Gas turbine helicopter engine with noise emission reduced through acoustical treatment of an eductor

CA 02602168 2007-09-18 1 Gas turbine helicopter engine with reduced noise emission by acoustic treatment of an ejector Background of the invention The invention relates to gas turbine helicopter engines.

As a result of the efforts made to reduce the noise generated by the rotation of the blades of helicopter rotors, the applicant has observed that the noise inherent in gas turbine engines becomes a significant component of the overall sound emission of helicopters.

A significant additional reduction in this noise emission could therefore be obtained by soundproofing the gas turbine engines themselves.

To this end, it is proposed in document EP 1 010 884 to provide the walls of a multichannel nozzle receiving the gases coming from the turbine with a coating capable of absorbing acoustic energy.

With regard to the sound frequencies to be attenuated, the coating is relatively thick, which is penalizing in terms of mass, especially since the coating must be made of a material capable of withstanding the temperature of the primary flow at the turbine outlet.

In addition, the thickness of the coating cannot allow the gas flow coming from the turbine to ensure effective suction of a possible secondary engine compartment cooling flow through a passage surrounding the nozzle, so that the acoustic treatment of the nozzle is not compatible with an ejector function.

OBJECT AND SUMMARY OF THE INVENTION The present invention aims to avoid the aforementioned drawbacks and to this end proposes a gas turbine helicopter engine having a combustion chamber, at least one turbine arranged downstream of the combustion chamber to receive combustion gases emitted by it, and a nozzle with a diffuser part connected downstream of the turbine and an ejector which has an upstream part surrounding the downstream end of the diffuser by providing with this end an outlet passage for a secondary flow of compartment cooling air motor and which extends downstream beyond the downstream end of the diffuser, the ejector having a wall formed at least partially by a CA 02602168 2007-09-18 2 acoustic attenuator suitable for attenuating sound frequencies generated by the or each turbine or by the combustion chamber.

In a gas turbine helicopter engine, the speed of ejection of gases from the turbine is reduced as much as possible to optimize the conversion of gas energy into mechanical energy.

The sound emission at the nozzle outlet is therefore essentially due to the rotating elements of the engine, in particular to the or each turbine which is the closest rotating element as well as to the combustion chamber.

The realization of the wall of the ejector to form an effective acoustic attenuator in a range of sound frequencies generated by the rotation of the or each turbine and / or by the combustion chamber therefore allows effective reduction of noise while benefiting from the current of secondary flow which protects the wall of the ejector against the hot gases coming from the turbine.

The acoustic attenuator can then be made of a light metallic material, for example based on titanium, that is to say of titanium or of a titanium alloy, so that the presence of the attenuator is not penalizing in terms of mass.

In addition, at the level of the ejector, there is space available to produce the acoustic attenuator in the form for example of a Helmholtz resonator several cm thick suitable for attenuating frequencies of a few hundred Hz. at a few kHz generated by the rotation of the or each turbine and by the combustion chamber, without penalizing the ejector function.

It may be noted here that the problem of soundproofing at the outlet of a gas turbine for a helicopter engine is completely different from that of soundproofing at the outlet of a gas turbine for an airplane engine.

In fact, with a gas turbine airplane engine, the aim is to produce thrust, and therefore to eject a large mass of gas at a high speed.

The noise generated is essentially a gas jet noise against which an acoustic attenuator in the wall of the flow channel would have no effect.

Brief description of the drawings The invention will be better understood on reading the description given below, by way of indication but not limiting, with reference to the appended drawings in which:

CA 02602168 2007-09-18 3 - figure 1 is a schematic view of a gas turbine helicopter engine;

- Figure 2 is a partial view showing a diffuser-ejector assembly of a helicopter engine, according to one embodiment of the invention.

Detailed Description of Embodiments Figure 1 schematically shows a gas turbine helicopter engine comprising a compressor stage 10 (for example centrifugal compressor) receiving outside air through an annular air inlet duct. 12, an annular combustion chamber 14 (for example with reverse flow) provided with injectors (not shown) supplied with fuel and with primary air flow coming from the compressor, a turbine 16 for driving the compressor 10 connected thereto by a shaft 18 and a power turbine 20 (for example single-stage) connected by a shaft 22 to a gear train providing mechanical power on a shaft output 24, the shafts 18 and 22 being coaxial.

In the downstream direction, that is to say in the direction (arrow F) of flow of the gases from the combustion chamber, the power turbine 20 is extended by a nozzle which comprises a diffuser 26 and an ejector 30, as shown in figure 2.

The diffuser 26 receives the hot primary gas flow coming from the power turbine 20 and contributes with the ejector 30 to slow down this flow to ensure that the energy of the gases passing through the turbine 16 is as much as possible converted into mechanical energy transmitted. on the output shaft.

In a known manner, the ejector is bent at least at its downstream end to prevent the outgoing gas flow from being directed along the axis of the engine, towards the rear of the helicopter.

At its upstream end part, the ejector 30 is fixed to the casing (not shown) of the motor and surrounds the downstream end of the diffuser 26, leaving therewith an annular gap 32.

A “cold” secondary air flow, used in particular for cooling equipment located in the engine compartment, flows around the combustion chamber, turbines 16, 20 and diffuser 26 assembly.

The diffuser has a straight shape and not a so-called daisy shape which is used when one seeks to promote the mixture CA 02602168 2007-09-18 4 of the primary and secondary flows.

Thus, the secondary flow escapes through the gap 32 by flowing along the internal face of the ejector 30. The extraction of the secondary flow is assisted by the suction effect produced by the primary flow. at the outlet of the diffuser.

On the internal side, the ejector has a wall formed by an acoustic attenuator 34 over at least part of the axial length of the ejector. The attenuator 34 may be formed from several successive adjacent parts for greater manufacturing convenience.

As shown in detail in FIG. 2, the acoustic attenuator can be formed by a set of adjacent cavities or cells 34a separated by walls 34b forming for example a honeycomb structure.

The walls 34b extend perpendicularly to the surface of the ejector between a bottom 34c formed by a rigid plate or waterproof sheet (not crossed by sound waves) and a front face 34d formed by a plate or sheet allowing sound waves to pass. .

The front plate or sheet 34d is for example perforated.

It would also be possible to use a porous plate or sheet permeable to the sound waves to be attenuated.

The depth of the cavities 34a (distance between the bottom and the front face) is chosen as a function of the wavelength of the sound waves to be attenuated.

The latter, produced essentially by the turbines 16 and 20 or the combustion chamber 14, have a frequency of a few hundred Hz to a few kHz, which implies a cavity depth (quarter wavelength) of several centimeters.

Due to the fact that the ejector, on the internal side, is swept by the “cold” secondary flow, the material constituting the acoustic attenuator can be chosen from a fairly wide range of materials, and therefore in particular from lightweight materials.

An example of a usable material is titanium.

Other metallic or non-metallic materials may of course be chosen.

To have the most effective attenuation possible, it is desirable for the attenuator to extend over the entire length of the ejector and for this length to be preferred over that of the diffuser.

It is possible to choose a ratio at least equal to 1 between the axial length of the ejector and that of the diffuser.

CA 02602168 2007-09-18 Although an attenuator structure of the type known under the name of Helmholtz attenuator has been described above, other types of sound attenuators can be used such as, for example foams or porous ceramic or metallic materials.