Abstract:
A fuel injector for a fuel spray nozzle of a gas turbine engine combustor is provided. The fuel injector has an annular flow passage which conveys fuel to a prefilming lip at an end of the flow passage. The fuel injector also has plurality of fuel distributor slots which are circumferentially spaced around and in fluid communication with the other end of the flow passage to deliver respective fuel streams into the flow passage. The slots are configured so that the fuel streams enter the flow passage at a swirl angle of at least 80° relative to the axis of the flow passage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is entitled to the benefit of British Patent Application No. GB 0820560.1, filed on Nov. 11, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a fuel injector for a fuel spray nozzle of a gas turbine engine combustor. 
     BACKGROUND OF THE INVENTION 
     Fuel injection systems deliver fuel to the combustion chamber of an engine, where the fuel is mixed with air before combustion. One form of fuel injection system known in the art is a fuel spray nozzle. Fuel spray nozzles atomise the fuel to ensure its rapid evaporation and burning when mixed with air. 
     An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray. 
     Efficient mixing of air and fuel results in higher combustion rates. It also reduces unburnt hydrocarbons and exhaust smoke (which result from incompletely combusted fuel) emitted from the combustion chamber. 
     Additionally, “lean burn combustion” is being developed as a way of operating at relatively low flame temperatures. The lower temperatures significantly reduce NOx emissions, but can necessitate the use of a pilot and mains fuel nozzle to avoid lean extinction at low engine powers. 
       FIG. 1  shows a schematic view of a fuel injection nozzle  10  which, in use, would be mounted on the upstream wall of a combustion chamber  100 . 
     The fuel injection nozzle  10  has a central axis  11 , and is in general circularly symmetrical about this axis. A pilot fuel injector  12  is centred on the axis, and is surrounded by a pilot swirler  13 . A mains airblast fuel injector  14  is concentrically located about the pilot fuel injector  12 , with inner and outer mains swirlers  15  and  16  positioned radially inward and outward thereof. 
     The mains airblast fuel injector has an annular flow passage or gallery  17 . Circumferentially spaced fuel distributor slots  19  deliver fuel to the fore end of the gallery. The fuel is then conveyed along the gallery to a prefilming lip  18  formed at the aft end of the gallery. An annular film of liquid fuel forms on the lip, and is entrained in and atomised by the much more rapidly moving and swirling air streams produced by inner mains swirler  15  and outer mains swirler  16 . 
     To achieve lean burn, the system not only incorporates pilot and mains fuel injectors, but also requires a relatively large amount of combustion air. To realise the low combustion temperatures the fuel must be well mixed with the air prior to combustion, hence creating uniform low flame temperatures. Non-uniform mixing prior to combustion can result in locally high combustion temperatures, and hence no reduction in NOx emissions. Low combustion efficiency in the lower temperature areas increases the engine&#39;s specific fuel consumption, and emissions of carbon monoxide and unburnt fuel. 
     Thus, it is desirable to improve the design of fuel injectors to achieve more uniform fuel-air mixing. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a fuel injector for a fuel spray nozzle of a gas turbine engine combustor, the fuel injector having: 
     an annular flow passage (or gallery) which conveys fuel to a prefilming lip at an end of the flow passage, and 
     a plurality of fuel distributor slots which are circumferentially spaced around and in fluid communication with the other end of the flow passage to deliver respective fuel streams into the flow passage; 
     wherein the slots are configured so that the fuel streams enter the flow passage at a swirl angle of at least 80° relative to the axis of the flow passage. 
     By “swirl angle” is meant the angle between the axis of the flow passage (which is typically coincident with the central axis of a fuel spray nozzle, of which the fuel injector is an element) and the direction of flow of a fuel stream as it enters the flow passage. 
     Advantageously, by swirling the fuel streams at a high swirl angle, the fuel streams can be merged earlier in the flow passage, producing a more circumferentially uniform fuel mass flow rate from the passage onto the prefilming lip. Indeed, preferably, the flow passage is configured so that the fuel streams merge in the flow passage to provide a circumferentially substantially uniform fuel mass flow at the prefilming lip. 
     A further advantage of the high swirl angle is that a shortened flow passage can be adopted, allowing a more compact and lighter fuel injector to be produced. 
     Preferably, in the circumferential direction, the ratio of the slot pitch (i.e. the distance between the centres of neighbouring slots) to the slot width at the narrowest point of a slot is at most 40. Preferably the ratio is at least 5, and more preferably at least 20. 
