WO2002036951A1

WO2002036951A1 - Fan noise reduction by control of nacelle inlet throat

Info

Publication number: WO2002036951A1
Application number: PCT/CA2001/001480
Authority: WO
Inventors: Man-Chun Tse
Original assignee: Pratt & Whitney Canada Corp.
Priority date: 2000-11-03
Filing date: 2001-10-23
Publication date: 2002-05-10

Abstract

A method for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct (10) of a jet engine (12) is implemented by a pneumatic boot (18) installed in the nacelle inlet duct (10) to form an adjustable inlet duct throat (22). The pneumatic boot (18) in an inoperative mode forms a part of the inlet duct wall (16) to satisfy requirements for jet engine performance under any flight conditions. In anoperative mode, the pneumatic boot (18) can be selectively and controllably inflated to form the adjustable throat (22), to change both the impedance of the system with respect to the noice propagation and the velocity of the inwardair flow. A maximum adjustment of the adjustable throat (22) can be reached to increase the velocity of the inward air flow to the sound speed to chokethe forward noise propagation from the nacelle inlet duct (10) when required, for example, in an aircraft landing situation.

Description

FAN NOISE REDUCTION BY CONTROL

OF NACELLE INLET THROAT

FIELD OF THE INVENTION

This invention relates to suppression of noise sound waves emitted from a jet engine, more particularly to the suppression of noise propagation in a direction opposite to the inward air flow through the nacelle inlet duct of a jet engine.

BACKGROUND OF THE INVENTION The suppression of noise radiated by jet engines is a problem that has been receiving increasing attention, particularly within recent decades. The gas turbine engine designer, and particularly the designer of such engines for aircraft propulsion, is faced with the dilemma of reducing engine noise with a minimum sacrifice of engine performance .

One source of engine noise, commonly denoted as inlet noise, is due to sound waves propagating in the forward direction through the inlet duct of a jet engine. The inlet noise is due to a number of sources. One of the sources of the inlet noise is generated by rotating turbo machinery itself, resulting from rapidly rotating blade rows disposed within the gas stream. The noise is affected by such parameters as blade rotational speed, blade-to-blade spacing, blade geometry, and also by the proximity of stationary hardware to such rotating blade rows, as in the case of an outlet guide vane arrangement and in typical multi-stage axial compressors and fans where stationary blade rows are alternated with rotating blade rows. Some of the noise generated in this manner can be absorbed and suppressed by means of acoustic or sound absorbent paneling disposed around the periphery of the nacelle enclosing the rotating the turbo machinery. Such sound absorbent material is well known in the art.

However, a significant percentage of the noise propagates forward from the gas turbine inlet duct due to the proximity of the fan or compressor to the inlet frontal plane and the lack of forward shielding in the forward direction. The problem, therefore, facing the gas turbine engine designer is to provide means for attenuating this forward propagating noise without incurring unacceptable engine performance penalties.

Several prior art attempts have been made to reduce inlet noise, in the main, the prior art has concentrated on mounting sound absorbent material on the air inlet duct wall or on the air inlet centerbody, such as examples described in United States patent 4,104,002 issued to Ehrich on August 1, 1978 and United States patent 4,759,513 issued to Birbragher on July 26, 1988. Because of the limited duct wall area within most conventional jet engines, such prior art attempts have often required the addition of structural members such as annular rings and vanes supported in spaced relationship with the inlet duct wall to increase the surface area upon which acoustically absorbent material can be applied or mounted, as described in United States patent 4,240,250, issued to Harris on December 23, 1980.

Another prior art attempt at reducing air inlet duct noise is described in United States patent 4,192,336, issued to Farquhar et al . on March 11, 1980. Farquhar et al. describe a noise-reducing jet engine air inlet duct contoured to control air flow such that noise sound waves propagating upstream from the engine toward the entryway of the air inlet duct are refracted toward the duct wall and/or engine inlet centerbody. The inlet duct wall and/or engine inlet centerbody are contoured to establish a cross-sectional duct region having substantial air flow velocity gradients in a direction transverse to the direction of the air flow. Starting at the inlet duct entryway, the contoured duct region includes a parallel or diverging section followed by a rapidly contracting section before a final diffusion section extending to the engine. The velocity gradients are created in the region of the contracting section. When the sound waves propagate forward through a throat formed between the contracting section and the diffusion section, the velocity gradients refract the sound waves toward the wall and/or engine centerbody of the air inlet duct to increase the impedance with respect to sound propagation.

