CA1208923A - Flow modifying device - Google Patents

Abstract

FLOW MODIFYING DEVICE
ABSTRACT OF THE DISCLOSURE

A swirler for a gas turbine engine combustor is disclosed for simultaneously controlling combustor flow rate, swirl angle, residence time and fuel-air ratio to provide three regimes of operation. A first regime is provided in which fuel-air ratio is less than stoichometric, NOx is produced at one level, and combustor flow rate is high. In a second regime, fuel-air ratio is nearly stoichometric, NOx production is less than that of the first regime, and combustor flow rate is low. In a third regime, used for example at lightoff, fuel-air ratio is greater than stoichometric and the combustor flow rate is less than in either of the other regimes.

Description

FLOW MODIFYING DEVICE
1. Field of the Invention This invention relates to flow modifying devices and particularly to a new and improved fluid flow modifying device in which the amount and direction of discharge of the fluid from the device can be varied.

2. Description of the Prior Art Many combustion chambers within gas turbine engines employ flow modifying devices, such as swirlers t to mix fuel and air and to aid in distributing the resultant mixture within the combustion chamber. The swirlers impart a swirling motion to the air. The swirling air increases the tendency of the fuel to atomize, causing better mixing and thus more efficient burning of th~ mixture in the combustion chamber.
However, many currently used swirlers have a fixed geOmetEy~ That is, the amount and the direction oE dischar~e, or swirl an~le, of air from the swirler i5 relatively constant, regardless of the amount of fuel which is injected into the combustion chamber.
For reasons of reducing objectionable gaseous emissions and improving combustor efficiency, it is desirable to be able to vary the amount of air which mixes with the fuel and to vary the swirl angle of the air as it leaves thè swirler. For example, when the engine is running at idle, it is preferable that there be a rich fuel-air mixture, that is, a high fuel to air ratio, in the primary com~ustion zone and tha-t the residence time of thè mixture within the primary zone be relatively long. The primary combustion zone comprises approximately the upstream third of the combustion chamber. Such a rich mixture reduces CO and HC emission levels at idle, and also enhances altitude relight capability. Correspondingly, with low airflow through'the combustor r as would occur with a rich mixture at idle, a higher swirl angle is needed to atomize the fuel prop'erly.
On the other hand r at high power operating conditions, it is preferable to have a lean fuel-air mixture, that is, a low fuel to air ratio, and to have a lower swirl angle in order to distribute the mixture more uniformly throughout the combustion chamber. This results in reduced NOx and visihle smoke emissions.
Furthermore, with a lean mixture at higher power conditions, less o~ a s~irl angle is required to properly atomize the fuel.
One approach which hàs been employed to vary the uel-air ratio is a two-stage, or double-annular, co~bustion sy~te~. In such a system, a pilot dome prod~lces a rich mixture for operation at idle engine conditions, while a second dome or mixing chute a5se~bly provides lean mixtures at hig~er power conditions.
~lthou~h SUC}l ~o-stage combustion systems are preferable to combustors employin~ single, fixed yeometry swirlers, they can be complex and expensive to fabricate,' and can add a significant amount of weight to the engine.
Anothèr approach'to varying the fuel-air ratio has heen the use of a shutter assembly for op~ning and closin~ air SCOOp5, the openings of which are normal to the flow'of compres'sed air from the compressor~ Such shutter assemblies, however, often have no positions æ~
~L~V~,~ ~

