FIELD OF THE INVENTION 
     The present invention relates to a fluid manifold and in particular to a fluid manifold for use in the combustion chamber of a gas turbine engine. 
     Industrial gas turbines are required to meet stringent emission levels. It is known that the production of NOx and CO is a function of the combustion flame temperature which in turn depends upon the combustor air inlet temperature, temperature rise of the combusting fuel/air mixture and the air inlet pressure. In order to limit the production of NOx and CO the temperature and residence time should be controlled. 
     Series staged combustion controls the levels of NOx and CO produced. In series staged combustion a number of separate combustion zones are used, each being fed by the previous stage. The sequential nature of the staging, whereby combustion products from upstream flames become mixed with downstream flames, provides added benefit to emissions since the undesired combustion emissions from upstream have a chance to be converted to the desired combustion products in the downstream flame. Further the higher the number of stages, the smaller the temperature range required for each stage resulting in the lowest maximum temperature attained by each stage and hence minimum NOx. Each stage provides lean combustion, that is combustion of fuel in air where the fuel to air ratio is low. 
     In multi-staged combustion chambers the fuel is premixed with air in separate premixing ducts for each stage. Fuel manifolds may be used to feed the fuel into the premixing ducts. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a fluid manifold which distributes a fluid such as fuel uniformly and which discharges the fuel at a velocity which enhances mixing of the fuel with air downstream of the manifold. 
     According to the present invention a fluid manifold comprises a plurality of annular chambers in flow series and having barriers therebetween, fuel being fed into the first annular chamber and circulates circumferentially therethrough before being passed through a barrier to the next annular chamber, the fuel circulating circumferentially around each annular chamber prior to its discharge through a final barrier which changes the velocity of the fuel flow discharging therefrom. 
     In the preferred embodiment of the present invention the fluid manifold has two annular chambers in flow series, the first annular chamber being larger than the second annular chamber. The fluid flow in the first annular chamber circulates in the opposite direction to the fluid flow in the second annular chamber. 
     Preferably the barrier between the first and second annular chambers has a plurality of apertures therein. The apertures in the barrier between the first and second annular chambers may be angled to increase the resistance to the passage of fluid from the first annular chamber to the second annular chamber. 
     The final barrier through which the fluid discharges from the manifold has a plurality of small apertures therein which decelerate the fluid. The apertures may be angled radially to remove any swirl from the fluid discharged from the manifold. 
     Preferably the fluid manifold is annular and may be used with a liquid or gaseous fluid. 
     A manifold in accordance with the present invention is for use in a combustion chamber of a gas turbine engine. In particular the manifold is for use in a combustion chamber having multiple combustion chambers in flow series. Each manifold is located in a premix duct of one stage of a multi-staged combustion chamber. The manifold is located by swirler vanes in the premix duct which swirl air, the swirled air mixing with the fuel discharged from the manifold in the premixing duct. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic view of a gas turbine engine having a combustion chamber and fuel manifolds in accordance with the present invention. 
     FIG. 2 is a cross-sectional view through the combustion chamber shown in FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view through one of the fuel manifolds shown in FIG.  2 . 
     FIG. 4 is a cross-sectional view through part of a fuel manifold in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An industrial gas turbine engine  10 , shown in FIG. 1, comprises in axial flow series an inlet  12 , compressor  14 , a combustion chamber assembly  16 , turbine section  18 , power turbine section  20  and an exhaust  22 . The turbine section  18  is arranged to drive the compressor section  14  via one or more shafts (not shown). The power turbine section  20  is arranged to drive an electrical generator  26  via a shaft  24 . 
     The combustion chamber assembly  16  is shown more clearly in FIG.  2  and comprises a plurality of tubular combustion chambers  30 . The axes of the tubular combustion chambers  30  are arranged to extend in a generally radial direction. The inlets to the tubular combustion chambers  30  are at their radially outmost ends and the outlets  60  at their radially innermost ends. 
     Combustion of the fuel is staged in three zones A, B &amp; C which are in flow series. To control the combustion flame temperature and hence NOx and CO levels fuel is premixed with air in separate premixing ducts  32 ,  36 , and  38  for each stage. 
     In the primary combustion zone fuel and air are mixed in a primary premix duct  32 . Fuel is also injected from a central injector  33  located in the upstream wall  31  of each tubular combustion chamber  30  just upstream of the exit of the primary premix duct  32 . Several fuel orifices (not shown) are distributed around the injector  33 . The number, size and location of the orifices are determined so as to provide best stability and combustion efficiency. A torch igniter  34 , which is lit by two spark ignitors (not shown), is provided in the centre of the central injector  33  of each tubular combustor  30 . A diffusion flame, initially lit by the torch ignitor  34 , is fuelled by the central injector  33 . The flame is contained in the primary zone A and stabilised by a recirculating flow generated by the primary premix ducts  32 . The diffusion flame is intended for starting and minimum power situations only ie. at power settings below 30% of maximum load. 
     For power settings between 30-50% of maximum load fuel is also injected into secondary premix ducts  36  forming a uniform mixture which begins burning in the secondary zone B. 
     Finally for power settings from 50-80% of maximum load fuel is also injected into a tertiary premix ducts  38  forming a uniform mixture which begins burning in the tertiary zone C. 
