US2726511A

US2726511A - Afterburners

Info

Publication number: US2726511A
Application number: US162723A
Authority: US
Inventors: Paul A Pitt
Original assignee: Solar Aircraft Co
Current assignee: Solar Aircraft Co
Priority date: 1950-05-18
Filing date: 1950-05-18
Publication date: 1955-12-13
Anticipated expiration: 1972-12-13

Description

P. A. PITT AFTERBURNERS Dec. 13, 1955 2 Sheets-Sheet 1 Filed May 18, 1950 R O T N E V m ATTORNEXS u Wm ATTORNEYS P. A. PITT AFTERBURNERS Dec. 13, 1955 2 Sheets-Sheet 2 Filed May 18, 1950 Pa a] xi Fifi United States Patent 6 AFTERBURNERS Paul. A. Pitt, San. Diego, Calif., assignor-to @0121" Aircraft Company, San Diego, Calif., a corporation of California Application May 18, 1950,. Serial No. 162,723

7 Claims. (Cl. 6ll-39.7 2)

This invention relates to improvements in combustion apparatus for burning fuel in a high velocity blast of air or gas, andhas particular-reference to a burner for continuously burning at high altitude a mixture of combustible'and combustion supporting elements in a high velocity stream to develop reactive forces in, and to increase the reactive forces of, the stream.

The subject invention is an improved burner of the type disclosed in co-pending application Serial No. 140,- 633, filed January 26, 1-950, by Robert E. Day, now Patent No. 2,701,444, issued February 8, 1955. These burners are presently used in jet engine assemblies for jet powered aircraft, and the improvement in the embodied invention resides in substantially increased burner, and therefore engine, performance at very high altitudes.

As explained in Patent No. 2,701,444, hereinbefore cited; the fuelmanifold in this type of burner is-equi'pped with gutters or flame holders which anchor pilotflames thereto. The pilot flames in turn mix withand ignite the fuel-exhaust gas mixture passing between the gutters to instigate the main combustion which increases the reac: tiveforces of the mixture. The flame front or upstream boundary of thismain combustion is normally located well forward in the jet engine tailpipe, not far downstream of' the fuelmanifold and gutters. However, I have discovered that as the altitude is increased the point of maximum heat release gradually moves further downstream fromthe gutters towards the after end of the tailpipe until considerable difli'culty is-experienced in obtaining effective burning at very high altitudes.

This difliculty is characterized by only partof the heat being released from the fuel before it leaves the tailpipe as the plane gains altitude. The further the intergutter flame front recedes the less complete is the combustion before ejection.

At very high altitude combustion probably ceases or the burning occurs outside of the jet engine nozzle. It will be understood that fuel burned after passing-through the jet nozzle contributes nothing toward driving the aircraft forward and represents only lost energy and an ultimate ineffectiveness of the jet engine afterburner. Blowout occurs becauseprimary combustion mustmake up for the lack of secondary combustion hence the fuel control continues to meter agreater quantity of fuel to the gutters which reaches a point where mixture is overrich and can no longer burn.

To overcome these problems encountered in the burners of the prior art the invention embodied herein pro vides a high altitude burner which employs a secondary flame anchoring gutter or flame holder downstream of the primary gutters as a flame anchor or stabilizerfor the main combustion. This novel construction enables a materially'improved afterburner performance at high altitude and atthe Sametime in no way impairs the excellent. performance of-this type of burner at lower'altituldes. Also, dueto the location and construction of this secondary flame anchoring gutter,- to be fully described hereinafter, very'little is addedto the totalresistance to flowthrough the burner, under non-afterburning conditions and at the same time combustion is: completed in a shorter distance, so that the tailpipe length downstream of the burner maybe desirably shortened. In addition, an important feature of this invention is that it is possible to burn nearly the maximum amount of fuel'with respect to the mass flow of air supplied, particularly at peak altitudes, thereby gaining the utmost in thrust. This very considerable increase in thrust represents a great improvement in the military effectiveness of an aircraft as this additional power may well mean the difference between winning and losing a fight, getting away or overtaking in pursuit.

With these and other considerations in view it is a prime object of this invention to provide a burner for jet engine afterburner assemblies. which will enable top engine performance at very high altitudes.

It is a further important object of this invention to provide a high altitude burner which enables top engine performance at low altitudes as well as high altitudes.

Another important object is to provide a. high altitude burner which presents a minimum of resistance to flow of exhaust gases through the burnerand only a negligible increase in back pressures.

