Diffuser for terrestrial or aviation gas turbine

A diffuser for a gas turbine engine, said diffuser being disposed between a last stage of a turbine and an exhaust casing, and comprising an outer annular wall and an inner annular wall together defining an annular passage for fluid that diverges in the flow direction of said fluid, at least one of the annular walls including a plurality of orifices leading from said annular passage to at least one collecting box leading to means for exhausting a fraction of said fluid so as to reduce the flow speed of said fluid in said annular passage.

BACKGROUND OF THE INVENTION

The present invention relates to the general field of diffusers for gas turbine engines of terrestrial or aviation type. It relates more particularly to diffusers placed between the turbine and the exhaust casing of a gas turbine engine.

The function of terrestrial or aviation gas turbines is to deliver power that is sufficient to drive either an alternator (terrestrial turbines) or a compressor (aviation turbines). To do this, a gas turbine takes a fraction of the energy of the hot compressed gases coming from the combustion chamber of the turbine engine and transforms it into mechanical energy. A turbine generally comprises a plurality of stages, each stage comprising a stator nozzle and a moving wheel placed after the nozzle for accelerating the flow of gas. The gas coming from the last stage of the turbine then feeds an exhaust casing.

The exhaust casing placed immediately downstream from the turbine is constituted by a diffuser and by casing arms which serve essentially to straighten the flow of gas at the outlet of a non-axial turbine and to pass cooling air for the internal portions of the engine. The diffuser serves to reduce the speed and increase the pressure of the gas coming from the last stage of the turbine. For this purpose, the diffuser generally comprises walls forming a passage for the gas, which walls diverge in the gas flow direction, as shown in U.S. Pat. No. 2,594,042.

An exhaust casing suffers from pressure losses which are typically proportional to the square of the speed of the gas at the leading edge of the casing arms. For example, for a terrestrial turbine, the gas reaches a speed close to Mach 0.6 at the outlet from the moving wheel of the last stage of the turbine. The diffuser enables this speed to be reduced to about Mach 0.45 at the leading edge of the casing arms, which leads to pressure losses of about 5%. Nevertheless, a gas speed of about Mach 0.45 still constitutes a value that is high. The slope of the walls constituting the diffuser must not exceed a certain value since otherwise there is a risk of boundary layers on said walls thickening. Thick boundary layers lead to separation, which harms the efficiency of the diffuser. Thus, when separation from the walls of the diffuser occurs, the aerodynamic section downstream therefrom is much smaller than its geometrical section, thus preventing the diffuser from performing its diffusion function. Furthermore, optimizing the turbine in terms of cost, mass, and performance generally leads to high loads per stage, giving rise to ever-increasing speed of the gas at the outlet from the last stage of the turbine.

OBJECT AND SUMMARY OF THE INVENTION

The present invention thus seeks to mitigate such drawbacks by proposing a gas turbine diffuser in which pressure losses are significantly reduced.

To this end, the invention provides a diffuser for a gas turbine engine, said diffuser being disposed between a last stage of a turbine and an exhaust casing, and comprising an outer annular wall and an inner annular wall together defining an annular passage for fluid that diverges in the flow direction of said fluid, wherein at least one of the annular walls includes a plurality of orifices leading from said annular passage to at least one collecting box leading to means for exhausting a fraction of said fluid so as to reduce the flow speed of said fluid in said annular passage.

As a result, the orifices made through at least one of the annular walls of the diffuser act via the collecting box to exhaust a fraction of the fluid passing through the annular passage, thus enabling the fluid flow speed in the annular passage to be reduced, and thus enabling pressure losses to be minimized. Any risk of boundary layers thickening on the walls of the diffuser and then separating is also eliminated. The collecting box(es) are also connected to at least one fluid exhaust channel. Advantageously, the diffuser further comprises suction means for controlling and monitoring a determined rate of flow for the fluid that is to be exhausted.

The orifices made through at least one of the annular walls may be holes or oblong slots that are substantially perpendicular to the wall or holes or oblong slots that are substantially inclined in the direction in which the fluid flows relative to the wall.

