US3032991A - Combustion sustaining means for continuous flow combustion systems - Google Patents

Description

May 8, 1962 J. w. VDOVIAK 3,032,991

COMBUSTION SUSTAINING MEANS FOR CONTINUOUS FLOW COMBUSTION SYSTEMS Filed. Oct. 1. 1959 F? v INVENTOR. 1 I El JO///\/ #4 war/4x1 QM QM United States Patent 32, 1 COMBUSTION SUSTAINING MEANS FOR CON- TINUOUS FLOW COMBUSTION SYSTEMS John W. Vdoviak, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Oct. 1, 1959, Ser. No. 843,725

Claims. (Cl. 60-4932) The present invention relates to a combustion sustaining means for continuous flow combustion systems and more particularly to a catalystic means for improving the stability or burning limits and providing ignition at high inlet temperatures.

In continuous flow combustion systems of the type encountered in turbojet afterburners and ramjets, difficulty has been encountered in sustaining combustion at oif design operation. At low fuel-air ratios combustion is often sporadic instead of stable as desired, while at low density, high velocity conditions the flame is easily extinguished due to combined efifects of quenching and low reaction rate. Catalytic ignitors have been suggested as a solution to these problems. However, problems inherent in the designs of the prior known catalytic ignitors have prevented utilization of their full potential. It has been determined that the mechanical design of the ignitor must provide a large localized surface area for contact with the combustion gases while overcoming or avoiding the problems of fouling, thermal shock and mechanical vibration. These problems are inter-related and their solution requires a balance between: catalyst surface and weight; gas composition, velocity and pressure; position of the catalyst within the combustion chamber; and the manner in which it is supported in the gas stream.

An object of the present invention is to provide a practical and efficient catalytic ignitor for use in continuous flow combustion systems.

The above object is realized in the present invention by provision of a multi-layer pad of fine-wire catalytic screen which is supported in a recirculation zone, such as the wake of a fiameholder, so that fuel bearing gases circulate through the multiple layers of the catalyst. The catalytic screen is mounted in firm intimate contact with a portion of the combustion system which operates at a lower temperature than the screen.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is an elevation view of the downstream surface of a V-gutter flameholder with the present invention positioned thereon;

FIG. 2 is a sectional view at an enlarged scale taken along line 22 of FIG. 1;

FIG. 3 is a perspective view of the catalytic ignitor illustrated in FIG. 2; and

FIG. 4 is a sectional view of an alternative mounting arrangement for the ignitor of FIG. 3.

As illustrated in FIG. 1, pads 11 of catalytic screen are secured to the downstream surfaces of a typical flameholder 12. The flameholder 12 is illustrated as including three concentric annular V-gutters '13, 14 and 15 with the individual pads spaced about the circumference of each to provide substantial contact with the fuel-air mixture circulating in the wake thereof. While a plurality of pads are shown, adequate ignition is provided by a single pad on each gutter.

Referring to FIGURES 2 and 3, each pad 11 is formed from a roll of fine-wire catalytic screen which is doubled upon itself to form two oppositely extending lobes 16 and 17 of generally elliptical cross-section which are joined by a central connecting portion 18. Locking strips resinate.

3,032,991 Patented May 8, 1962 19 and 21 are positioned one on either side of the connecting portion 18. Threaded studs 22 and 23 are then secured to the locking strips, as by welding, etc., to secure the locking strips in position on the connecting portion. In order to increase the rate of heat transfer between the ignitor assembly and the somewhat cooler V-gutter, the locking strips are spot-welded together at intervals along their length. In this case the studs can be secured to only the upstream locking strip. The resulting ignitor assembly is secured to the downstream surface of a V-gutter between arms 24 and 25. The studs are inserted through aligned openings in tubular member 26 which forms the leading edge of the V-gutter. A heat resistant nut 27 is then threaded onto the protruding end of each stud and tightened to provide the proper amount of torque needed to maintain the ignitor assembly in position.

An alternative form of mounting for conventional V- gutters is illustrated in FIG. 4. In this arrangement a pad 11 is incorporated into an ignitor assembly similar to that of FIG. 3. The resulting ignitor assembly is secured to one of the arms 30 and 31 of the V-gutter. The assembly is secured tightly to the arm to achieve a good contact therebetween.

