Fuel-injection arrangement for a gas turbine combustor

Abstract: A fuel-injection arrangement for a gas-turbine engine has distributed longitudinally along a pre-chamber portion (51) of the combustion chamber a series of fuel-outlet holes (38; 32, 34,36; 60, 62, 64, 66, 68) configured such that a radial component of momentum of the fuel exiting the holes varies along the series of holes. To achieve this the holes are preferably differently sized along the trailing edge. Advantageously, the holes at the very upstream end of the pre-chamber portion have the smallest diameter, the size thereafter progressively increasing along the series. The size distribution may vary either continuously, or in stepped fashion. The direction of exit (46) of the fuel from the outlets is preferably radial towards the central axis (22) of the swirler (14). The variable-sized holes may be employed in a swirler (14) upstream the main combustion chamber region (52) and/or in an intermediate region (50) between the swirler and the main chamber region.

Description: The invention concerns a fuel-injection arrangement for a combustor of a gas-turbine 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. Figure 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 Figure 2 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 distibution of the fuel along the axial length of the swirler. EP-A-0 747 636 relates to a low-emulsion combustion system in which a lean premix combustion mode is enabled by an upstream "dome" part of the combustor in which the vanes of a fixed axial swirler have internal radial fuel passageways leading to radially spaced fuel injection outlets for distributing the fuel more uniformly across the air flow into the combustor. In accordance with the present invention, there is provided insert the winding of claim 1 without reference numbers. 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. An embodiment of the invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 is a sectional view of part of a gas-turbine engine incorporating a conventional swirler; Figure 2 shows the swirler of Figure 1 in both side- and end-elevations; Figure 3 is a view of a gas-turbine engine corresponding to that of Figure 1 and showing a dynamic aspect of the fuel-air mixture inside the swirler; Figures 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; Figures 5(a) and 5(b) show two alternative fuel-outlet size distribution profiles for the swirler of the present invention; Figure 6 shows an embodiment of the swirler according to the invention in which fuel is supplied to the swirler by way of fuel posts, Figure 7 is an end-view of the swirler according to the invention including radially oriented fuel outlets, and Figure 8 is a partial view of Figure 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. The operation of the swirler according to the invention is now explained with reference to Figure 3. In Figure 3, which shows the same engine arrangement as in Figure 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 Figures 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 afore-mentioned 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 Figure 4. In Figure 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 Figure 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 (Figure 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 Figure 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 fuel-jet 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 l 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 Figures 5(a) and 5(b), respectively. In the case of Figure 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 Figure 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 Figure 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 Figure 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 maximise the radial component of velocity of the fuel. An example of such an arrangement is shown in Figure 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 Figure 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. Figure 8 shows stepped holes 60, 62, 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.

Claims: A gas turbine combustor having; a longitudinal axis (22) extending in a streamwise direction with respect to combustion flow therethrough, a main chamber region (52), a pre-chamber region (51) upstream of the main chamber region, and a fuel injection arrangement comprising at least one series of fuel-injection outlets (38; 32, 34, 36; 60, 62, 64, 66, 68) arranged in axially spaced-apart relationship, with respect to the longitudinal axis (22) of said combustor in the pre-chamber region (51) characterised in that the fuel-injection outlets are arranged to discharge the fuel jets into the pre-chamber region with radially inward components of momentum that are greater in magnitude at the upstream end of the at least one series of outlets than at the downstream end of the at least one series of outlets. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 1, wherein said fuel-injection outlets are arranged to discharge the fuel jets with radially inward components of velocity that are greater in magnitude at the upstream end of the at least one series of outlets than at the downstream end of the at least one series of outlets. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 2, wherein sizes of said fuel-injection outlets vary from smallest at the upstream end of the at least one series of outlets to largest at the downstream end of the at least one series of outlets. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 3, wherein said variation in outlet size in said at least one series of outlets is monotonic . A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 3, wherein said variation in outlet size in said at least one series of outlets is a stepped variation. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 3, wherein said variation is linear over at least one part of said series of outlets. A gas turbine combustor having a fuel-injection arrangement as claimed in any one of the preceding claims, wherein said outlets are configured such that the fuel jets exit said outlets in a substantially radial direction (46). A gas turbine combustor having a fuel-injection arrangement as claimed in any one of the preceding claims, wherein said outlets are substantially equally spaced apart in the axial direction. A gas turbine combustor having a fuel-injection arrangement as claimed in any one of the preceding claims, wherein said outlets are disposed in a swirler portion (14) of said pre-chamber region. A gas turbine combustor having a fuel-infection arrangement as claimed in any one of claims 1 to 8, wherein said outlets are disposed in an intermediate portion (50) of said pre-chamber region between a swirler portion thereof and said main-chamber region. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 9, wherein said swirler portion comprises a plurality of vanes (16), said at least one series of outlets being incorporated into each of at least some of said vanes at a trailing edge (24) thereof. A gas turbine combustor having a fuel-injection arrangement as claimed in Claim 10, wherein said outlets (68) are disposed in a wall of said intermediate portion. A gas turbine combustor having a fuel-injection arrangement as claimed in any one of Claims 1 to 10, wherein said outlets are provided in fuel posts (40) situated in said pre-chamber region.

Inventors: ALKALBIE HISHAM SALMAN, ALKALBIE, HISHAM SALMAN
Applicants: EUROP GAS TURBINES LTD, EUROPEAN GAS TURBINES LIMITED
Application year: 1998
Main classification: F23R3
IPC codes: F23R3, F23C7