GB2039359A - Gas turbine combustion chamber - Google Patents

Abstract

In a gas turbine combustion chamber 2 the fuel and air mix in the initial burning zone 21 with an equivalence ratio of unity at idle and a very rich mixture at higher powers with additional air for combustion being added downstream of the initial burning zone in such a way as to assure very rapid and complete mixing for completion of the burning in a short secondary burning zone 34. <IMAGE>

Description

SPECIFICATION Burner construction The regulations governing allowable emissions from airborne turbine engines have necessitated significant changes in the combustion system in order to meet the 1979 level allowed. Further revisions of the combustors to meet the lower proposed 1971 level are necessitating further modification of the combustor construction particularly to meet both idle and high power levels established. A significant problem is a revision of the combustorwithout changing the length of the burner thereby to avoid extensive engine design changes, both in new engines and in engines currently in use.

A feature of the invention is an initial burning zone in which under all operating conditions the mixture is rich for an equivalence ratio of about one at idle and richer at higher engine speeds with a secondary burning zone downstream of the initial burning zone in which additional air is mixed quickly and thoroughly for complete combustion in a short time interval. Another feature is a constructions to simulate the so-called axial staged combustor but without the need for a secondary fuel injection nozzle and with the desired result of low emissions.

Another feature is a zoned combustion chamber in which the initial burning zone, except at idle, is a reducing combustion and occurs at a relatively low temperature to minimize NOx emissions, and with the secondary burning zone downstream of the initial burning zone arranged to add additional air and mix thoroughly with the effluent from the initial burning zone to establish complete combustion, and with the secondary burning zone long enough to assure consumption of any carbon, carbon monoxide, hydrocarbons and amines produced in the initial burning zone by the breakdown of the fuel therein.

According to the invention, the nozzle is an aerating or an air blast nozzle in which the fuel is atomized and mixed with air when introduced into the initial combustion zone, and only enough additional air is admitted to this zone to establish a unity equivalence ratio at idle. The additional air is introduced only by a swirler closely surrounding the nozzle and the portion of the combustion chamber wall surrounding and estabishing the initial combustor zone is imperforate, except for a small amount of cooling airwhich does not take part in the combustion thereby providing for no additional air entry and maintaining the desired equivalence ratio.Downstream of the initial combustorzone, and spaced far enough so that additional air supplied thorugh plunged and/or unplunged holes in the chamber walls will not be recirculated to any significant extent to the initial burning zone, is the secondary burning zone, and here the added air is mixed as completely as possible with the gases in the zone to assure rapid complete combustion of any combustible gases in the chamber. Additional dilution air holes may be supplied downstream of the secondary burning zone for exit temperature control of the gases.

The foregoing and other objects, features, and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawing, wherein: Figure lisa sectional view through a burner construction embodying the invention.

Figure 2 is a sectional view, on a larger scale, through the fuel and air nozzle.

Figure 3 is a diagram of the combustion in the secondary burning zone along line 3-3 of Figure 1.

Figure 4 is a diagram of the combustion in the secondary burning zone along line 4-4 of Figure 1.

Figure 5 is a plot of staged burning performance.

Figure 6 is a plot of burning performance by the present device.

Figure 7 is a diagram of the initial burning zone fuel and airflow.

Referring to Figure 1, the annular combustion chamber 2 is positioned within a burner case 4 having inner and outer walls 6 and 8 which form a burner duct for the gases from the compressor discharge, not shown, at the left of Figure 1 and the turbine inlet, also not shown, at the right of Figure 1.

Within this burner case, the combustion chamber is made up of inner and outer walls 10 and 12, connected at their upstream ends by an end cap 14 in which are positioned a row or assembly of fuel nozzles 16, all in a single transverse plane, and only one of which is shown. Each nozzle is closely surrounded by a spacer ring 18 between which and the nozzle are swirl vanes 20. All the added air for combustion in the initial combustion zone 21, that portion of the combustion chamber directly downstream of the end cap, is supplied by the nozzles and by the control swirler. The portion of the walls 10 and 12 that surround the initial burning zone are imperforate, as shown, except for a small amount of wall cooling air that does not take part in the combustion process, so that no additional air enters this zone except past the swirlers 20.Fuel is supplied to the nozzles through one or more supply pipes 22 in brackets 24 carried by the upper end of the outer wall 8 and supporting the fuel nozzle at its outer end.

Extending forwardly from the inner and outer walls 10 and 12 are sheet metal rings 26 and 28 that converge in an upstream direction to form a hood 30 surrounding the nozzle and the swirlervanes.

