Patent Publication Number: US-8984889-B2

Title: Combustor for a gas-turbine engine with angled pilot fuel nozzle

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2008/063435, filed Oct. 8, 2008 and claims the benefit thereof. The International Application claims the benefits of Great Britain application No. 0721577.5 DE filed Nov. 2, 2007. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a combustor for a gas-turbine engine. 
     BACKGROUND OF INVENTION 
     It is a common desire in gas-turbine technology to increase the efficiency of combustion of the fuel-air mixture employed in such engines. It is also desirable to be able to obtain a good mix between fuel and air, which is not greatly dependent on the load, which is placed on the engine. 
     Various measures have been invented to enhance the fuel-air mixing process. One such is disclosed in U.S. Pat. No. 4,991,398, issued to assignee United Technologies Corporation.  FIGS. 1(   a ) and  1 ( b ) show a basic principle of a technique described in this patent. An annular combustor  10  has disposed at a dome end  12  thereof a number of fuel nozzles  14 . The nozzles  14  are circumferentially spaced apart in two rows—a first, radially inner row  16  and a second, radially outer row  18 , referred to a longitudinal axis  20  of the combustor. The nozzles of one row interleave with those of the other row, so as to create a triangular configuration shown as G in  FIG. 1(   a ). Each of the nozzles has its own swirler device and the directions of the swirl in each case are shown by the arrows  24 . Combustion air is introduced into the combustion chamber at an axially intermediate point  22 . 
     As can be seen in  FIG. 1(   b ), the fuel spray  26  (nominally in the form of a cone) emanating from the nozzles in row  18  interleaves with the fuel spray  28  emanating from the nozzles in row  16 , the result being a mutual reinforcement of the two flows. Because of this reinforcement, the fuel tends to be distributed more uniformly throughout the combustion chamber, and fuel-air mixing is more intense. Indeed, intense combustion starts quite close to the dome within distance C 1 , increasing the axial extent of the intense burning to distance E 1 , before the combustion becomes diluted. This enables the combustor to be made shorter in length, thereby saving space and weight. 
     A development of the arrangement just described is set out in U.S. Pat. No. 6,360,525, issued to Alstom Gas Turbines Ltd. In this patent (see  FIGS. 2(   a ) and  2 ( b )) an annular combustor  30  has a similar arrangement of nozzles to that in U.S. Pat. No. 4,991,398, but this time the dome end  32  of the combustor is made in two radially adjacent sections, namely sections  32   a  and  32   b , which are arranged at an angle  34  to each other at a mid-point  36  of the combustor. The two rows of nozzles  38  are provided in respective sections  32   a ,  32   b  and, since these sections are inclined with respect to the normal to the longitudinal axis  36 , the spray cones  40 ,  42  emanating from the nozzles are radially inclined toward each other in an interleaving manner. The longitudinal centrelines  41 ,  43  of the spray cones leaving the nozzles cross each other at an axial point  44 , which forms a circle over the whole annular combustor. 
     This mutual inclination of the two flows radially toward each other creates a stronger mixing action between the flows than is the case in the  FIG. 1  arrangement. This, in turn, enhances the uniformity of combustion in the primary combustion zone. In addition the distance C 1  in  FIG. 1(   b ) is reduced, enabling the combustor to be further shortened in the axial direction. 
     SUMMARY OF INVENTION 
     In accordance with the invention there is provided a combustor for a gas-turbine engine, comprising: a burner head; a combustion chamber disposed downstream of the burner head; a swirler means for creating a swirling flow of air in the combustion chamber, and a fuel nozzle disposed in the burner head for supplying fuel to the combustion chamber; said fuel nozzle being disposed in the burner head such as to give rise to a first angle of exit of the fuel from a downstream face of the burner head of &gt;±0° with respect to a longitudinal axis of the combustor, said first angle lying in a first plane passing through the longitudinal axis, and to give rise to a second angle of exit of the fuel from said downstream face of &gt;±0° with respect to said first plane, said second angle lying in a second plane orthogonal to said first plane. 
