FUEL INJECTOR ARRANGEMENT FOR A COMBUSTION APPARATUS

This invention relates to a fuel injector arrangement for a combustion apparatus utilising

fluid fuel. It is concerned particularly, but not exclusively, with such an injector arrangement

for a turbine, especially a gas turbine, but it is also suitable for use with liquid fuels and in

combustion apparatus other than turbines.

Environmental considerations per se, as well as legislation in the environmental field

mean that it has become essential to ensure that the levels of pollutant emission from all combustion apparatus and in particular the emission ofthe nitrogen oxides (NO.) is reduced to as low levels as possible. One way of reducing such emission is to ensure efficient mixing of the air/fuel mixture to thereby obtain efficient combustion. Various injection and mixing

arrangements have been described which purport to give efficient mixing but in practice it has been found difficult to ensure such mixing over the wide range of fuel-air ratios encountered in practice, and especially ratios encountered under low-power conditions, i.e. close to flame

extinction.

It is an object of the present invention to provide a fuel injection arrangement, e.g. for

a turbine, which gives efficient mixing and combustion over a wide range of fuel-air ratios and

thereby ensures low NO _x emissions over a range of operating conditions.

According to the invention there is provided a fuel injector arrangement for a combustion apparatus, comprising at least one passage for the flow of fluid, said passage being of

substantially annular cross-section, being defined by a radially inner wall and a radially outer

wall and having an inlet region and an outlet region, wherein the inlet region incoφorates a

plurality of vanes adapted to modify a flow pattern of a fluid entering said inlet region such that

fluid passing from the inlet region to the outlet region, has a composite flow pattern having both an axial component and a component rotational about the longitudinal axis of the passage.

In a preferred arrangement the vanes are adapted to impose a rotational component about

said longitudinal axis on at least part of a substantially axially flowing incoming fluid.

It is further preferred that the vanes are adapted to provide a controlled radial variation in said axial and rotational components.

Each vane may be set at an angle to a longitudinal axis of the injector arrangement.

Further, each vane may be provided with a root, a mid-region, and a tip, the root and tip

both being of reduced width relative to the mid-region whereby an existing axial flow of fluid entering said inlet region continues substantially unaffected at the root and tip but is affected as

it flows past the mid-region; it is particularly contemplated that progressively between the root

and the mid-region and between the tip and the mid-region, the fluid is given a rotational component whereby the fluid is caused to spiral along the passage.

In one embodiment it is provided that, in use, air enters the inlet region and fuel enters

the annular passage at at least one position between the inlet region and the outlet region; fuel may enter the annular passage through at least one hole in a wall of the annular passage, and/or

fuel may enter the annular passage through at least one hole in a said vane.

Each vane may have a straight leading edge and a curved trailing edge and the trailing

edge may be of convex or concave form as viewed in the direction of an axial fluid flow

component.

Alternatively the leading edge may be curved.

The trailing edge and/or the leading edge of each vane may have a corrugated surface formation.

It is envisaged that each vane may have a crescent shaped cross-section, and the sides of each vane may be curved axially along the annular passage.

The inlet region ofthe annular passage may be provided by a disc formed with slots and in this arrangement each vane may be provided by a wall between adjacent said slots.

In particular, each wall may extend substantially radially and the radially inner and outer

walls of each slot may be straight or curved.

The annular passage may surround a central axial bore for the flow of fuel and/or air, in

use, and the central bore may incoφorate at least one vane to give a rotational component to the

flow of fluid fuel and/or air therethrough; means may be provided for injecting fuel substantially tangentially into the central bore. The rotational component of flow through the central bore may be counter to or in the same rotational direction as the rotational flow

component of fluid flow in the annular passage.

The central bore is preferably arranged, in use, to provide the fuel/air mixture for a pilot

flame, the annular passage providing the fuel/air mixture for a main flame; in use the main

flame and pilot flame may coalesce to give a flame of crown-shaped formation.

The outlet region of the annular passage may be provided by a component which acts to

give a coanda jet flow of air to improve flame stability.

The annular flow passage may be formed by spaced surfaces of two components with the radially inner component being formed with the said central bore.

Embodiments ofthe invention will now be described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 shows a part section of one embodiment of a fuel injector arrangement according to the invention;

Figure 2a, 2b respectively show a side view and an end view of a vane for use in the

injector arrangement of Figure 1 ;

Figure 3a shows part of the upstream face of a disc of an injector arrangement according to the invention and Figure 3b shows a section on line A-A of Figure 3a;

Figure 4a shows part of the upstream face of an alternative disc and Figure 4b shows a

section on line B-B of Figure 4a;

Figure 5 shows a face of an alternative disc;

Figure 6 illustrates a flame formation produced in a fuel injector arrangement according

to the invention.