intermedia.te the open and closed positions~ Furthermore, while they may vary the'amount of air entering the com~ustion chamber, they fail to provide a corresponding variation in the swirl angle of the air. Another drawback of shutter ass'emblies in which the openings of the scoops are disposed normal to the air flowing from the compressor is that the compressed air exerts heaYY
5tresses directly against the elements of the shutter assembly. In order to avoid leakage and prevent damage, the elements must be fabricated so as to withstand such stresses, which'can in turn result in increased cost and wei~ht~ ' In view of the a'bove problem, it is therefore an object of the present invention to provide a Elow modifying device, particularly adaptable to a combustion ch.amberl which can vary the amount of air discharged from it, and therefore the fuel-air ratio, to improve combustor efficiency and reduce undesirable gaseous emissions.
20. Another object of the present invention is to provide a flow modifying device in which the direction of dischar~e, or swirl angler of the air can ~e varied in relation to the amount of air which is dischar~ecl Erom the device in orde.r to improve fuel-air mixing and distribution.
Yet another object of the present invention is to provide a ~low mod`ifyin~ device which is relatively simple and inexpensive~ ' Still another ob'ject of the present invention is

3~ to provide a flo~ ~odifyin~ device having eIements arranged so that, when' the device is employed i~ a combu:stion chamber located in the path of a flow of' compres~sed air, the elements of the device are substanbially protected fr~m stres's-es exerted by the :
compressed air.

~æo~æ3 SUMM~RY.OF THE INVENTION
A swirler for a gas turbine engine combustor is disclosed~ for simultaneousl~ contr-olling combustor flow rate, swirl angler res'idence time'and fuel-air ratio to provide three regimes of'operation. A first regime is provided in which.'fuel-air ratio i5 less than stoichiometric, NOx is produced at one level t and combustor flow rate is high. In the second regime, fuel-air ratio is nearly stoichiometric, NOx production 0 i5 less than that of the first reyimel and combustor flow rate i5 low. In a third re~imer used for example at lightoff, fuel-air ratio is greater than stoichiometric and th.e combustor flow rate is less than in either of the other regimes~
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B~IEF DESCRIPTION OF ~HE DRAWINGS
This invention ~ill be better understood from the following description taken in con~unction with the accompan~ing draw.ing r wherein:
FIGURE 1 is a fragmentary cross-sectional view of a combustion c~mber and a swirler incorporating eatures of the present invention.
FIGURE 2 is a cross-sectional view of a swirler taken along linas 2-2 of Figure 1.
FIGURES 3 through 5 are fragmentary cross~-sectional views of the swirler taken along lines 3-3 of Figure 2 and showing different relative positions of the plate and vane assembly.
FIGUR~ 6 is a schèmatic view of a gas turbine engine combustor.
FIGURE 7 is a plot of combustor inlet .~emperature as a function of compres'sor pressure ratioO
FIGURE 8 is a plot of NOx production as a function of fu~l-air ratio.
FIGURES 9,,10,. and ll are'plots of emissions versus combu~tor'inlet temperature.

