Patent Publication Number: US-6216466-B1

Title: Fuel-injection arrangement for a gas turbine combustor

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a fuel-injection arrangement for a combustor of a gasturbine engine, and in particular a fuel-injection arrangement enabling reliable performance at low load conditions of said engine. 
     Provision is made in gas turbine engines to inject fuel into a region upstream of the main combustor region of the engine for mixing with air and eventual burning in the main combustor region. 
     FIG. 1 shows part of a gas-turbine engine comprising a combustion chamber  10 , a fuel-inlet head  12  and a radial swirler  14  disposed therebetween. The swirler  14 , which is commonly used in gas turbine engines as a mixing device to mix fuel and air for supply to the combustion chamber, is configured as illustrated in FIGS. 2 a  and  2   b  and comprises a series of vanes  16  equally spaced around a circumference of the swirler, the vanes forming a corresponding series of passageways  18  for the flow of mixing air  20  through the swirler from a radially outer to a radially inner region thereof. 
     The vanes are shaped and disposed such as to impart to the incoming air a tangential component, whereby the air is caused to “swirl” around the longitudinal axis  22  of the swirler, the air also being caused to exit the swirler at a downstream region thereof and enter the combustion chamber  10  (see arrows  21 ). 
     Along the trailing-edge region  24  of the vanes  16 —i.e. trailing-edge in terms of air flow through the vane arrangement—are conventionally disposed a series of fuel outlets  26  fed from a fuel inlet conduit  28  connected to the fuel head  12 . The outlets or holes  26  are of uniform diameter and are evenly spaced axially along the trailing edge. Use of such holes evenly spaced along at least most of the length of the trailing edge promotes better mixing of fuel and air by making for a uniform distribution of the fuel along the axial length of the swirler. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a fuel injection arrangement for a gas turbine combustor, comprising at least one series of fuel-injection outlets arranged in spaced-apart relationship, referred to a longitudinal axis of said combustor, in a pre-chamber region of said combustor upstream of a main-chamber region thereof, said series of outlets being such as to provide, in use, a longitudinal variation in a radial component of momentum of fuel jets exiting said outlets. The variation in radial component of momentum preferably takes the form of a variation in a radial component of velocity, which may achieved by arranging for the outlets in the series to be of varying size. 
     The outlets may be smallest in an axially upstream portion of said pre-chamber region and the variation in outlet size in said series may be monotonic referred to said longitudinal axis. 
     Said variation may be a continuous variation or alternatively a stepped variation. It may be linear over at least a part of said series of outlets. 
     The outlets, which may be substantially equally spaced, may be configured such that a direction of fuel jets exiting said outlets is substantially radial. 
     The outlets may be disposed in a swirler portion of said pre-chamber region, and/or they may be disposed in an intermediate portion of said pre-chamber region between a swirler portion thereof and said main-chamber region. In the former case, where said swirler portion comprises a plurality of vanes, said series of outlets may be incorporated into each of at least some of said vanes at a trailing edge thereof. In the latter case, the outlets may be disposed in a wall of said intermediate portion. Alternatively, the outlets may be provided in fuel posts situated in said pre-chamber region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example only, with reference to the drawings, of which: 
     FIG. 1 is a sectional view of part of a gas-turbine engine incorporating a conventional swirler according to the prior art; 
     FIGS. 2 a  and  2   b  shows the swirler of FIG. 1 in both side- and end-elevations according to the prior art; 
     FIG. 3 is a view of a gas-turbine engine corresponding to that of FIG.  1  and showing a dynamic aspect of the fuel-air mixture inside the swirler according to the prior art; 
     FIGS.  4 ( a ),  4 ( b ) and  4 ( c ) are side views of the swirler showing a velocity profile for the fuel-air mixture at upstream-end, two-thirds from upstream-end and downstream-end axial points, respectively, of the swirler; 
     FIGS.  5 ( a ) and  5 ( b ) show two alternative fuel-outlet size distribution profiles for he swirler of the present invention; 
     FIG. 6 shows an embodiment of the swirler according to the invention in which fuel is supplied to the swirler by way of fuel posts, 
     FIG. 7 is an end-view of the swirler according to the invention including radially oriented fuel outlets, and 