Figure 7 is a part sectional view of a further fuel injector according to the invention.

The embodiment of Figure 1 shows a fuel injector arrangement 10 to which are fed

supplies of fuel and air for mixing to give a combustible mixture for combustion in a combustion chamber 30 to which the fuel injector arrangement is attached; it is also envisaged that 30 may represent a precombustion chamber with combustion occurring further downstream. The

arrangement 10 has a generally cylindrical body 11 having its longitudinal axis identified by broken line 40 and comprising a first part 13 and second part 14, the first body part 13 being formed with a central cylindrical axial bore 12. The first part 13 has a generally frustoconical external form being tapered in the direction towards the combustion chamber 30 but with its

outer surface 15 also being curved to provide a concave surface as shown. The body part 14 is of overall externally cylindrical form but it has an inner surface 16 curved in convex manner such that the thickness of the wall 17 of body part 14 increases from a region 18 of minimum thickness towards a region 19 of maximum thickness and then decreases towards a region 20

of intermediate thickness at which region 20 the body part 14 is seciued to the upstream end of

the combustion chamber 30.

The surfaces 15, 16 have similar curvatures with the concave surface 15 of the body part

13 facing the convex surface 16 of the body part 14 between the regions 18, 19 thereof, whereby

a passage 23 is formed therebetween, the passage 23 having a substantially annular cross-section but with its bounding longitudinal walls formed by the curved surfaces 15, 16 so that the

diameter of the annular passage decreases in an axial downstream direction, i.e. its distance from

axis 40 decreases in the downstream direction.

In the embodiment thus far described the body parts 13, 14 are formed as separate

components which are suitably secured to adjacent elements to form the passage 23

therebetween, as shown. In other embodiments it may be possible for the body parts 13, 14 to

be formed integrally, e.g. by casting with suitable supporting/interconnecting means between

the parts 13, 14. Furthermore it is envisaged that the body parts 13, 14 may be formed and/or positioned and/or interconnected such that instead of a single annular passage a plurality of separate annular passages are formed around the central bore 12.

The fuel injector arrangement 10 receives air and fuel and mixes them in such a manner as to form a lean mixture for efficient combustion with low NO -.production.

For the purpose of such mixing, there are provided in the or each annular passage 23 a plurality of vanes 25 adapted and arranged to give to fluid passing through the passage 23 a composite flow pattem having both an axial component and a component rotational about the

longitudinal axis of passage 23, bodi components varying in a controlled fashion in the radial

direction; the disposition of passage 23 effectively means that each vane 25 is set at an angle to

the longitudinal axis 40 of the injector.

Figures 2a, 2b shows views of one possible form of vane 25. In Figure 2a_which

represents an overall view of a vane 25 from one side, the leading edge 51 of the vane is seen to be straight and the trailing edge 52 is curved. Figure 2b shows an end-on view looking into

the leading edge 51 of the vane 25 and it can be seen that in cross-section the vane is of

substantially crescent shape having a concave side 53 and a convex side 54, both sides 53, 54

extending between the root 55 and the tip 56 of the vane. As shown, the root and tip each have

a width which is only sufficient to ensure reliable attachment to surfaces 16, 15 respectively, but

where fuel passages are provided in the tip/root of the vanes (see below), the root tip will, of

course, be wider. The sides 53, 54 are curved also in an axial direction so that the vane 25 has

a formation curved in two dimensions. As shown, the positioning of the vane 25 in the passage 23 is such that the concave surface 53 is angled towards the inlet end 71 of the passage 23, but

positioning the vane with the convex surface 54 angled towards the inlet end of the passage is

also envisaged.

As indicated above, the vanes 25 give a composite flow pattern to the air/fuel mixture leaving the passage 23. There are a number of possible ways of introducing air and/or fuel into

the passage whereby this flow pattern is obtained.

In one example, the region 60 upstream of the body 11 is supplied with compressed air by a compressor driven by the turbine. Fuel in gaseous form and for main operation, e.g. engine

on load, may be introduced into the passage 23 through bores in the vanes whose exits are formed in the concave and/or convex sides of the vanes 25 and/or through holes (e.g. in fuel

posts) in the surfaces 15, 16 defining the passage 23 and/or through a fuel post adjacent inlet 71

within or just outside passage 23. For fuel in liquid form, introduction would normally be

through atomiser holes in the surfaces 15, 16 only. Typically, for pilot operation (e.g. engine start-up and low-power operation) and in the case of either gas or liquid fuel, fuel is introduced

into the central bore 12 as will be further described later.

