratio of the first width portion WP1 over the second width portion WP2 remains approximately <NUM> at all points along the line H, such that the first side wall 22C and the second side wall 22D are both symmetrical about the line H at all radial points thereon. The pipe outlet <NUM> in <FIG> is a symmetric elliptogon shape. In an alternative possible shape for the pipe outlet <NUM> that is asymmetrical about the line H, the ratio of the first width portion WP1 over the second width portion WP2 may be between <NUM> and <NUM>, such that the asymmetry of the pipe outlet <NUM> may be on either side of the line H. Referring to <FIG>, the pipe outlet <NUM> is asymmetric about the width line WL. The shape or contour of the pipe outlet <NUM> is different on each radially-opposite side of the width line WL. The radially outer wall 22B and the radially inner wall 22A are not symmetrical about the width line WL.

Although the cross-sectional profile <NUM> at the pipe outlet <NUM> has a shape different from the oblong or pure ellipse cross-sectional shape of a conventional pipe, the cross-sectional profile <NUM> may have the same area and/or same parameters as a conventional oblong or pure ellipse cross-sectional shape. Referring to <FIG>, a reference cross-sectional profile 27R is defined in the plane normal to the pipe center axis <NUM> at the same location along the pipe center axis <NUM> as the pipe outlet <NUM>. The reference cross-sectional profile 27R is shaped as an ellipse and defines a reference width WR along a major axis and a reference height HR along a minor axis. The shape of the pipe outlet <NUM> is different than the ellipse shape of the reference cross-sectional profile 27R. Despite the differences in shape, the reference width WR of the reference cross-sectional profile 27R is equal to the width of the pipe outlet <NUM>, represented in <FIG> by the width line WL. In some designs for diffuser pipes <NUM>, the width of the flow passage <NUM> is an important design parameter affecting the aerodynamic performance of the diffuser pipe <NUM>. Therefore, by equating the width of the cross-sectional profile <NUM> to the width of a conventional elliptical cross-sectional shape for a pipe, the designer of the diffuser pipe <NUM> is able to better benchmark the aerodynamic performance of the diffuser pipe <NUM> against a conventional "elliptical" diffuser pipe. Furthermore, the circumferential envelope of the diffuser pipe <NUM> may be the same as that of a conventional "elliptical" diffuser pipe because their widths are the same, such that no reconfiguration or redesign of engine components near the diffuser pipes <NUM> may required. In an embodiment, the maximum height, measured along a general radial line to the center axis <NUM>, of the cross-sectional profile <NUM> is equal to the maximum height HR of the reference cross-sectional profile 27R.

Referring to <FIG>, the area of the cross-sectional profile <NUM> at the pipe outlet <NUM> is equal to the area of the reference cross-sectional profile 27R. Thus, despite the cross-sectional profile <NUM> at the pipe outlet <NUM> having a shape different from the oblong or pure ellipse cross-sectional shape of a conventional pipe, the aerodynamic performance of the diffuser pipe <NUM> may be compared or benchmarked against that of a conventional "elliptical" diffuser pipe because their cross-sectional areas are the same. Furthermore, the radial and circumferential envelope of the diffuser pipe <NUM> may be the same as that of a conventional "elliptical" diffuser pipe because their cross-sectional areas are the same, such that no reconfiguration or redesign of engine components near the diffuser pipes <NUM> may be required. It can thus be appreciated that the cross-sectional area of the diffuser pipe <NUM> is not changed compared to a conventional diffuser pipe, just its shape. The cross-sectional width of the diffuser pipe <NUM> may also remain the same as the cross-sectional width of a conventional diffuser pipe. Referring to <FIG>, a center of area CA of the cross-sectional profile <NUM> at the pipe outlet <NUM> is closer to the radially inner wall 22A than a center of the reference area CRA of the reference cross-sectional profile 27R is closer to its radially inner wall. The cross-sectional profile <NUM> thus has a "lower" or "dropped" (i.e. disposed closer to the center axis <NUM>) center of area CA than the center of the reference area CRA, despite both cross-sectional profiles <NUM>,27R having the same area.

Referring to <FIG>, the radially inner wall 22A has a compound curvature. The radially inner wall 22A in the cross-sectional profile <NUM> shown is made up of two or more curves with different radii that bend the same way and are on the same side of a common tangent. Thus, the radius of curvature RIW of the radially inner wall 22A varies, or does not remain constant, between the side walls 22D.

