Abstract:
A fuel injector comprising: a prefilmer; a plurality of discrete fuel sources each arranged to supply fuel to a surface of the prefilmer; wherein the prefilmer comprises circumferential dispersion structure which, in use, spreads the fuel in a circumferential direction as it passes from an impingement point on the surface of the prefilmer to a downstream edge of the prefilmer.

Description:
The present invention relates to a fuel injector, and particularly but not exclusively to a fuel injector having a prefilmer which provides a uniform circumferential fuel distribution. 
     BACKGROUND 
       FIGS. 1 and 2  show a conventional fuel injector  2 . The injector  2  comprises a pilot injector  4  and a pilot swirler  6  for swirling air past the pilot injector  4 . A main injector  8  is concentrically positioned around the pilot injector  4  and the pilot swirler  6 . An inner main swirler  10  and an outer main swirler  12  are disposed on concentrically inner and outer sides of the main injector  8 . 
     An inner annular member  14  is located between the pilot swirler  6  and the inner main swirler  10 . Similarly, an outer annular member  16  is located between the inner main swirler  10  and the outer main swirler  12 . 
     The main injector  8  comprises a plurality of discrete fuel sources (not shown) which are spaced around the circumference of an outer surface of the inner annular member  14 . As indicated by the dashed lines, the fuel sources direct jets of fuel towards an inner surface of the outer annular member  16 , which forms a prefilmer  18 . Alternatively, the fuel may be placed on the prefilmer  18  using a series of discrete slots located around the circumference of the prefilmer  18 . 
     The fuel flows over the surface of the prefilmer  18  prior to being shed from a downstream edge  20  into the swirling airflows. This allows effective atomisation of the fuel. 
     In an alternative arrangement, the fuel may be supplied to the prefilmer using an annular gallery. Such a gallery supplies a circumferential (i.e. non-discrete) film of fuel onto the prefilmer, and thus creates a uniform circumferential distribution of fuel. 
     In certain applications, it is desirable to use an injector comprising discrete fuel sources as described above. In order to obtain a circumferential distribution comparable to that provided by an annular gallery, it is desirable to use a larger number of discrete jets. However, there is a limit on the minimum jet hole size in order to prevent blockage from debris and fuel cracking (oxidative coking). Consequently, this limits the number of jets which can fit around the circumference of the injector and also limits the uniformity of the circumferential distribution of the fuel film on the prefilmer. 
     Accordingly, the present invention seeks to provide a discrete fuel source-type injector which has a more uniform circumferential fuel distribution. 
     STATEMENTS OF INVENTION 
     In accordance with an aspect of the invention, there is provided a fuel injector comprising: a prefilmer; a plurality of discrete fuel sources each arranged to supply fuel to a surface of the prefilmer; wherein the prefilmer comprises a circumferential dispersion structure which, in use, spreads the fuel in a circumferential direction as it passes from an impingement point on the surface of the prefilmer to a downstream edge of the prefilmer. 
     The present invention may provide a more uniform fuel distribution at the downstream edge of the prefilmer. 
     This may allow the fuel injector to use a smaller number of discrete fuel sources. Consequently, the construction of the fuel injector may be simpler resulting in reduced manufacturing cost. Furthermore, the fuel injector may be more reliable since there are fewer fuel sources which may become blocked. In addition, using fewer fuel sources may allow the sources to be located at a lower radius. This may reduce the heat load to the fuel wetted transport passages and reduce the risk of coking. 
     Alternatively or in addition, the improved fuel distribution may allow the prefilmer to be made shorter. This may therefore lead to the fuel injector and surrounding components being shorter, lighter and cheaper to manufacture. 
     The circumferential dispersion structure may comprise one or more surface formations. 
     The circumferential dispersion structure may comprise a plurality of radially convex portions (i.e. ribs) spaced around the circumference of the prefilmer and separated from one another by a plurality of troughs (i.e. flutes). 
     Each discrete fuel source may be arranged so that the impingement point on the surface of the prefilmer is located at a peak of one of the convex portions. 
     The convex portions and troughs may extend from the impingement point to the downstream edge. 
     The convex portions and troughs may taper such that the cross-section of the prefilmer approaches circular towards the downstream edge of the prefilmer. 
     The cross-section of the prefilmer at the downstream edge may be circular. 
     The circumferential dispersion structure may comprise a plurality of protruding walls (i.e. ribs) or recessed channels (i.e. flutes) which channel the fuel toward a circumferential direction. 
     Each protruding wall or recessed channel may form a U-shaped profile or a V-shaped profile. 
     The impingement point may be located at the centre of the U-shaped profile or the V-shaped profile. 
     The plurality of protruding walls or recessed channels may be grouped together in sets of protruding walls or recessed channels, with each set comprising a plurality of protruding walls or recessed channels fanning from the impingement point. 
     The circumferential dispersion structure may be asymmetric. 
     The discrete fuel sources may be fuel supply slots or fuel supply jets. 
     The discrete fuel source may form a pilot injector or a main injector. 
     The fuel injector may be used in a gas turbine engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example, to the following drawings, in which: 
         FIG. 1  is a cross-sectional view of a conventional fuel injector in an axial direction; 
         FIG. 2  is a cross-sectional view of the fuel injector of  FIG. 1  in a radial direction; 
         FIG. 3  is a cross-sectional view of a fuel injector in accordance with an embodiment of the invention in an axial direction; 
         FIG. 4  is a cross-sectional view of the fuel injector of  FIG. 3  in a radial direction; 
         FIG. 5  is a developed view of a prefilmer in accordance with another embodiment of the invention; and 
         FIG. 6  is a developed view of a prefilmer in accordance with another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 3 and 4 , a fuel injector  102  in accordance with an embodiment of the invention comprises a pilot injector  104  and a pilot swirler  106  for swirling air past the pilot injector  104 . A main injector  108  is concentrically positioned around the pilot injector  4  and the pilot swirler  106 . An inner main swirler  110  and an outer main swirler  112  are disposed on concentrically inner and outer sides of the main injector  108 . 
