GB2026622A

GB2026622A - Blade for Fluid Flow Machine

Info

Publication number: GB2026622A
Application number: GB7917565A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1978-07-08
Filing date: 1979-05-21
Publication date: 1980-02-06

Abstract

The blade, e.g. a fan blade (10) in a ducted fan gas turbine engine, is designed to absorb noise impinging on it. To this end the blade (10) is hollow with a cellular core (22) and a part or the whole of the concave working face at least of the blade is sound permeable so that sound energy can enter the cells for absorption. Energy absorption may be by resonance and frictional damping effects or by means of fibrous or porous sound absorbing material disposed within the cells. <IMAGE>

Description

SPECIFICATION Rotor Blades for Fluid Flow Machines This invention relates to rotor blades for fluid flow machines, and particularly, but not exclusive ly, to fan blades for ducted fan gas turbine aeroengines, the fan blades having particular value in reducing forward arc noise from the engine.

Forward arc noise for current front fan gas turbine aeroengines is comprised of noise from a number of sources, for example: interaction between the wakes of the fan blades and the stators downstream of the fan blades, such as the inlet guide vanes of the core engine or the outlet guide vanes for the fan air stream in the fan duct; other fan-related noise such as that due to interactions between the fan blade surfaces and unsteady flow in the fan air intake; and noise from the core engine compressor. Some of this forward arc noise, when within the fan duct and intake, propagates directly upstream through the plane of rotation of the fan rotor, and some propagates upstream in rotating (spiral) modes due to the rotation of the fan rotor.

A straightforward solution to the general problem of forward arc noise is to apply sound absorbing linings to the internal wall surfaces of the fan duct and fan air intake. This approach has its limitations, however, in that there is obviously an upper limit to the area available for such linings, and also in that their uses increases the cost and weight of the engine. Further, not all noise from the sources referred to radiates in directions which cause it to impinge on the wall linings, and therefore much noise tends to radiate freely from the fan air intake.

In an attempt to intercept some of this noise, it has been proposed to incorporate further sound absorbing surfaces upstream or downstream of the fan rotor in the form of "splitters", these surfaces being radially or axially disposed with respect to the rotational axis of the fan rotor.

However, such measures not only increase weight and cost, they also contribute extra internal aerodynamic drag and thus reduce the overall efficiency of the engine.

In contrast to the above, the present invention provides fluid flow rotary machines in general, and ducted fan gas turbine aeroengines in particular, with extra sound absorbing areas over and above those which can be provided on the internal wall surfaces of the fluid flow ducts of the machines, but this is done at no increase in the weight of the machine and also conceivably at no increase in its cost. These extra sound absorbing surfaces within the flow ducts can be used to intercept noise propagating directly upstream as well as noise propagating upstream in rotating modes.

In its widest sense, the present invention comprises a fluid flow rotary machine rotor blade specifically designed and configured to absorb noise impinging thereon.

According to the present invention, a rotor blade for use in the flow duct of a fluid flow machine has an aerofoil-shaped working portion with concave and convex working faces and comprising, at least in part, a rigid skin surrounding and bonded to an appropriately shaped cellular core of honeycomb or other spaced cell type of structure, at least part of the aerofoil's rigid skin being sound-permeable, whereby sound energy in the flow duct can enter the core cells of the aerofoil portion and be dissipated therein by resonance or other sound energy absorbing effects.

For maximum effectiveness in absorbing noise, it is preferred that if possible the rigid skin be sound permeable on both the concave and convex working faces of the aerofoil. Where this cannot be achieved, it is generally preferable that the rigid skin be sound permeable only on the concave face of the aerofoil.

In the case in which the rigid skin is sound permeable on both sides of the aerofoil, the cellular core should comprise two sets of core ceils, the first set communicating with the flow duct via the sound-permeable skin on the concave side of the aerofoil and the second set communicating with the flow duct via the sound-permeable skin on the convex side of the aerofoil, there being no communication between the first and second sets of cells. In the case in which the rigid skin is sound-permeable on only one side of the aerofoil, there need only be one set of cells provided.