8~

DESCRIPTION OF THE PREFERRED EMBODIMENT
Turning now to a co~sideration of the drawing, and in particular to Figure 1, there is shown the upstream portion of a combustion chamber lcombus~or~ 20 in a gas turbine engine. A mixture of air and fuel enters and is burned within the combustion chamber 20.
The energy of the resulting exhaust gases is extrac-ted to perform work~ such as to rota~e'a ~urbine tnot shown).
The fuel for combustion is introduced from the pressurized fuel nozzle 21. As the fuel exits the fuel nozzIe 21, it is mixed with air in the swirler 22 and the resulting mixture enters the combustion chamber 20 to be burned. The swirler-22 impar~s a swirling motion to the air flowing through it and ~us to the fuel emitted ~rom the fuel nozzle 21 which mixes with the air causing atomization of the fuel' and thereby promoting better mixing.
As shown in Figure 6, incoming air enters a plenum 22B~ The air can exit the plenum only at ~hree locations: through the swirler 22 of the present invention, through yenturis 38 (which can provide a swirling up5tream in the opposite direction to that provided by the present invention~, or through dilu-tion holes 22D. Thus, an increase in airflow through one exi-t location must result in a decrease in airflow through one of t.he others. The present invention allocates airflow a~ong these three exits in a manner which will become clear in the followin~ discussion.
The present invention comprises a flow modiying 3G device, such as the swirler 22, which receives at least a porti~n of its fluid from a generally radial direction and discharges that `fluid in a generally axial direction and which can vary the amount and direction of the . . .
dischàrge'of thè ~luid, such as air, flowing through it.
By "ràdial" it is meant in a direction generally ~2~8~3 13DV-~41 perpendicular to the swirler longitudinal axis, the axis bein~ depicted by the dashed line 27. By "axial" it is meant in a direction gen~rall~ parallel to the swirler longitudinal axis ~1. A radially displaced axis 27a is shown in Figure 1 and in end vie~ in Figure 2. The radially displaced axis 27a is parallel to the longitudinal axis 27 and serves a reference function which ~7ill be described later more fully.
In a particular embodiment of the invention, a first element, as can be seen in Figures 1 and 2, comprises an annular, radially aligned plate 23 and a plurality of axially ex~ending channels 24. Preferably, and as can be seen in Figure 3, the portions 23a o~ the plate 23 circumerentially adjacent each of the openings 24 include at least one radially extending surface 25a or 25b which lies in a plane angled from the longitudinal axis 27:of the swirler 22. These portions 23a are termed vanes. As will be seen later, in certain relative positions of the first and second elements, the surfaces 2~ 25a and 25b establish the sw.irl angle imparted to the air as it exits the swirler 22. Thus, the angle which the suraces 25a and 25b make with the displaced axis 27a is determined by the degree of swirl desired. As can be seen in Figure 3, the preferred cross-sectional shapes oE
~S the portions of the plate 23 circum~erentially adjacent each channel 24 is that of a hexagon, tl~at is, -three sets o~ paxallel and opposite radially extending suraces, 25a and 25b, 26a and 26bl and 28a and 28b~
As can be seen in Figures 1 and 2, the second ele~ent .is substantially annular and comprises a Yane assembly 29 including a plurality of radially exten~ing vanes.30 ~hich are interconnected at the radially inner and outer~ ends to annular memb-ers 31 and 32 respectivelyr Thè vanes:3a.are so disposed .that:an axially extending chànn;el:33 is defined between: each pair of vanes.

~%01!~!~23 As can best be seen in Figure 3~ the radially extendin~ surfaces 34 and 35 define the channels 33 and the angle which these ~urfaces make with the displaced axis 21a of the swirler determines' at least partially the swirl angle imparted: to the air as it exits the swirler 22. This angle should thus be predetermined according to ~he degree of swirl desiredO For reasons to be explained hereinafter, the distance between the surfaces 34 and 35 o adjacent vanes 30 is substantially the same'as the width o~ the surface 28a of the plate 23, and the suxfaces 34 and 35 of the vanes 30 are parallel to the surfaces 26a and 26h of the plate 23~
AS can be seen in Figure 1, the swirler 22 includes a hollow hub 36 which is generally annular.
The'upstream portion of ~he hub 36 extends generally radially, lying in a plane perpendicular to the swirler longitudinal axis 27~ The hub 36 is curved such that the downstr.eam portion, which is disposed radially i.nward of the plate 23 and the vane assembly 29, and which hub can be integral or attached with the plate 23, extends generally axially. The vane assembly 29 and the upstream portion of the hub 36 define an annular radially ~acing air inlet 37 through which a portion oE
the air for combustion enters the swirler.
~5 The fact that the air enters the variable portion o~ the swirler 22, that is r the vane assembly 29 and plate 23`portion, from a radial direction xather than axially is advantayeous because the vane assembIy and plate are thereby pro.tected by the upstream portion o~
the hùk 36 from the stresses` which`would be exerted by a direct flow of compressed air against ~hem. 'The upstream portion of the hub 36 can include as inteyral or attachèd with:it a radially aligned annular disc 39 Fuel ior combustion exits. the`fuel' nozzIe 21~ which extends through a gap in the'annular disc 39 o~ the 12~ 3 13DV-8~12 _ ~ _ upstream portion of the hub 36, and ~lows through the hollow interior of the hub 36 prior to entering the combustian chamber. The swirler can also include a plurality o~ fluid duots, such as the Yenturis 38~ in the annular disc 33 o* the upstream portion of the hub 36r through which air enters from a generally axial direction and mixes with fuel~ Thus, with this arrangement, initial mixing of air and fuel occurs in the interior of the hub 36 as air from the venturis 38 .lO mixes with fuel from the fuel nozzle 21. As this mixture then exits the hub 36, it is further mixed with air from the radial air inlets 37 after it flows through the vane assembly 29 and the plate 23. It is the amount . of the direction of dischar~e of the second source of air, that is, the air entering the swirler radially and flowing through the vane assembly 2~ and plate 23, which the present invention can vary.
Varying of the amount and direction of dischaxge, or swirl angle, of air from the swirler 22, is acco~plished by positioning, preferably rotatably, the second element, such as the vane assembly 29, relative to the first element, such as the plate 23. The vane asse~bly 29 is rotatably mounted on the swirler hub 36.
Means Eor positioning ~he second element preferably ~5 compri.se at least on.e actuatable drive arm 40 connected to the seconcl element, as can be seen in Figure 1 and 2 The radially outer portion of the drive arm 40 is connected to means which impart motion to the drive arm.
For example, the dri~e arm 40.can be connected to a unison ring 41 through a spherical bearing 42. The unison ring 41 can be connected with other dri~e arms 4Q
associated with other swirlers in the combustion section of the engine such that all o~ the drive arms will be moved together~
The radially inner end ;of the dri~e arm ~O.is J ~ ~

