AXIAL CENTRIFUGAL COMPRESSOR WITH SCALABLE RAKE ANGLE

Abstract

A mobile wheel for a centrifugal or axial centrifugal compressor for a turbine engine, that includes a base on which a plurality of vanes are attached at their feet and extend between a leading edge and a trailing edge, the trailing edge being inclined at the base of the vane by a slope angle relative to the middle plane extending through the base thereof in the rotation direction of the compressor. The trailing edge has at the head of the vane a slope angle smaller than the slope angle at the vane base.

Description

The field of the present invention is that of turbomachines and more particularly that of turbomachine compressors comprising at least one centrifugal or mixed (axial-centrifugal) compression stage.

Aeronautical turbine engines and in particular turbine engines for helicopters generally comprise a compressor of the centrifugal or mixed type as a compression stage of the gas generator. These compressors are formed by moving wheels or impellers which have a substantially axial air inlet and a more radial outlet, the blades being fixed on a base in the form of a bell which serves as a guide for the air flow and diverts it from the inlet direction toward the outlet direction. The flow thus circulates between this base and a compressor casing; it is compressed and propelled by the various blades of the impeller.

The head of the blading is free and positioned opposite the casing, or cover, which closes the air duct. The trailing edge is rectilinear in the prior art and may be inclined with respect to the meridian plane of the impeller passing through the foot of the blade. The angle which it makes with this meridian plane is designated by the term rake angle, or slope angle of the trailing edge, and constitutes one of the relevant geometric parameters for describing a centrifugal impeller blading.

The choice of a significant slope angle (of the order, for example, of 20°) at the trailing edge of a centrifugal impeller is a known means of reducing the stresses at the blade foot which are due in particular to the bending of the blading under centrifugal force. It thus allows a higher-performance design of the compressor, and makes it possible in particular:

• • to reduce, at equal speed of rotation, the thickness of the blading, and hence that of the shell of the base, and thus to significantly reduce the mass and the inertia of the rotor,
  • or else, at equal mass and inertia for the impeller, to increase the speed of rotation.

Document U.S. Pat. No. 5,730,582, which describes a centrifugal impeller for a turbomachine compressor, is known, for example. In this impeller, which has the shape of a helix inscribed in a cylinder, the slope angle is constant all along the trailing edge.

The consequence of choosing a significant slope angle, with a rectilinear trailing edge, is that the head of the blading is inclined by the same angle as the foot with respect to this meridian plane. In the case of accidental contact between the blading and the cover, if the trailing edge is inclined in the direction of rotation of the impeller, the head of the blade has a tendency to auto-engage in the metal, which risks causing significant degradation.

Also known is document U.S. Pat. No. 4,819,884, which relates to a ventilation device comprising an impeller in which the slope angle continuously increases from the foot of the blades toward their head and in which the head is held in a plate, thus avoiding any risk of the blades engaging in the casing of the device.

The aim of the present invention is to overcome these disadvantages by providing a centrifugal or mixed axial-centrifugal compressor which does not have some of the disadvantages of the prior art and, in particular, which has a trailing edge with a significant slope angle at its foot, without there being produced a phenomenon of auto-engagement of the head of the blade in the case of contact thereof with the cover.

Accordingly, the subject of the invention is a moving wheel for a centrifugal or mixed axial-centrifugal compressor of a turbomachine, comprising a base on which a plurality of blades extending between a leading edge and a trailing edge are fixed by their feet, said trailing edge being inclined at the foot of the blade by a slope angle with respect to the meridian plane which passes through it at its foot, in the direction of rotation of said compressor, characterized in that the trailing edge has at the head of the blade a slope angle β which is less than its slope angle α at the blade foot.

Since the slope angle at the head is smaller, the corner formed by the trailing edge and the head of the blade moves more tangentially with respect to the casing and is therefore less capable of penetrating therein in the case of accidental contact.

Preferably, the slope angle β of the trailing edge at the blade head is close to zero, or even zero. Even more preferably, the slope angle β of the trailing edge at the blade head is negative. These configurations provide greater assurance that the corner at the head of the trailing edge cannot become implanted in the casing in the case of accidental contact.

In one particular embodiment, the curvature of the trailing edge is constant between the foot and the head of said trailing edge. The trailing edge thus has the shape of an arc of a circle.

In another embodiment, the curvature of the trailing edge is continuously variable between the foot and the head of said trailing edge. The trailing edge thus has the shape of a helix.

The invention also relates to a turbomachine comprising a compressor having one of the characteristics described above. Finally, it relates to an aircraft equipped with such a turbomachine.

The invention will be better understood, and other aims, details, characteristics and advantages thereof will emerge more clearly, in the course of the following detailed explanatory description of an embodiment of the invention given by way of purely illustrative and nonlimiting example, with reference to the appended schematic drawings, in which:

FIG. 1 is a perspective view of a centrifugal compressor impeller according to an embodiment of the invention;

FIG. 2 is a front view of a trailing edge of a centrifugal compressor blade according to the prior art, and

FIG. 3 is a front view of a trailing edge of a centrifugal compressor blade according to an embodiment of the invention.