     The nozzle may be configured such as to give rise to a generally cone-shaped spray of fuel entering the combustion chamber, an angle between a surface of the fuel cone and the downstream face being &gt;0°. 
     The combustor may be a can-type combustor and the fuel nozzle may be disposed radially offset from the longitudinal axis of the combustor. 
     The first angle may be such that the fuel cone is inclined toward the longitudinal axis of the combustor. 
     There may be a plurality of the fuel nozzles. 
     The combustor may be an annular combustor comprising a plurality of the fuel nozzles disposed in circumferentially spaced apart manner. At least one of the fuel nozzles may be a pre-filmer device comprising a fuel duct, a swirler and a pre-filmer element, all of which are disposed at the first and second angles. At least one of the fuel nozzles may be a pressure-swirl injector device comprising a swirler and a fuel duct, both of which are disposed at the first and second angles. At least one of the fuel nozzles may be an air-blast injector device comprising two or more coaxially disposed swirlers and filming elements, all of which are disposed at the first and second angles. 
     One or more further fuel nozzles may be disposed axially downstream of the fuel nozzles, the fuel nozzles being disposed such as to direct a flow of fuel toward respective further fuel nozzles. 
     The combustor may further comprise one or more further fuel nozzles disposed in the burner head radially inwardly or radially outwardly of the fuel nozzles, the fuel nozzles being disposed such as to direct a flow of fuel toward respective further fuel nozzles. 
     The fuel nozzle in the various embodiments of the invention may be a pilot-fuel nozzle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with the aid of the appended drawings, of which: 
         FIGS. 1(   a ) and  1 ( b ) are, respectively, a radial section and a partial longitudinal section through a known annular combustor; 
         FIGS. 2(   a ) and  2 ( b ) are, respectively, a partial longitudinal section and a partial radial section through a further known annular combustor; 
         FIGS. 3(   a ) and  3 ( b ) are, respectively, a longitudinal section through a known can-type combustor and a radial section through a radial swirler employed in this combustor; 
         FIG. 4(   a ) is a simplified longitudinal section through the combustor of  FIGS. 3(   a ) and  3 ( b ), while  4 ( b ) is a longitudinal section through a combustor in accordance with a first embodiment of the present invention; 
         FIGS. 5(   a )- 5 ( d ) are perspective and plan views of four different injector configurations in the combustor of  FIGS. 4(   a ) and  4 ( b ); 
         FIGS. 6(   a ) and  6 ( b ) are results of a computer simulation of two different configurations of the combustor in accordance with the first embodiment, while  FIGS. 6(   c ) and  6 ( d ) are simplified representations of  FIGS. 6(   a ) and  6 ( b ), respectively; 
         FIGS. 7(   a ) and  7 ( b ) are perspective and longitudinal sections, respectively, of a combustor in accordance with a second embodiment of the present invention; 
         FIG. 8(   a ) is a longitudinal section through a variant of a combustor in accordance with the second embodiment, while  FIGS. 8(   b )- 8 ( d ) represent various component parts of this combustor; 
         FIGS. 9(   a )- 9 ( c ) are partial longitudinal sections of an annular combustor in accordance with a third embodiment of the present invention, in three variants thereof; 
         FIGS. 10(   a )- 10 ( d ) are partial radial sections showing four different injector configurations in the combustor of  FIGS. 9(   a )- 9 ( c ), and  FIG. 10(   e ) is a partial radial section showing the effect of the configuring of the injectors in the manner set forth in  FIGS. 10(   a )- 10 ( d ); 
         FIG. 11  is an axial section through a pressure-swirl type of injector that may be employed in a combustor in accordance with the present invention; 
         FIGS. 12(   a ) and  12 ( b ) are longitudinal and radial sections, respectively, of an air-blast type of injector that may be employed in a combustor in accordance with the present invention; 
         FIGS. 13 and 14  are partial longitudinal sections of an annular combustor in accordance with the present invention and including, respectively, axially and radially staged injectors; and 
         FIGS. 15(   a ) and  15 ( b ) are longitudinal and radial sections, respectively, of a silo combustor employing injector tilt. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Turning now to  FIG. 3(   a ) this is a longitudinal section through a can-type combustor described in U.S. Pat. No. 6,532,726, issued to Alstom Gas Turbines, Ltd, as assignee.  FIG. 3(   b ) is a radial section through a radial swirler employed in this combustor. The combustor comprises a burner head  50  and a combustion chamber  52 . The combustion chamber  52  narrows down into a pre-chamber  54  before being connected to a downstream face  56  of the burner head. A radial swirler  58  is disposed intermediate the burner face  56  and prechamber  54 , and a number of pilot-fuel nozzles  60  and main-fuel nozzles  62  are provided in circumferentially spaced-apart manner in the burner head. In use, liquid fuel is supplied via the pilot-fuel nozzles  60 , this fuel being mixed with compressed air entering the passages  64  of the swirler. The mixture is ignited by means of an igniter  66 . Once the engine load has increased to a given level, main fuel is supplied via the main-fuel nozzles  62 . This main fuel is then regulated to provide in the region of 95% of the total engine fuel requirement. The ignited flame is shown in  FIG. 3(   a ) as a flame front F and a flame front-face FF adjacent the burner face  56 . 
