US2613758A - Baffle type muffler with sound absorbing material - Google Patents

Description

Oct.h14, 1952 D. J.,w. cuLLuM BAFFLE TYPE MU'T'F'LER WITH SOUND ABSORBING MATERIAL Filed Dec.'

I NVENTOR ,lllli l l l Irllllllvlxll j DOUGLAS J.

W ULLUM BY ATTORNEY Patented Oct. 14, 1952 ABSORBING MATERIAL Douglas Jack Wayin Cullum, London, England Application December 19, 1949, Serial No. 133,810

In Great Britain October 14, 1948 1 Claim. (C1. 181-50) In the construction and testing of jet propulsion engines,- such as in research and test workshops, the engines are suitably supported on their benches orequivalent with their jet nozzles directed towardsan aperture or respective apertures in the wall of the workshop in order that the iiarnes and products of combustion of the jets shall be discharged into the outside atmosphere. A great noise is caused by the jets during test, and it is known to employ a so-called detuner arrangedoutside the building with its mouth located in or passing through the wall aperture to receive the issuing jet and partially absorb and minimise the sound. The aperture is sometimes provided with a pair of doors which are opened to allow the detuner to be run up so as to dispose its mouth in proximity 'to the engine during use. These detuners are usually in the form of a perforated cylindrical tube concentrically mounted within another tube to form an annular space which is lagged with sound-absorbing material.

`The present invention is an improvement in noise-suppressing apparatus for the foregoing purpose, and has for its objects to provide optimum operating conditions for the engine while suppressing the noise, and to absorb the sound of the issuing jet to a greater degree than is attained by known detuners.

According to the present invention there is provided noise suppressing apparatus for use in testing jet engines comprising a detuner tube having an interior hollow pocket-like baille, projecting so as to be in the path of the gas stream and having its mouth directed towards the entrance to the tube. In practical embodiments of the invention the said baille is a hollow perforated or equivalent structure of metal or other suitable material supported substantially concentrically within the detuner tube and arranged so that there is an annular space between its wall and the inner wall of the tube at all points between the mouth and rear end of the baille. Preferably a part or the whole of the baille, in longitudinal section, is progressively smaller towards its rear end. To form a complete lobstruction across the detuner tube the edge of the mouth of the baille, such as a ilange, is iixed in contact with the inner wall of the detuner so that its body is spaced from the inner wall of the detuner tube. A part or the whole of the detuner and, where an inner sound absorbing core is used, as hereinafter described, at least the section of the detuner tube which embraces this core, is constructed with an outer jacket which is lagged interorly, e. g. by rock t 2 wool, to provide sound absorption, and the lagging is protected by a perforated metal liner tube.

In the use of the improved device the flames and combustion products from the jet exhaust are directed into the interior of this pocket-like baille and escapethrough the perorations into the space between the baiile and the surrounding detuner wall. The baille may be of conical, pyramidal or semi-spherical form, or may be cylindrical of smaller diameter than the inner Wall of the jacket and with an out-turned flange at the mouth for attachment to the jacket, orit may be any suitable'combnationof these shapes, e. g. cylindrical with a spherical or conical end. Instead of a single pocket it may be an assembly ofpockets in laminated array, slightly spaced to give passage for thegases, either with or without sound absorbing material interposed between the plates.

One baiiie is preferably disposed at or near the exit end of the detuner, and may be associated with a core supported within the detuner to lie longitudinally along the centre thereof, said core preferably having pointed deflector ends, one of which terminates within the saidbaille. Said inner core is a hollow member containing a suitable sound-absorbng packing and the wall of the body thereof is perforated. The flames and combustion products transverse the space between the core and wall of the detuner, eventually entering the baille and passing along the space between the end of the core and the wall of the baille.

The accompanying drawing illustrates diagrammatically an embodiment including the various features of the invention.

