US3587232A - Combustion devices - Google Patents

Abstract

A FLAME STABILIZER COMPRISES AN ANGLE SECTION FIXED WITH ITS APEX POINTING UPSTREAM. AT EACH OF ITS DOWNSTREAM EXTREMITIES IS A BIMETALLIC PLATE, THE TWO PLATES EXTENDING DOWNSTREAM IN THE DIRECTION OF FLOW AND PARALLEL TO EACH OTHER. A PILOT BURNER IN THE FORM OF A FUEL SPRAY BAR EXTENDS TRANSVERSELY OF THE STABILIZER WITHIN THE ANGLE SECTION IN WHICH THERE ARE SMALL HOLES TO ADMIT AIR TO THE BURNER. THE INNER FACES OF THE BIMETALLIC PLATES ARE OF NICKEL ALLOY HAVING A HIGH COEFFICIENT OF EXPANSION AND THE OUTER FACES ARE OF A REFRACTORY METAL SUCH AS NIOBIUM OR MOLYBDENUM HAVING A LOW COEFFICIENT OF EXPANSIONS. THE DIFFERENTIAL EXPANSION OF THE PLATES

UNDER THE EFFECT OF HEAT CAUSES THESE TO BEND OUTWARDLY TO INCREASE THE FRONTAL AREA OF THE STABILIZER AND CAUSE INCREASED TURBULENCE.

Description

United States Patent [72] Inventor William Dean Bryce Farnham, England [21] Appl. No. 823,442 [22] Filed May 9, 1969 [45] Patented June 28, 1971 [32] Priority May 16, 1968 [33] Great Britain [31] 23298/68 [73] Assignee Minister of Technology in Her Britannic ajestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England [54] COMBUSTION DEVICES 10 Claims, 1 Drawing Fig.

[52] US. Cl 60/39.72, 29/1955, 60/261, 431/350 [51] Int. Cl E02k 3/10 [50] Field of Search 60/39.72, 261; 29/194, 195.5, 198 (1nquired);43l/350 [56] References Cited UNITED STATES PATENTS 2,332,416 10/1943 Waltenburg 29/1955 2,563,270 8/1951 Price 60/261 2,578,197 12/1951 Mudge 29/195.5 2,679,137 5/1954 Probert 60/3972 2,770,096 11/1956 Fox 60/3972 2,805,192 9/1957 Brenner.... 29/198 2,918,794 12/1959 Hurd 60/3972 2,974,488 3/1961 Eggers... 60/3972 3,057,048 10/1962 Hirakis 29/198 Primary Examiner-Douglas Hart Att0rneyStevens, Davis, Miller and Mosher ABSTRACT: A flame stabilizer comprises an angle section fixed with its apex pointing upstream. At each of its downstream extremities is a bimetallic plate, the two plates extending downstream in the direction of flow and parallel to each other. A pilot burner in the form of a fuel spray bar extends transversely of the stabilizer within the angle section in which there are small holes to admit air to the burner. The

inner faces of the bimetallic plates are of nickel alloy having a high coefficient of expansion and the outer faces are of a refractory metal such as niobium or molybdenum having a low coefiicient of expansion. The differential expansion of the plates under the effect of heat causes these to bend outwardly to increase the frontal area of the stabilizer and cause increased turbulence.

COMBUSTION DEVllCES The present invention relates to combustion devices wherein a flame stabilization zone is formed in a swiftly moving gas stream by the separation of flow from the surface of a body disposed within the stream.

In aircraft gas turbine jet propulsion engines, extra power can be developed by burning additional fuel in an afterburner. It has long been the practice in such installations to use fixed baffles to produce stable turbulent (or recirculation) zones in their wakes wherein flames will maintain themselves. Such baffles offer undesirable resistance to flow during such time as afterburning is not in operation. One of the more common forms offixed baflle is a V-shaped gutter having its apex pointing'upstream of the flow. The extreme simplicity of the arrangement tending to outweigh adverse considerations. By setting the arms to an included angle of the order of 45, or slightlyless according to flow velocity, a reasonable optimum between resistance and the provision of an adequate recirculation zone can be obtained.