     The present invention, in a first embodiment thereof, retains the use of only one of the pilot nozzles shown in  FIGS. 3(   a ) and  3 ( b ) and changes its configuration in a manner shown in  FIGS. 4(   a ) and  4 ( b ).  FIGS. 4(   a ) and  4 ( b ) are a simplified representation of  FIG. 3(   a ), in which the radial swirler  58  and pre-chamber/combustion chamber combination,  54 / 52 , are shown, but not the burner head  50 . Emerging from the pilot nozzle  60  is a cone-shaped spray of pilot fuel  70 . This spray is injected into the base of a recirculating-air region of the burner, shown by the dashed lines  72 . The recirculation region is a region of the combustor, in which the combustion products lose their momentum and are drawn back into the radially central part of the combustor by a low-pressure region created by the swirling action of the swirler. It is noted that the pilot-fuel nozzle  60  is positioned off-centre with respect to the longitudinal axis  74  of the burner. This is beneficial for the control of the nozzle temperature. However, in the design shown in  FIG. 4(   a ), which corresponds to the  FIG. 3  arrangement, the off-centred nature of the spray can result in a large proportion of the spray missing the recirculation region, where the main combustion reaction takes place. This has the drawback that the resultant unreacted fuel-spray droplets leave the combustor in the form of unburnt hydrocarbons. 
     The solution provided by the present invention is to reconfigure the nozzle  60  so as to direct the cone-shaped spray toward the longitudinal axis  74  of the burner and combustor. To achieve this, the duct  76  forming part of the nozzle is angled, as shown in  FIG. 4(   b ). Either only the end portion of the duct adjacent the burner face is angled, in which case the rest of the duct can follow a path normal to the burner face (see duct portion  76 ′), or the whole duct is angled (see dotted line portion  76 ″). 
     Examples of the possible orientations that may be assumed by the spray cone are shown in  FIGS. 5(   a )- 5 ( d ). The unslanted spray cone of  FIG. 4(   a ) is shown in  FIG. 5(   a ), while the slanted spray cone of  FIG. 4(   b ) is shown in  FIG. 5(   b ). In  FIG. 5(   a ), the major axis of the cone lies along the y-axis, while the burner face lies along the x-axis of an x, y co-ordinate system. In  FIG. 5(   b ), on the other hand, the major axis of the cone lies at an angle α to the y-axis, whereby the spray from the nozzle is directed toward the centre of the recirculation region. 
     In addition to a tilting of the nozzle toward the longitudinal axis of the combustor, the nozzle is also subjected to rotation in a plane orthogonal to the plane in which the x, y co-ordinates lie, namely the plane of the burner face. This is illustrated in  FIGS. 5(   c ) and  5 ( d ). In  FIG. 5(   c ), the cone is rotated so that it makes an angle β with the x-axis in the same direction as the swirl direction of the air emanating from the swirler, whereas in  FIG. 5(   d ) the angle β is in the opposite direction, i.e. against the swirl direction. In what follows, rotation of the cone with the swirl direction will be called “positive swash”, while rotation against the swirl direction will be called “negative swash”. Incidentally, the angle α in  FIGS. 5(   c ) and  5 ( d ) is greater than that in  FIG. 5(   b ), although it could just as well have been smaller or the same. 
