GB2236804A - Compound nozzle - Google Patents

Abstract

A compound nozzle consists of a primary passage 22 for conveying a first part of a fluid to be ejected and a secondary passage 23 surrounding the said first passage 22 and communicating with the ambient. The secondary passage 23 is effective to divert a second part of the fluid along a path 26 so as to induce a flow of fluid from the ambient, and is also effective to direct the induced fluid such that in use it encases the first part of the fluid as it leaves the primary passage 22 of the nozzle. The arrangement reduces the noise generated by fluid leaving the nozzle, and may be used in nozzles producing jets for cleaning or cooling, and also in the propulsion of aircraft or boats. In modifications, a shroud may surround part of the induced ambient flow, and the cross-sectional area of the surrounding passage may be variable. <IMAGE>

Description

rwrod Nozzle This invention relates to a fluid nozzle with a means for reducing the level of noise generation by exhaust gases issuing from it.

Nozzles are used for directing and accelerating the release of a fluid Typical applications include the cleaning of machine tools, cooling by jet impingement, and the propulsion of vehicles such as boats and aircraft.

Jets issuing from nozzles are known to be noisy, the level of noise being dependent upon the mass of fluid within the jet and upon the velocity of the jet. A significant part of this noise is generated at the interface between the jet and its surrounding ambient fluid where there is a rapid variation in axial velocity with distance from the centre of the jet. This rapid change in velocity generates viscous shear stresses which in turn generate high levels of noise The present invention seeks to provide a compound nozzle wherein noise due to the velocity gradient normal to the major axis of the nozzle is reduce Various means are known for reducing exhaust noise.One known group of devices seeks to promote the rate of mixing between the jet and its surrounding ambient fluid with the intention of reducing the distance over which high levels of shear stresses exist and thereby reduce the level of noise. Such devices, however tend substantially to reduce the momentum flux from the jet and so make it less effective for its intended purpose.

Other devices are known eg. from GB 878,191; Q -A-2,207,708 and GB 1,207,194 where use is made of the phenomenon wherein when a fluid jet issues from a nozzle, it entrains a sheath of low velocity air from ambient around its circumference. Such devices are only partially successful owing to the distance required in order to entrain a sufficient quantity of ambient air to be effective for the purpose of noise attenuation. Furthermore, at some supersonic nozzle operating conditions the static pressure of the air issuing from the nozzle can be greater than ambient such that entrainment into this region is very ineffective.

Further, it is known from GB 1,521,495 and WO-A-83/03281 to draw ambient air into the core of the jet before it exits from the nozzle by way of ducts traversing through the stream of the jet, but such devices are complex, involve excessive pressure losses and increase the overall weight of the device.

According to the present invention there is provided a compound nozzle consisting of a primary passage for conveying the major portion of the fluid to be ejected and means for diverting into a secondary passage around the primary passage a minor portion of the fluid, said means being effective to induce a flow of fluid from the ambient and direct it eventually to encase the said major portion as it leaves the said primary passage.

Preferred embodiments of the invention will now be described by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic view of fluid flow issuing through an unsilenced and silenced nozzle, respectively; Figure 2 is a longitudinal section through a silenced compound nozzle according to a first enoodiment of the invention; Figure 3 is a section through the compound nozzle of Figure 2, shown while adjusted for full boost operation; Figure 4 is a graph of frequency against noise level measured from a nozzle such as is shown in Figures 2 and 3; Figure 5 is a section through an alternative embodiment of a compound nozzle according to the invention wherein the minor passage of fluid and its associated entrained fluid are partially encased by a shroud;; Figure 6 shows a further alternative arrangement of the compound nozzle, in longitudinal section, considered to be advantageous for propulsive purposes; and - Figure 7 is a cross-section of the nozzle taken on the plane indicated by the lines X-X illustrated in Figure 6.

Referring first to Figure 1 of the drawings, the upper diagram (as viewed) shows a nozzle 10 not equipped with any silencing means and a jet of fluid 11 issuing from it. The line 12 indicates the velocity of the jet 11 and as can be seen, the gradient changes from 0 to 900 and back again to 0 very suddenly, whereby a high level of noise results. The lower diagram shows that by some means unillustrated in Figure 1 ambient air 13 is induced to flow around the main jet 11, resulting in a more rounded velocity profile 12', i.e. in a less steep velocity gradient resulting in appreciable noise reduction.

