Compressor cover for turbine engine having axial abutment

A centrifugal compressor for a turbine engine, including a cover with an upstream end and a downstream end; a casing presenting an upstream edge and a downstream edge; and a bladed impeller mounted to rotate in the casing. The cover covers the blades of the impeller to define an outside surface of a gas-flow passage extending between the upstream and downstream edges of the casing, while being fastened to the upstream edge of the casing via its upstream end while its downstream end remains free. The cover further includes an abutment limiting axial movement of its downstream end relative to the downstream edge of the casing while the compressor is in operation.

BACKGROUND

The present invention relates to the field of gas turbines, in particular those to be found in turbomachines, and by way of non-limiting examples but not only in the turbine engines of helicopters or in the turbojets for airplanes.

The present invention relates more particularly to the compression stage of such gas turbines that constitute the main power plant of an aircraft.

Still more precisely, the present invention relates to a centrifugal compressor of a turbine engine, the compressor comprising:a cover including an upstream end and a downstream end;a casing presenting an upstream edge and a downstream edge; anda bladed impeller mounted to rotate in said casing;

said cover being designed to cover the blades of the impeller so as to define an outside surface of a gas-flow passage extending between the upstream and downstream edges of the casing, being fastened to the upstream edge of the casing via its upstream end while its downstream end remains free.

Conventionally, the compressor is placed between a fresh air inlet and a combustion chamber, the role of the compressor being to compress the fresh air entering into the gas turbine and to convey the compressed air into the combustion chamber in order to be mixed with fuel.

Furthermore, it is known that an impeller comprises a plurality of blades extending generally radially from an impeller hub, which hub is fastened to a rotary shaft of the gas turbine.

Thus, the gas stream initially enters into the casing of the compressor via an upstream inlet, and then flows along a gas-flow passage defined between an outside surface defined by the cover and an inside surface defined by a surface of the impeller hub, while being compressed and driven in rotation about the axis of the impeller prior to being exhausted through a downstream outlet of the compressor, it being specified that the terms “upstream” and “downstream” are taken relative to the flow direction of the gas in the gas-flow passage through the compressor.

Generally, the stream of compressed gas leaving the impeller then penetrates into a diffuser prior to entering into the combustion chamber.

It can thus be understood that the cover defines the outside surface of the gas-flow passage, with the inside surface of the passage being formed by a surface of the impeller hub from which the blades extend.

In order to control the thermomechanical behavior of the cover, its downstream end is generally left free, i.e. it is not fastened to the downstream edge of the casing.

This configuration serves to avoid the cover being secured in a statitically overdetermined manner which would have the potential of damaging control over the clearances between the impeller and the cover.

Nevertheless, that solution is not perfect: certain degraded behaviors of the compressor, such as pumping or other unstable phenomena, for example, can appear and can lead to sudden variations of pressure within the impeller of the compressor.

Insofar as the downstream end of the cover is free, it will be understood that it can deform slightly as a result of pressure variations inside the compressor, and that such deformation might lead to the cover coming into contact with the blades of the impeller. When the pressure inside the compressor drops below that existing outside the cover, then the cover tends to deform so as to come into contact with the blades of the impeller. This deformation may also be due to vibration.

Naturally, it is extremely harmful both for the cover and for the impeller if the cover comes into contact with the blades of the impeller, where such contact might seriously damage the compressor.

Such a phenomenon may also occur when the gas turbine is being operated under extreme conditions.

One solution to the problem is to increase the clearance that exists between the cover and the blades of the impeller. Nevertheless, such a solution presents the drawback of reducing the efficiency of the compressor, and consequently of diminishing the performance of the gas turbine.

BRIEF SUMMARY

An object of the invention is therefore to propose a cover that makes it possible to avoid contact with the blades of the impeller during degraded operation of the compressor.

The invention achieves its object by the fact that the cover further includes an abutment for limiting the axial movement of its downstream end relative to the downstream edge of the casing while the compressor is in operation.

Preferably, the abutment is placed at the downstream end of the cover.

By means of the abutment in accordance with the invention, axial movement of the downstream end of the cover is limited.

The downstream end of the cover and the downstream edge of the casing are arranged in such a manner that when the downstream end of the cover comes into abutment against the downstream edge of the casing, clearance still remains between the blades of the impeller and the cover, whereby contact is advantageously avoided.

Preferably, the cover is mounted so as to leave a calibrated amount of axial clearance between the downstream end of the cover and the downstream edge of the casing.

Advantageously, the preferably annular abutment forms a radial extension that extends from the downstream end of the cover. This extension thus extends orthogonally relative to the axis of the impeller when the cover is in place. In a variant, the abutment is constituted by a plurality of radial tongues.

The abutment thus radially covers a circumferential portion of the edge of the casing.

Preferably, the downstream end of the cover also includes an axial extension forming an annular rim suitable for lying almost flush with the downstream edge of the casing when the cover is in place.

An advantage of this axial extension is to provide better guidance for the flow of air downstream from the impeller.

A calibrated small amount of radial clearance is thus provided between the downstream end of the cover and an inside end of the downstream edge of the casing so as to limit sudden changes of shape in the air passage, where such changes are harmful to the efficiency of the compressor.