A further object is to provide a high altitude burner which anchors the main combustion of the fuel-exhaust gas mixture within the tailpipe of the jet engine.

A still further object is to provide a simply constructed high altitude burner which is both dependable and highly efficient over a very great range of altitude.

Another object is to provide a burner in which combustion is more rapidly completed, permitting use of shorter stack sections.

Other objects and advantages will be apparent from the following description in conjunction with the accompanying drawings and from the appended claims.

The accompanying drawings, in which like reference numeralsare used to designate similar parts throughout, illustrate the preferred embodiment for the purpose of disclosing the invention. The drawings, however, are not to be taken in a limiting or restrictive sense, since it will be apparent to those skilled in the art that various changes in the illustrated construction may be resorted to without in any way exceeding the scope of the invention.

In thedrawings:

Figure 1 isa side elevation of a jet engine afterburner assembly;

Figure 2 is an enlarged front elevation of the burner embodying the invention, taken along line 22 of Fig ure 1;

Figure 3 is a section of the burner of Figure 2, taken along line 3-3 of Figure 2, showing the primary and secondary pilot flame patterns;

Figure 4 is a section of the burner of Figure 2, taken along line 44 of Figure 2; and

Figure 5 is a graph illustrating the relationship between heat generated during combustion vs. distance downstream from primary gutter at various altitudes.

Referring to Figure 1, reference number 20 generally indicates a jet engine after burner assembly such as that described in Patent No. 2,701,444, comprising a diffuser 24, a burner housing or shell 25 and a tailpipe 2,6. The diffuser 24 and tailpipe 26 are secured to cylindrical housing 25 through a pair of peripheral welded. flanges 27' and 28 by means of bolts 31 and

complementary flanges

32 and 33 welded to sections 24 and 26', respectively.

Since the basic burner assembly mounted within'the burner housing 25is' described in detail as to construction and operation in Patent No. 2,701,444 the burner will be only generally described herein. Referring to Figure 2, it will be seen that the fuel distribution grid, indicated generally at 34, and a grid formed by a compound carburetion and combustion gutter, indicated generally at 35, are disposed within the burner housing 25. Both grids are sufiiciently smaller in diameter than housing 25 to provide an annular area 30 for the passage of a layer of protective gases between the burner structure and the housing which prevents serious overheating of the housing when large volumes of fuel are burned. The fuel distribution grid 34 comprises an intercommunicating network of tubes or pipes, some of which are arranged concentrically within the housing 25 and others radially. In the illustrated embodiment the pipes forming the fuel distribution grid are coplanar. The grid 34 includes a diametrically extending generally vertical pipe 36 and a generally horizontal pipe 37 which extend to and communicate with a circular pipe 38. Two other

circular pipes

41 and 42, spaced inwardly and concentric with the pipe 38, connect the

pipes

36 and 37 at other points. A series of equiangularly spaced radial pipes 43 bridge the space between the outer circular pipe 38 and the adjacent circular pipe 41, except that there are no such radial pipes in the places already occupied by the vertical and

horizontal pipes

36 and 37, since the

pipes

36 and 37 already connect the

circular pipes

33 and 41 at those locations. All of the pipes forming the fuel distribution grid 34 have openings 44 facing generally upstream into the exhaust or other gases, i. e. towards the diffuser 24 of the afterburner assembly.

The fuel distribution grid 34 is mounted within the cylindrical housing or shell 25 in the following manner. At points coaxial with the vertical and

horizontal pipes

36 and 37, four tubular axial extensions 45 are welded to the outer circular pipes 38. Three of the extensions 45 have a slip fit into a streamlined tubular socket 46 having an integral peripheral flange 47 that is welded to shell 25. Thus, while the fuel distribution grid is rigidly located within the housing 25, it is free to expand and contract with temperature changes without imposing stresses on the housing that would tend to deform it.

Fuel is fed to the grid 34 at or near the bottom thereof by means of an inlet pipe 53 which passes through a stuffing box 54, Figures 2 and 3, into the interior of the shell 25 and is connected to the extension 45 at the lower end of the vertical pipe 36. The inlet pipe 53 provides the fourth radial support for the fuel distribution grid 34, and the stuffing box 54 provides for any axial movement of the inlet pipe 53 caused by expansion or contraction of the grid 34. The fuel thus enters the fuel distribution grid 34 at the bottom thereof and rises within the intercommunicating network of pipes, filling it to the top and causing any gas within the pipes to be completely displaced.