The initial burning zone, the downstream limit of which is about at the dotted line 32 is thus essentially formed by an imperforate dome made up of the end cap and the imperforate portions of the chamber walls extending downstream therefrom so that the only air in the initial burning zone is that introduced with the fuel and that through the very small control swirler 20 which is designed to modify the combustion process but not to contribute to flameholding as does a conventional swirler.

Below the initial burning zone is the secondary burning zone 34 and in this area the walls 10 and 12 have air admitting holes of a size to add a substantial amount of air, more than enough for complete combustion. As shown, there is a first row of plunged holes 36 and a second row of plain or unplunged holes 38. The plunged holes assist in directing the air entering these holes to penetrate and mix with the gasses in this zone throughout the cross-sectional area of the zone. As shown these plunged holes 36 are preferably larger than the holes 38 such that a significantly larger quantity of air enters the first row of holes. The arrangement of the rows of holes 36 and 38 is such that all the effluent leaving the initial burning zone is adequately mixed with fresh air and that jet flameholders are produced to complete the combustion.After burning is substantially complete in the secondary burning zone dilution air for cooling the gases in the combustion chamber as they approach the turbine inlet is added by one or more rows of dilution holes 40 downstream of the secondary burning zone air holes. The spacing of the dilution holes is such as to assume complete oxidation of the effluent from the initial burning zone in the secondary combustion zone while the gases are hot enough to assure this combustion. The secondary zone terminates substantially at line 41 about at the line of the holes 40.

The holes 38 are staggered with respect to the holes 36 and the holes 40 are in line with the holes 36 and thus staggered with respect to the intervening row 38. Those well skilled in the art can appreciate that similar results can be achieved with alternative arrangements of the air holes.

In making an effective burner it has been found that, the length of the initial burning zone from the nozzle to the centerline of the row of holes 36 is preferably from 1 to 1.5 times the annular height of the end cap and the length of the secondary burning zone, from the row of holes 36 to the centerline of the row of holes 40 is greater than 0.5 times the annular height of the end cap. With this arrangement it is expected that burning will occur as desired with a minimum of objectionable emissions over the entire range of engine operation.

The nozzle is an aerating or air blat nozzle and may be of any well-known construction. One form of nozzle for this purpose is shown in Figure 2. In this arrangement the nozzle has a center body 42 having a conical inner end 43 and connected to a surrounding ring 44 by swirl vanes 46. This ring 44 has a fuel chamber 50 therein from which fuel is discharged between conveying flanges 52 and 54 on the ring. A second ring 56 surrounding ring 44 and spaced therefrom by swirl vanes 58 has an inturned toroidal flange 60 to direct air flowing over the vanes 58 toward and into the fuel spray from the chamber 50.

Thus air flowing over the vanes 46 and air over the vanes 58 supply two annular flows of air to atomize and mix with the annular discharge of fuel therebetween. The result is very effective mixing of the air and fuel at all fuel flows. The entire fuel flow for the full range of engine power from idle to maximum power is supplied through this row of nozzles.

In operation the nozzle or nozzle assemblage and surrounding swirl passage are so proportioned that at idle there is an equivalenve ratio of substantially unity thereby assuring complete combustion of the fuel in the initial burning zone. The nozzle as described above, injects the fuel in such a way as to avoid any impingement of fuel on the relatively cool dome and this is assisted by the nozzle swirler, which may be fitted with the toroidal deflector 60, Figure 2, or be of the radial inflow type, which minimizes the flow of fuel or a mixture of fuel and air from the nozzle outwardly onto the dome. This procedure reverses the conventional primary zone recirculation pattern, which is directed toward the burner centerline and gives the initial burning zone a recirculation pattern which flows from the burner centerline outward toward the liners.This flow is shown in Figure 7 in which the fuel and air flow from the nozzle 16 is shown by the arrows 70 to be nearly axial and as combustion occurs the flow is radially outward from the centerline of the chamber 2 with recirculation toward and forwardly between the walls and the axially flowing fuel.

As the engine power is increased by throttle advance more fuel is introduced than can be burned in the initial burning zone, and with a resulting reducing atmosphere. When this occurs the initial burning zone vaporizes the excess fuel which cannot be burned and it and the reaction products pass down into the secondary burning zone where added air is introduced through the holes 36 and 38 so as to mix thoroughly with the gases in the burner and results in completed combustion. The result is a minimizing of underirable emissions over the entire range of the engine.

It is believed that the emissions of unburned hydrocarbons at idle are generally caused by the chilling effect of liner cooling air and the relatively cold metal around the periphery of the conventional primary zone. By using the nozzle swirler to keep the injected fuel spaced from the periphery of the initial burning zone the chilling effect is minimized.