Finally, the invention also provides a gas turbine, in particular for a helicopter, that includes one or more compressors in accordance with the present invention.

DETAILED DESCRIPTION

FIG. 1Ais an overall section view of a helicopter turbine engine10that is well known.

In this example, the turbine engine10is constituted by a gas turbine that comprises a compressor12, also referred to as a compression stage, an air inlet14for admitting fresh air into the compressor12, and a combustion chamber16in which combustion takes place of a mixture of a fuel and the air compressed by the compressor12.

The turbine engine10also includes a turbine18connected to a bladed impeller20of the compressor12via a shaft22, which turbine18is set into motion by the stream of burnt gas leaving the combustion chamber16and serves to drive the impeller20in rotation.

Finally, the turbine engine10also includes a free turbine24that is driven in rotation by the stream of gas leaving the turbine18, said free turbine serving to drive the rotors of the helicopter (not shown) in rotation.

The bladed impeller20, of the centrifugal impeller type, is well known from elsewhere. It comprises a hub26from which there extend radially a plurality of blades28that may present shapes that are curved, with the radial ends thereof being contained in a geometrical envelope that has the shape of a hyperboloid of revolution. The impeller20also presents an axis of rotation A and the term “axial” is used relative to said axis.

Furthermore, the compressor12includes a casing30that preferably forms a component part of the casing of the turbine engine10.

The casing30is the structure that holds together the elements of the compressor; in this respect, the impeller20is mounted to rotate in the casing30.

The casing30presents an upstream edge32and a downstream edge34, it being specified that the terms “upstream” and “downstream” are considered relative to the flow direction of the gas stream inside the compressor20. The flow direction is represented by arrows F in the various figures.

FromFIG. 1B, it can be understood that the gas stream F enters into the bladed impeller20axially via an upstream inlet33and leaves it radially via an outlet35close to the downstream edge34of the casing30prior to penetrating into a diffuser36. The downstream edge34of the casing30is constituted by an upstream edge of the diffuser36in this example.

It can be understood that the gas stream flows between the blades28of the impeller20in a gas-flow passage38extending from the upstream edge32to the downstream edge34of the casing30.

It can also be seen that the passage38is defined between a surface26aconstituted by the hub26, from which hub the blades28extend, and a cover40defining an outside surface of the passage38.

In other words, the cover40covers the blades28of the impeller20so that it extends between the upstream edge32of the casing and the downstream edge34of the casing30while fitting substantially to the shape of the above-mentioned geometrical envelope. In other words, the clearance between each of the blades28and the cover40is small.

More precisely, the cover40has an upstream end40aand a downstream end40b, the upstream end40abeing fastened to the upstream edge32of the casing via a fastener member42, while the downstream end40bis free.

In other words, the downstream end40bof the cover40is not fastened to the downstream edge34of the casing30.

In contrast, it can be seen that the downstream edge34of the casing30extends the downstream edge40bof the cover40with continuity.

Insofar as the cover40is fastened to the casing solely by the upstream edge32, it can be understood that it is free to deform, essentially at its downstream edge40bthat is free.

With reference toFIG. 2, which shows a detail of a turbine engine of the invention, there follows a description of a cover100of a centrifugal compressor200in accordance with the present invention, the other component parts of the turbine engine10being identical to those described above and carrying the same reference numbers.

As can be seen inFIG. 2, compared with the prior art, the downstream end100bof the cover100of the invention further includes an abutment102forming a radial extension that extends orthogonally relative to the axis A of the impeller20.

This abutment102, which is preferably annular, serves to limit the axial movement of the downstream end100bof the cover100. In a variant shown inFIG. 3, the abutment is constituted by a plurality of radial tongues102a.

For this purpose, the abutment102has a contact face103suitable for bearing against the downstream edge34of the casing30if the downstream end100bof the cover100flexes towards the blades28of the impeller, thereby preventing the cover100from deforming any further, and thus advantageously avoiding any contact between the cover100and the blades28of the impeller20.

In normal operation, axial clearance Ja is ensured between the contact face103and the downstream edge34of the casing30.

As can be seen inFIG. 2, the downstream end100bof the cover100also includes an axial swelling104that extends in the opposite direction to the contact face103. This axial swelling presents an annular shape and serves to reinforce the mechanical strength of the abutment102, which is subjected to mechanical stress when it comes into contact with the downstream edge34of the casing30.

Furthermore, the downstream end100balso includes an axial extension106in the form of an annular rim that is designed to come substantially flush with the downstream edge34of the casing30. More precisely, small radial clearance Jr is provided between this annular rim106and the downstream edge34so as to prevent the stream of gas being disturbed in the gap that exists between the downstream end100bof the cover100and the downstream edge34of the casing30.

Preferably, the annular rim106is arranged in such a manner as to present a radial height that is greater than the height of the trailing edges of the blades.

Preferably, the inside surface of the cover100, beside the impeller, is covered in an abradable material, known from elsewhere, in order to avoid damaging the cover and the blades in the event of them coming into contact.