Nevertheless baffle resistance can be critical in determining the total amount of afterburning which can be employed, and an afterburner system having a lower pressure loss when in the nonoperating condition is desirable, provided that the penalties arising out of other considerations are not too severe. The present invention is concerned with means directed toward this end, as is British prior copending Application. No. 36855/67, now British Pat. No. l,l84,379 and the corresponding U.S. Pat. No. 3,504,49! to LB. Wigg.

A combustion device according to the invention for installation in gas flow ducts and the like includes at least one member arranged to distort with the change in temperature and supported in such manner that increase in temperature within a predetermined range will distort the member so as to define at least part of a stable combustion zone in a duct.

Preferably, the member is a composite of materials having different coefficients of expansion.

An-embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawing which represents a longitudinal section of a flame stabilizer.

The stabilizer has a nose portion 1 which comprises a generally V-section channel member with the extremities 2 of its side webs cranked inwardly to lie parallel with each other. Bimetallic plates 3,4 are riveted to the extremities 2 and extend parallel to each other in a cantilever fashion as extensions of the said extremities (i.e. they are directed away from the apex of the nose portion). The plates 3,4 each comprise one sheet of a metal having a relatively high coefficient of linear expansion, and one sheet of a refractory metal having a significantly lower coefficient ofexpansion, the two sheets being attached to each other on mating faces by suitable known methods. A nickel based alloy is a suitable metal for the firstmentioned sheets and niobium, or molybdenum, for the refractory sheets. (A typical nickel alloy has a coefficient of linear expansion of the order of 16 l per C, and that for niobium is about 7.5' per C.) The first-mentioned sheets face towards each other and the refractory sheets form the outer surfaces of the stabilizer. in some cases it may be expedient toapply an oxidation resistant coating to the sheets.

The stabilizer is intended to be mounted in a gas flow duct with the apex of the nose portion 1 directed towards oncoming flow, the direction of such gas flow being indicated by the arrows B in the drawing.

A tube 5 is located within the nose portion ll lying in the angle of the apex and extending therealong. Formed in the surface of the tube is a row of holes (of which one is shown at 6) extending axially along the tube, the holes being generally directed towards the space formed between the plates 3,4. A row of holes (of which one is shown at 7) extends along of the channel member extremities 2 adjacent to the tube 5. (There may be differing arrangements of holes in the tube 5 and in the nose portion 1 --the foregoing being merely typical.)

The tube 5 is connected to a fuel supply (not shown), fuel passing into the tube being discharged through the holes 6 in its surface, as indicated by the arrow C. Air from the main flow through the duct in which the stabilizer is mounted will pass through the holes 7, as indicated by the arrow A to mix with the fuel and the resultant mixture, when ignited by known means, will burn substantially within the confines of the stabil' izer. The heat of combustion raises the temperature of the bimetallic plates 3,4 and the consequent differential expansion of the constituent sheets causes the plates to bend from their respective attachment points away from each other to take up a configuration as indicated in a dotted line on the drawing. The flame stabilizer as a whole thus becomes in effect a V-shaped gutter having arms set to an included angle of approximately 30".

Deflection of the plates will lead to flow over the stabilizer which hitherto will have been substantially laminar, being induced to break away and form a turbulent zone between the plates 3,4. Fuel introduced into the main flow, as from nozzles 8 located upstream of the stabilizer, and ignited will burn in the said zone where the recirculation existing therein will promote mixing between the hot burnt products and incoming mixture to maintain stable combustion.

The heat produced as described immediately above will possibly be adequate to maintain the distortion of the bimetallic plates 3,4 and may even cause them to deflect further. At this stage it is envisaged that use of the pilot burner might be discontinued, though this would be dependent on the operating conditions prevailing and other pertinent factors.

Likewise, the heat release from the combustion of fuel from the nozzles 8 could in certain circumstances cause deflection of the plates 3,4 without the need for pilot burning (i.e. the burning of fuel discharged from the tube 5). However, in general, unless ignition of fuel from the nozzles 8 can be carefully controlled, unstable burning conditions would be likely to occur until such time as the plates had deflected to approximately their intended configuration. In view of the relatively long time constant possessed by bimetallic constructions generally, this is most likely to result in the plates not reaching their configuration (or not consistently so). Such uncertainties would, of course, be largely unacceptable and thus the steadystate heating obtainable by the use of a pilot burner is at least desirable for reliable operation.

