In an aircraft engine, particularly a large high-bypass turbofan, it may be necessary for the best operation of the engine to bleed air pressure from within the compressor. To do this, a servo-controlled valve can be provided in ducting leading from the compressor casing to the fan by-pass duct. Normally, there would be an array of several bleed valves spaced around the axis of the engine. Also, two or more valves or valve arrays may be provided at different stages of the compressor. The different valves or valve arrays are opened and closed, sometimes modulated, i. e. set to give a required flow rate between on and off by and engine management system. The valves are controlled by this system along with other engine parameters to optimise the operation of the engine for different operating conditions.

The release of high pressure air into the fan by-pass duct can create considerable noise and, at least in relation to valves which may open at low altitude when the aircraft is taxiing, taking off or landing, sound attenuation is required.

In National Advisory Committee for Aeronautics Technical Note 3187, The Near Noise Field of Static Jets and Some Model Studies of Devices for Noise Reduction by Leslie W Lassiter and Harvey H Hubbard, it was found that the noise level resulting from an air stream emitted from a nozzle could be reduced by placing a mesh screen in the airflow. The noise reduction was dependent on the distance between the jet and the mesh. Specification WO 03/046358 in the name of the Applicant provided means for reduction of noise from an air bleed valve through the provision of an attenuator over the valve, said attenuator being provided with an array of perforations positioned to control flow in such as way as to reduce noise.

According to one aspect of the invention there is provided apparatus for the release of pressurised fluid through an opening comprising an attenuator member extending over the opening and operable for controlling the flow of said fluid and thereby attenuate sound associated therewith characterised by a further attenuator member positioned for receiving said fluid upstream of the first-mentioned attenuator member.

According to a second aspect of the invention there is provided air bleed apparatus for a turbo fan engine, the apparatus comprising duct means having an entrance port for connection to a compressor stage of said engine and an exit port for connection to a by-pass duct of said engine, the apparatus further comprising an air bleed valve mounted in said duct means and a first and a further attenuator member positioned in series between said valve and said exit port.

According to a third aspect of the invention, there is provided apparatus for the release of pressurised fluid through an opening comprising an attenuator member extending over the opening for receiving said fluid, the attenuator member having a plurality of perforations extending therethrough for passing said fluid and the periphery of the attenuator comprising a plurality of re-entrant relieved portions arrayed around said periphery.

According to a fourth aspect of the invention, there is provided air bleed apparatus for a turbofan engine, the apparatus comprising duct means having an entrance port for connection to a compressor stage of said engine and an exit port for connection to a by-pass duct of said engine, the apparatus further comprising an air bleed valve mounted in said duct means and an attenuator member positioned between said valve and said exit port, the attenuator member comprising a perforated plate for receiving and passing air from the valve and a plurality of re-entrant relieved portions around the periphery of the plate.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a sectioned view of an air bleed valve arrangement for an aircraft engine; Figure 2 is a perspective view from above the air bleed valve used in the Figure 1 arrangement; Figure 3 is a perspective view of another perforated plate.

As shown in Figures 1 and 2, an air bleed arrangement for a high-bypass turbofan aircraft engine may comprise duct means 1 leading from the compressor casing 2 to the by-pass duct inner surface 3. The duct means includes the body 4 of a"bullet-type"air bleed valve. It is called a"bullet-type"valve because it comprises a valve member 6 having a portion 7 which generally smoothly reduces in diameter in the direction facing the flow of the air from the compressor casing 2 through the valve 4. The valve member 6 is mounted on a shaft 8 by bolt 5 that is supported by a bearing arrangement 9 including spider arrangement 10. The spider 10 is fixed to the body 4 above the valve member 6. The valve member 6 is hollow and forms a servo chamber sealed by sealing ring 11 which is held in spider arrangement 10.

The valve member 6 is normally pushed by air pressure against a spring 12 so that the periphery 13 of the member 6 engages a matching sealing portion 14 of the valve body 4.

This closes the valve 4. To open the valve, a counter pressure is applied to the interior of the valve member via inlet port 30 and ducts 31.

When the valve is open, air flows at high pressure as shown by arrows 15 and tends to form a jet or plume with the maximum velocity above the valve member 6.

Above the valve 4, there is a hat shaped attenuator member 16, the top of which is provided with a number of holes 17 to allow the air flow to pass into the by-pass duct 3.

The hat shaped attenuator member 16 may benefit from the features described in WO 03/046358, the disclosure of which is incorporated herein by reference. The same configuration of holes and metal foam may be used. However, this is not essential to the present invention.

