System for ventilating a combustion chamber wall

A system for ventilating a combustion chamber wall in a turbomachine is disclosed. The system includes a centrifugal compressor, a diffuser, a combustion chamber, and an annular case arranged radially between the combustion chamber and a downstream end-piece of the diffuser. The annular case includes a radially external wall for the guidance of an air flow coming out of the diffuser, and a radially internal wall delimiting with the end-piece of the diffuser a channel for the air coming out of the diffuser to pass through.

The present invention relates to a system for ventilating a combustion chamber wall in a turbomachine comprising a centrifugal compressor and a diffuser supplying an annular combustion chamber with air.

BACKGROUND OF THE INVENTION

In a conventional manner, the annular combustion chamber of the turbomachine is situated in an annular space delimited by an external casing, this casing supporting the diffuser which comprises an annular end-piece which extends downstream and along the rotor or impeller of the centrifugal compressor.

DESCRIPTION OF THE PRIOR ART

The air coming out of the diffuser is intended, primarily, to enter the combustion chamber and be mixed with fuel and then burnt, and secondarily, to travel round the combustion chamber and to supply primary and dilution orifices of the chamber and of the air injection means for ventilating and/or cooling a turbine situated downstream of the combustion chamber (see for example document U.S. Pat. No. 5,555,721).

The air that travels round the chamber passing between the latter and the end-piece of the diffuser is not guided and is subjected to turbulence, separations of flow and considerable pressure losses that reduce the performance of the turbomachine.

A small portion of the air flow coming out of the compressor does not enter the diffuser and flows in a radial annular passageway formed between the impeller of the compressor and the end-piece of the diffuser, this air being relatively hot because of its viscous shearing between the impeller and the end-piece of the diffuser.

SUMMARY OF THE INVENTION

The object of the invention is in particular to improve the performance of these ventilation systems and to prevent the disadvantages of the prior art.

Accordingly it proposes a system for ventilating a combustion chamber wall in a turbomachine comprising a centrifugal compressor supplying, via a diffuser, an annular combustion chamber, the diffuser comprising an annular end-piece with a substantially L-shaped section that extends downstream and along the impeller of the centrifugal compressor in order to form a radial annular passageway for the flow of air communicating at its radially external end with the outlet of the compressor, wherein an annular case is arranged radially between the combustion chamber and the end-piece of the diffuser and extends axially from the diffuser to the vicinity of the outlet of the combustion chamber and of the downstream end of the end-piece of the diffuser, this case comprising a radially external wall of revolution for the guidance, along a radially internal wall of the combustion chamber, of an air flow coming out of the diffuser, and comprising a radially internal wall of revolution delimiting, with the end-piece of the diffuser, an annular channel for air coming out of the diffuser to pass through.

The case according to the invention ensures a stable air flow, without separation and with minimal pressure losses along the internal wall of the combustion chamber, which allows an optimal supply of the air injection means and of the primary and dilution orifices of the internal wall of the chamber.

The air that flows between the case according to the invention and the end-piece of the diffuser also cools and ventilates the end-piece of the diffuser which can therefore absorb a portion of the heat generated by the shearing of the air coming out of the compressor and passing between the impeller of the compressor and the end-piece of the diffuser.

Preferably, the radially external wall of the case extends substantially parallel to the internal wall of the combustion chamber, and the radially internal wall of the case extends substantially parallel to the end-piece of the diffuser.

The walls of revolution of the case advantageously comprise pressure-balancing orifices in order to limit their deformations in operation.

In a preferred embodiment of the invention, the case comprises an upstream cylindrical rim and a downstream cylindrical rim centered and supported on the diffuser and on an annular ring of the internal wall of the combustion chamber, respectively.

The rims of the case may be attached by shrink-fitting to the diffuser and to the annular ring of the internal wall of the combustion chamber, respectively.

The upstream cylindrical rim of the case is advantageously crenellated and forms orifices supplying, with air coming out of the diffuser, the air passageway channel along the end-piece of the diffuser.