In forming the pad 11 a strip of fine-wire catalytic screen is formed into a roll with a circumference of approximately three inches. The length of the strip is selected such that there will be between five and ten layers of screen in the roll. The result is that the roll is tightly wound with the layers being in close, intimate contact with each other. The roll is then doubled upon itself, to form the lobes 16 and 17 and the central connecting portion 18, and the locking strips attached as described above. In this connection it may be desirable for some applications to double the roll of screen from one edge so as to form only a single lobe instead of two lobes. The dimensions of the catalyst pad are relatively unimportant except that it has been determined that the ignition delay time increases as the exposed surface area of the pad decreases and that an increase in the number of layers of screen decreases the problem of fouling. The number of layers of screen in a pad will depend to some extent upon the application, but in general, more layers of screen would give more rigidity and increased endurance and the added surface would increase the resistance to fouling. The increased rigidity would enable the pad to resist the heavy buifeting of gas loads and mechanical vibrations. To increase endurance sharp bends in the catalytic screen at the mounting should be avoided, likewise the locking strips should have no sharp edges in contact with the screen.

The catalytic effect of the ignitor is reduced when the catalytic screen fouled by exposure to low concentrations of unburned fuel. Exposure of the fouled ignitor to a zone of active combustion will reactivate or unfoul the catalyst. Accordingly, reactivation is automatic, occurring with each ignition.

Initiation of the catalytic action requires activation of the catalyst material. This can be accomplished by exposure of the catalytic screen to a hot combustion gas stream, i.c. by some separate source of ignition, or by pretreatment of the screen with a light coating of liquid platinum bearing compound such as platinum or rhodium These coatings rapidly reduce to active metal upon exposure to a hot (approximately 1000 F.) oxidizing atmosphere.

The catalyst material used in the present invention is an alloy of 90% platinum and 10% rhodium, however other materials such as platinum or alloys of 90% platinum and 10% iridium, platinum and 20% rhodium, or platinum and 10% ruthenium give satisfactory results. The screen is 80 mesh-.003 wire, such as is commonly used in the chemical industry for the oxidation of ammonia in the production of nitric acid. Heavier wire screen can be used, but it causes an increase in the ignition delay time due to the decrease in localized surface area.

While the present invention has been described in connection with an afterburner or ramjet burner it also has application to the main combustor of a gas turbine engine. For such an application a catalytic ignitor such as shown in FIG. 3 would be mounted in the main combustor and the same considerations would apply as in the case illustrated. That is the ignitor should be located in a recirculation zone and should be firmly secured to a relatively cool portion of the combustor to enhance the trans- 'fer of heat from the pad or screen.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A catalytic ignitor adapted for use in a continuous flow combustion system comprising: a catalyst pad formed from a tightly wound roll of a plurality of layers of fine-wire catalytic screen, the roll being doubled upon itself to form at least one lobe of generally elliptical crosssection, each of the layers of said plurality being in close, intimate contact with an adjacent layer; and means secured to the doubled portion of the roll for mounting the ignitor in a recirculation zone within the combustion system.

2. A catalytic ignitor adapted for use in a continuous flow combustion system comprising: a tightly wound roll of fine-wire catalytic screen, the screen being doubled upon itself to form at least one lobe of generally elliptical cross-section, said one lobe having at least five tightly packed layers of screen; and means secured to the doubled portion of the roll for mounting the ignitor in a recirculation zone Within the combustion system.

3. A catalytic ignitor adaptedfor use in a continuous flow combustion system comprising: a tightly wound roll of fine-wire catalytic screen, the screen being doubled upon itself to form twooppositely extending lobes of generally elliptical cross-section, each individual layer of screen being in close contacting relationship with another layer; a pair of locking strips, one on each side of the screen intermediate the lobes; and mounting means connecting the locking strips and extending outwardly therefrom.

4. A catalytic ignitor adapted for use in a continuous flow combustion system comprising: a catalyst pad formed from a tightly wound roll of a plurality of layers of fine-Wire catalytic screen, the roll of screen being doubled upon itself to form two oppositely extending lobes of generally elliptical cross-section, the layers of said plurality being in a close contacting relationship with each other; the lobes being joined by a central connecting portion; a pair of locking strips, one on each side of the central connecting portion, said locking strips being spotwelded together; and mounting means connected to at least one of said locking strips.

5. A catalytic ignitor adapted for use in a continuous flow combustion system comprising: a catalyst pad formed from a tightly wound roll of a plurality of layers of fine-wire catalytic screen, the roll of screen being doubled upon itself to form two oppositely extending lobes of generally elliptical cross section, the layers of said plurality being in close contacting relationship with each other; a pair of locking strips, one on each side of the pad intermediate the lobes, said locking strips being spot-welded together; and mounting means connected to at least one of the locking strips.

References Cited in the file of this patent UNITED STATES PATENTS 2,194,081 Bock Mar. 19, 1940 2,526,657 Guyer Oct. 24, 1950 2,607,663 Perry et al Aug. 19, 1952 2,648,190 Maisner Aug. 11, 1953 2,858,672 Clark Nov. 4, 1958 2,964,907 Toone Dec. 20, 1960 FOREIGN PATENTS 696,756 Great Britain Sept. 9, 1953 735,570 Great Britain Aug. 24, 1955 758,371 Great Britain Oct. 3, 1956

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