     The effects of this “tilt” and “swash” technique are shown in  FIGS. 6(   a ) and  6 ( b ), which are the results of a computer simulation. The simulations include the burner face  56  and nozzle  60  (the latter can be seen in  FIG. 6(   a ) only), and the fuel exiting the nozzle can be seen being caught up in the swirling action created by the swirler. The lighter coloured swirl lines represent liquid fuel droplets, which are smaller than those represented by the heavier coloured swirl lines. It can be seen how the heavier (i.e. larger) droplets are less affected by the swirl action and therefore tend to keep to the areas around the burner axis, while the lighter droplets are flung further afield.  FIGS. 6(   c ) and  6 ( d ) correspond to  FIGS. 6(   a ) and  6 ( b ), respectively, but in simplified form, in which just the outline shape of the fuel-air mixture is shown. In  FIGS. 6(   a ) and  6 ( c ) the swash angle is positive, whereas in  FIGS. 6(   b ) and  6 ( d ) it is negative. It can be clearly seen that there is significantly more mixing of the fuel droplets with the incoming air in  FIGS. 6(   b ) and  6 ( d ) than in  FIGS. 6(   a ) and  6 ( c ). However, in both cases there is improved fuel dispersion throughout the pre-chamber. This increases the efficiency of the combustion process, since the variability in the concentration of fuel in the central recirculation zone and in the main combustor dome expansion regions is reduced. Furthermore, the burning processes in the combustor dome recirculation regions are more circumferentially uniform, resulting in a longer combustor life. 
     Positive and negative swash bring different advantages in terms of combustor performance. As already mentioned, positive swash will result in poorer dispersal of fuel compared with the negative swash. However, this can be beneficial for conventional or non-premixed combustor designs, since the poorer dispersal will ensure that there are locations where a high fuel concentration will exist. This, in turn, will provide an anchor for the flame. An application where this might prove useful is an aeroengine gas-turbine combustor application, where a situation of high water-ingestion may occur (e.g. the aircraft flies through heavy rain). In this event, it is still possible to achieve a degree of stability of the flame. On the other hand, negative swash is beneficial for low-emissions combustor designs. This is because the high shear and disruption of the fuel spray in the combustor gas stream will improve the fuel-to-air mixing process and minimise locations of high fuel-concentration. High concentrations of fuel can give rise to stoichiometric burning, which in turn produces high emissions of NOx. 
     Returning momentarily to  FIG. 5(   b ), in this diagram an angle γ is shown, this being the angle between the surface of the cone and the burner face. It is preferred that this angle not equal 0°, because a 0° angle will result in large amounts of fuel coming into contact with the combustor surfaces, which is not desirable, since such fuel cannot contribute to combustion and also tends to form carbon or “coke” deposits on the burner face. 
     Although the nozzles in the first embodiment have been described in connection with the supply of pilot fuel, they may equally be nozzles for supplying main fuel. 
     A second embodiment of the invention will now be described with reference to  FIGS. 7(   a ) and  7 ( b ).  FIG. 7(   a ) is a development of the first embodiment, in which, instead of employing just one nozzle, two or more are used in conjunction with each other. In the  FIG. 7(   a ) example there are three such nozzles approximately equidistantly spaced around the burner face. Each of the nozzles is inclined toward the centreline (longitudinal axis) of the burner/combustor arrangement by the same tilt angle, but at the same time the nozzles are rotated by the same swash angle. This is shown in a simplified longitudinal section in  FIG. 7(   b ), where the arrows rising obliquely from the burner face represents the fuel flow toward the central region of the combustor. Incidentally,  FIG. 7(   b ) could also be interpreted as a two-nozzle system, in which case the nozzles will normally be spaced 180° apart. 
     The nozzles in  FIGS. 7(   a ) and  7 ( b ) are plain nozzles formed in the burner head, along the lines of those shown in the known combustor arrangement of  FIGS. 3(   a ) and  3 ( b ). That is, they are not associated with their own swirler or, e.g., pressure injection mechanism. 