Carried by the fuel distribution grid 34 and disposed downstream thereof, or within the portion of the housing 25 adjacent the tailpipe 26, is the compound carburetion and combustion gutter network or grid 35. This grid 35 is arranged transversely of the housing 25 as is the fuel distribution grid 34, and in the illustrated embodiment is planar and parallel to the fuel distribution grid. The grid 35 is made up of several portions, substantially identical in cross-section as appears in Figures 3 and 4, and is positioned directly downstream of several elements of the pipe forming the fuel distribution grid 34. It comprises an outer compound gutter ring generally indicated at 55, and an inner compound gutter ring generally indicated at 56, downstream of the circular

fuel distribution pipes

38 and 41, respectively. As shown in crosssection in Figure 3, each compound gutter ring comprises a pair of concentric, spaced

walls

57 and 58 rigidly connected by a continuous transverse trough shaped common member or connecting wall 61 which divides the compound gutter into a downstream facing combustion gutter or trough 62 andan upstream facing carburetion gutter or trough 63. The outer and inner compound gutter rings 55 and 56 are connected by four equiangularly spaced radial compound gutters 66 of similar crosssection, that lie directly downstream of the vertical and horizontal

fuel distribution pipes

36 and 37. The compound radial gutters 66 are provided with connecting members 61a dividing the gutters into downstream facing combustion gutters 62a and upstream facing carburetion gutters 63a.

The compound gutter grid is mounted upon the fuel distribution grid in such a manner as to allow relative differential expansion and contraction therebetween. This is accomplished as by means of pairs of yokes 67, Figures 2 and 3, and 4, that have a sliding fit on

pipes

36 and 37 of the fuel distribution grid. The yokes extend towards and embrace the outer walls of the radial compound gutters 66, to which they are rigidly secured through pins 68.

Those portions of the vertical and horizontal

fuel distribution pipes

36 and 37 upstream of the radial compound gutters 66 having downstream directed fuel discharge openings 69, Figures 3 and 4, that discharge pilot fuel downstream into the carburetion gutter section 63a of the compound gutters 66. The

circular pipes

38 and 41 also have downstream directed fuel discharge openings 71, Figure 3, that discharge pilot fuel into the circular carburetion gutters 63 of the compound gutter rings and 56, respectively. This fuel may be target atomized when it strikes

common members

61 and 61a in the compound gutter system that divides the carburetion gutter from the combustion gutter. These

members

61 and 61a have openings 72 and 72a respectively therethrough to permit the passage and impacting of the atomized fuel against

walls

57 and 58 and 66 and into the

combustion gutters

62 and 62a where it burns to form an igniting and combustion sustaining flame. Instead of projecting the pilot fuel downstream, openings 69 and 71 may also be arranged to discharge the pilot fuel upstream in addition to the fuel discharge through openings 44 so that it is carried by the high velocity gases into the carburetion gutters, giving adequate pre-combustion mixing time and improved high altitude operation.

Since the exhaust gases are traveling through the housing 25 at speeds of over three hundred miles per hour, a portion thereof enters the

carburetion gutters

63 and 63a wherein it circulates turbulently and mixes thoroughly with the atomized fuel within the gutter, thereby also preheating the fuel, and the mixture passes through the openings 72 into the

combustion gutters

62 and 62a striking against the

hot walls

57 and 58 and 66 by reason of the inclination of the connecting

walls

61 and 61a and the holes 72 and 72a.

A pair of spark plugs 73 and 74 are mounted in reinforced sections of the housing 25 adjacent to the bottom thereof, one on each side of fuel pipe 53, with their electrodes extending into the combustion gutter 62 of the outer ring 55, as shown in Figure 2. These spark plugs are one example of means which can be used to ignite the mixture in the compound gutter, which once ignited provides a pilot flame that courses rapidly through the outer, inner and radial combustion gutters 62 to establish a pilot flame grid which continues to burn until he fuel supply to the fuel distribution grid 34 is cut 0 This pilot flame pattern which is established and anchored at its forward end by the

combustion gutters

62 and 62a is illustrated diagrammatically at in Figure 3, and will be referred to hereinafter as the primary pilot flame. As explained in detail in Patent No. 2,701,444 this pilot flame serves to initiate the main combustion in the fuel-exhaust gas mixture passing between the compound gutters. However, as pointed out hereinbefore, these primary flames lengthen more and more and the point of maximum heat release of the main combustion retreats farther and farther downstream at increasingly high altitudes with ultimate blowout.