. g _ preferably connected to the vane assembly 29 through a hinge 43~ The use of a hinge 43 permits the vane assembly 29 to be rotated even when there is an axial dimensional mismatGh between the vane assembly 29 and the unison ring 41. As sho~n in Figure 2, the hinge 43 can include shims 44 to permit presetting of the circumferential position of the drive arm 40 to thereby synchronize the position of that drive arm with other drive arms which might be connected with the unison ring 41.
The swirler 22 is connected wit~ the upstream end of the combustion chamber 20 by an appropriate means, su~h as by welding or ~olting flanges 45, extending from the plate 23, to a lin~r 47 of the combustion chamber.
Likewise, the unison ring 41 can be supported ~y any suitable means, such as by a roller bearing 48 and support bracket 46. This embodiment of the 10w modifying operates as follows:-Figure 3 shows the swirler in its open position.
The vane assembly 29 is positioned such that the surfaces34 and 35 of -the vanes 3~ are aligned with the surfaces 26a and 26b respectively of ~he plate 23. Thus, the channels 33 of the vane assembly 29 are aligned with the channels 24 of the plate 23 such that the maximum amoun-t ~5 o~ air passes through them~ The direction that the air will Elow as it is discharged from the slots 33 and openings 24, ~hat is, its swirl angle~ is determined by ~he an~le that the surEaces 34 r 35, 26a~ and 26br which ar~ preexably parallelr make with the displaced axi~ 27a.
Figure 4 shows the ~ane as~em~ly 29 after it has been rotatably positioned to an intermediate position.
Part of the air flow`ing through each of the channels 33 of the vane assembly 29 impinges upon and is turned by a surface 25b of the plate 23. `This part of the air causes the`xemainder of th~e air flowing through the channel 33 to also be turned and flow across the adjacent surface 25a~
Figure 5 shows. the vane assem~Iy 29 af.ter it has been rotatably positioned to the' closed position. The surfaces 2ga of the pl.ate 23'block the channels 33 such that suhstantially no air can flow through the channels 33 or channels 24. When the vane assembly 29 is in the closed position, the.only air entering the combustion chamber 20 through the swirler 22 would be tha~ flowing from the venturis 38 through the int~rior of the swirler hub 36, a~ can be seen in Figure lr or through the dilution holes 22D in Figure 6.
Accordingly, an in~ention has ~een described in which a register plate valve for throttling axially flowing air is incorpôrated into a combustor in a gas turbine engine. The'second pla~e 29 of the valv~ as shown in Figures 3-5 includes a plurality of vanes 30 which are positioned in a radial array as shown in Figure 2. The vanes 30 resemble parallelograms in cross sections as shown in Figure 3. The distance 75 between adjacent faces 34 and 35 at a given r'adius such as radius 78 in Figure ~ does not change in the downstream direction~ That is, distance 75a in Figure 3 equals downstream distance 75br so that the width of the channel ~5 33'does not change in the downstream directi.on. Faces 34 and 35 make a first swirl angle 80 with the radially d'isplaced longitudinal axis 27a~ This angle 80 i5 preferabIy within the approximate range of lS to 30 degxees.
The first plate 23'contains a radial array of vanes ~3A as shown in Figure 2 which'are hex'agonal in cross section as shown in Figures 3-5. Opposite faces of the hexagonal cross sectionæ~ are parallel. (That is, faces 25a an~ b are parallel`.,:faces 2Ga and b are parallel'r and faces 2ga and b 'are parallel). Faces 26a ~2~ 2~