     This embodiment of the invention may be also applied to combustors having a number of nozzles greater than three. For example, combustors with as many as twelve nozzles are not uncommon. As applied to the present invention, these nozzles would be as shown in  FIG. 7(   a ), that is approximately equally spaced around the burner surface and subjected to approximately the same tilt and swash angles. 
     In both the  FIG. 7  arrangement and its variant comprising a greater number of nozzles, the nozzles may be brought into service either together, or in sequence one after the other or in groups. When used sequentially, they may provide a staged combination of pilot and/or main fuel jets, controlled in dependence on the engine load, for example. When used all together simultaneously as either main or pilot fuel nozzles, the jets from the nozzles will interact with each other to create an enhanced mixing action between the liquid fuel and the air from the swirler in a radially central region of the combustion chamber. 
     A further variant of the invention as applied to a can-type combustor is shown in  FIG. 8(   a )- 8 ( d ).  FIG. 8(   a ) represents a can-type combustor comprising a burner base  71 , a swirler  73  and a combustor chamber  75  connected in series. The burner base  71  per se is shown in  FIG. 8(   b ). Although shown hatched, the base is not completely solid, but contains passages for the supply of main and pilot fuel, etc. Accommodated in the burner base is a nozzle unit  77 , which is shown in perspective view in  FIG. 8(   c ). Liquid fuel is passed through the stem of the nozzle unit  77  and emerges from individual nozzles  79  formed in the nozzle face attached to the stem (see  FIG. 8(   d )). As with the  FIG. 7  arrangement, these nozzles are inclined at at least a tilt angle, as can be seen from the orientation of the spray cones shown in  FIG. 8(   d ). A combination of the interaction of neighbouring spray cones and the swirling air exiting the swirler  73  creates minor vortices, as shown by the arrows in  FIG. 8(   d ). This provides more localized areas of mixing across the radial extent of the combustion chamber, leading to greater uniformity of mixing at the downstream end of the combustion chamber. It should be noted here that, although some of the spray cones appear directed toward the combustion-chamber wall, in practice little spray reaches this wall, since it is caught up in the spray of adjacent, inwardly facing nozzles. 
     Although not shown in  FIG. 8(   d ), it is possible to employ swash as well as tilt in this arrangement, in order to enjoy some of the benefits arising from the use of swash in the earlier-described arrangements. In this case, the swash angles of the outwardly facing nozzles and those of the inwardly facing nozzles would be chosen, either by experiment or by computer simulation, or both, to provide the desired mixing behaviour overall. In order to obtain swash, it is necessary to employ nozzles which are offset from their respective swirler axes, after the manner of the first embodiment shown in  FIGS. 4 and 5 . The tilt and swash angles would correspond to those shown in  FIG. 5  (where the swash angle is designated as angle β). 
     Instead of all of the nozzles shown in  FIG. 8(   d ) being used simultaneously, as described above, they may be staged, as explained in connection with  FIG. 7 . Also, the tilt and/or swash angles of individual nozzles or groups of nozzles in both the  FIG. 7  and  FIG. 8  arrangements (and in the variant of  FIG. 7)  may be varied, in order to create a particular mixing effect. Again, the individual angles are readily determinable by experiment and/or computer simulation. 
     A third embodiment of the invention is illustrated in three variants thereof in  FIGS. 9(   a )- 9 ( c ).  FIGS. 9(   a )- 9 ( c ) show a longitudinal section of an annular-type combustor  80 , in which a series of injectors is arranged around the dome of the combustor.  FIGS. 10(   a )- 10 ( d ) show a portion of the annulus in a radial section thereof, the portion including just two injectors in each case. 
       FIG. 9(   a ) uses as the injector a pre-filmer arrangement, in which each of the injectors is constituted by a nozzle  82  contained within a swirler  84 . Adjacent the swirler on a downstream side thereof is a prefilmer device  86 . The swirler and the prefilmer device are both annular in shape. In use, fuel is injected into the cylindrical space defined by the inside surface of the prefilmer device  86 . The conical fuel spray emerging from the nozzle  82  strikes the inside surface of the prefilmer device and continues as a modified conical shape into the combustor itself. Air flowing from the swirler  84  is directed into the fuel stream and mixes therewith, at the same time helping to create secondary atomization at the lip  88  of the prefilming device. 