The rigid skin of the aerofoil portion of the blade may comprise sheet metal having perforations therein to provide the required degree of permeability to sound. Alternatively, the rigid skin may comprise a sintered metallic material or a fibre-reinforced synthetic resin material, these materials being porous, perforated, or otherwise permeable to sound to the required degree.

In the case where the blade is specifically constructed to absorb sound energy by resonance effects, at least some of the core cells may be acoustically interconnected with their adjoining cell or cells in such a way as to provide a larger volume for resonance than just one cell can provide.

In the case where the blade is constructed specifically to absorb sound energy by sound energy absorbing effects other than resonance, the core cells may be fully or partially filled by a sound-absorbing material such as mineral fibre matt, metallic felt, or foamed plastic.

The fluid flow machine may be a ducted fan gas turbine engine, and in the case of such an engine having a front fan, the rotor blade is advantageously a front fan blade. The invention includes a ducted fan gas turbine engine fitted with rotor blades according to the invention.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a ducted fan gas turbine aeroengine incorporating a fan blade according to the invention; Figure 2 shows the fan blade in greater detail, together with some adjacent engine structure; Figure 3 shows a view on section line Ill-Ill in Figure 2 of the fan blade together with one of its immediate neighbouring blades; Figure 4 shows another embodiment of the invention, the illustration being part of a crosssection of a fan blade seen in a view similar to that of Figure 3.

The drawings are not to scale.

In Figure 1, the aeroengine 1 has a front fan 2 which rotates in fan casing 4. The fan casing 4 is coaxial with and partially encloses the core engine 6 which as is well known comprises compressors, combustion chamber(s) and turbines (not detailed). The core engine 6 provides the power to drive the front fan rotor 2 and also produces some jet thrust by expansion of combustion products through the propulsion nozzle 8.

Figure 1 shows diagrammatically part of the fan casing 4 broken away to reveal a fan blade 10.

In Figure 2, part of the surface of the fan blade has also been removed to show the internal structure.

In Figure 2 it will be seen that the fan blades, of which 10 is a representative example, are situated upstream of two sets of stators, namely the inlet guide vanes 12 of the core engine 6 and the outlet guide vanes 14 in the bypass duct 5.

Vanes 12 are nearer to the fan 2 than vanes 14.

In such arrangements it is found that the noise in an arc forward of the fan air inlet 7 contains a prominent tone generated at the 2nd harmonic of fan blade passing frequency. This tone is primarily due to the interaction between the fan blade wakes and the core engine inlet guide vanes 12.

Bypass outlet guide vanes 14 also contribute to the tone, but less so than vanes 1 2 because they are further away from the fan 2, thus allowing the wakes more time to dissipate.

Some other forward arc fan-related noise is due to interactions between the fan blade aerofoil surfaces and unsteady flow in the fan air intake 7.

Again, this noise is related to blade-passing frequency but has a wider band-spread due to the inherent characteristics of unsteady flows.

Further noise comes from the compressor of the core engine 6; for example, that due to the interaction between inlet guide vane 12 and the first stage of compressor blades (not shown).

In order to combat the noise from these various sources, the internal wall surfaces of the fan duct 5 and the fan air intake 7 have sound absorbing linings 16 and 1 8. Although these linings are effective in absorbing noise which impinges on them, some noise travels upstream without impinging on the linings 1 6 and 18 and thus, in the prior art, this noise radiates freely from the intake 7. However, in our invention, part of this noise is intercepted by noise-absorbing fan blades comprising a fan rotor 2.

As shown in Figures 2 and 3, the aerofoil portion of one of the noise-absorbing blades 1 0 comprises a rigid skin 20 fabricated from sheet metal, such as titanium, and a rigid cellular metallic honeycomb core structure 22. The skin 20 surrounds, and is bonded to, the honeycomb core 22, which besides helping to fulfil the objects of the invention, also contributes to the strength and rigidity of the blade. The skin 20 on the concave side 21 of the aerofoil is perforated with many small holes 24, of which only a token number are shown. These perforations 24 render the skin 20 permeable to sound and allow sound energy impinging on the perforated portions of the skin to enter the core cells for absorption by resonance and frictional damping effects. The skin of the convex side of the aerofoil is not perforated and acts as an impervious rigid backing for the honeycomb cells.