With reference to FIG. 1, there can be seen a centrifugal compressor 1 comprising a base 2, in the form of a bell, on which compressor blades 3 distributed uniformly over the periphery of the base 2 are fixed. The base 2 terminates at its upper part, which corresponds to the inlet plane of the air to be compressed, in a cylinder forming a hub 4 for the compressor, and, at its lower part, in a disk 5 oriented radially with respect to the axis of rotation of said compressor. The blades 3 are held by the base 2 at their feet 6; the heads 7 of these blades are free and rotate inside a compressor casing (not shown). In the direction of flow of the air stream, the blades extend between a leading edge 8 situated at the hub 4 and a trailing edge 9 situated at the disk 5.

With reference now to FIG. 2, there can be seen a trailing edge 9 of a compressor blade 3, according to the prior art. This trailing edge is, at its foot, inclined with respect to the meridian plane which passes through it, in the direction of rotation, by a slope angle α. This trailing edge 9 is rectilinear, that is to say that it has the same inclination, equal to its value α at the blade foot, over all its height, from the foot to the head of the trailing edge 9.

By contrast, in FIG. 3, the trailing edge 9 has a shape which is changeable along its height. It is inclined by a slope angle α in the direction of rotation, at the foot 6 of the blade 3, which is defined as in the prior art, but this angle becomes progressively smaller in the upward direction along the trailing edge so as to reach a value β at the head 7 of the blade 3, which is less than α. This value β may be zero or even negative, that is to say that the head of the blade is then inclined in the opposite direction to the direction of rotation of the impeller.

The proposed invention therefore consists in choosing, at the trailing edge 9, a large slope angle α at the blade foot 6, then in applying a curvature to the blading 3 so as to find a smaller, or even zero or negative, slope angle at the head of the blading. The presence of a large slope angle α at the blade foot 6 makes it possible to reduce the bending stresses at the foot, whereas the small slope angle β at the head 7 reduces the risks of auto-engagement of the blade 3 in the casing in the case of accidental contact between the head 7 of the blade at the trailing edge 9 and the casing.

The production of such a profile at the trailing edge 9 is based on the point machining method which allows 3 D blading geometries; by contrast, the flank machining previously used restricted the possibilities by imposing ruled surfaces (that is to say consisting of a stack of straight lines) for the design of the profile of the blades.

This modification of the geometry of the profile is not manifested by a significant aerodynamic impact. The use of blades 3 according to the invention even contributes, in a surprising manner, rather to an increase in the performance of the compressor, resulting from the reduction in the slope angle along the height of the trailing edge 9.

The dual reduction of the stresses in the blading and of the auto-engaging nature of the head of the blade, by application of the invention, have been verified by calculation on actual blading.

The invention therefore allows a saving in mass and inertia or, with equal mass and equal inertia, an increase in the speed of rotation without degrading the behavior in the case of contact between the blade 3 and the casing.

Many variants can be envisioned for the change in the curvature of the blade 3 at the trailing edge 9 between its foot 6 and its head 7. The curvature may, for example, be constant, the trailing edge having the shape of an arc of a circle, or else be continuously variable, the profile of the trailing edge then having the shape of a helix. The curvature between the foot and the head of the blade is chosen according to the impact that it has on the aerodynamic performance of the compressor.

Although the invention has been described in relation to a particular embodiment, it is clearly obvious that it is not in any way restricted thereto and that it encompasses all the technical equivalents of the means described and combinations thereof where these fall within the scope of the invention.

Claims

1-8. (canceled)

9. A moving wheel for a centrifugal or mixed axial-centrifugal compressor of a turbomachine, comprising:

a base on which a plurality of blades extending between a leading edge and a trailing edge are fixed by their feet, the trailing edge being inclined at a foot of the blade by a slope angle α with respect to the meridian plane that passes through it at its foot, in a direction of rotation of the compressor,

wherein the trailing edge has at the head of the blade a slope angle β that is less than its slope angle α at the blade foot.

10. The moving wheel as claimed in claim 9, in which the slope angle β of the trailing edge at the blade head is close to zero, or is zero.

11. The moving wheel as claimed in claim 9, in which the slope angle β of the trailing edge at the blade head is negative.

12. The moving wheel as claimed in claim 9, in which curvature of the trailing edge is constant between the foot and the head of the trailing edge.

13. The moving wheel as claimed in claim 9, in which curvature of the trailing edge is continuously variable between the foot and the head of the trailing edge.

14. A compressor comprising a moving wheel as claimed in claim 9.

15. A turbomachine comprising a compressor as claimed in claim 14.

16. An aircraft comprising a turbomachine as claimed in claim 15.