     The whole of the injector arrangement, including the nozzle  82 , swirler  84  and prefilming device  86 , is inclined at an angle to the dome wall, as shown. This inclination also forms an angle δ with a line parallel to the longitudinal axis of the combustor annulus. The longitudinal axis is represented by the line  92 , which lies parallel to the line  90  passing through the radial mid-point of the annulus section (the distances are not shown to scale). Consequently, the fuel-air mixture is directed toward a part of the combustor, which will provide for an enhanced combustion. More specifically, the tilting shown as angle δ influences the recirculation zones around the area when the fuel is injected into the combustor from the nozzle  82  and prefilmer  86 . This, in turn, can provide increased stability or modulate any combustor-driven dynamics (including acoustic pulsations in the whole combustor system. Note also that the spray from the nozzle  82  and prefilmer  86  can be injected radially off-centre between the outer and inner combustion-chamber walls, instead of substantially on-centre, as shown. 
     The inclination of the injector assembly  82 - 88  corresponds to the positive tilt configuration shown in  FIG. 10(   a ). In this configuration the major axes  96  of the fuel cones  98  pass through the centre of the combustor annulus, which is the afore-mentioned longitudinal axis  92  of the combustor. Where necessary, in order to enhance the combustion process, the tilt angle may be negative, as shown in  FIG. 10(   b ). Again, the major axes  96  will pass through the longitudinal axis  92 . Where, as mentioned in the last paragraph, the fuel injection point is radially off-centre between the inner and outer walls of the annular combustion chamber, the tilt angle may be deliberately chosen so as to direct the spray cones more toward the radial centreline between the two combustion-chamber walls, in the manner of the first embodiment ( FIG. 4) . 
     In addition to using tilt, the swash described earlier in connection with the first embodiment may be employed. This is illustrated in  FIGS. 10(   c ) and  10 ( d ), in which it can be seen that the spray cones originate from a point (the nozzle exit point), which is offset from the longitudinal axes  99  of the respective swirlers, as in the manner of the first embodiment ( FIGS. 4 and 5) . In  FIG. 10(   c ) positive tilt is combined with positive swash (angle ε 1 ), while in  FIG. 10(   d ) positive tilt is combined with negative swash (angle ε 2 ), the swash angles being relative to the unswashed major axes  96 . 
     The spray profile in the case of positive swash ( FIG. 10(   c )) is shown in  FIG. 10(   e ).  FIG. 10(   e ) is an end-view corresponding to that of  FIG. 10(   c ), but from the opposite side. Thus, the nozzles themselves are shown as item  93 , while the spray profile is shown as item  95 . The distribution shown by dashed lines is the distribution applicable to the untilted and unswashed nozzles, whereas that shown by the solid lines applies to the tilted and swashed nozzles. The skewing of the distribution is clearly apparent, and can be beneficial in as much as it can adjust the combustor exit profile, i.e. the fuel-air mixing behaviour as viewed over the whole radial extent of the combustor at its output end. 
     The advantages of positive and negative swash in this embodiment are the same as those mentioned earlier in connection with the first embodiment and apply also to the second embodiment. 
     Two variants of the  FIG. 9(   a ) arrangement are illustrated in  FIGS. 9(   b ) and  9 ( c ). In  FIG. 9(   b ) the prefilmer-type injector is replaced by a so-called pressure-swirl injector arrangement  97 . An example of a pressure-swirl injector is depicted in  FIG. 11 . This pressure-swirl injector, which is derived from US 2006/0042254, filed in the name of Yoshida, Shouhei, et al., comprises a liquid-fuel nozzle  100  and a combustion burner  102 . The burner  102  includes a swirler  104 , an air nozzle  106 , a further swirler  108  and a guide ring  110 . The liquid-fuel nozzle  100  comprises a nozzle tip  112  in communication with a swirl chamber  114 , a nozzle cover  116 , a nozzle stay  118  and an outlet  120 . The dome end of the combustion chamber  80  in  FIG. 9(   b ) is shown in  FIG. 11  as region  122 . The operation of this particular injector is described in US 2006/0042254, which is incorporated herein by way of reference, and will not be gone into in detail here, except to say that the air nozzle  106  directs the air passing through it toward the axis of the liquid-fuel nozzle  100 , forming a space around the outlet  120 , through which liquid fuel is injected from the nozzle  100  into the combustion chamber  122 . This enables carbonaceous deposits on the surrounding surfaces of the liquid-fuel nozzle outlet to be suppressed regardless of the operating conditions of the combustor. 