and 26b define an angle with the displaced axis 27a which is the same as angle 80~ Thus, when the first and second plates 23 and 29 are positioned as shown in ~igure 3, the ~aces 26b and 35 are aligned along line 83 and faces 26a and 34 are aligned along line 85 (that is, the respectlve Eaces are colinear ~i~h thb corresponding lines 83 or 85.) In such a caset a continuous channel including channel s-ubparts 24 and 33 is defined by these faces. The air flowing through the channel is imparted a swirl angle determined by anyle 80. A gap 88 is sho~n between ~he two plates 23 an~ 29, but this is illustrative only. The gap is actually of the order of one thousandth inch and no 'appreciable airflow travels alon~ the gap in the directions-of arrows 90. The register plate throttle is preferably dimensioned so that approxima~ely fifteen per'cent of the air entering the combustor does so through`t'nis throttle, as positioned in Figure 3. The remainder enters through venturis 38 and dilution holes 22d of Figure 6.
~he operating regime shown in Figure 3 and just described is used during takeoff and cruise conditions of aircraft flight. The regime used for idle conditions is shown in Figure 4. In Figure 4I the first plate ~3 has been rotated so that the vanes ~3a partial].y obstruct ~S the channels 33'of tl~e second plate 29. In such a case, the swirl angle of the air is dominated by the angle 95 which faces ~5a and b make with the displaced axi5 27a.
Faces ~Sa ~nd b define a ~low channel and are parallel in cross section~ The angle 95 is preferably within the approximate range of 50 to 7Q de~rees.
Under the conditions o~`Figure 4, a large swirl angle 95 is imparted to the'air and consequently a larger residence time'of the air in the combustor is imparted' as` comparea with the residence time of Figure 3.
This larger residen'ce time'pr'omotes fuller combustion 8~i2~