     An inner annular member  114  is located between the pilot swirler  6  and the inner main swirler  110 . Similarly, an outer annular member  116  is located between the inner main swirler  110  and the outer main swirler  112 . 
     The main injector  108  comprises a plurality of discrete fuel sources which are spaced around the circumference of an outer surface of the inner annular member  114  (not shown). As indicated by the dashed lines, the fuel sources direct jets of fuel towards an inner surface of the outer annular member  116 , which forms a prefilmer  118 . 
     The fuel flows over the surface of the prefilmer  118  prior to being shed from a downstream edge  120  into the swirling airflows. This allows effective atomisation of the fuel. 
     As shown in  FIG. 4 , the prefilmer  118  has a generally cylindrical cross-section defined by a plurality of radially convex portions  122  separated from one another by a plurality of troughs  124 . This profiled shape of the prefilmer  118  acts as a circumferential dispersion structure, as will be described in more detail below. 
     The discrete fuel sources are arranged such that the jets of fuel contact the prefilmer  118  at peaks of the convex portions  122 , as indicated by impingement point  126 . Accordingly, the convex portions  122  cause the fuel to be dispersed from the impingement point  126  in a circumferential direction towards the adjacent troughs  124 . The convex portions  122  therefore create a more uniform circumferential fuel distribution at a downstream edge  120  of the prefilmer  118 . 
     The cross-section of  FIG. 4  is taken through an upstream portion of the prefilmer  118  at or adjacent to the impingement point  126 . The convex portions  122  and troughs  124  may extend from the upstream portion to the downstream edge  120 . Alternatively, the convex portions  122  and troughs  124  may taper such that the cross-section of the prefilmer  118  transitions to circular towards the downstream edge  120 , with the cross-section of the prefilmer  118  being circular at the downstream edge  120 . 
       FIG. 5  shows another embodiment of a prefilmer  218  which uses an alternative circumferential dispersion structure. 
     In this embodiment the circumferential dispersion structure comprises a plurality of walls or channels  228  which channel the fuel in a circumferential direction. Where a plurality of walls are used, these protrude from the surface of the prefilmer  218  (as shown in cross-section (i) of  FIG. 5 ). On the other hand, where a plurality of channels are used, these are recessed into the body of the prefilmer  218  and thus lie below the surface of the prefilmer  218  (as shown in cross-section (ii) of  FIG. 5 ). 
     The plurality of walls or channels  228  are grouped together in sets, with each set comprising a plurality of walls or channels  228  fanning from (or a point adjacent to) the impingement point  226  on the surface of the prefilmer  218 . In other words, in each set the walls or channels  228  have ends which are collocated at a point, and which extend from this point towards the downstream edge  220  at different angles. 
     Accordingly, the fuel enters channels formed between adjacent walls  228  or the channels  228  themselves at the impingement point  226 . The fuel is directed by the walls or channels  228  in order to disperse the fuel in the circumferential direction as it passes over the prefilmer  218  to the downstream edge  220 . At the downstream edge  220 , the fuel has been dispersed to create a more uniform circumferential fuel distribution, thus occupying the voids between adjacent fuel jets. 
       FIG. 6  shows another embodiment of a prefilmer  318  which uses walls or channels  328  as a circumferential dispersion structure. 
     In this embodiment a plurality of U-shaped walls or channels  328  are provided on the surface of the prefilmer  318 . Again, where a plurality of walls are used, these protrude from the surface of the prefilmer  318  (as shown in cross-section (i) of  FIG. 6 ), and where a plurality of channels are used, these are recessed into the body of the prefilmer  318  and thus lie below the surface of the prefilmer  318  (as shown in cross-section (ii) of  FIG. 6 ). The walls or channels  328  are arranged such that the base of the U-shape is toward the downstream side of the prefilmer  318 . 
     The impingement point  326  of each fuel jet is located at the centre of one of the U-shaped walls or channels  328 . Accordingly, the wall or channel  328  directs the fuel away from the impingement point  326  so as to disperse the fuel in the circumferential direction as it passes over the prefilmer  318  to the downstream edge  320 . At the downstream edge  320 , the fuel has been dispersed to create a more uniform circumferential fuel distribution, thus occupying the voids between adjacent fuel jets. 
     Although the walls or channels  328  have been described as being U-shaped, they could alternatively have a V-shaped profile or other shape which disperses the fuel in a circumferential direction. 
     The present invention may alternatively employ a series of discrete slots located around the circumference of the prefilmer  118 ,  218 ,  318  to place fuel onto the surface of the prefilmer  118 ,  218 ,  318 . Accordingly, the term “impingement point” may have width, but the fuel sources still provide discrete supplies of fuel to the circumferential dispersion structure. 
     Although shown as being symmetrical, the circumferential dispersion structure provided by the convex portions  122  and troughs  124 , and walls or channels  228 ,  328  may alternatively be asymmetric in order to allow fuel impingement on the prefilmer with a swirl angle. 
     Although the invention has been described with reference to a prefilmer for a main injector, it could also be applied to a prefilmer for a pilot injector.