     As with the  FIG. 9(   a ) case, the whole of the injector  97  is inclined by tilt and, where desired, swash with respect to the combustor  80 . 
     The second variant employs an air-blast injector arrangement, which is shown purely representationally in  FIG. 9(   c ) as item  130 .  FIGS. 12(   a ) and  12 ( b ) give an example of such an injector arrangement and are taken from U.S. Pat. No. 6,662,565, issued to Brundish, K. D., et al. The injector comprises a nozzle  130  having an inner swirler  132 , an inner fuel filmer  134 , an air filmer  136 , an outer swirler  138 , an outer fuel filmer  140  and an outermost swirler  142 . Fuel supply channels  144  and  146  supply fuel to the inner and outer fuel filmers, respectively. In use, the air passing through the swirlers interacts with the fuel to atomize the latter and provide two separate air-fuel flows into the combustor, to which the injector is mounted. In the illustrated example the two flows are main and pilot flows. Other designs are available, which cater for one or the other of these flows. 
     As with the  FIGS. 9(   a ) and  9 ( b ) arrangements, the variant shown in  FIG. 9(   c ) may employ swash as well as tilt. In addition, the fuel nozzles may either all be of the same type, or be of different types. 
     The use of tilt and, as desired, swash can be extended to an axially staged or radially staged annular combustor.  FIGS. 13 and 14  show an example of these. In  FIG. 13  a secondary fuel bank  150  is included along with the primary fuel bank  152 , which may take the form of any of the variants shown in  FIGS. 9(   a )- 9 ( c ) and is located at the dome end  156 . By tilting the primary injectors  152  toward the radially outer wall of the combustor in the manner shown, the ignition of the secondary fuel bank  150  can be enhanced, without the need for complex cross-firing tubes or special igniters. As with the  FIG. 9  embodiment, swash may also be employed along with tilt, in order to improve combustion. 
       FIG. 14  shows an annular combustor, in which two rows of injectors  154 ,  158  are disposed spaced radially apart on the dome part  156  of the combustor. The injectors  154  and  158  may be interleaved, as shown in  FIG. 1 , or they may be disposed directly opposite each other. Whichever configuration is used, the injectors of one row are inclined toward the injectors of the other row. This likewise helps to improve the cross-firing between the injector banks, as in the  FIG. 11  case. 
     In both the  FIG. 13  and  FIG. 14  arrangements, the injector banks  152 ,  154  and  156  are inclined as shown without requiring the dome end  156  of the combustor to be similarly inclined. Thus, these arrangements are similar to those shown in  FIGS. 9(   a )- 9 ( c ). Also similar to  FIGS. 9(   a )- 9 ( c ), the additional injector banks may all be of the same type, or of different types. 
     Tilt may also be used with the injectors of a silo-type combustor. A simplified representation of such a combustor is shown in  FIG. 15(   a ).  FIG. 15(   b ) is an end-view of the same combustor. The injectors  160 ,  162 , from which the fuel cones can be seen to emanate, are configured in two concentric rows—an outer row including injectors  160  and an inner row including injectors  162 . The injectors of one row are staggered with respect to the injectors of the other row, in order to promote the generation of recirculation zones (see arrows  164 ). The injectors are also tilted, cross-firing between the injectors being thereby improved. As in previous embodiments, the nozzles  160 ,  164  may also be rotated through a swash angle, provided the nozzles are offset from the longitudinal axes of their respective swirlers.