Such contouring should, of course, establish the required velocity gradient while simultaneously providing adequate inward air flow to ensure proper engine operation. As a matter of fact, when the concern is proper engine operation, it. is neither practical nor desirable to design a nacelle inlet contour to substantially suppress or even completely choke the noise propagating outward through the inlet duct .

It is well known in the aircraft industry to use inflatable boots to change a surface contour of the aircraft. The inflatable boot is generally formed with a deformable sheet of woven cloth substrate and a fluid-impervious, resilient coating. The inflatable boot is attached to aircraft surfaces, particularly to the leading edge of the aircraft wings so that the inflatable boot can be expanded to produce a deformation of the surface sufficient to detach and dislodge accumulations of ice on the ice accreting surface. This technology has been developed especially for deicing and examples are described in United States patent 4,687,159, issued to Kageorge on August 18, 1987, United States patent 4,706,911 and United States patent 4,826,108, both issued to Briscoe et al . on November 17, 1987 and May 2, 1989 respectively. Nevertheless, this technology has never been developed as a solution for noise attenuation, particularly relating to j et engines .

The aircraft industry has continuously sought solutions for reducing noise propagating from the inlet duct of a jet engine without sacrificing overall engine performance. Therefore there is a need for developing a new technology to adjust the inlet duct contour of a jet engine depending on different requirements for noise suppression and engine performance in different flight conditions .

SUMMARY OF THE INVENTION

It is the primary object of the present invention to selectively reduce aerodynamically induced noise propagating from the inlet of a jet engine without sacrificing overall engine performance.

It is another object of the present invention to provide an adjustable inner contour of the nacelle inlet duct to selectively suppress aerodynamically induced noise propagating from the inlet of a jet engine when it is required.

It is a further object of the present invention to provide a feasible solution to substantially choke the noise propagation from a nacelle inlet duct of a jet engine using an adjustable inlet throat when required.

In general terms, according to the present invention, a method for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct of a jet engine comprises a step of selectively changing an inner contour of the nacelle inlet duct during the jet engine operation to adjust both the impedance thereof with respect to the noise propagation and the velocity of inward air flow to affect the noise propagation.

The selective change of the inner control of the nacelle inlet duct is preferably conducted by selectively and controllably injecting compressed air into a pneumatic boot installed within the nacelle inlet duct forming an inflatable annular surface of the nacelle inlet duct. The velocity of the inward air flow is preferably increased to a sound speed to choke the noise propagation when required.

In one embodiment of the present invention the method comprises a step of providing an adjustable inlet throat installed in the nacelle inlet duct, preferably formed by an inflatable annular surface located at an inlet throat location for selectively and controllably reducing a cross-sectional throat area of the inlet duct, to increase both the impedance at the throat with respect to the noise propagation and the velocity of the inward air flow. In accordance with another aspect of the present invention, there is an apparatus for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct of a jet engine. The apparatus comprises an inflatable annular surface adapted to be operatively secured to the inlet duct to form an adjustable inlet throat for selectively and controllably reducing a cross-sectional throat area of the inlet duct to increase both an impedance at the throat with respect to the noise propagation and the velocity of the inward air flow. The apparatus preferably comprises an pneumatic boot adapted to be selectively and controllably inflated with compressed air to form the inflatable annular surface.

