Turbomachine test bench with active noise control

Abstract

A test bench for turbomachine comprising: an installation zone for turbomachine; an active system for attenuating the noise emissions produced by the turbomachine. The active system includes an attenuation zone with emitters such as loudspeakers; a first microphone placed downstream of the turbomachine; and a second microphone placed downstream of the attenuation zone. The system reduces the turbomachine waves on the basis of the measurements from the first microphone and from the second microphone. The invention also proposes a method for attenuating the noise emissions from the turbomachine tested in the test bench.

Claims

1. A test bench for an aircraft turbojet, said test bench comprising: an installation zone for an aircraft turbojet; and an active system for attenuating sound waves emitted by the aircraft turbojet, the active system including a first microphone placed downstream of the installation zone of the aircraft turbojet, and an attenuation zone for attenuating the sound waves of the aircraft turbojet with an emitter for emitting attenuation sound waves; wherein the active system further comprises a second microphone placed downstream of the attenuation zone, and the active system being configured to attenuate the sound waves of the aircraft turbojet as a function of measurements from the first microphone and from the second microphone, wherein the attenuation zone is situated between the first microphone and the second microphone, the bench further comprising a tube for collecting the airflow propelled by the aircraft turbojet downstream of the installation zone for the aircraft turbojet, the attenuation zone being arranged along the tube, the bench further comprising two bulkheads defining a chamber for the emitter, the first microphone and the second microphone being outside of the chamber.

2. The test bench according to claim 1, wherein the first microphone is placed upstream of the tube and upstream of the attenuation zone.

3. The test bench according to claim 1, wherein the second microphone is placed downstream of the tube.

4. The test bench according to claim 1 further comprising a structural wall downstream of the tube, the second microphone being closer to the tube than to the structural wall, the structural wall being perpendicular to the main elongation of the test bench.

5. The test bench according to claim 1 further comprising a passage for a flow from the turbomachine, the first microphone and the second microphone being placed in the passage.

6. The test bench according to claim 1, wherein the first microphone and the second microphone are axially spaced from each other by a distance longer than or equal to 16 feet (5 meters).

7. The test bench according to claim 1, wherein the turbomachine has an axis of rotation and the installation zone for turbomachine includes a central axis coaxial to the axis of rotation of the turbomachine, at least one or each microphone being placed on the central axis.

8. The test bench according to claim 1, wherein the turbomachine has an axis of rotation and the installation zone for turbomachine includes a central axis that is one of coaxial or parallel to the axis of rotation of the turbomachine, at least one or each microphone being offset by less than 3 feet (1 m) relative to the central axis.

9. The test bench according to claim 1 further comprising a straight corridor in which the installation zone for turbomachine and the second microphone are arranged.

10. A test bench for an aircraft turbojet, said test bench comprising: an installation zone for an aircraft turbojet; an active system for attenuating sound waves emitted by the aircraft turbojet, the active system including a first microphone placed downstream of the installation zone of the aircraft turbojet, and an attenuation zone for attenuating the sound waves of the aircraft turbojet with an emitter for emitting attenuation sound waves; a second microphone placed downstream of the attenuation zone; a tube for collecting the airflow propelled by the aircraft turbojet downstream of the installation zone for the aircraft turbojet, the attenuation zone being arranged along the tube; two bulkheads defining a chamber for the emitter; and chimneys with passive systems for attenuating a noise propagating out of the test bench, the second microphone being directly below one of the chimneys outside of the bulkhead.

11. The test bench according to claim 10, wherein the first microphone is placed outside the chimneys.

Description

DRAWINGS

(1) FIG. 1 shows a turbomachine test bench according to various embodiments of the invention.

(2) FIG. 2 is a diagram of a portion of the test bench of FIG. 1 according to various embodiments of the invention.

(3) FIG. 3 illustrates a flow chart of the method for attenuating noise emissions according to various embodiments of the invention.

DETAILED DESCRIPTION

(4) In the description that will follow, the terms inner and outer refer to a positioning relative to the axis of rotation of a turbomachine. The axial direction corresponds to the direction along the axis of rotation of the turbomachine. The radial direction is perpendicular to the axis of rotation. The upstream and the downstream are with reference to the main direction of the flow in the turbomachine.

(5) FIG. 1 shows, in a simplified manner, a test bench 2 for an engine 4, more particularly a test bench 2 for a turbomachine 4, in particular for an aircraft turbojet 4. The test bench 2 can, in various instances, accommodate a complete aircraft, or at least a part of an aircraft.