It will be noted from Figure 3 that the concave side 21 of the aerofoil faces both rearwardly (downstream) and towards the convex side of adjacent blade 11 (across-stream). It can thus intercept not only sound waves 26 travelling upstream from the core engine 6 and stators 12 and 14, but also sound waves 28 and 30 respectively generated at or reflected from the convex surface of the adjacent blade 11.

Due to considerations relating to the strength and rigidity of the honeycomb core, there is obviously an upper limit on the size of its individual cells. Further, the trailing edges 32 and the tips 34 are regions of the blade where the blade thicknesses, and hence the depths of the cells, are relatively small. Hence, in order to absorb frequencies present in the fan duct and intake more effectively by resonance, it may be necessary to interconnect at least some of the core cells with adjoining cells to form groups or "nests" of cells, such as the shaded cell group 36, which together act to absorb a wider range of frequencies than just one of the cells could do on its own. The interconnection of the cells provides a larger volume for resonance.

If it is desired that the core cells should act predominantly as tube-type resonators, then there should be as many perforations 24 as possible in registration with each cell. However, if it is desired that the core cells act predominantly as Helmholtz-type resonators, then preferably only one perforation should be in registration with each cell.

If necessary, it would be possible to arrange the cells and perforations so that some cells would act predominantly as tube-type resonators, whilst others would act predominantly as Helmholtz-type resonators. This would increase the range of frequencies which could effectively be absorbed. The two different types could be intermixed with each other over the extent of the aerofoil, or certain regions of the aerofoil could consist entirely of one type of resonator, whilst the other regions could consist of the other type.

For example, high frequency noise coming from core engine 6 (Figure 2) may be best dealt with by tube-type resonators, and therefore the radially inner region of the aerofoil of blade 10 could advantageously consist entirely of this type, whilst the radially outer region of the aerofoil could consist of Helmholtz-type resonators for absorbing lower frequency noise.

As a further alternative, the resonator cells in any one fan blade could be all of the same type, but different blades could have different types, e.g. each alternate blade could have tube-type resonator cells and the other blades could have the Helmholtz type. Note that such an arrangement would superimpose further tones (related to fan speed) on the overall noise radiated from the fan inlet, because of the fluctuations in the amount of noise absorbed in any particular part of the plane of rotation of the fan blades due to the passage of blades qf differing absorptive and reflective characteristics. Thus, although the overall radiated noise level would be lower than for non-absorbing blades, the usefulness of the technique would depend on the acceptability to the ear of the perceived tones.

As an alternative to the use of resonance effects to absorb noise, the core cells could be wholly or partially filled by a bulk sound-absorbing material such as mineral (glass or ceramic) fibre, metal felt or plastic foam. These materials are well known as sound absorbers and require no further description. If such materials are used, it is of course desirable that there be as many skin perforations per cell as possible. The use of bulk sound-absorbing materials instead of resonance effects produces blades with a wider noise absorption bandwidth. In order to further optimise the noise absorbing qualities of the blades, it may be desirable to provide blades in which some of the core cells absorb noise by resonance effects, and some absorb noise by means of bulk absorber material.It would of course also be possible to provide some blades with core structures adapted for resonance and other blades with core structures containing bulk absorber materials.

Because the fan blades are rotating, the noise generated at their surfaces propagates upstream in a rotating (spiral) mode. The sound waves have appreciable components of movement in the plane of the fan rotor, for example the sound wave 28 in Figure 3. Of course, noise is generated on both sides (convex and concave) of the aerofoil and is to some extent reflected back and forth between adjacent blades, see for example sound wave 30. It is therefore evident that a greater proportion of the rotating mode noise could be absorbed if the fan blades were able to absorb noise at both their convex and concave faces.

