US 2664702 A
Description (OCR text may contain errors)
P. LLOYD ET AL Jan. 5, 1954 COOLED FLAME TUBE 2 Sheets-Sheer 1 Filed July'29, 1948 Jan. 5, 1954 P. LLOYD ET AL COOLED FLAME TUBE 2 Sheets-Shem 2 Filed July 29, 1948 lnvenlorJ gA/forueys having any substantial penetrative power transverse to the hot gas stream.
By way of example, various forms of the invention are described below with reference to the accompanying drawings, in which:
Figure 1 is an axial section of one form;
Figure 2 is an axial section of a modification of Fig. 1.
In Figure 1, a high temperature gas duct in the form of a combustion chamber l of circular section is arranged co-axially within an outer air casing 2 and is provided with a hollow baffle or cap 3, having its open larger end facing downstream and forming in combination with the duct I an annular cooling air entry 4.
The cap 3 encloses a primary combustion zone 5 into which fuel is injected through a burner t, and combustion is initiated by means of a spark plug 24.
The cap 3 is enclosed within a convergent tubular scoop i of frusto-conical form, having its upstream narrower end open to form an entry aperture 8. The wall of the chamber I has an upstream tubular extension la, enclosing said scoop 1 which defines a metering orifice and passage for the entry of primary air, while the extension la forms with scoop 1 a passage for a supply of cooling air through the entry 4.
The casing 2 is also provided with an extension 2a, similar to the extension la, and has an entry portion 2b into which is led a fast moving airstream, as for example from the compressor of a gas turbine. This airstream is divided by the extensions la, 2a and the scoop E, into the three parts, of which one part forms an annular air cooling stream entering the chamber i through the aperture 4, a second part flows through the annular space between the casing 2 and the duct i, while the remainder passes through the cap 3 to the primary combustion zone 5.
The dimensions of the cap 3, entry i and chamber l are such that in operation at the design point, the velocity of the air at the entry s and the velocity of the hot gas flowing downstream from the primary combustion zone 5 are, in the region of the aperture 4, sufficiently closely matched to enable these two streams to flow as separate streams for a substantial distance downstream of the entry so as to maintain an envelope of cool air as a separate non-turbulent flow between the hot gas stream and the inner wall of the chamber 1. Since, however, the velocity of these two streams cannot be matched over a wide range of operating conditions, the velocities are preferably matched at the conditions existing at the maximum operating temperature as a breakdown of the cooling air layer at lower temperatures can be generally accepted.
The cap 3 constitutes the flame stablising baflie generally used in such cases and is provided with an axial air entry 9 for primary air, this entry being of suitable dimensions to pass the required air flow.
The cap 3 may also be provided with swirl vanes ill for swirling the air passing through entry 9 and circular apertures H and slots i2 may also be provided in the wall of the cap 3 for the entry of further primary air.
Downstream of the cap 3 are provided apertures 25 (which may also be scoops or stub pipes) in the wall of chamber 1 for the admission of further air from casing 2 to form a zone I! where turbulence and mixing take place.
In Fig. 1 the fuel is injected downstream CO! axially into the chamber l, but the fuel may alternatively be injected upstream, for example by suitably adapting the structure disclosed in co-pending application No. 609,532, new Patent No. 2,529,506.
It may also be possible in some cases to modify the arrangement described by dispensing entirely with a separate flame tube or inner chamber downstream of the bafiie.
It is desirable to cool the chamber 5 downstream of the point at which the initial cooling air flow through the entry 4 ceases to be effective, since at high temperatures the cooling effect of the additional air normally introduced for further stages of combustion may be insuiilcient; moreover any turbulence so produced will promote heat exchange between the hot gases and the chamber wall, and is therefore disadvantageous. For this purpose additional flows of cooling air may be provided similar to that aiready described, and having at their regions of introduction into the chamber l a velocity comparable to that in the outermost zone oi the chamber at these regions.
Such additional cooling air may be introduced through radial flow entries arranged and shaped so as to avoid the formation of well defined air-- streams of substantial penetrative power transverse to the hot gas stream, thus forming over the inside of the chamber wall a relatively stable or non-turbulent insulating air layer which mixes only slowly with the hot gases due to the differing velocities and by difiusion rather than by trans verse penetration.