(6) The test bench 2 forms an infrastructure, a construction. It comprises a passage 6 with an entrance 8 and an exit 10. The passage 6 can comprise a corridor 12, essentially elongated. Its length can be greater than or equal to 200 feet (60 m). The length of the corridor 12 enables an airflow 14 to circulate in a straight line, while limiting the formation of vortices detrimental to the quality of the test.

(7) In order to limit the resistance to the flow through the corridor 12, in particular the resistance opposing the entry of an airflow 14 into the turbomachine 4, the corridor 12 can have a passage cross section larger than or equal to 540 ft.sup.2 (50 m.sup.2). It should be noted that the airflow 14 passing though the test bench 2 can be caused by the turbomachine 4 itself during its test phase. The passage cross section, or free cross section, can be measured upstream of the installation zone 16 of the turbomachine 4. The passage cross section can be preserved over at least a quarter of the length of the corridor 12, in various instances over the majority.

(8) The installation zone 16 can be a fastening zone for the turbomachine 4. It can be equipped with a fastening arm 18 to which the turbomachine 4 is fastened during its test. The arm 18 can extend vertically from the ceiling of the corridor 12, in the manner of a column or a post. The arm 18 allows the turbomachine 4 to be mounted with an offset, and to center the latter relative to the middle of the corridor 12, relative in particular to a central axis 19 of the corridor 12. The centring is vertical and horizontal.

(9) The corridor 12 can be delimited by vertical chimneys (20; 22), as entrance 8 and exit 10. They allow an admission of air and an emission, both vertical, and elevated relative to the corridor 12. In order to reduce noise nuisance, they can include acoustic baffles 24, or acoustic plates 24, allowing the sound waves to be passively absorbed.

(10) Complementary devices 26 can be present as entrance 8 and exit 10 in order to avoid reversals of flow that would disturb the test conditions. The U-shape arrangement is not indispensable; other configurations, for example without chimneys, can be envisaged. One single chamber can form the passage. The test can be made in open air.

(11) At the junction between the upstream chimney and the corridor 12, the bench 2 is equipped with a row of deflector plates 28. They allow the air descending from the inlet chimney 20 to be reflected in a horizontal direction. They extend horizontally, and cross the entire corridor 12. They have curved profiles. At the entrance to the corridor 12, the bench 2, in various instances, has a grid 30 making it possible to intercept debris likely to disturb the test and to damage the turbomachine 4.

(12) Downstream of the turbomachine 4, the bench 2 includes a tube 32 for collecting the airflow 14 propelled by the turbomachine 4, including its exhaust gasses. The mouth of the tube can form a funnel, an upstream cone converging in a downstream direction. The collector tube 32 assists with absorbing the noise generated during the test. The collector tube 32 is placed horizontally and, in various instances, comprises a diffuser 34 at its outlet. It can form a tube with a perforated wall. This diffuser 34 is also known by the term blast basket. The diffuser 34 can be in the outlet chimney 22.

(13) The collector tube 32 can be held in the bench 2 by means of at least one bulkhead 36, in various instances, by two bulkheads 36. These bulkheads 36 extend vertically and transversely in the corridor 12. One of them can form a separation between the corridor 12 and the outlet chimney 22. They form sealed separations, that make it possible to contain the flow 14 from the turbomachine 4.

(14) In order to divert the flow from the collector tube 32, and from the optional diffuser 34, a cone 37 can be placed in the extension of the collector tube 32. It can be fastened to a vertical wall at the end of the corridor 12. Its point can coincide with the central axis 19.

(15) In order best to process the noise emissions, the test bench 2 is equipped with an active system 38 for attenuating noise emissions. The active system 38 processes the emissions produced by the turbomachine 4, and can also take into account the phenomena of waves reverberating against the walls of the test bench 2. In effect, the walls of the corridor 12 offer large reflective surfaces.

(16) Although the active system 38 is shown in association with the tube 32, it can also be envisaged to embody an active system for attenuating noise emissions without a tube, or distanced from the collector tube 32.

(17) FIG. 2 shows a portion of the test bench 2 of FIG. 1. Appearing there are the turbomachine 4, a section of the corridor 12, the collector tube 32, the diffuser 34 and the active system 38.

(18) When operating, the turbomachine 4 produces sound waves 40, or engine waves. They are pressure waves considered to be harmful, and this in more than one respect. The turbomachine waves 40 can be wide band waves. Such a wide band wave can comprise components having frequencies ranging between 10 Hz and 4000 Hz, or between 80 Hz and 800 Hz. The noise level of these turbomachine waves 40 can exceed 140 dB at the turbomachine 4, which risks disturbing the environment of the test bench 2.