DETAILED DESCRIPTION OF AN EMBODIMENT

InFIG. 1, there can be seen a diffuser10disposed immediately downstream from a moving wheel12of a last stage of a gas turbine, where “downstream” is in the flow direction of a gaseous fluid coming from said turbine and marked by arrow F. A casing arm14serving in particular to straighten the gas flow is mounted downstream from the diffuser10.

The diffuser10has an outer annular wall16aand an inner annular wall16bso as to form an annular passage18for the gas from the turbine. The walls16aand16bare arranged in such a manner that the annular passage18diverges in the gas flow direction F so as to reduce the flow speed and increase the pressure of the gas passing therethrough. The outer wall16adiverges while the inner wall16bis substantially parallel to the axis (not shown) of the engine fitted with this diffuser. It is also possible to devise a diffuser in which the inner wall16bdiverges (relative to the fluid) while the outer wall16ais parallel to the axis of the engine.

In the invention, the diffuser10has a plurality of orifices20through its outer annular wall16aand/or its inner annular wall16b, the orifices leading from the annular passage18to at least one collecting box22leading to means for exhausting a fraction of the gas passing through the annular passage.

InFIG. 1, only the outer wall16ais fitted with orifices20. The orifices20shown are holes that are substantially inclined in the flow direction F of the gas relative to the outer wall16a. It is also possible for the orifices20to be substantially perpendicular to the outer wall16aand/or to the inner wall16b(FIG.2).

In a second variant shown inFIG. 1A, the orifices20may be in the form of a plurality of oblong slots extending over an angular sector of the outer wall16a. These slots may likewise be substantially perpendicular or substantially inclined in the flow direction F of the gases relative to the outer wall16a.

It yet another variant (not shown), the orifices20may be constituted by one or more slots of the “scoop” type having upstream and downstream walls that are radially offset. Chamfered slots of this type provide better guidance for the gas being directed towards the exhaust means.

A single annular box22may be provided for collecting the gas that is to be exhausted from all of the holes20, or else a box, e.g. a cylindrical box, may be provided for each orifice20(or for a plurality of orifices) so as to ensure that the flow of gas to be exhausted is more uniform.

The gas collecting box or boxes22are preferably connected to at least one gas exhaust channel24. One or more exhaust channels24may be provided per box22. When the inner wall16bof the diffuser is provided with orifices20, the channel(s)24may pass along the casing arms14in order to exhaust the gases outside the diffuser.

According to an advantageous characteristic of the invention, the diffuser further comprises suction means26for sucking out the fraction of the gas that is to be exhausted. These suction means26may be constituted by a pilot valve, a pump, a compressor, or any other system enabling a desired flow of gas to be sucked out. Thus, it is possible to control and monitor a determined rate of flow of gas that is to be exhausted.

Nevertheless, if it turns out to be unnecessary to control the rate of flow of the gas for exhausting, then the gas passing through the orifices20formed in the outer wall16aand/or the inner wall16bmay lead directly to the outside of the diffuser without passing via collecting boxes and evacuation channels for the gas. Under such circumstances, the pressure difference between the annular passage18and the outside of the diffuser suffices to suck out gas through the orifices20.

FIG. 2shows a diffuser of the invention applied to a double-flow aviation gas turbine engine. The diffuser10is disposed immediately downstream from a moving wheel12of a last stage of a gas turbine. The outer and inner walls16aand16bof the diffuser define a first diverging annular passage18for the gas coming from the turbine. This first passage18is commonly referred to as a “hot flow” passage. An additional wall16cis placed coaxially around the walls16aand16bof the diffuser, thereby defining a second annular passage28for air sucked in by the fan (not shown) of the engine. This second passage28is referred to as being the “cold flow” passage.

In the invention, the inner wall16bhas a plurality of orifices20leading from the first annular passage18into at least one collecting box22connected to at least one gas exhaust channel24. The exhaust channel(s)24pass along the casing arms14mounted in the first annular passage18and via casing arms30mounted in the second annular passage28. The diffuser may also comprise suction means26for sucking out the fraction of gas that is to be exhausted.