Turning now to Figure 2, there is shown a compound nozzle 20 according to the invention which comprises an inlet passage 21 leading to a division of flow between a primary passage 22 and a smaller, concentric secondary passage 23. Access to passage 23 is via an annular connecting duct 24 or a plurality of angularly spaced apart ducts and an annular passage 25 which also serves as a nozzle to the stream of secondary flow 26. This stream of flow 6 suffers a considerable change of direction by a convex wall 27 or step of a shaped body 28 designed with a substantially convex portion according to the aerodynamicists art in order to create a substantial drop in static pressure to a level below that of the surrounding ambient. Fluid 9 from the ambient is induced towards the convex wall 27 and its generally axially directed continuation wall 29.Along the wall 29 a momentum is transferred from the stream 26 until there is a gradual reduction in streamwise velocity with distance from the wall 29 and there is no longer a severe (significant) depression at the wall, say at the point 30 which may readily be determined by a person skilled in the art of aerodynamics. From point 30 a surface 31 tapering radially inwardly directs the induced stream such that it forms a sheath which totally or near totally encases the jet 32 emerging from the exit of passage 32 at the end 37. A shutter 33, axially slidably displaceable by means not shown, is illustrated in its inoperative position in which the secondary passage is open.

Figure 3 illustrates the shutter 33 moved longitudinally such that it blocks off the passage 23. Thus all the high pressure fluid continues along passage 22 eventually to emerge from exit 37 in order to provide maximum boost or propulsion from the jet.

Figure 4 is a graph of octave band centre frequency in Hz against octave band sound pressure level. The full and broken lines correspond, to the condition of the nozzle illustrated in Figures 2 and 3, respectively.

Figure 5 shows an alternative embodiment according to the invention wherein a shroud 48 is used to form a wall of a tertiary fluid flow passage 46. The shroud 48 may also serve as a location for materials with sound attenuating properties and for directing the stream 26 and its entrained ambient air 9 towards the exit 37.

Figure 6 illustrates a longitudinal section through yet another alternative embodiment according to the invention, wherein part of the external surface of the nozzle comprising items 27, 29, 30 and 31 are located on a slide 48 which can be moved longitudinally towards and away from outlet passages 25 to vary the quantity of the mass flow of stream 26; in the extreme case, the slide 48 can close off the passage 25 wholly to prevent flow of the stream of fluid 6. The exit to passage 22 is now replaced by a nozzle of variable area 44 which can be adjusted in accordance with the longitudinal movement of slide 48 in order to control the level of pressure in the passage 21.

Struts 45 support the casing of primary passage 22.

Claims (9)

1. A compound nozzle consisting of a primary passage for

conveying a first part of the fluid to be ejected and a secondary passage surrounding the said first passage and communicating with the ambient, said secondary passage being effective to diverting a second part of the fluid along a path including itself so as to induce a flow of fluid from the ambient, and also being effective to direct said induced fluid such that in use it encases the said first part of the fluid as it leaves the primary passage of the nozzle.

2. A compound nozzle as claimed in claim 1, wherein said path includes a body having an aerodynamic surface and being longitudinally translatable to to adjust the area of, or to close off, the said secondary passage.

3. A compound nozzle as claimed in claim 1 or 2, wherein a variable area nozzle is provided to control the cross-sectional area at or close to the plane of exit of the stream of fluid from the primary passage.

4. A compound nozzle according to claim 2 or claim 3 when dependent on claim 2, wherein said body has an upstream convex, flow-deflecting wall or step portion, a generally axial centre portion and a radially inwardly tapering downstream portion.

5. A compound nozzle as claimed in claim 1 wherein the secondary passage is effective to divert a second flow of fluid into a tertiary passage open at its inlet end such that in use fluid from the ambient enters this tertiary passage in addition to or as a replacement for the said path.

6. A compound nozzle as claimed in any preceding claim, wherein means, which may be the said body, are provided in the secondary passage which reduce the velocity of the induced flow in proportion with the perpendicular distance away from the primary passage of the nozzle.

7. A compound nozzle as claimed in any preceding claim, wherein means, which may be the said body and a nozzle, are provided in the secondary passage which reduce the static pressure of the pressurized fluid exiting from the primary passage.

8. A compound nozzle as claimed in claim 1 or claim 2, wherein obturating means are provided to control the flow crosssection of the secondary passage to any desired value from zero to a predetermined maximLnL

9. A compound nozzle substantially as herein described with reference to and as shown in Figures 2 and 4, Figure 5 or Figures 6 and 7 of the accompanying drawings.