of fuel at idle~ The thro-ttle valve is preferably dimensioned so that, at idle, under the conditions of -Figure 4, about five percent of the combustor airflow is supplied by the register plate throttle and the remainder is supplied by venturis 38 and dilu-~ion holes 22d in Figure 6.
The operating regime of Figure 5 is used during engine ignition (i.e., '~lighto f")~ The register plate throttle closes off all airflow to provide a very rich fuel mixture.
Some important aspects of the present invention are now discussed with reference to Figures 6, 7, and 8.
Figure 7 is a plot o combustor inlet temperature as a function of enyine'ccmpressor ratio~ NOx production is a function of this temperature. The three operating conditions corresponding to Figures 3 and 4 are indicated in Figure 7 of Figures 3'and 4 being abbreviated as "F3" and "F4".
Figure 8 is a plot of NOx production as a ~unction of combustor fuel-air ratio. NOx production peaks at the'stoichoimetric ratio, which is approximately .067 by weight. The stoichoimetric ratio is that at which the air present contains exactly t~e amount of oxygen needed to completely burn fuel in-to carbon dioxide and water vapor, One explanakion Eor this peak at the s-toichoimetric ratio is that the combustor temperature tends to be highest at this ratio and consequently,,since NOx production is t~mperature-sensitive, NOx production i5 also highest.
In considexing Figure 7 anA 8 to~ether,,one sees that,,under the regime'o~ ~igure 3'(~akeoff or cruise~
combustor inlet temperature is ~uch higher ~han at idle, and so NOx production tends to be'similarly higher for this-reason'. Hawevèrl as discu~sed in connec-tion with Figure'3, use`of'the regime o` Figure 3'results in a high airflow rater low .residence time, and lean fuel-air ratios within the combustor as shown by point 98a in Figure 8. Consequently, despite the higher combustor inlet temperature, the lowered residence time subjects atmospheric nitrogen to hi.gh temperatures for a short time, thus promoting lessened N5x production. Also, as illustrated in Figure 8, the lean ~uel-air ratio results in a low NOx production rate. Calculations made in conjunction with`exper`imental e~idence indicates that 1~ the regime of Fiyure 3 results in NOx production of about 15 to 20 pounds per thousand pounds of fuel, compared to 40 to 50 lbs/1000 lbs~ fuel for con~entional combustion systems.
During idler using the regime of Figure ~, several factors affect NOx production~ (At idler fuel control means kno~n in the art reduces fuel suppliea to the combustGrs.) The lowered combustor inlet temperature, as shown in Figure 7 r tends to reduce NOx production.
~Iowever, t~e engine`at idle is preferably run at a fuel-air ratio whi`ch is at or near stoichiometric, as shown by point 98 in Figure 8. This tends to increase NOx production. Further, the increased swirl angle of F.igure 4 tends to increase residence time in the combustor as does the reduced air~low under idle ~5 condi.tions~ Both of these factors tend to increase NOx production, in exposing atmospheric nitrogen to high combustor temperatu.res for longer times. However~
calculation a~d experiment indicate that the use of the ~egime`in Figure ~ results in reduced NOx production of about three to ~our pounds NOx per thousand pounds of fuel. The lowe`red combustor inlet` temperature o~ the idle c~nditions of Figure 4 is thus, in a sense,; the`
dominating factor in NOx pxoductiont despite the upward influence on NOx productlon~iof:low flow rater high swirl angle, stoic~iiometric ratio, and incr`eased residence ~%~8!~2;3 13D~-~412 -- 1~ --time, all of which are associated with the regime of Figure 4.
In addition, the stoichiometric fuel-air ratio, the hi~h'swirl an~le and the increased residence time S of Figure 4 serve to promote-more co~plete combustion, thereby reducing carbon monoxide (Co) and hydrocarbon (HC) production.
Some of these performance characteristics of the present invention are further illustrated in Figures 9-11. Figure 9 is a plot of carbon monoxide (CO) produced versus combustor inlet temperature tT3), Figure 10 is a plot of hydrocar~on emissio~s (HC~
versus combustor'inlet temperature and Figure 11 is a plot of NOx emissions versus combustor inlet temperature. In all three plots, thé gas quantity on the vertical axis (CO, HC, or NOx) has units of pounds of the gas produced per t~ousand pounds of fuel burned.
In the three plots, the per~ormance of the present invention, based on computations taken from experimental evidence, is lab~led~l'invention," while thè performance of a typical prior art combustor is labeled "prior art."
The three plots are considered self-explanatory.
One of the principal merits of the present invention lies in the pro~ision of three selectable ~5 positions of the register plate throttle. These are s~own in Figures 3-5. These three positions provide two separate s~irl an~les and ~hxee separate aperture settings determined by the degree o~ obstruction of the channel 33 by the vane~ 23 of the plate 23.' Thus, the airflow rate (in pounds per second) is simultaneously controllabIe with~the s~irl angle. Further, the register plate throttle'ser~es` to shift airflow from the'combustor dome to the dilution holes without the use o~ other components of ~ariable ~eometry.
In the prefer'red-embodIment, operation'of the 13D~8412 register plate throttle is restricted to one of the three regimes shown in ~igures 3~5, and no others.
For example, a regime intermediate those of Figures 3 and 4 is not contemplated. Therefore, once designed r the combustor is configured to operate in either a first regime having a first airflow and first swirl angle, in a second regime ha~ing a second airflow ~nd second swirl angle, or a third regime having zero airflow and no swirl angle~
It is to be understood that this invention is not limited to the particular embod.iment disclosed, and it is intended to cover all modifications coming within the true spirit and scope of this invention as claimed.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A swirler for a gas turbine engine combustor, comprising:
a register plate throttle for simultaneously adjusting combustor flow rate, swirl angle, residence time, and fuel-air ratio for producing three regimes of combustion, including:
(a) a first regime in which fuel-air ratio is less than stoichiometric, NOx produced is at a first NOx level, and combustor flow rate is at a first flow rate;
(b) a second regime in which fuel-air ratio is closer to stoichiometric than in the first regime, NOx production is at a second level less than the first NOx level, and combustor flow rate is substantially less than the first flow rate; and (c) a third regime in which fuel-air ratio is greater than stoichiometric and combustor flow rate is less than either flow rate of (a) or (b), in which the register plate throttle comprises two axially adjacent arrays of vanes such that (d) one of the arrays defines a first swirl angle, (e) the other of the arrays defines a second swirl angle similar to the first swirl angle and defines a third swirl angle, whereby the first swirl angle of (d) chiefly determines the swirl angle of the combustor air in the first regime of (a) and the third swirl angle of (e) chiefly determines the swirl angle of the combustor air in the second regime of (b).