     Preferably, the ratio of the annular flow passage length in the axial direction to the slot width in the circumferential direction at the narrowest point of a slot is at most 20, and more preferably at most 10 or 3. 
     Preferably, the fuel distributor slots open to an upstream wall of the annular flow passage, the slots being further configured so that on entry into the flow passage the fuel streams retain contact with the upstream wall. Typically, the upstream wall is perpendicular to the axis of the flow passage. In this case, by retaining contact with the wall, at least the edges of the fuel streams have 90° swirl angles. However, other arrangements are possible. For example, the upstream wall may have a serrated, rippled or saw-tooth profile in the circumferential direction such that portions of the wall at the exits of the slots are at an angle of less than 90° (but at least 80°) to the axis of the flow passage, whereby the fuel streams can enter the flow passage at a corresponding swirl angle and still retain contact with the wall. 
     By keeping the fuel streams in contact with the upstream wall of the flow passage, rapid merging of the flow streams can be achieved. Further, two phase flow in the passage can be reduced or eliminated. 
     To retain contact between the fuel streams and the upstream wall of the flow passage, each slot may have: 
     a first section in which a pressure surface and an opposing suction surface constrain the respective flow stream to flow at a predetermined angle relative to the axis of the flow passage, and 
     a second section in which the suction surface is blended to said upstream wall so that the Coand{hacek over (a)} effect causes the respective flow stream to retain contact with the upstream wall. 
     The predetermined angle may be at least 70°. The predetermined angle may be at most 85°. 
     Preferably, the pressure surface is absent from the second section. This can help to discourage expansion of the fuel stream, which might otherwise tend to counter the Coand{hacek over (a)} effect. 
     The flow passage may be a cylindrical annulus. Alternatively, the flow passage may be a frustoconical annulus which expands from the fuel distributor slots to the prefilming lip. Configuring the fuel distributor slots, so that the fuel streams merge early in the flow passage, allows relatively simple passage geometries to be adopted. Advantageously, such geometries can allow fuel to drain fully from the passage when the flow of fuel is stopped. This helps to prevent trapped fuel coking in and blocking the passage when the main fuel is stopped (staged) below full engine power and the engine operates with pilot fuel only. 
     Preferably, the fuel injector is an airblast fuel injector. 
     A further aspect of the invention provides a fuel spray nozzle having the fuel injector according to the previous aspect. For example, the fuel injector may be a mains fuel injector, with the nozzle further having a radially inwards pilot fuel injector. 
     A further aspect of the invention provides a gas turbine engine combustor having the fuel spray nozzle of the previous aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic longitudinal cross-sectional view of a fuel injection nozzle; 
         FIG. 2  shows the fuel stream as predicted by computational fluid dynamics (CFD) for a 20° sector of the gallery of the mains injector of a nozzle such as that shown in  FIG. 1 , the gallery having at its fore end the outlet of one of eighteen equally circumferentially spaced fuel distributor slots; 
         FIG. 3  shows non-uniform fuel spray from a prefilming lip of a mains injector; 
         FIG. 4  shows the fuel stream predicted by CFD for a modified gallery relative to that of  FIG. 2 , the modified gallery having a change of direction forcing the fuel stream to impinge on a wall of the gallery; 
         FIG. 5  shows the calculated divergence angle between the two sides of a fuel stream required to cause adjacent streams to meet at the exit from a gallery of a given axial length plotted against the swirl angle of the fuel stream; 
         FIG. 6  is a schematic plan view of a typical conventional fuel distributor slot; 
         FIG. 7  shows longitudinal cross-sections through the bottom parts of mains fuel injectors having respectively (a) a parallel-walled cylindrical gallery and (b) an expanding frustoconical gallery; 
         FIG. 8  is a schematic plan view of a fuel distributor slot having a geometry for producing 90° swirl; and 
         FIG. 9  is a schematic plan view of a fuel distributor slot having a geometry for producing less than 90° swirl. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before discussing the invention it is helpful to provide more detail of other fuel injector arrangements. 
     The mains fuel injector of a pilot and mains fuel nozzle passes typically 85% of the fuel and air, and is thus the dominant emissions source. In a fuel injection nozzle such as that shown in  FIG. 1 , a relatively large diameter mains fuel prefilming lip, and correspondingly large annular flow passage (gallery), is generally needed to deliver such a high percentage of the fuel and air. The large diameter can result in a correspondingly wide spacing of the fuel distributer slots which deliver fuel to the fore end of the gallery. For example, the fuel slot pitch to width ratio in the circumferential direction may be 30:1. In the gallery, the fuel streams delivered by the distributor slots spread sideways. Desirably, the spread should be enough to fill the annulus circumferentially, and hence create a circumferentially uniform mass flow rate onto the prefilming lip, as required for low emissions. 