In a stabilizer as already described having an overall undeflected width of one-fourth inch (i.e. as presented to an oncoming flow) with plates 0.050 inch thick and 2.8 inches long (in the direction of flow) the free end of each plate can be deflected outwardly three-eighths inch by a temperature increase of 400c., to give a new overall width of l inch. Thus, the resistance (and resultant pressure loss) when the stabilizer is not in operation is of the order of one-fourth of that of a fixed V-gutter of comparable performance. Deflection of the plates with the materials envisaged will usually be proportional to the square of their length divided by their thickness. Reduction in thickness results in reduced length for a given deflection (with consequent weight reduction) but the effects of gas bending loads, possibility of distortion and stress limita tions must be taken into consideration and also questions of manufacture.

The stabilizer is envisaged as being installed in, for example,

an afterburner duct between the exhaust of a gas turbine and a propulsion nozzle, in a bypass duct of a turbofan engine, or other similar arrangement where, in general, relative shortperiod thrust boosting is required.

In such cases, and particularly in an afterburner, the stabilizer is likely to be subjected to considerable temperatures when it is not itself in operation.

To ensure that the plates extend substantially in the direction of gas flow (asshown in the drawing) 'during such conditions as when an associated engine is operating but without afterbuming, the plates will be initially set in the cold condition so that they bend towards each other and the differential expansion resulting from the temperature increase involved immediately above will cause them to straighten out. It may be desirable to fit spacers or other means to prevent the plates from bending more than a predetermined amount so as, for instance, to avoid undue stresses.

Also for reasons of stress, the bonding of the sheets forming the plates should preferably be efiected in a manner which will give no thermal strain when the plates are undistorted (i.e. at the temperature of the gas stream to which a stabilizer will be subjected in its nonoperating condition). Roll bonding or explosion bonding of the sheets can be carried out under nonnal workshop conditions and the plates then rolled at the temperature at which stress relief is desired. Such rolling is not readily practicable where riveting is used to bond sheets together and such a construction is thus less desirable. The effects of stress concentration and the expansion coefficient of rivets also render this construction generally less satisfactory.

The geometry of a bimetallic stabilizer as heretofore described is not particularly well suited to a curved configuration (an annular arrangement, for example) but a series of straight stabilizer segments can be used to give a close approximation. Alternatively, straight stabilizers could be arranged in parallel, radial or grid fonnations to give a requisite duct coverage.

In the case of an afterburner used with mixed flow from a bypass engine there is usually a substantial temperature gradient between the core and circumference of flow through the afterburner duct since the mixing process between the hot and cold streams rarely approaches completeness.

In order to use the same bimetallic materials throughout, it might be necessary to vary the stabilizer geometry somewhat in different parts of the duct, including, possibly providing stops at the free ends of plates situated in cooler regions to prevent closing of the pilot zones more than is desirable.

I claim:

1. A combustion device comprising a plurality of members each composed of a plurality of metallic materials bonded together and having different coefficients of linear expansion, a first metallic material having a predominantly nickel content and a second metallic material being predominantly a refractory metal, said members supported in such manner that increase in temperature within a predetermined range will cause them to distort in different directions to define at least part of a zone wherein stable combustion can be induced to take place.

2. A combustion device according to claim 1 in which the refractory metal is niobium.

3. A combustion device according to claim 1 in which the refractory metal is molybdenum.

4. A combustion device according to claim 1 wherein said members are supported cantilever fashion in such manner that increase in temperature will induce the free ends of said members to deflect away from each other.

5. A combustion device according to claim 1 further comprising integral heating means.

6. A combustion device according to claim 1 wherein said members are supported on a channel member and define therewith a space, the overall width of which will vary with change of temperature.

7. A combustion device according to claim 6 wherein said plurality of members are attached at one end to an extremity of said channel member, fonning an extension of said extremity.

8. A combustion device according to claim 7 further comprising heating mcans disposed within said space.

9. A combustion device according to claim 8 in which the heating means comprises a perforated tube connected to supply fuel to the space.

10. A combustion device according to claim 8 provided with holes in the channel member for the admission of air to the space.

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