The present invention advantageously provides a feasible solution to selectively adjust a cross-sectional throat area of the nacelle inlet duct of a jet engine, thereby substantially reducing the noise propagation, even substantially choking the noise propagation when required. Other advantages and features of the present invention will be better understood with reference to a preferred embodiment as described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus generally described the nature of the invention, reference will now be made to the drawings by way of illustration showing the preferred embodiment in which:

Fig. 1 is a front schematic view of a nacelle inlet duct of a jet engine, illustrating a preferred embodiment of the present invention; Fig. 2 is a longitudinal cross-sectional schematic view of Fig. 1; and

Fig. 3 is a longitudinal cross-sectional schematic view of another embodiment of the present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figs. 1 and 2 illustrate a section of a jet propulsion engine, including a nacelle inlet duct 10 structured in accordance with the present invention. In Fig. 2, the nacelle inlet duct 10 is located upstream of a jet engine assembly 8 including, for example, a turbofan engine 12 housed in a fan cowl 14. The nacelle inlet duct wall 16 is generally defined by a portion of the fan cowl 14 that extends forward of the turbofan engine 12. The wall 16 may be continuous with the remainder of the fan cowl 14 or may be a separate structure joined to the over all engine assembly 8 by suitable means such as bolts, for example. The engine assembly 8 may take various forms other than the turbofan engine 12 and cowl 14 illustrated in Fig. 2. In a jet engine not equipped with a turbofan, for example, the compressor unit of an engine structure may be directly located downstream of the nacelle inlet duct 10. In any case, the nacelle inlet duct 10 of the present invention usually has a circular internal cross-sectional shape as illustrated by the wall 16 in Fig. 1, and directs air into the engine.

A pneumatic boot 18 is installed in the nacelle inlet duct 10 at the throat region and connected to a compressed air source (not shown) through a plurality of nozzles 20 located in the wall 16 and circumferentially spaced apart from one another. The pneumatic boot 18 can be made using the technology and materials described by Kageorge in United States patent 4,687,159 and Briscoe et al. in United States patents 4,706,911 and 4,826,108.

In an inoperative mode in which no compressed air is introduced to the pneumatic boot 18, the pneumatic boot 18 forms an annular flat surface 18c, as shown by the dashed line, contoured substantially to the wall 16 of the nacelle inlet duct 10 to meet the general requirement for air flow in engine operation under any flight conditions. The annular space between the surface 18c and the wall 16 in the drawing is exaggerated for clearer illustration only. In practice, the surface 18c is formed as a part of the' wall 16.

In an operative mode in which compressed air is injected through the nozzles 20 into the pneumatic boot 18, the pneumatic boot 18 is inflated and expands inwardly and radially, to form a throat 22 denoted in Fig. 2 by orthogonal dashed line 22 which defines an area of minimum duct cross-section. The pneumatic boot 18, when inflated, smoothly defines a transitional upstream surface 18a and a transitional, downstream surface 18b. The transitional upstream surface 18a starting from an upstream point of the inlet duct wall 16 forms a section of the inner contour of the nacelle inlet duct 10 that contracts rapidly toward the axial center line of the nacelle inlet duct 10 so that the velocity of the inward air flow increases rapidly when passing through the inlet section defined by the annular transitional upstream surface 18a, and reaches a maximum velocity at the throat 22. The contour of the transitional upstream surface 18a is smoothly designed to provide a gentle guidance for the inward air flow without separation. A transitional downstream surface 18b extends generally with a gentle gradient from the throat 22 until it is smoothly attached to the inlet duct wall 16 at a downstream point, thereby defining continuous inlet duct cross-sectional areas increasing gently toward the fan blades to recover inlet pressure without flow separation when the inward air flow passes through the throat 22. Optionally, longitudinal ridges (not shown) may be provided on the transitional downstream surface 18b or other various contours of the transitional downstream surface 18b can be provided as long as a gentle guidance for the inward air flow without separation is provided.