(19) In response, the active system 38 includes an attenuation zone 42 where sound waves 46 are emitted to counter the turbomachine waves 40. This attenuation zone 42 groups together with the emitter 44 capable of emitting sound waves 46, that oppose the passage, the propagation of the turbomachine waves 40. For example, the attenuation sound waves 46 can come in opposition of phase against the turbomachine waves 40 to reduce their amplitudes. The mixture, the sum of their fluctuating pressure fields, tends towards the cancellation.

(20) The emission 44 can comprise noise sources, such as loudspeakers. Even though a single source might be sufficient, several noise sources can be used. The noise sources can be spaced around the tube 32, in one or several circular rows. The emitter 44 can act through the tube 32, or thanks to openings made in the tube 32. The emitter 44 can be directly exposed to the turbomachine waves 40, and hence to the exhaust gasses from the turbomachine 4. The emitter 44 can be placed in the corridor 12. They can be between the bulkheads 36. The latter can form a chamber for protecting the emitter 44, combined with the tube 32.

(21) The active system 38 furthermore includes a first microphone 48, such as an upstream microphone; and a second microphone 50, such as a downstream microphone, that are both connected to a control unit 52. The active system 38 for attenuating noise is configured to reduce the emissions on the basis of the measurements from the first microphone 48 and from the second microphone 50 placed downstream of the former.

(22) Thanks to the measurements from the microphones (48; 50), the control unit 52 defines an electric signal for powering the emitter 44 so that they produce waves 46 for reducing the amplitude of the outgoing noise from the tube 32. The control unit 52 defines an electric correction signal, while taking account of the respective positions of the microphones, the positions of the emitters 44. The speed of the flow caused by the turbomachine 4 can also be considered. Corrections can be made as a function of the physical environment. Consequently, the outgoing noise from the chimney as outlet is reduced. In addition, the action of the acoustic baffles located there is simplified.

(23) The first microphone 48 is placed facing the turbomachine 4, it makes it possible to measure the sound outgoing from it. It is immersed at the same time in the flow propelled by the turbomachine 4, but also in its sound field. The first microphone 48 is arranged upstream of the attenuation zone 42. It is placed upstream of the tube 32, but it can also be placed inside as an alternative.

(24) The second microphone 50 is placed downstream of the attenuation zone 42, for example outside the tube 32. However, it can be arranged inside as a variant. The second microphone 50 makes it possible to check the effect of the correction. If necessary, the control unit 52 modulates the correction to be made as a function of the measurements taken by the second microphone 50. The control unit 52 can increase or reduce certain components of the wide band signal it defines.

(25) The test bench 2 includes different items of equipment 54. For example, sensors, supporting structures. During operation, these items of equipment 54 vibrate because of the oscillating nature of the acoustic pressure existing in the test bench 2. These vibrations can reduce the durability of the item of equipment, and potentially destroy it. This phenomenon can be observed when the turbomachine waves 40 are at the frequency specific to the item of equipment 54 and at a harmonic. In order to counter this phenomenon, the attenuation waves 46 can be tailored to correct this phenomenon. The collector tube 32 can perform as the item of equipment.

(26) FIG. 3 shows a flow chart of the method for attenuating, or method of controlling, the noise emitted in the direction of the environment from a test bench in which a turbomachine is tested. The test bench can correspond to that described with reference to FIGS. 1 and 2.

(27) In various embodiments, the method can comprise the following steps:

(28) (a) test 100 of the turbomachine; and

(29) (b) emission 102 of attenuation sound waves tailored to oppose the sound waves emitted by the turbomachine during the step (a) test 100.

(30) During the step (a) test 100, the item of equipment is excited into vibration because of the noise emissions produced by the turbomachine. The item of equipment can vibrate at its specific frequency. It can also vibrate at one or several harmonics of its specific frequency. The vibration performance of the item of equipment can include several accumulating frequencies, including its specific frequency and one or several harmonics.

(31) During the step (b) emission, the active attenuation system emits in response sound waves at the specific frequency of the item of equipment. During the step (b) emission, the attenuation waves reach the item of equipment in opposition of phase compared with its vibration during the step (a) test 100. Advantageously, the attenuation sound waves comprise a complex signal with several accumulating frequencies. The different frequencies can have different amplitudes. The definition of attenuation sound waves can be based on the measurements from the first microphone and from the second microphone. The complex signal can be tailored to the reverberations. A learning phase or a calibration phase can make it possible to refine the noise attenuation that the active system provides.