The case is advantageously formed of two half-shells juxtaposed transversely, which makes the case easier to install in the turbomachine. A cylindrical ring may be engaged axially and attached for example by welding to the downstream cylindrical rim of the case in order to keep the two half-shells assembled.

The invention also relates to a turbomachine, such as an aircraft turbojet or turboprop, which comprises a ventilation system as described above.

Finally the invention relates to a case for a system for ventilating a combustion chamber wall in a turbomachine, which comprises two coaxial walls of revolution extending one inside the other, the upstream ends of these walls being connected to one another by an axial cylindrical rim oriented upstream, and the downstream ends of these walls being connected to one another by an axial cylindrical rim oriented downstream.

According to an embodiment of the invention, the radially external wall of the case is substantially frustoconical, and the radially internal wall of the case has a substantially U-shaped section. These walls advantageously comprise orifices in order to balance pressure in operation.

In order to make it easier to install, the case is formed of at least two half-shells that are juxtaposed transversely and that can be kept assembled by a cylindrical ring engaged axially and attached for example by welding to the downstream cylindrical rim of the case.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made first toFIG. 1which represents a portion of a turbomachine, such as an aircraft turbojet or turboprop, comprising, from upstream to downstream, in the direction of flow of the gases inside the turbomachine, a centrifugal compressor10, a diffuser12and a combustion chamber14.

The inlet18of the centrifugal compressor10is oriented upstream, substantially parallel to the axis of the turbomachine, and its outlet20is oriented outward, substantially perpendicularly to the axis of the turbomachine, and is aligned with a radial portion22of the diffuser12. The diffuser has a shape that is generally bent at 90° and comprises an axial outlet portion24that is connected to the radial portion22and that extends along the external casing28and opens at its radially downstream end on the outside of the inlet of the combustion chamber14.

The diffuser12is supported by the external casing28which externally surrounds the compressor10, the diffuser12and the combustion chamber14.

The diffuser12comprises a substantially cylindrical upstream ring30attached by appropriate means of the screw-nut type to the external casing28.

The diffuser12also comprises a downstream annular end-piece26with a similarly L-shaped section that comprises an upstream radial portion36that extends inward from the inlet22of the diffuser, and a substantially cylindrical downstream portion that extends downstream from the radially internal end of the radial portion36and comprises at its downstream end an annular flange40for attachment to air injection means42for ventilating and/or cooling in components situated downstream of the combustion chamber14.

The radial portion36of the end-piece of the diffuser extends downstream of the rotor of the centrifugal compressor, also called the “impeller” along and at a short distance from the latter in order to form a radial annular passageway70communicating at its radially external end with the outlet20of the centrifugal compressor.

The combustion chamber14comprises two coaxial walls of revolution44,46extending one inside the other and connected at their upstream ends to a chamber-bottom wall48, these walls44,46and48delimiting between them an annular enclosure into which fuel is brought by injectors (not shown).

The radially external wall44of the chamber is attached at its downstream end to the external casing28, and the radially internal wall46is connected at its downstream end to a frustoconical ring52that comprises, at its radially internal end, an internal annular flange54for attachment to the aforementioned air injection means42.

The air injection means42comprise an annular duct64that opens outward at its radially upstream end, downstream of the flange40of the end-piece and upstream of the flange54of the ring52, respectively, and at its downstream end axially downstream. The upstream end portion of the means42comprises orifices66for means of the screw-nut type to pass through that extend through corresponding orifices in the flanges40and54.

A small portion of the air flow coming out of the centrifugal compressor10enters the radial passageway70formed between the impeller of the compressor and the end-piece26of the diffuser. This air (arrow72) is subjected to considerable shearing forces caused by the rotation of the impeller close to the end-piece26of the diffuser, which generates a rise in temperature of the air and a heating of the impeller and the end-piece.

The air coming out of the diffuser12(arrow74) supplies partly the combustion chamber14(arrows76) and partly an internal annular stream78and an external annular stream80traveling round the combustion chamber14(arrows82).

The external stream80is formed between the external casing28and the external wall44of the chamber, and the air that enters this channel is used to cool and/or ventilate components, not shown, situated downstream of this chamber.