For this purpose, as shown in Fig. l, the wall of the chamber I may be provided downstream the mixing zone with a very large number of small circular apertures l3, or by a relatively smaller number of transversely elongated apertures i i, and in either case throttling means be provided to reduce the amount and velocity of air entering through these apertures.
In Fig. 1 throttling is effected by means of a flanged annular plate 26 suitably perforated to admit the desired air flow downstream of the plate, the whole of this admitted flow entering progressively in the downstream direction into the chamber I.
As an alternative, the chamber 4 may be formed towards its downstream end 29 by a wall of metal gauze or of porous material, as for example, sintered metal, to permit substantially uniform entry of cooling air by diffusion from the casing 2 into the chamber I. The chamber i and casing 2 are relatively tapered towards their downstream ends to provide a gradual reduction in the cross-sectional area of the annular space between them and to maintain a uniform flow of air therethrough.
As a further alternative for cooling the downstream parts of a combustion chamber use may be made of an arrangement wherein part of the airstream is first led to a locality downstream of the most upstream region at which it is required to enter the combustion chamber and is then led in the reverse direction along the outer surface of the combustion chamber to cool said surface being then allowed to enter the combustion chamber to mix with the gases of combustion.
Such an arrangement is shown in Fig. 2, which is similar to Fig. 1, except that between the casing 2 and the chamber l is inserted an annular shield 15, which forms, together with chamber i, an annular space I6 open at its downstream end and closed at its upstream end.
Beyond this downstream end the casing 2 is partly closed by a stop plate 26 and an apertured flanged annular plate 26 similar to that in Fig. 1.
Air flows downstream through the casing 2, and due to the presence of the plates 26 and 28 a part of this air is reversed in direction so as to flow upstream through space [6, entering chamber through entry apertures 25 to form a mixing zone I? as in Fig. 1.
Downstream of the mixing zone, as in Fig. 1, the wall of the chamber I may have small circular apertures l3 or transversely elongated apertures l4, and may also have a tapered outlet 29 conforming in shape to the tapered downstream end of the casing 2. Alternatively, the chamber I may be provided at its downstream region with a wall of metal gauze or porous material as described in connection with Fig. 1.
In combustion apparatus in which fuel is to be burnt in a fast-moving airstream, and comprising an outer tubular casin having an inlet for the stream, a tubular flame tube within the casing, a hollow baffle centrally located in one end of the flame tube defining a primary combustion zone, and means for injecting fuel into said zone, said bafiie being apertured to provide an inlet for part of said stream as primary combustion air to said zone, and said flame tube having an outlet at its other end, whereby the flame tube defines a path for a stream of hot combustion gases; means for directing a layer of cooling air along the inside of the flame tube wall, said means comprising a tubular member enclosin the baffle and projecting for a short distance into the end of the flame tube and defining therewith an annular gap providing a path from the casing into the flame tube for a stream of cooling air,
the gap being so dimensioned in relation to the conditions inside and outside the flame tube in the conditions of operation, that the velocity of said stream of cooling air through the gap is substantially the same as the velocity of said stream of hot combustion gases.
WALTER FREDERICK SHILLING.
RAYMOND ARTHUR BOULTER.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,382,564 Haverstick Aug. 14, 1945 2,396,068 Youngash Mar. 5, 1946 2,398,654 Lubbock et a1 Apr. 16, 1946 2,404,335 Whittle July 16, 1946 2,446,059 Peterson et a1. July 27, 1948 2,447,482 Arnold Aug. 24, 1948 2,458,497 Bailey Jan. 11, 1949 2,470,184 Pfenninger May 17, 1949 2,547,619 Buckland Apr. 3, 1951 2,581,999 Blatz Jan. 8, 1952 FOREIGN PATENTS Number Country Date 433,631 Great Britain Aug. 19, 1935 539,069 Great Britain Aug. 27, 1941 579,424 Great Britain Aug. 2, 1946 588,086 Great Britain May 14, 1947 OTHER REFERENCES The Oil Engine and Gas Turbine, January 1950, pages 300 and 301. (Article therein entitled Developing Marine-Type Combustion Chambers.)