The emissions of carbon monoxide, at idle, are believed to be caused generally by relatively low flame temperatures resulting from the lean combustion of the primary zone. By introducing all the idle combustion air through the fuel nozzle and the surrounding small control swirler, and by operating the initial burning zone with imperforate walls for sufficient distance and at unity equivalence ratio sufficient flame temperatures and distance is allowed to consume almost completely all the carbon monoxide produced.

At powers above idle, it is believed that the low NOx emissions obtained are a result of (1) the reaction of atmospheric nitrogen in the reducing atmosphere of the initial burning zone with simple hydrocarbons formed from the fuel to form nitrogen compounds such as HCN, CN, NH, NH2, NH3 due to oxygen deficiency rather than NOx; and (2) the unburned fuel is vaporized, is thoroughly mixed with the products of combustion consisting of carbon monoxide together with the nitrogen compounds above noted in the initial burning zone and is carried into the secondary burning zone where excess air is added through the holes 36 and 38.

The air flowing through the holes 36 penetrates substantially to the middle of the burner and is mixed quickly and thoroughly with a portion of the unburned gaseous fuel components. The remaining portion of the unburned gaseous fuel components which flows between the air jets from holes 36 is mixed quickly and thoroughly with the air added through holes 38. The streams of air through holes 36 and 38 function as aerodynamic flameholders and thereby complete combustion of all the remaining fuel.

As shown in Figure 3, the air entering the holes 36 from both inner and outer burner walls penetrates in streams 62 to the center of the burner where the streams are diverted lateraly into stream 64 along the centerline of the burner. From this figure it is clear that the holes 36 in inner and outer walls are desirably opposite each other. Although some portion of the above noted gaseous fuel components are mixed with these streams the remaining portion flows between the radial streams 62 in pockets 66.

The second row of holes 38 are smaller than and staggered with respect to the holes 36 to enter these pockets 66. The air entering these holes need not penetrate as far as the air from the first row 36 as this air need penetrate only into these pockets 66 established by the streams 62 and 64. The position of these holes is shown in Figure 4 and the air streams 68 therefrom are centrally of the pockets 66 and midway between the adjacent streams 62.

Since the holes 38 are plain or unplunged, the depth of air penetration from these holes is less and does not extend beyond the limits of the pockets.

The aerodynamic blockage and vortex formation around the streams 62 and 68 produces and aerodynamic flameholder in the secondary burning zone.

The purpose and result of this mixing and flameholding by these streams is quickly to transfer the combustion in the secondary burning zone from the reducing atmosphere of the fuel rich initial zone (above stoichiometric equivalence ratio) to lean oxidizing atmosphere (below stoichiometric equivalence ratio) where there is an excess of oxygen for complete combustion of the gaseous fuel components.

It is expected that, by reason of the effective mixing in the secondary zone, substantially all combustion will be completed by the time the gases reach the row of holes 40. Thus these holes serve primarily as dilution holes functioning to admit air to reduce the gas temperature downstream from these holes to a temperature level and distribution suitable for the turbine.

It has been found that smoke emissions, most serious at high power are a minimum if the fuel injector air plus the air past the swirlers 20 is more than about 10 or 11% of the total air flow. It has been found important to introduce the air in the immediate vicinity of the fuel introduction and well mixed with the fuel so that the equivalence ratio in the fuel-air spray is everywhere close to the fuel-air equivalence ratio in the initial burning zone. This is achieved with the aerated or air blast fuel injector as shown in Figure 2 where air is introduced on both sides of the fuel. To prevent local excessively rich mixtures being formed immediately downstream of the toroidal flange or deflector 60, a small part of the air is introduced through the small control swirler 20.

The effect of the present concept is compared to a staged burner in Figures 5 and 6. In Figure 5, which shows the operation of a burner having two rows of fuel nozzles, one for low power, both for higher powers, the NOx emissions increase as the engine power increases along the line 72 as fuel is supplied by the primary nozzles only. When the secondary nozzles are cut in at the point 74 and power increases the rate of NOx emissions is represented by the line 76. Thus by a staged construction the emissions at higher powers are significantly less than would be without the secondary nozzles or the staging effect as the emissions would otherwise increase along an extension of curve 72, the dotted line 78.