The throat 22 formed in the nacelle inlet duct 10 functions in several aspects of noise suppression. The impedance of the nacelle inlet duct 10 with respect to the sound propagation is determined directly by the contouring thereof and the inward air flow condition which is also affected by the contouring of the nacelle inlet duct 10. The noise sound waves generated by the turbofan 12 or the compressor (not shown) propagates through the nacelle inlet duct 10, as indicated by the arrows S in a direction generally opposite to the inward air flow F. The throat area of the nacelle inlet duct 10 will cause a transmission loss of the sound wave energy. The transmission loss is affected by various parameters, such as the ratio of the outlet cross-sectional area to the inlet cross-sectional area of the throat, the ratio of the conical length to the sound wave length. If other parameters are maintained unchanged, the transmission loss will increase when the inlet cross-sectional area decreases, which is the minimum inlet cross-sectional area defined by the throat 22 in this embodiment of the invention. When the inward air flow condition is considered, the sound waves propagate away from the engine assembly in a manner similar to that which would be experienced within a conventional inlet duct until they reach the throat 22 since the velocity of inward air flow is relatively uniform within the inlet duct section defined by the annular transitional downstream surface 18b and the downstream portion of the inlet duct wall 16. Upon reaching the throat 22, however, the sound waves encounter the inward air flow having flow velocity gradients established by the transitional upstream surface 18a. When the inward air flow passes through the contraction section defined by the transitional upstream surface 18a, the velocity of the inward air flow will vary as a function of the transverse distance from the inlet duct wall 16, with the portion of the inward air flow adjacent to the inlet duct wall 16 in the vicinity of the throat 22 travelling at a substantially higher velocity than the portions of the air flow located closer to the axial center line of the nacelle inlet duct 10. These velocity gradients refract a substantial portion of the sound waves toward the inlet duct wall 16. The noise energy of the refracted sound waves impinges on both the transitional upstream surface 18a and the upstream portion of the inlet duct wall 16, as illustrated by dashed arrows SI. The transitional upstream surface 18a and the inlet duct wall 16 absorb a portion of this noise energy and reflect the remaining portion. The reflected portion of the energy causes scattering of the sound wave travelling through the nacelle inlet duct 10 thereby providing additional noise suppression. It is optional to attach a sound absorbent lining 24 to the inlet duct wall 16. The sound absorbent lining is formed of any one of several well known materials adapted to absorb acoustic energy.

It is therefore concluded that the impedance of the nacelle inlet duct 10 with respect to the noise propagation can be selectively and controllably changed by changing the minimum duct cross-sectional area defined by the throat 22. The pneumatic boot 18, according to the embodiment of the present invention, provides a feasible solution to form an adjustable throat 22 whereby the impedance of the nacelle inlet duct 10 with respect to the noise propagation can be conveniently increased or decreased as required by selectively and controllably inflating the pneumatic boot 18.

Furthermore, it is well known that when sound waves in a lined duct propagate through a steady air flow, the effect is usually to increase the attenuation of the sound if the sound is travelling in an opposite direction to the air flow. Several mechanisms contribute to this phenomenon, the most obvious being the convection affect of the air flow itself. The sound waves are carried along in the steady flow and the time available for the pressure alternations of the sound to interact ^• with the acoustic lining of the duct is multiplied by a factor of 1/ (1+M) , where M is the Mach number of the air flow in the direction of the sound propagation. If the velocity of the steady air flow is equal to the speed of sound, the attenuation of sound travelling upstream against the air flow is very large: M=-l for this situation. Theoretically the sound propagation is choked by the air flow.

By examining the structure of Fig. 2 the inward air flow F can be generally considered as a steady flow which is caused by both the suction generated by the turbofan 12 and the forward movement of the aircraft. The noise sound wave generated by the jet engine assembly 8 propagates in the direction opposite to the inward air flow F through the nacelle inlet duct 10, so that the sound attenuation contributed by the convection effect of the inward air flow can be increased if the velocity, of the inward air flow F increases. As discussed above, the velocity of the inward air flow F will increase when the inward air flow F reaches the minimum duct cross-sectional area defined by the throat 22. Therefore, it is possible to increase the velocity of the inward air flow F at the throat 22 to the sound speed if the adjustment range determined by the pneumatic boot 18 is large enough, whereby when it is desired, the pneumatic boot 18 can be inflated to a maximum extent to increase the velocity of the inward air flow F to the sound speed to choke the noise propagation. Of course, this condition is not desirable for engine performance and therefore is only selectively provided for a period of time when a significant suppression of forward noise propagation of the jet engine is required such as in an aircraft landing situation.