In Figure 4, this objective is realised by providing fan blade 34 with a metal skin 36 which is perforated on both sides of the aerofoil. In order to avoid any communication between the concave ("pressure") and convex ("suction") sides of the blade via the honeycomb core structure 58, the core comprises two separate sets of core cells 40 and 42. The cells 40 receive sound via the perforations on the concave side of the aerofoil and the cells 42 receive sound via the perforations on the convex side of the aerofoil, there being no communication between cells 40 and cells 42. In this embodiment, the two sets of cells required are conveniently produced by making the core structure 38 as two layers of honeycomb cells arranged back-to-back so that they share a common impervious backing wall 44 which can also conveniently form a strengthening spine for the blade.This double-layer structure for the honeycomb core is only possible in cases where the fan blade is thick enough (a) to allow individual cells adequate depth and volume for resonance to the frequencies to be absorbed, or, in the case of noise absorption using bulk absorbers, (b) to allow an adequate depth of bulk absorber material to be incorporated in the cells.

Trends towards providing turbofan engines with fan rotors comprising fewer blades each having a greater thickness and degree of twist between base and tip regions than present day blades mean that the embodiment shown in Figure 4 may find particular application to future fan or "prop-fan" blades.

- Comments made with respect to Figures 2 and 3 concerning the size of the core cells, interconnection of core cells (provided that for the embodiment discussed in relation to Figure 4 this only occurs between cells of the same set), choice of the number of skin perforations per cell, and choice of the mode of noise absorption to be utilised, also apply to the embodiment shown in Figure 4.

More generally, within the limits imposed by the strengths of the materials used and the required rigidity and aerodynamic characteristics of the fan blades, the honeycomb cell size, the nature of any bulk absorber material and the size and number of the perforations in the skin of the aerofoil are chosen to give the best absorption of the noise frequencies present in the duct.

Although the rigid skin of the aerofoil has been described as being composed of conventional sheet metal, it could instead conceivably be composed of a suitable sintered metallic or fibrereinforced synthetic resin sheet material, provided that such material has adequate strength and rigidity and is perforated or otherwise rendered permeable to sound to the required degree.

Although the invention has been described as applied to the blades of a fan rotor, it could also be applied to stator blades, such as the vanes 12 and 14 in Figure 2.

Claims

1. A rotor blade for use in the flow duct of a fluid flow machine, said blade having an aerofoilshaped working portion with concave and convex working faces and comprising, at least in part, a rigid skin surrounding and bonded to an appropriately shaped cellular core of honeycomb or other spaced cell type of structure, at least part of the aerofoil's rigid skin being sound-permeable, whereby sound energy in the flow duct can enter the core cells of the aerofoil portion and be dissipated therein by resonance or other sound energy absorbing effects.

2. A rotor blade according to claim 1 in which the rigid skin is sound permeable at least on the concave working face of the aerofoil.

3. A rotor blade according to claim 1 in which the rigid skin is sound permeable on both sides of the aerofoil and the cellular core comprises two sets of core cells, the first set communicating with the flow duct via the sound-permeable skin on the concave side of the aerofoil and the second set communicating with the flow duct via the sound permeable skin on the convex side of the aerofoil, there being no communication between the first and second sets of cells.

4. A rotor blade according to any one of claims 1 to 3 in which the rigid skin of the aerofoil portion of the blade comprises sheet metal having perforations therein to provide the required degree of permeability to sound.

5. A rotor blade according to any one of claims 1 to 3 in which the rigid skin of the aerofoil portion of the blade comprises a sintered metallic material or a fibre-reinforced synthetic resin material, said materials being porous or perforated to provide the required degree of permeability to sound.

6. A rotor blade according to any one of claims 1 to 5 which is constructed to absorb sound energy by resonance effects, at least some of the core cells being acoustically interconnected with their adjoining cell or cells.

7. A rotor blade according to any one of claims 1 to 5 in which the core cells are at least partially filled with a sound absorbing material of a fibrous or porous nature.

8. A rotor blade substantially as described in this specification with reference to and as illustrated in Figures 2 and 3 or Figure 4 of the accompanying drawings.

9. A ducted fan gas turbine engine fitted with rotor blades according to any one of claims 1 to 8.

10. A ducted fan gas turbine engine substantially as described in this specification with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.