       FIG. 2  shows the fuel stream spread as predicted by computational fluid dynamics (CFD) for a 20° sector of a gallery  17  having at its fore end the outlet of one of eighteen equally circumferentially spaced fuel distributor slots  19 . Within the gallery there is two phase flow of fuel and air. The fuel stream  20  spreads with a divergence of about 2° at either side. However, by the aft end of the gallery, due to the wide spacing of the slots around the gallery, the streams have not spread sufficiently to fill the gallery.  FIG. 3  shows the non-uniform fuel spray from the prefilming lip which undesirably results. 
     One option is to modify the shape of the gallery to encourage better circumferential spread of the fuel streams.  FIG. 4  shows the fuel stream predicted by CFD for a modified gallery which has a change of direction forcing the stream  20  to impinge on a wall of the gallery. The impingement causes the stream to spread further than in the unmodified gallery of  FIG. 2 . However, a uniform circumferential mass flow rate at the gallery exit is still not achieved. 
     Possible further modifications to achieve uniform circumferential mass flow are (a) to lengthen the gallery between the fuel distributor slots and the prefilming lip and (b) to adopt a more complicated gallery geometry. However, these add cost, size and weight. 
     Further, as a result of engine staging operations the mains fuel is not always flowing. That is, to achieve high combustion efficiencies, the nozzle sometimes flows fuel through the pilot injector only. In this case, the fuel in the mains gallery should drain away completely to prevent stagnant fuel thermally degrading in the gallery and forming coke. Successive mains staging events (which can occur many times per flight) can cause such coke deposits to grow, until eventually the gallery may become partially or completely blocked. As incomplete mains fuel draining tends to occur in more complicated gallery geometries, this mitigates against the adoption of such geometries. Stagnant mains fuel upstream of the gallery remains cooler due to the closer proximity of pilot fuel passages, and coking is therefore not such a problem in these locations. 
     The two phase flow in the mains gallery illustrated in  FIGS. 2 and 4 , even if eliminated by the time the fuel reaches the prefilming lip, can itself lead to fuel coking. This is because the gallery walls are only cooled by the mains fuel. Consequently those portions of the walls that are not wetted by the main fuel will be hotter than the wetted portions. In some circumstances, the wall temperature at the edge of a fuel stream can be high enough to break the fuel down to coke, and hence gradually block the gallery. 
     Thus, according to the present invention, a different approach is taken to encourage the fuel streams in the mains gallery to provide a uniform circumferential mass flow rate at the gallery exit. Trigonometric calculations using a typical fuel gallery geometry show that, for a gallery and fuel slot arrangement as shown in  FIG. 2 , in which each fuel stream diverges by about 2° at either side, swirling the fuel streams by 80° degrees or more can cause the streams to meet at the gallery exit. For example,  FIG. 5  shows the calculated divergence angle between each side of the fuel stream required to cause the streams to meet at the exit from the gallery plotted against the swirl angle of the fuel stream produced by the distributor slot. One plot in  FIG. 5  is for a set of calculations in which there are eight equally spaced slots, and the other plot is for a set of calculations in which there are twelve equally spaced slots. In both cases, however, the calculations show that a swirl angle of about 80° degrees or more is needed to cause the streams to meet. In contrast, typical conventional fuel distributor slots, as illustrated in  FIG. 6 , produce swirl angles of only about 30° degrees or 60° degrees. The dashed arrow indicates the direction of the fuel stream flowing from the slot into the gallery. The swirl angle is indicated θ. 
     Although, generating a higher swirl angle can cause the fuel streams to meet in the gallery, which is an improvement over the fuel flows illustrated in  FIGS. 2 and 4 , there may still be significant variation in fuel mass flow rate between the centrelines of the streams and the edges of the streams. Also it is desirable to eliminate two phase flow early in the gallery. Thus preferably 90° of swirl is generated in at least part of each flow stream to encourage the fuel streams to meet as early as possible in the gallery. 