This invention generally can be applied to any type of nacelle inlet ducts for aircraft engines. Fig. 3 shows another embodiment of the present invention incorporated into a typical nacelle inlet duct 30 that has a permanent throat 32 defined by the contour of the inlet duct wall 34 and located in an axial position in the nacelle inlet duct 30 more forward than the throat 22, formed in the embodiment as shown in Fig. 2. The parts similar to those of the embodiment shown in Figs. 1 and 2 are indicated by the same numerals and not redundantly described. The pneumatic boot 18 is installed in the nacelle inlet duct 30 at the permanent throat region. When the pneumatic boot 18 is inflated the cross-sectional area of the throat 32 is reduced. The minimum cross-sectional area defined by the inflated boot 18 and the permanent throat 32 may be superposed. However, they can be located differently, as shown in Fig. 3 in which the ^' minimum cross-sectional area defined by the pneumatic boot 18 is indicted by numeral 36. This embodiment functions similarly to the previously described embodiment in terms of adjusting impedance of the system with respect to the sound wave propagation, and choking the sound wave propagation when required.

The embodiment of the present invention described above provides a conceptual but feasible solution for an adjustable throat of a nacelle inlet duct of a jet engine to selectively and controllably change the impedance of the system with respect to the noise propagation with an extensive adjustment range from a least noise suppression level for any desired flight conditions, to a maximum noise suppression level to choke the noise propagation for a selected period of time when it is required. Modifications and improvements to the above-described embodiment of the invention without departing from the spirit of the , invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore is intended to be limited solely by the scope of the appended claims.

Claims

CLAIMS :

1. A method for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct of a jet engine, comprising a step of selectively changing an inner contour of the nacelle inlet duct during the jet engine operation to adjust both an impedance thereof with respect to the noise propagation and a velocity of the inward air flow to affect the noise propagation.

2. A method as claimed in claim 1 wherein the selective change of the inner contour of the nacelle inlet duct is conducted by selectively and controllably injecting compressed air into a pneumatic boot installed within the nacelle inlet duct forming an inflatable annular surface of the nacelle inlet duct.

3. A method as claimed in claim 1 comprising a step of increasing velocity of the inward air flow to a sound speed to choke the noise propagation.

4. A method for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct of a jet engine, comprising a step of providing an adjustable inlet throat installed in the nacelle inlet duct for selectively and controllably reducing a cross-sectional throat area of the inlet duct, to increase both an impedance at the throat with respect to the noise propagation and a velocity of the inward air flow. ι

5. A method as claimed in claim 4 comprising a step of enabling the adjustable inlet throat to reduce the cross-sectional area, thereby increasing the velocity of the inward air flow to a sound speed to choke the noise propagation.

6. A method as claimed in claim 4 comprising a step of providing an inflatable annular surface at an inlet throat location in the inlet duct to form the adjustable inlet throat.

7. A method as claimed in claim 1 wherein the inflatable annular surface includes a smoothly defined transitional surface upstream of the inlet throat to provide a gentle guidance for the inward air flow without separation when the surface is inflated.

8. A method as claimed in claim 1 wherein the inflatable annular surface includes a transitional surface downstream of the inlet throat defining continuous inlet duct cross-sectional areas increasing gently towards fan blades to recover inlet pressure without flow separation when the surface is inflated.

9. A method as claimed in claim 4 comprising a step of selectively and controllably providing compressed air to inflate the inflatable annular surface.

10. An apparatus for selectively reducing noise propagation in a direction opposite to an inward air flow through a nacelle inlet duct of a jet engine, comprising: an inflatable annular surface adapted to be operatively secured to the inlet duct to form an adjustable inlet throat for selectively and controllably reducing a cross-sectional throat area of the inlet duct to increase both an impedance at the throat with respect to the noise propagation and a velocity of the inward air flow.

11. An apparatus as claimed in claim 1 wherein the inflatable annular surface includes a smoothly defined transitional upstream surface to provide a smooth guidance for the inward air flow without separation when the surface is inflated.

12. An apparatus as claimed in claim 1 wherein the inflatable annular surface includes a transitional downstream surface defining continuous inlet duct cross-sectional areas increasing gently towards fan blades to recover inlet pressure without flow separation when the surface is inflated.

13. An apparatus as claimed in claim 1 comprising a pneumatic boot adapted to be selectively and controllably inflated with compressed air to form the inflatable annular surface .

14. An apparatus as claimed in claim 1 wherein the inflatable annular surface forms a part of a designed inlet contour when no compressed air is introduced into the pneumatic boot, thereby providing a smooth guidance to the inward air flow without separation in a given flight condition.