As. has also been explained hereinbefore, at very high, altitudes, the. main combustion flame front will have. retreatd so far downstream that at least a portion. of: the. fuel-gas mixture, will pass out of the jet engine noz le unburned, representing lost energy as far as. concernsthe, propelling of the aircraft. To eliminate this. high altitude problem this invention, in the embodiment disclosed, provides a secondary combustion gutter ring 82, Figures 2,- 3, and. 4, located downstream of, and spaced radially between, the compound gutter rings 55. and 56. The secondary combustion gutter ring 82 has a, V-shaped cross-section, although it is within the scope of, the invention that the gutter ring be of any suitable cross-section. The gutter ring is mounted with its open. side facing, downstream, as is best illustrated in Figure. 3., and is carried by the compound gutter grid 35; by meansoffour pairsof channel members 84 welded or otherwise suitably fastened: at their forward or upstream ends to the four radial compound gutters 66, Figures 2,, 3, and. 4. The channel members 84 are secured at their other ends to four pairs of brackets 85. in. alignment therewith which are welded to the back of the gutter ring 82. The brackets 85 are cut away as shown in Figure. 3 to fit closely over the back of the V-shaped. gutter 82 to enable them to be securely welded thereto. Brackets 8,5 and channel members 84 are fastened together by some suitable means as bolts 86 which rigidly locate the gutter 82 while at the same time allowing it to be easily removed if desired, and also allowing for relative differential expansion and contraction between the gutter 82 and the compound gutter grid 35.,

Referring now to Figure 3, it will be seen that the secondary gutter ring 82 has anchored thereto a secondary. pilot flame, or stabilizing flame which at very high altitudes prevents the main combustion flame front from retreating too far downstream in the tailpipe, and keeps it instead. Well upstream in the immediate area of the compound gutter grid} 35. This secondary pilot flame indicated at, 87 in Figure 3 isinitiated in the following manner. The main stream of fuel-exhaust gas mixture which. passes between the primary compound gutters has. a. tendency to eddy about and recirculate in he secondary gutter 82 as indicated by the arrows 88. This eddying and recirculation gives the mixture a greater chance, to mature, towards its ignition point, and this maturing process is also aided by the direct radiant heat of the primary pilot flame 80 and by the fact thatthe primary flame pattern has caused the secondary gutter 82 to be substantially heated. The combination of these factors, i. e. the opportunity for rapid maturing of the fuel-gas mixture plus the proximity of the mixture to high heat, aids the bridging of flame into this area, forming the secondary flame 87. In actual Practice, the whole process of ignition to form a secondary plane takes place at high altitudes with extreme rapidity, probably in IADQ of a second or less. Furthermore, once the flame is initiated in the gutter, it will remain a, stable pilot flame as long as the after-burner is in operation since the eddying and recirculation of the fuel-gas mixture which would tend to decrease the stability of the flame becomes less due to localized burning and expansion.

Once the secondary pilot flame is formed, the main combustion flame front no longer tends to retreat downstream but will in effect be stabilized or anchored well upstream approximately in the area indicated by the dash lines 89.. Thisoccurs because the secondary pilot flame 87 causes the fuel-exhaust gas mixture passing betweentheprimary gutters to mature rapidly well upstream and be ignited by the adjacent primary pilot flame 80,. or by the secondary pilot flame itself. Thus, the secondaryflame in a sense plugs the gap inthe primary flame-pattern, which gaps assume a largeimportance at high altitudes. Itshould be pointedout here that at altit des. of, lower tha 40,000., feet. h process inst. descrihedin onn ction. withthe ignition f thesecondarv pilot fl m 1 P bably oe-s no oc r to a substantial degree since. at, the lower altitudes higher absolute, pres: sures. Wi hin. he af b rner a se he fuel-gas mixture to mature and burn, more rapidly s that the. maincom: bustion. flame. front is retained in the. immediate ar aor upstream of the secondary gutter 82.

It is of considerable importance that the addition of the secondary gutter 82 to the burner adds, very little to the total resistance to flow of gases through the burner, under n nr f rburning. ndi ions. duev to. the fact that it is located downstream of the nflain.v com,- pound gutter grid 35, and is of less. projected area than that at the primary gutters. It might appear that the secondary gutter could also be placed in the sameplane as the

primary gutters

62 and 62a to Stabilize the main combustion at high. altitudes, but doing this, would, in,- crease the per cent of coplanarsolid matter in the, afterburner and therefore materially increase the. resistance to. flow of the exhaust gases through the afterburner.