Just above the valve 4 there is provided an additional attenuator member 20 fixed to support pillars 25, by bolts 21. This achieves further noise attenuation benefits. The additional attenuator member 20 is positioned in the path of the airflow above the valve and takes the form of a perforate plate 22 of domed form. Alternatively, the perforate plate could be flat. It has a number of small holes 23 to allow the required level of airflow through the plate. The perforate plate 22 is positioned between the valve and the attenuator member 16. At least in the example shown, the plate 22 is spaced from the outlet end 26 of the valve 4 by the lengths of the pillars 25, i. e. the plate 22 is not fixed in close contact with end 26.

Figure 3 shows an example of an alternative form of the additional attenuator member. This member comprises a flat or domed plate 40 with perforations similar to those of place 22 but its outer periphery is formed with an array of re-entrant relieved portions 41, i. e. indentations or notches of any suitable shape. In the example shown, the plate has a serrate edge formed by v-shaped indentations, so that it is reminiscent of a"pastry-cutter"implement. The presence of the serrations 41, the separation of the plate 40 from the outlet end 26, and the domed shape of the plate 40 both separately and in combination contribute to an improved noise attenuation performance.

For preference the plate 22 and the plate 40 are each designed i. e. the shape of the plate, its position, and the number and size of the perforations and serrations are chosen, so that the presence of the plate will not unduly reduce the airflow passing through the duct means 1 from the compressor casing 2 to the bypass duct.

Alternatively, the plates 22 and 40 may affect, e. g. reduce, the airflow but, as will be noted from the experimental results below, when the airflow is adjusted (by suitably controlling valve 4) so that the airflow is the same when the plate 22 or 40 is or is not present there is a reduction in noise is not just due to a reduction of airflow caused by the presence of the plate 22 or 40.

As a further preferred embodiment, the plates 22 an 40 are each designed, e. g. the shape of the plate, its position and the number and size of the perforations are chosen, to provide a real reduction in the noise levels which is not just due to a reduction of airflow caused by the presence of the plate, i. e. so that for a given airflow, the noise is less when the plate is present than when the plate is absent.

Noise testing has been conducted with a series of valves and attenuation means fitted for comparison of their effectiveness in reducing noise. Testing was carried out on bleed valves of a type fitted to a large high bypass ration turbofan aircraft engine such as would be found on a twin-engined, narrow bodied civil aircraft to measure noise levels. Noise level reductions over the base-line of Example 1 are given in Table 1. Noise measurements were taken at an equivalent position under equivalent conditions for each valve configuration tested for equal fluid flow rates through the valves.

Example 1-"Slieve/Sleeve"valve tested to give base line noise level. Such valves give more acceptable noise levels than bullet valves but are prone to contamination and are, therefore, not reliable in service. Base line noise level for this slieve/sleeve valve was measured as 84 dBA Example 2-"Bullet"valve of the type shown in Figure 1 without a perforate plate fitted.

Example 3-Bullet valve of Example 2 with flat perforate plate mounted above valve body in airflow. Perforate plate had a thickness of 3.2 mm and an array of holes with diameter 2.26 mm. Example 4-Bullet valve of Example 2 with domed perforate plate mounted above valve body in airflow. Perforate plate had a thickness of 3.2 mm and an array of holes with diameter 2.26 mm. Plate was domed to a radius of 129.5 mm.

Example 5-Bullet valve of Example 2 with domed perforate plate with serrated outer periphery mounted above valve body in airflow. Perforate plate had a thickness of 3.2 mm and an array of holes with diameter 2.26 mm. Plate was domed to a radius of 129.5 mm.

Outer periphery of plate had serrated form. Height of serrations was 11.4 mm including a radius of 1.5 mm at the root of the serration and a radius of 1.0 mm at the tip of the serration.

Serrations were positioned around the periphery with a 15° separation. Valve Example No. Change over base noise level of Example 1 2 Increase of 4-5 dBA 3 Reduction of 0.5-1. 5 dBA 4 Reduction of 3-4 dBA 5 Reduction of 6-8 dBA The attenuator member 16 could comprise differentially distributed apertures and apertures of different sizes as disclosed in patent specification number WO/046358. Also as disclosed therein, there could be a layer of metallic foam adjacent the attenuator member. The foam could be arranged, e. g. by having a graded density and/or thickness, to provide different porosity at different zones of the member. Often the preference will be for the porosity to be greater further away from the central zone of the plate. Different porosity can achieve different airflow rates or sound attenuation levels for different applications/aircraft and thus provide greater flexibility in attenuation design.