     The secondary and tertiary premix ducts  36  and  38  each supply a fuel/air mixture to the burner in a ring of jets which penetrate and mix with the gases from the upstream stages. To create the discrete jets, each premix duct  36  and  38  ends with aerodynamic partitions  39 , wedges, whose base form part of the combustor liner. 
     Swirlers  40  are provided at the inlet to the secondary and tertiary ducts  36  and  38 . The swirlers  40  are of an efficient aerodynamic design to accelerate the air passing therethrough. The swirlers  40  have a counter-swirling configuration so that they produce a shear layer which gives good mixing of the air and fuel. 
     The inlet to the secondary and tertiary ducts  36  and  38  is at a large radius from the burner centerline. To minimise the length of the ducts  36  and  38  the most vigorous method was sought to mix premix gas with the air. Vigorous mixing is achieved by injecting the fuel at a low velocity into the shear layer formed between the air flows from the counter-rotating swirlers  40 . 
     The partition between the counter-rotating swirlers  40  is a manifold  41  to which premix fuel is supplied. The external profile of the manifold  41  has been optimised to give the air passing over it the best aerodynamic performance. The internal configuration of the manifold  41  is designed to even out any non-uniformities in the fuel flow and feed the fuel in a uniform manner into the premix duct  36 . 
     The internal configuration of a fuel manifold  41  in accordance with the present invention is shown in more detail in FIGS. 3 and 4. Each manifold  41  consists of a large annular chamber  42  fed by one of more feed pipes  43 . The feed pipes  43  are attached to the circumference of the manifold  41  at an angle. In the preferred embodiment of the present invention three feed pipes  43  are attached to the circumference of the manifold  41 . Each feed pipe  43  is attached at the same angle relative to the tangent of the circle at the point of attachment and the feed pipes  43  are equally spaced around the circumference. In this way the fuel flowing into the annular chamber  42  is given a strong circumferential motion. Each feed pipe makes an angle of no more than forty five degrees so that the majority of the fluid momentum goes into the circular motion rather than raising the local static pressure. 
     The dimensions of the annular chamber  42  are such that relative to the speed of the circulating flow the fuel residence time is several times greater than the time it would take the average fuel element to circumnavigate the chamber  42 . In the preferred embodiment of the present invention the fuel swirls around the circumference of the large annular chamber  42  four-eight times. 
     On an interior wall  45  of the manifold, opposite where the feed pipes  43  are attached, are numerous identical and equally spaced exit passages  44 . The holes  44  are orientated at an angle such that the flow entering one of the holes  44  makes a turn of about one hundred and thirty five degrees. Due to this acutely sharp turn the passage  44  entrance has an effective flow area much smaller than its geometric area. In this way the problem of making many holes with very small identical flow areas is diminished. Furthermore this orientation makes the holes less sensitive to being located near a feed pipe  43  since any momentum hitting a hole from a nearby feed pipe  43  will be perpendicular to the hole direction. 
     The fuel passes from the large annular chamber  42  thorough the passages  44  in interior wall  45  to a smaller annular chamber  46  where it circulates in the opposite direction. The fuel then passes though small radial holes  47  in a wall  48  to a plurality of separate cavities  49 . Fuel is fed from the cavities  49  to etched diffusers  50  and discharges from the trailing edge of the fuel manifold  41  into the premix duct  36 . 
     The largest pressure drop in the fuel manifold  41  takes place between the two chambers  42  and  46  and the radial holes  47  in the wall  48  discourage swirl at the entry to the diffusers  51 . The slits  49  decelerate the fuel and is discharged from the trailing edge of the fuel manifold  41  into the premix ducts. 
     The fuel discharges from the trailing edge of the manifold  41  and is mixed with the counter-swirling flows of air from the swirlers  40 A,  40 B located in the inlet of the premix ducts  36  and  38 . 
     The fuel/air mixture from the premix ducts  36  and  38  penetrates and mixes with the gases from the upstream combustion stages. 
     The sequential nature of the staging, whereby combustion products from upstream flames become mixed with downstream flames provides added benefit to emissions since the undesired combustion emissions from upstream have a chance to be converted to the desired combustion products in the downstream flame. 
     Although the manifolds have been described for use in a combustor  30  of a gas turbine engine it will be appreciated that they could be used in any application where a circular uniform distribution of a fluid, either gaseous or liquid, is required. 
     It will be further appreciated by one skilled in the art that a manifold  41  in accordance with the present invention may have any number of annular chambers  42  and  46  provided a uniform circumferential distribution of the fluid is achieved. If several annular chambers are to be implemented to obtain the required uniformity the passages  44  and  47  should decrease in size and increase in number with each stage. The manifold residence time and the pressure drop should decrease with each stage. Circumferential flow is eliminated in the last stage by making its feed holes radial. Similarly the manifold  41  could decelerate or accelerate the fluid discharged therefrom.
A fluid manifold ( 41) comprises two annular chambers ( 42 and 46) in flow series and having a barrier ( 45) therebetween. A fluid is fed into the first larger annular chamber ( 42) and circulates circumferential therethrough before being passed through a plurality of apertures ( 44) in the barrier ( 45) to the second smaller annular chamber ( 46). The fluid circulates circumferentially through the second chamber ( 46) in the opposite direction prior to its discharge through apertures ( 50) in a final barrier ( 48). The apertures ( 50) in the final barrier ( 48) decelerate the fluid discharging therefrom to enhance mixing of air with the fluid downstream of the manifold ( 41).