     FIG. 8 is a partial view of FIG. 3 showing the use of the variable-sized outlets according to the invention in an intermediate portion of a pre-chamber region of the combustion chamber. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     The operation of the swirler according to the invention is now explained with reference to FIG.  3 . In FIG. 3, which shows the same engine arrangement as in FIG.  1  and includes a prior-art swirler, it can be seen that, in operation, in a radially central region of the swirler  14  there is a body of fuel and air  23  rotating around the swirler axis  22  moving in a direction away from the swirler and toward the combustion chamber  10 . This rotating body can be likened to a spinning tube with an effective tube wall consisting of an air/fuel mixture and having a thickness “T” and turning in corkscrew fashion. In this central region of the swirler three airflow velocity components can be identified: an axial component (U) pointing in a direction parallel to the swirler axis  22 , a radial component (V) normal to the swirler axis  22 , and a tangential component (W) about the swirler axis  22 . 
     In a gas turbine combustor of the type shown in FIGS. 1 and 3, the combustion flame has an upstream flame face in the region of the swirler back-face  30  and a downstream flame face in or towards the combustion chamber facing the swirler. As engine load decreases and with less fuel supplied, the downstream flame face withdraws progressively to the upstream face so that at minimum operating load (or on engine starting) there exists only a small pilot flame which is located in the swirler region. Typically, the upstream flame-face zone is a fuel-weak region and without some means of fuel supplementation to this region the pilot flame would tend to extinguish at low-load settings. This is because in a fuel-weak mixture the flame spreads to find fuel and in so doing is weakened, to the point at which extinction of the flame occurs—so-called “weak extinction”. One reason for the region being fuel-weak is that the aforementioned tube wall acts as a barrier to the incoming fuel-air mixture from the swirler. Furthermore, inside the so-called tube is a counter-flowing mass of partly burnt (and therefore fuel-weak) combustion gases drawn from the combustion chamber. 
     One known way of supplementing the provision of fuel to the pilot flame under these circumstances is to inject fuel directly into the region from a fuel injector means situated at the back-face of the swirler. Such a method is generally effective in sustaining a flame at low-load settings, but has the drawback of adding to the overall constructional complexity of the combustor assembly. 
     The present invention provides a swirler which enhances the radial momentum of the fuel jets leaving the fuel outlets in the afore-mentioned fuel-weak region at the upstream end of the swirler. This has the effect of enabling the fuel jets at that part of the swirler to penetrate through the “tube” wall, thereby to supplement the fuel supply to the pilot flame within the “tube”, thus maintaining the stability of the flame at low load settings without the need for supplementary fuel provision. 
     The preferred way of increasing radial momentum according to the invention is to increase the radial velocity of the fuel jets- This enhancement of radial-velocity component reinforces an existing velocity characteristic of the swirler which can be seen by reference to FIG.  4 . In FIG.  4 ( a ) a typical profile graph of velocity components as a function of radial distance from the swirler axis for the fuel-air mixture exiting the swirler at an axial position adjacent the swirler back-face  30  is shown. It can be seen that the radial component is the largest component at this point and the axial component the weakest. By contrast, at the downstream face of the swirler (see FIG.  4 ( c )) the radial velocity component is the weakest and the tangential component is the strongest. At an intermediate position, e.g. two-thirds of the way from the upstream end-face  30  (FIG.  4 ( b )), the tangential component is already well established and the radial component is not significantly greater than in the downstream-end case shown in FIG.  4 ( c ). 
     For the jets of fuel nearest the pilot flame to actually reach the flame, they must penetrate through the “tube” wall and must therefore have sufficient radial momentum. It is of benefit that the radial velocity of the airflow is already greatest in this area, but it is not strong enough by itself to carry fuel through to the flame. Even when the additional radial momentum given by the fuel jets is taken into account, there is not sufficient energy to breach the wall if the conventional swirler design is used. 
     The invention takes the step of sizing the holes nearest the upstream end  30  smaller than those in the mid- and end-region, which increases the velocity of the fueljet passing through those holes. This increase in velocity produces a corresponding increase in the momentum flux ratio, which is defined as: 
     