In this drawing l is the jet engine exhaust which is located opposite an aperture in a Wall 2, through which aperture is projected the leading end 3 of the detuner tube while this leading end may take any suitable form. It may be telescopically mounted in a main part 3a which leads to an entrance 3b of frusto-conical interior. The detuner part 3b extends into a second tubular part 4 which consists of a jacket having an interior sound absorbing lining which is protected by a perforated liner 5. Near the rear or exit end of the detuner part 4 the interior passage is obstructed by conical baille plate 6 which is perforated and which has a mouth flange secured to an annular shoulder 1 formed on the inner wall of the detuner partit. The baffle mouth is co-extensive with the perforated liner 5, with the apex of the cone directed towards the rear. The baffle, in conjunction with the rear internal Wall 8 of the detuner, forms a diverging annular passageway 9 through which the gases pass to atmosphere.

Increased sound absorption is provided by the core I0 which is a hollow perforated metal cylinder of circular cross section coaxial with the detuner part d and lled with rock wool or other suitable sound absorbing material. The core is of smaller diameter than the inner wall of the detuner tube so as to provide an annular gas channel II leading to the baille 6, the ends I2, I3 of the core being conical deflectors which guide the gases in the required direction, for which purpose they are advantageously plain and not perforated, the rear end I3 being located within the baffle 6 as shown. The pointed end of the baille B may be blanked off by a plate I4 which may or may not be perforated. The tip of the cone in rear of the plate I4 may be imperforate. I

Although any suitable cross section may be given to the detuner 4, the core IU and the baille Ii, it is advantageous to adopt a circular section for all three, giving the core a torpedo shape with conical ends as shown, and the baille a conical shape, but the angle of the cone of the baille need not necessarily correspond with the angle of the point of the core end I3; in fact as shown it is preferable that the angle of the baille is more acute than the angle of the core point whereby the annular space l5 between core and baffle gradually increases in cross section towards the outlet.

For utmost erhciency, the area of the annular space 9 outside the baille and between the baille and detuner wall taken at any plane normal to the axis of the detuner should be not less than (and preferably is greater than) the superficial area in the aggregate of all the perforations in the baffle between said lplane and the base of the baille, this area relationship being particularly applicable when a conical baille is employed as shown.

I claim:

Noise suppressing apparatus for use in testing jet engines, comprising a cylindrical muffler tube, an interior lining of sound absorbing material for said tube, a protective perforated liner for said material, said tube with its sound absorbing lining and perforated liner comprising a gas entrance portion of frusto-conical section providing greater thickness of sound absorbing material at the entrance end of said portion than at the remote end of said portion, an intermediate portion wherein the annular space for the sound absorbing material between the tube and the liner is of uniform cross-section along the entire length of the intermediate portion, and a gas discharge portion wherein the liner flares outwardly towards the outer end of the tube providing a diminishing thickness of sound absorbing material between the tube and the liner, a hollow conical perforated baille located within said gas discharge portion with the mouth of the baille directed towards the entrance portion of the tube and co-extensive with the outer end of the intermediate portion of the tube, an imperforate conical cap attached to the rear end of the frustoconical baille, an imperforate plate closing the outer end of said baille, a perforated hollow core supported co-axially within the intermediate portion of the tube and extending from inside the entrance end of the intermediate portion to inside the mouth of said baille and forming with the intermediate portion an annular gas channel leading to the mouth of the baille, imperforate conical deflectors closing the ends of the core, and a filling of sound absorbing material within the core.

DOUGLAS JACK WAYTH CULLUM REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,067,200 Shlosberg July 8, 1913 1,709,333 Webb Apr. 16, 1929 1,811,762 Schnell June 23, 1931 1,847,830 Hills Mar. 1, 1932 2,017,744 Bourne Oct. 15, 1935 2,373,231 Demuth Apr. 10, 1945 2,514,996 Faust July 11, 1950 2,523,260 Campbell Sept. 26, 1950

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