Fluid manifold

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).

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.

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 & 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 40A, 40B 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.