     90° swirl allows the individual streams to merge early and flow together for a significant distance in the gallery, allowing the fuel mass flow rate to become circumferentially uniform by the time it reaches the gallery exit, and hence to provide a circumferentially uniform mass flow onto the prefilming lip. 90° swirl can also eliminate two phase flow and hence the hot walls that can cause fuel coking. It also does not require a complex geometry for the gallery. Indeed, only a relatively short gallery may be needed, as shown in  FIGS. 7(   a ) and ( b ), which are longitudinal cross-sections through the bottom parts of respective mains fuel injectors. In  FIG. 7(   a ), fuel distributor slot  29  outlets to a parallel-walled cylindrical gallery  30 . In  FIG. 7(   b ), fuel distributor slot  29  outlets to an expanding frustoconical gallery  30 . Such galleries can completely eliminate the coking of trapped fuel during staging. 
     A fuel distributor slot  29  having a geometry for producing 90° swirl is shown in  FIG. 8 . The slot has a pressure surface  31  and a suction surface  32 . At the inlet to the slot the pressure surface makes an angle of typically between 70° and 85° relative to the axial direction of the fuel nozzle. This angle is maintained by the pressure surface into a central section of the slot. At the inlet to the slot, the suction surface has a radius R 1 . Following that, in the central section, the suction surface adopts the same angle to the axial direction of the slot as the pressure surface, i.e. the central section is parallel-walled. The radius R 1  helps prevent flow separation at the inlet, while the parallel-walled central section promotes a uniform flow velocity at a predetermined angle within the slot parallel to the pressure and suction surfaces. The length of the parallel-walled central section is typically between one and three times the slot width in that section. 
     The following section of the slot  29  provides an outlet to the gallery  30  at the upstream wall  33  of the gallery. At the outlet, the pressure surface  31  has a relatively small radius R 2 . The suction surface  32 , on the other hand, has a radius R 3  which blends to the upstream wall over a significantly longer distance. The uniform flow velocity produced by the central section of the slot encourages adherence of the flow to the radius R 3  of the suction surface. Further, the flow adheres to the radius R 3  by the Coand{hacek over (a)} effect, and hence as the suction surface blends to the upstream wall the edge of the fuel stream contacting the wall achieves 90° of swirl. 
     To encourage the fuel stream to retain contact with the upstream wall  33 , the pressure surface  31  does not extend to oppose R 3 . Further R 3  should be sufficiently large. Thus the pressure surface has a relatively small blend radius R 2  to the upstream wall. Indeed, the radius R 2  could be replaced by a square end that achieves a similar length reduction in the pressure surface. Preferably, R 3  starts on the suction surface  32  at at least 0.5 slot widths downstream of the end of the pressure surface to ensure that the fuel flow is not diffusing (expanding) when it starts to flow around R 3 , as such diffusion would oppose the flow adhering to R 3 . 
     With at least the edge of the fuel stream exhibiting 90° of swirl into the gallery, there is rapid convergence of the fuel streams and a relatively uniform circumferential fuel flow rate at the gallery exit to the prefilming lip. Indeed, it may be possible to reduce the length of the gallery while maintaining the uniform flow. This simplifies manufacture of the injector, and promotes complete drainage of the gallery when the flow of mains fuel is staged. 
       FIG. 9  is a schematic plan view of a fuel distributor slot having a geometry for producing less than 90° swirl. The same reference numbers indicate features equivalent to those indicated in  FIG. 8 . In the geometry of  FIG. 9 , the upstream wall  33  of the gallery has a serrated, rippled or saw-tooth profile in the circumferential direction. The suction surface  32  blends to a portion of upstream wall which is angled at less than 90° (but at least) 80° to the axis of the gallery. However, the large size of blend radius R 3  still causes the flow to adhere to the radius R 3  by the Coand{hacek over (a)} effect and thence to the upstream wall  33 . 
     Thus, the edge of the fuel stream exhibits less 90° of swirl into the gallery. However the spreading of the stream can still cause it to converge with adjacent streams to provide relatively uniform circumferential fuel flow. 
     To summarize, the 90° of swirl at the fuel distributor slot exit can achieve the following: 
     elimination of two phase flow in the uncooled gallery. Development of regions of stagnant air in the gallery and corresponding high gallery wall temperatures can thus be avoided, which in turn prevents coking of fuel on the hot walls. 
     circumferentially uniform fuel mass flow exiting the gallery onto the prefilming lip, which reduces emissions in lean burn combustors. 
     circumferentially uniform fuel mass at a relatively short distance from the outlets of the distributor slots, which allows the gallery to be shortened, facilitating a compact and light mains injector. 
     allows adoption of a simple gallery geometry that does not trap fuel when the mains fuel stops flowing. This eliminates gallery blockage due to coking of trapped fuel after mains staging events, thereby maintaining combustion efficiency during engine operation. 
     While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention as claimed.