Referring to <FIG>, the compound curve of the radially inner wall 22A has a first curved portion 22ACP1 joined to one of the side walls 22D and having a first radius of curvature RC1, a second curved portion 22ACP2 joined to the other of the side walls 22D and having a second radius of curvature RC2, and a third curved portion 22AI between the first and second curved portions 22ACP1,22ACP2 having a third radius of curvature RC3. The first and second curved portions 22ACP1,22ACP2 are similarly curved, such that the first radius of curvature RC1 is approximately equal to the second radius or curvature RC2. The third curved portion 22AI is curved differently from the curvature of first and second curved portions 22ACP1,22ACP2, such that the third radius of curvature RC2 is different from the first and second radii of curvature RC1,RC2. The third curved portion 22AI defines an apex point of the radially inner wall 22A, which is the point on the radially inner wall 22A that is furthest from the center axis <NUM>. The first and second curved portion 22ACP1,22ACP2 each define proximal points of the radially inner wall 22A, which are the points on the radially inner wall 22A that are closest to the center axis <NUM>. At each of the apex and proximal points, tangents to the radially inner wall 22A are define which are parallel to the width line WL. Thus, in the shape of the pipe outlet <NUM> shown in <FIG>, at least the radially inner wall 22A has multiple and different radii of curvature. Still referring to <FIG>, the radius of curvature ROW of the radially outer wall 22A remains substantially constant, or is a simple curve, throughout its length between the side walls 22D. The radius of curvature ROW of the radially outer wall 22B does not define a compound curve.

Other elliptical polygon shapes for the cross-sectional profile <NUM> are possible and within the scope of the present disclosure. For example, and referring to <FIG>, the elliptical polygon shape of the cross-sectional profile <NUM> of <FIG> is flipped or inverted. In <FIG>, the radially inner wall 22A of the cross-sectional profile <NUM> has a curvature greater than the curvature of the radially outer wall 22B. The disclosure above related to the curved radially inner and outer walls 22A,22B of <FIG> applies mutatis mutandis to the curved radially inner and outer walls 22A,22B of <FIG>. Such an inverted shape for the cross-sectional profile <NUM> in <FIG> may provide the same stiffening structural benefits to the diffuser pipe <NUM> that are described above. Another possible elliptical polygon shape for the cross-sectional profile <NUM> is shown in <FIG>, in which the radially inner wall 22A is curved outwardly relative to the pipe center axis <NUM>.

Yet another possible elliptical polygon shape for the cross-sectional profile <NUM> is described with reference to <FIG>. Both the radially outer wall 22B and the radially inner wall 22A of the cross-sectional profile <NUM> are curved. The cross-sectional profile <NUM> has a peanut or kidney shape, in that part of the radially inner wall 22A protrudes toward the radially outer wall 22B. More particularly, and referring to <FIG>, the radially inner wall 22A is a compound curve that has an indented portion 22AI (corresponding to the third curved portion 22AI described above) that extends toward the radially outer wall 22B. The indented portion 22AI is a local depression in the radially inner wall 22A. The indented portion 22AI is symmetrical about the line H defining the height of the tubular body <NUM>. The indented portion 22AI includes a peak point PP that is on a radial line from the center axis <NUM> and on the line H. The indented portion 22AI includes the portions of the radially inner wall 22A that are positioned furthest from the center axis <NUM>. Referring to <FIG>, the radially inner wall 22A has two lateral portions 22AL (corresponding to the first and second curved portions 22ACP1,22ACP2 described above) disposed on opposite circumferential sides of the indented portion 22AI. The lateral portions 22AL are positioned closer to the center axis <NUM> than the indented portion 22AI. The lateral portions 22AL are symmetrical about the line H. The disclosure above related to the compound curve of the radially inner wall 22A and the radii of curvature ROW,RIW,RC1,RC2,RC3 of <FIG> applies mutatis mutandis to the compound curve of the radially inner wall 22A of <FIG>. Referring to <FIG>, the first and second side walls 22C,22D are free of any indentations. As the fluid flow F separates on the radially inner side of the diffuser pipe <NUM> after the bend portion <NUM>, the peanut or kidney shape provided by the indented portion 22AI of the radially inner wall 22A may have positive aerodynamic effects on the fluid flow F by containing or confining low momentum flow in the "valleys" of the lateral portions 22AL.

Referring to <FIG>, the shape of the cross-sectional profile <NUM> at the pipe outlet <NUM> is an elliptical polygon (sometimes referred to as an elliptogon), and more specifically is a kidney shape. The pipe outlet <NUM> is not oblong, where an oblong shape is an elongated rectangle or oval with parallel sides. The shape of the pipe outlet <NUM> is not oval. The shape of the pipe outlet <NUM> is different from a shape defined by two semi-circles with the same radius spaced apart and interconnected by parallel lines. The shape of the pipe outlet <NUM> has all curved lines represented by the radially inner and/or outer walls 22A,22B and side walls 22D. The shape of the pipe outlet <NUM> is free of parallel lines. Some conventional pipes, in contrast, have oblong, elliptical or symmetrical cross-sectional shapes along the downstream region of the diffuser pipe.