The internal stream78is formed between the chamber-bottom wall48and the radial portion22of the diffuser, and the air that enters this stream is divided into a flow that supplies the air injection means42and a flow that passes through the primary and dilution orifices (not shown) of the chamber.

In the prior art, the air that flowed in the internal stream78was not guided and was subjected to turbulence, separations of flow and considerable pressure losses reducing the performance of the turbomachine.

The system according to the invention makes it possible to remove these disadvantages by creating channels86,88for the flow of stable air between the end-piece26of the diffuser and the internal wall46of the chamber by means of an annular case arranged radially between the end-piece26and the combustion chamber14.

In the example represented inFIGS. 1 and 2, the annular case90comprises a substantially frustoconical radially external wall of revolution92which extends inward downstream, and a substantially U-shaped radially internal wall of revolution94.

The external wall92extends substantially parallel to the internal wall46of the chamber and at a distance from the latter for the guidance of a portion96of the air flow82which travels round the chamber via the inside. The air in this channel86is channeled by the external wall92of the case and the wall46of the chamber, which makes it possible to prevent separations and limit the turbulence and pressure losses.

The U-shaped internal wall94of the case comprises an upstream annular portion98that is parallel to and at a short distance from the radial portion36of the end-piece and that is connected via its radially external end, with the upstream end of the external wall92, to a cylindrical rim104oriented upstream. The wall94also comprises an intermediate cylindrical portion100that extends around and at a short distance from the end-piece26, and a substantially frustoconical downstream annular portion102that extends downstream and outward around the air injection means42, and that is connected via its downstream end, with the downstream end of the external wall92, to a cylindrical rim106oriented downstream.

The upstream rim104of the case is engaged from downstream in an annular groove108opening downstream and formed at the radially internal end of the inlet22of the diffuser, and the downstream rim106is mounted on a cylindrical rim110oriented upstream of the ring52of the chamber. The groove108and the rim110make it possible to support and center the case, as will be described in greater detail below.

The upstream rim104of the case is of the crenellated type and comprises notches or slits112alternated with solid portions114and evenly distributed about the axis of the case (FIG. 2). The notches112form orifices for air to pass through and their axial dimension is greater than the axial dimension of the groove108of the diffuser so that a portion116of the air flow82traveling round the chamber via the inside can pass through these notches and flow between the internal wall94of the case and the end-piece26of the diffuser in order to cool them and then supply the injection means42.

This air116makes it possible in particular to ventilate and cool the end-piece26of the diffuser that is exposed in operation to considerable temperature rises of the air72sheared between the impeller of the compressor and the end-piece26. This end-piece26can therefore absorb a large proportion of the heat generated by this shearing and in this manner limits the heating of the impeller of the compressor and increases its service life.

The case90is formed of at least two half-shells117(FIG. 2) that are juxtaposed transversely and, by a cylindrical ring118, are kept engaged on the downstream cylindrical rim106of the case and attached for example by welding to this rim (FIG. 1).

This case90is mounted in the turbomachine as follows: After the diffuser12has been mounted on the centrifugal compressor10and before the injection means42and the combustion chamber14are assembled to the end-piece26of the diffuser, the two half-shells117separated from one another are brought transversely on either side of the end-piece26and juxtaposed, then the ring118is engaged axially and attached to the downstream rim106of the case. The rim104of the case90is then fitted into the groove108of the diffuser. The injection means42are assembled to the flange40of the end-piece of the diffuser, then the chamber is in turn moved upstream and attached via its ring52to the injection means42, the upstream rim110of this ring52being engaged radially inside the downstream rim106of the case in order to center it.

The case90may be removed by removing the ring118and repeating the aforementioned operations in the reverse direction.

The case90is attached by hot shrink-fitting in operation in the groove108and on the rim110. For this, the rims104,106of the case have, at ambient temperature, internal diameters that are less than the internal diameter of the groove108and the external diameter of the rim110, respectively. The shrink-fitting of the case in the groove108and on the rim110is carried out by heat expansion in operation.

The walls of revolution92and94of the case preferably comprise through-holes (represented in dashed lines at120inFIG. 2) in order to balance the pressures inside and outside the case.