. Afurther important. advantage of the downstreamloca:

tion of the secondary gutter 82 is, that the added sec? ondary pilot flame 87. causes far less. back pressure in this location than if it were i the same plane. as the primary pilot flame, also there. is greater heat. available tomature the mixture.

he. graph of Figure 5 hasbeen included inv the drawings to give a visual presentation of approximately what takes place during combustion in theafterburner tail: pipe at various altitudes. The curves of this graph are plotted from data obtained from. experimental operation in. a test. cell with simulated altitudes. The. abseissas of the curvesare interms of distance downstream fromprimary gutter, while the ordinates arein terms of B. t.. u. prcducedduring combustion-in the tailpipe. Thesolid line curvesindicate the B. t. u. yielded by theprimary and secondary combustion. gutters together and. are 1,81- belled' Total." The curves in. phantomv lines indicate B. t. u. produced. by theprimary gutter alone, and. the curves in dash lines indicate B. t. u. that would, be produced by the secondary gutter alone. It will be understood. from. the. foregoing disclosure that. combustion. does not generally occur in the secondary gutter. alone; for this reason, coordinates for the secondary gutter curves were necessarily obtained by subtracting the valuesof the ordinates for the primary gutter alone from. th values of; ordinates for the primary and secondary gutters together. The curves illustrate the B. t. 11. produced by the primary gutter alone, the secondary gutter alone and the total. of primary. and secondary gutters at sea level (S. L.), and at simulated 40,000 feet; andthe. total of primary andsecondary B. t. u. at simulated 30735,000 feet.

, It will beobservedthat the total B. t. u. produced bysthe primary and secondary gutters together is.greater in every case, than, the B. t. u. produced bythe primary alone. This, of course, is an added advantage of the high alti= tude secondary burner. However, the most important point to be brought out by this graph is illustrated by the peak points and 92 of the primary gutter curve at 40,000feet and the primary and secondary total curve at 40,000 feet, respectively. These peak points represent thepointof maximum combustion efliciency or maximum B.. t. u, yield. It will be noted that peak point 90 of the primary curve. is slightly to the left or upstream of the peak 92 of the total curve at 40,000 feet. However, as explained hereinbefore, this-point recedes down the tailpipe at increasing altitudes above 40,000 feet. Thus if it were practical. to include a series of graphs or many additional primary curves on the illustrated graph, the. peak point would. be seen to drift. gradually tothe right as the altitude increased. In each case-v the total. effective B. t. u. made available by the burner with: out the additional secondary gutter is indicated by he area under the primary curve and the additional thrust made available by the secondary gutter is represented in area under the secondary curves. Thus it will be seen that as altitude increases and the maximum combustion rate of the primary recedes downstream, the secondary plays a larger and larger part in release of the available B. t. u. within the tailpipe Where it will add to the thrust of the plane. The vertical line 94 intersects the principal abscissa at a point equal to the length of the afterburner tailpipe. Area under the curves to the right of line 94 represents lost energy while area under the curves to the left of line 94 represents energy contributing to the propulsive thrust desired.

One further point which should be noted in connection with this graph is that as the altitude increases from sea level to 40,000 feet and above, the total B. t. u. produced by the primary and secondary gutters gradually decreases. This is a normal characteristic of a jet engine-afterburner combination and is due to the fact that as altitude is increased less mass of air is pumped through the system because of reduced air density. This does not mean, however, that the speed of the aircraft is necessarily reduced, since the lowering of B. t. u. output is in greater or less degree compensated for by the general decrease in resistance offered to the aircraft as it moves through the air.

Thus it will be understood that the invention embodied herein provides an efiicient and dependable means of obtaining top afterburning performance at very high altitudes by stabilizing or anchoring themain combustion flame front well upstream in the afterburner tailpipe, thus eliminating any possible loss of thrust augmentation due to the fuel-gas mixture passing unburned through the tailpipe. At the same time this increased afterburner performance is obtained with but a negligible increase in back pressures and resistance to the flow of gases during nonafterburning, and as the combustion is completed more rapidly, the tailpipe may be shortened for a given fuel consumption rate, or the amount of fuel burned may be increased, thus increasing the capacity of a given prior stack length. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by the United States Letters Patent is:

I. In a combustion apparatus for increasing the mass and velocity of a confined stream of gases moving at high velocity; a duct having an inlet and outlet for said gases; a first grid of fuel distribution pipes adjacent the inlet of said duct for discharging fuel into said duct; a second grid between said first grid and the outlet of said duct comprising a carburetion gutter facing said first grid to receive fuel therefrom and a combustion gutter colinear with said carburetion gutter facing the outlet of said duct; said carburetion gutter and combustion gutter having common concentric side walls separated by a perforated cross-wall forming a communicating passage way between said gutters to allow fuel to pass from said carburetion gutter to said combustion gutter; and an additional combustion gutter facing downstream in said duct between said second grid and said outlet.