Compressed air or, possibly, a preformed air/fuel mixture enters the inlet end 71 of

passage 23 with an essentially axial flow pattem. The root 55 and tip 56 position of each vane

25 have, as previously explained, a reduced width or chordal dimension relative to the mid-

region ofthe vane and the shape of the vanes at the root and tip allows an existing axial flow of

air to continue substantially unaffected. However, progressively between the root 55 and tip 56

of the vane and the mid region of the vane 25 the air is affected as it flows past the mid-region.

Specifically the flow directed by the centre parts of the vane is given a rotational flow component whereby air is caused to spiral along the passage 23. The composite flow receives fuel from the holes in the vanes 25 and/or surfaces 15, 16 as it progresses and engenders

thorough mixing to provide a fuel/air mixture without isolated fuel-rich pockets or substantial zones of retarded flow whereby efficient combustion without flash-back may be obtained.

By means of careful consideration of the aerodynamics of the vanes and the associated components, pre-ignition, which is a common feature of prior-art pre-mixing burners, is avoided.

In the embodiment of Figures 1, 2a and 2b the volume 60 effectively constitutes a simple

chamber from which air flows at low velocity, but it would also be advantageous to have means

adjacent the inlet 71 of passage 23 to ensure forced axial flow of air thereinto. Such means

could comprise inter alia a suitably dimensioned annulus or tube constituting an axial extension

of passage 23 at its upstream end which acts to guide air directly to the passage inlet.

In further altemative embodiments illustrated by Figures 3a, 3b and 4a, 4b composite

swirling flow into the passage 23 is achieved by means of a disc 71 formed with an annular array of slots 72 therethrough, which slots 72 have a radial dimension corresponding to that of the

passage 23 and act collectively as the inlet region of the passage 23. In these embodiments each

slot may be visualised as being skewed as it extends through the disc; each vane 25 may then

be visualised as the similarly skewed radially extending wall between a pair of adjacent slots.

By appropriate contouring ofthe slots, the desired contouring ofthe vanes is achieved to ensure

the composite flow pattem.

In the embodiment of Figure 3a, 3b the dotted lines 73, 74 indicate the form of such matching contoured surfaces of a slot at the downstream outlet; they can be seen to provide a curved surface angled with respect to the downstream face ofthe disc 71 and having a convex

surface 75 facing the upstream side. Respective axial inclined passages 76, 77 at root and tip act to provide the axial flow component with the angled curved part of the vane in the middle region thereof providing the rotational flow component.

In this embodiment the radially inner and outer walls of the slot are shown as straight whereas in the embodiment of Figures 4a, 4b these walls have a curve corresponding to the

curves of the surfaces 15, 16 of passage 23.

Figure 5 shows a further arrangement with sickle-shaped slots, with the vanes/walls

therebetween formed similarly.

As described above, the annular passage 23 gives a fuel/air mixture with a composite flow pattem. The central bore 12 may be utilised in a number of ways depending on the precise

application but it is particularly envisaged that an intimate mixture of air and fuel may be formed therein for low emission pilot flame production or to stabilise combustion during low power

operation. Furthermore, better mixing may be obtained if the flow of fluid in the central bore

12 is given a rotational component, e.g. by the use of vanes 86 in the bore 12, (see Figure 1) or

by utilising an arrangement whereby the fuel is caused to be injected into the bore tangentially,

e.g. from a fuel gallery or galleries. In either case optimum mixing will generally be assured if the rotational direction of flow of fluid exiting central bore 12 is counter to that exiting passage

23.

Figure 6 illustrates the downstream end of the injector arrangement 10 where the fuel/air mixture exiting the central bore 12 forms a pilot flame 90 and the fuel air mixture exiting the annular passage forms a main flame 91 of annular form surrounding the pilot flame 90, to give an overall flame of crown-shape, which is particularly stable. The streams of air/fuel mixture respectively flowing in passage 12, 23 may be arranged to have the same air/fuel ratios or different ratios. More specifically, NO _x production is minimised if the fuel/air ratios are the same at high firing temperatures, whilst different fuel/air ratios at low firing temperatures will

assist in maintaining combustion stability.

In the modification of Figure 7, the exit region of the injector unit 10 has an additional

component 101 associated therewith which may be integral with body part 14 but will generally,

and as shown, be a separate component. The mounting of component 101 is such mat there will

be at least one axial gap 102 forming a radially extending channel between component 101 and

the downstream end of body part 14. The body part 14 is formed with a lip 103 whereby air

flow is directed on exit from radial to axial flow to give a coanda effect jet flow - this will act to prevent flame creep. Each gap may be realised by means of a radial groove in component 14

and/or component 101.

Further modifications of the fuel injector are envisaged, which are specifically aimed at

improving air/fuel mixing. Thus the inlet 71 of passage 23 may have crescent or part-circular

vanes to control air flow entering the passage 23. Further the vanes 25 may be provided with

a corrugated traihng edge and/or leading edge to create vortices to assist the mixing process.