Referring to <FIG>, the peanut or kidney shape may be governed by the ratio of a height HIP of the indented portion 22AI over a height HCS of the cross-sectional profile <NUM>. The height HIP of the indented portion 22AI is measured along a line being substantially radial to the center axis <NUM>. The line extends from a first tangent at an inflection point of the curved lateral portions 22AL, to a second tangent at an inflection point of the curved indented portion 22AI. The height HCS of the cross-sectional profile <NUM> extends between the second tangent and the apex point AP of the radially outer wall 22B. For the cross-sectional profile <NUM> shown in <FIG>, a ratio of the height HIP of the indented portion 22AI over the height HCS of the cross-sectional profile <NUM> may be between <NUM> and <NUM>. The height HCS of the cross-sectional profile <NUM> is thus always greater than the height HIP of the indented portion 22AI. Where the value of the ratio is zero, the radially inner wall 22A have very little curvature. Where the value of the ratio is greater than zero, the range of ratios helps to ensure that the radially inner wall 22A is curved and contributing to the desired peanut or kidney shape of the cross-sectional profile <NUM>. The peanut or kidney shape of the cross-sectional profile <NUM> may take other forms as well. For example, in one non-limiting example, the radially outer wall 22B also has an indented portion 22AI. For example, in another non-limiting example, the cross-sectional profile <NUM> is inverted, such that only the radially outer wall 22B has the indented portion 22AI.

Referring to <FIG>, the cross-sectional profile <NUM> is symmetric about the line H defining a height of the tubular body <NUM>. The outlet <NUM> in <FIG> is a symmetrical kidney shape. In an alternative possible shape for the cross-sectional profile <NUM> that is asymmetrical about the line H, the ratio of the first width portion WP1 over the second width portion WP2 may be such that the asymmetry of the cross-sectional profile <NUM> may be on either side of the line H. Referring to <FIG>, the cross-sectional profile <NUM> is asymmetric about the width line WL. The shape or contour of the cross-sectional profile <NUM> is different on each side of the width line WL. The radially outer wall 22B and the radially inner wall 22A are not symmetrical about the width line WL.

Referring to <FIG>, the area of the cross-sectional profile <NUM> is equal to the area of the reference cross-sectional profile 127R that is elliptical and which has the same width WL and the same height measured along the line H. Thus, despite the cross-sectional profile <NUM> having a shape different from the oblong or pure ellipse cross-sectional shape of a conventional pipe, the aerodynamic performance of the diffuser pipe <NUM> may be compared or benchmarked against that of a conventional "elliptical" diffuser pipe because their cross-sectional areas are the same. Furthermore, the radial and circumferential envelope of the diffuser pipe <NUM> may be the same as that of a conventional "elliptical" diffuser pipe because their cross-sectional areas are the same, such that no reconfiguration or redesign of engine components near the diffuser pipes <NUM> may be required. Referring to <FIG>, a center of area CA of the cross-sectional profile <NUM> is closer to the radially inner wall 22A than a center of the reference area CRA of the reference cross-sectional profile 127R. The cross-sectional profile <NUM> thus has a "lower" or "dropped" (i.e. disposed closer to the center axis <NUM>) center of area CA than the center of the reference area CRA, despite both cross-sectional profiles <NUM>,127R having the same area.

Although <FIG> and <FIG> show a single cross-sectional profile <NUM>,<NUM> at a particular location along the diffuser pipe <NUM>, it will be appreciated that the cross-sectional profiles <NUM>,<NUM>, as well as any other cross-sectional shapes or contours disclosed herein, may also be present at other locations along the length L of the diffuser pipe <NUM>. Referring to <FIG> and <FIG>, the elliptogon and/or kidney or peanut cross-sectional profiles <NUM>,<NUM> are present at every point of the pipe center axis <NUM> from the bend portion <NUM> to the pipe outlet <NUM>. More particularly, the cross-sectional profiles <NUM>,<NUM> are present along all points of the length L of the tubular body <NUM> from an upstream end 28A of the bend portion <NUM> to the pipe outlet <NUM>. The upstream end 28A is the extremity of the bend portion <NUM> closest to the radially-extending first portion <NUM>. The cross-sectional profiles <NUM>,<NUM> thus extend along all of the length of the bend portion <NUM> and the second portion <NUM> to reinforce or stiffen these portions of the diffuser pipe <NUM>. The cross-sectional profiles <NUM>,<NUM> may start at the bend portion <NUM> and "blend" or become more pronounced (e.g. greater curvature to the radially outer wall 22B, greater height HIP of the indented portion 22AI, etc.) in the direction of the fluid flow F along the pipe center axis <NUM> toward the pipe outlet <NUM>. In an embodiment, the cross-sectional profiles <NUM>,<NUM> are not present in the radial, first portion <NUM> of the diffuser pipe <NUM> since there may be no bending or flexion moment acting on the first portion <NUM>. In an embodiment, the cross-sectional shape along the first portion <NUM> is elliptical. In an embodiment, the cross-sectional shape of the inlet <NUM> is circular.