       
         Momentum flux ratio=ρ F V F   2 ρ A V A   2    
       
     
     where 
     ρ F  is fuel density 
     V F  is fuel velocity 
     ρ A  is air-wall density 
     V A  is air-wall velocity. 
     The fuel-jet holes are reduced to a size giving a value of V F  sufficient to yield a momentum flux ratio of greater than unity, which will then ensure penetration of the fuel through the wall. The hole size required varies according to wall density and will therefore be different for each engine combustor configuration. The hole size may be obtained by application of the following formula: 
     
       
         d F =k y max . (Momentum flux ratio) ½   
       
     
     where 
     d F  is the diameter of the fuel jet, 
     y max  is maximum fuel-jet penetration required, and 
     k is a constant. 
     The constant k is arrived at empirically by making incremental adjustments to an actual system, and for a typical system might lie in the region of 1.25. 
     The size of the holes varies progressively over the length of the trailing edge of the vane, the distribution being either continuous, i.e. each hole along the edge being larger than the previous one, or stepped, i.e. hole size varies in discrete jumps. These two cases are illustrated in FIGS.  5 ( a ) and  5 ( b ), respectively. In the case of FIG.  5 ( b ) three small holes  32  are shown on the lefthand side of the diagram, likewise three holes  34  of an intermediate size, and finally two large holes  36 . By contrast, in FIG.  5 ( a ) all holes  38  are of different diameters. It goes without saying that these representations are exemplary only, and the numbers of holes and their distribution will vary considerably in practice and depending on the application. 
     Whereas it has been assumed in the description of the invention so far that fuel will be introduced into the vanes themselves, so that the fuel outlets are holes formed in the vanes, it is also possible to employ fuel posts to carry the fuel into the swirler. Such a scheme is shown very schematically in FIG. 6, where two posts  40  connected to the inlet conduit  28  extend into the swirler in the area just inside the trailing edge  24  of the vanes. Holes are formed in these posts as they were in the vane-fed scheme shown, for example, in FIG. 5, and the dimensions of the holes are, as already explained, different over the length of the post. 
     It is preferable to arrange the fuel outlets so that the fuel passing through them is aimed as near as possible towards the central axis  22  of the swirler in order to maximize the radial component of velocity of the fuel. An example of such an arrangement is shown in FIG. 7, in which each vane is fed with fuel along a conduit  42  lying roughly parallel to a median, approximately tangential, axis  44  of the vane, the conduit  42  then changing direction by approximately 90° to lie roughly in a radial direction  46  oriented towards the axis  22  of the swirler. The line of exit of the fuel may, however, in practice lie anywhere between the median line  44  and the radial line  46 . 
     The fuel outlets may be allocated to each vane of the swirler, or alternatively may be restricted to some vanes only, e.g. every other vane. 
     Although the invention has been described in connection with its implementation in a swirler, it is also possible to incorporate the variable hole-sizing technique in the combustor pre-chamber wall region shown as  50  in FIG. 3, where there may still be an effective rotating body of fuel-air mixture having a wall thickness T nearby. The whole pre-chamber region  51  thus comprises both the swirler region  14  and the afore-mentioned region  50  intermediate the swirler and the main-chamber portion  52  of the combustion chamber  10 . 
     The present inventive fuel-injection technique may be incorporated into either the swirler, or the intermediate chamber area  50 , or both. FIG. 8 shows stepped holes  60 ,  2 ,  64 ,  66 ,  68  in both areas. The use of fuel posts to supply the fuel applies equally to the swirler portion  14  and to the intermediate portion  50  and, where the present inventive fuel-injection technique is employed in both portions, an extended length of post can be used in simple manner. Where, alternatively, the variable-sized fuel outlets are incorporated into the wall of the intermediate portion  50  rather than in adjacent fuel posts, fuel may be supplied to those outlets either from an extension of the fuel-gallery system supplying the swirler outlets, or from some additional system, whichever is convenient. 
     Where the invention is applied to the intermediate portion  50  only, mixing of fuel and air upstream of the intermediate portion may be by means of a swirler or by any other appropriate method.