When utilizing the quasi staging of the present concept, in which only the single set of nozzles is used but in which the arrangement of the burner is as above described so as to accomplish the result of staging without the extra injector nozzles, the low power emissions are represented by the line 72' and the high power emissions by the line 76'. This shows that the results previously accomplished by staging, that is the use of two or more spaced rows of injector nozzles, is accomplished by the present invention with only a single row of injector nozzles for the entire power range, but with the burner so con structed as to obtain the beneficial effects of a staged construction.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that other various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims (16)

1. A burner construction including: a duct; an inlet end cap in said duct; inner and outer burner walls extending downstream from said end cap in spaced relation to the duct, and establishing initial and secondary burning zones between the walls; an air blast nozzle positioned in said end cap for supplying fuel and air over all power ranges from idle to maximum power; a swirler surrounding said nozzle and so proportioned that the air entering this swirler at idle will establish with the fuel and air from the nozzle an equivalence ratio of about unity in the burner adjacent the end cap; and rows of holes in the burner walls at a substantial distance from the end cap, said walls being imperforate from the cap to said rows of holes, the first row of holes being plunged holes to improve the penetration of air entering the burner through these holes.

2. A burner as in claim 1 in which the first row of holes is spaced from the end cap a distance from 1 to 1.5 times the crosswise dimension of the cap.

3. A burner as in claim 1 in which the successive rows of holes are staggered with respect to one another.

4. A burner as in claim 1 in which the first row of holes is plunged and the second row is plain or smaller.

5. A burner as in claim 1 in which there are three rows of holes and the third row is in line with the first row and the spacing between the first and third rows is greater than 0.5 times the crosswise dimension of the end cap.

6. In the operation of a burner construction so as to reduce emissions such as CO, NOx, C, and unburned hydrocarbons the steps of: providing a burner having a closed upper end and imperforate side walls for a distance from the end cap defining an initial burning zone and rows of holes in the walls beyond the initial burning zone to establish a secondary burning zone; providing an air blast nozzle assembly in said closed upper end; introducing fuel and air through said closed upper end in such proportion that at idle operation the fuel and air in the initial burning zone has an equivalence ratio of about unity and at all powers above idle the equivalence ratio is greater than unity to provide a reducing atmosphere; and introducing additional air to the burner in the secondary zone through said holes in such an amount as to produce quickly an equivalence ratio less than unity for completion of combustion in an oxidizing atmosphere.

7. A process of operation as in claim 6 including the step of supplying the additional airfrom both inner and outer walls through the first row of holes in such as manner as to create aerodynamic flamerholders meeting substantially midway of the burner.

8. A process as in claim 7 with the added step of introducing the air through the second row of holes in staggered relation to the streams from the first holes and to a less penetration in the burner.

9. In the operation of a burner construction for an engine so as to reduce emissions such as CO, NOx, C and unburned hydrocarbons the steps of: providing a single row of air blast fuel nozzles in a single transverse plane at the upstream end of the burnerforthe injection of all the fuel into an initial burning zone at all engine powers; burning the mixture of fuel and air in this zone at substantially stoichiometric at idle and above stoichiometric at all other powers; and rapidly adding air to and mixing it with the fuel and air in the burner in a secondary zone immediately below the initial zone so that the completion of the burning occurs below stoichiometric.

10. The process of claim 9 including the step of supplying substantially all the air for combustion in the initial burning zone through the air blast nozzle.

11. The process of claim 9 including the step of injecting the additional air into the secondary zone through large holes in the burner to create aerodynamic flameholders in this zone.

12. In the operation of a burner construction for a engine so as to reduce undesirable emissions in the effluent gas the steps of: injecting all the fuel for all power ranges through a single set of air blast nozzles located in the upper end of the burner with the quantity of air supplied in the nozzle such that at all powers the fuel and air mixture is at or above stoichiometric; causing burning in an initial burning zone in the burning with no added air so that the combustor is always at or above stoichiometric; and adding more air for combustion to the burner immediately below the initial zond in a secondary burning zone in such a way as to form aerodynamic flameholders extending across the burner and to cause further burning at below stoichiometric.

13. The process of claim 12 including the step of making the portion of the burner walls surrounding the initial burning zone substantially imperforate so that substantially all the air for initial combustion is that supplied through the nozzles.

14. The process of claim 12 including the step of providing at least two rows of staggered holes in the burner walls surrounding the secondary zone, the first row being plunged, to cause the rapid flow of large quantities of air into the fuel and air mixture in the secondary zone to form aerodynamic flameholders and mixers.

15. The process of claim 12 including the step of cusing the fuel flow to be generally axially along the centerline of the burner thereby to cause a recirculation pattern which flows radially away from the centerline and forwardly between the centerline and the walls.

16. The invention substantially as hereinbefpre described with reference to and as illustrated in the accompanying drawings.