<FIG> compare the Mach number of the fluid flow F through diffuser pipes <NUM> having different cross-sectional shapes. The cross-sectional shape of the diffuser pipe in <FIG> is elliptical, and thus resemble the shape of a conventional diffuser pipe. The cross-sectional shape of the diffuser pipe <NUM> in <FIG> is elliptical polygonal, and is defined by the cross-sectional profile <NUM>. The cross-sectional shape of the diffuser pipe <NUM> in <FIG> is elliptical polygonal with a peanut or kidney shape, and is defined by the cross-sectional profile <NUM>. <FIG> show that the fluid flow F at various sections along the diffuser pipe is substantially the same for all three cross-sectional shapes, suggesting that the cross-sectional profiles <NUM>,<NUM> disclosed herein may stiffen the diffuser pipe <NUM> without negatively impacting its aerodynamic performance when compared to an "elliptical" diffuser pipe.

Referring to <FIG> and <FIG>, there is disclosed a method of stiffening (i.e. reducing the flexion) of the diffuser pipe <NUM> which is cantilevered at its inlet <NUM> to the casing of the impeller <NUM>. The method includes providing a cross-sectional profile <NUM>,<NUM> to the diffuser pipe <NUM> in its bend and/or axial portions <NUM>,<NUM>. The cross-sectional profile <NUM>,<NUM> is defined by a curved radially outer wall 22B and has symmetry about the line H defining a height of the diffuser pipe <NUM>.

Referring to <FIG> and <FIG>, there is disclosed a method of stiffening (i.e. reducing the flexion) of the diffuser pipe <NUM> which is cantilevered at its inlet <NUM> to the casing of the impeller <NUM>. The method includes providing a cross-sectional profile <NUM>,<NUM> to the diffuser pipe <NUM> in its bend and/or axial portions <NUM>,<NUM>. The cross-sectional profile <NUM>,<NUM> is defined by a curved radially inner and outer walls 22A,22B, where the radius of curvature ROW of the radially outer wall 22B is different from the radius of curvature RIW of the radially inner wall 22A.

The diffuser pipes <NUM> disclosed herein may have dimples, which are extrusions or impressions in one of the radially inner and outer walls 22A,22B, in addition to the cross-sectional profiles <NUM>,<NUM> disclosed herein. Dimples may give the diffuser pipe <NUM> a different natural frequency such that it is out of the range of dynamic frequencies during operation of the engine <NUM>, thereby contributing to tuning the diffuser pipe <NUM> modes out of running range at high speeds. When employed with dimples, the cross-sectional shapes disclosed herein may help to reduce risks of the diffuser pipe <NUM> cracking due to high cycle fatigue (HCF).

Claim 1:
A centrifugal compressor (<NUM>) for an aircraft engine (<NUM>), the centrifugal compressor (<NUM>) having an impeller (<NUM>) and a compressor diffuser, the compressor diffuser comprising a plurality of diffuser pipes (<NUM>) disposed circumferentially about a center axis (<NUM>) of the compressor diffuser, the center axis (<NUM>) extending in an axial direction, a diffuser pipe (<NUM>) of the plurality of diffuser pipes (<NUM>) extending from an inlet (<NUM>) of that diffuser pipe (<NUM>) to an outlet (<NUM>) of that diffuser pipe (<NUM>) and increasing in cross-sectional area from the inlet (<NUM>) to the outlet (<NUM>), the outlet (<NUM>) opening in the axial (<NUM>) direction, wherein at least the outlet (<NUM>) is defined by a radially inner wall (22A), a radially outer wall (22B), and side walls (22C,22D) joining the radially inner wall (22A) to the radially outer wall (22B),
the centrifugal compressor being characterised in that, both the radially inner wall (22A) and the radially outer wall (22B) are curved, a radius of curvature (ROW) of the radially outer wall (22B) being different from a radius of curvature (RIW) of the radially inner wall (22B), and in that, the diffuser pipe (<NUM>) is cantilevered at the inlet (<NUM>) to a casing of the impeller (<NUM>).