2. A combustion apparatus comprising a duct having an inlet and an outlet for admitting and discharging gases at high velocity; at first grid of fuel distribution pipes; a second grid having a pair of interconnected colinear carburetion and combustion gutters; and an intermediate interconnected combustion gutter secured to said second grid downstream thereof.

3. A combustion apparatus comprising a duct having an inlet and an outlet for admitting and discharging gases at high velocity; a first grid of fuel distribution pipes; a second grid having inner and outer concentric rings comprised of colinear carburetion and combustion gutters, said inner and outer concentric rings joined together by radial members comprised of colinear carburetion and combustion gutters; and an additional combustion gutter, secured to said radial members of said second grid downstream thereof and radially spaced between said pair of concentric rings of said second grid.

4. In a combustion apparatus for increasing the mass and velocity of a confined stream of gases moving at high velocity; a duct having an inlet and outlet for said gases, said duct including a mixing zone adjacent said inlet and a combustion zone downstream of said mixing zone and adjacent to said outlet; a distributor for adding and substantially evenly distributing fuel to the gases passing through said duct, comprising a grid of spaced fuel supply pipes arranged in said mixing zone transversely of said duct, certain of the pipes forming said grid having outlets discharging fuel downstream towards said combustion zone and others of said pipes having outlets discharging fuel upstream and away from said combustion zone; a second grid in said combustion zone comprising a series of carburetion gutters disposed transversely therein with their open ends facing those pipes in said fuel distributor having downstream facing openings, said second grid including a series of combustion gutters colinear with said carburetion gutters but having their open ends facing downstream, said carburetion gutters having communicating means with said combustion gutters to permit fuel from said carburetion gutters to enter said combustion gutters; and a third combustion gutter grid facing downstream in said combustion zone supported in spaced relation downstream of said second grid.

5. In a combustion apparatus for increasing the mass and velocity of a confined stream of gases moving at high velocity therethrough; a duct having a mixing zone and a combustion zone for said gases; a first grid in said mixing zone for distributing fuel to the gases passing therethrough; a second grid downstream of said first grid comprising a carburetion gutter to carburet fuel from said first grid and a combustion gutter having igniting means therein; and a third grid downstream of said second grid comprising a combustion gutter only; whereby mixed fuel and gases supplied to said combustion gutter from said carburetion gutter become ignited producing a primary pilot flame, and mixed fuel and gases eddying about the combustion gutter of said third grid become ignited producing a secondary pilot flame, said pilot flames in turn igniting the remaining mixed gases and fuel passing through said grids to cause main combustion of said mixture thereby forming a combustion pattern of main combustion in zones intermediate pilot flame zones in the combustion zone of said duct.

6. In a combustion apparatus comprising a duct having an inlet and outlet for admitting and discharging gases moving at high velocity, said duct including a mixing zone adjacent said inlet and a combustion zone downstream of said mixing zone and adjacent said outlet; a first grid in said mixing zone for distributing fuel to the gases passing therethrough; and a second grid for igniting and maintaining combustion of the mixed gases and fuel, comprising carburetion gutters facing upstream towards the inlet side of said duct, and combustion gutters facing downstream towards the outlet side of said duct and having igniting means therein; and a third grid downstream of said second grid comprising a combustion gutter only; whereby mixed gases and fuel entering said second grid are ignited to produce a plurality of interconnected primary pilot flames extending downstream of said second grid, and mixed fuel and gases entering said combustion gutter of said third grid are ignited to produce a secondary pilot flame extending downstream of said third grid,

said primary pilot flames serving to continuously ignite the remaining mixed gases and fuel passing through said grids to cause main combustion of the mixture, and said secondary pilot flame serving to anchor the main combustion flame front in the approximate area of the third grid at high altitudes.

7. A combustion apparatus comprising a duct having an inlet and an outlet for admitting and discharging gases at high velocity; a first grid of fuel distribution pipes; a second grid having inner andouter concentric rings comprised of colinear carburetion and combustion gutters, said inner and outer concentric rings being joined together by radial members comprised of colinear carburetion and combustion gutters; and an additional combustion gutter downstream of said first and second grids and radially spaced between said pair of concentric rings of said second grid.

References Cited in the file of this'patent UNITED STATES PATENTS