2. In a gas turbine engine combustor, the improvement comprising first and second parallel, mutually rotatable register plates through which axially directed airstreams flow, both plates including a radial array of vanes which define channels between adjacent vanes, the channels in the first plate defining a first swirl angle and the channels in the second plate defining both a second swirl angle similar to the first and a third swirl angle, wherein:
(a) alignment of the plates in a first predetermined position causes air flowing through them to (i) acquire a swirl angle chiefly determined by the first swirl angle and (ii) acquire a first flow rate;
(b) alignment of the plates in a second predetermined position causes air flowing through them to (i) acquire a swirl angle chiefly determined by the third swirl angle and (ii) acquire a second flow rate lesser than the first flow rate;
(c) alignment of the plates in a third predetermined position causes airflow through them to terminate.

3. In a register plate throttle which surrounds a fuel injection nozzle, the improvement comprising:
(a) means for providing a first, relatively low, swirl angle at a relatively high airflow regime;
(b) means for providing a second, relatively high swirl angle at a relatively low airflow regime; and (c) means for terminating airflow in a third airflow regime.

4. A method of operating a gas turbine engine comprising the following steps:
(a) using a register plate swirler which surrounds at least one fuel nozzle in at least one combustor, providing a relatively high airflow therethrough at a relatively low swirl angle for (i) providing a fuel-air mixture for combustion in the combustor having a fuel-air ratio which is less than stoichiometric and (ii) providing a relatively reduced residence time of the fuel-air mixture in the combustor;
(b) usinq the register plate swirler, providing a relatively low airflow therethrough at a relatively high swirl angle for (i) providing a fuel air mixture for combustion in the combustor having a fuel-air ratio which is greater than stoichiometric for less-than-peak NOx production, (ii) providing a relatively increased residence time of the fuel-air mixture in the combustor; and (c) using the register plate swirler, terminating airflow therethrough for providing a rich mixture to the combustor.