Abstract:
Disclosed is a gas turbine fuel injector and swirler assembly, including: a delivery tube structure arranged on a central axis of the fuel injector and swirler assembly, a first fuel supply channel arranged in the delivery tube structure, a shroud surrounding the delivery tube structure, swirl vanes arranged between the delivery tube structure and the shroud, a radial passage in each swirl vane, communicating with the first fuel supply channel, a set of apertures open between the radial passage and the exterior surface of said each swirl vane, wherein a second fuel supply channel is arranged in the delivery tube structure extending to a downstream end of the delivery tube structure and a mixer with lobes for fuel injection is arranged at the downstream end. Further disclosed is an assembly method for assembling a fuel injector and swirler assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a fuel injector and swirler assembly for a gas turbine. Further, the invention relates to a combustion system. The invention also relates to a gas turbine. In addition, the invention relates to an assembly procedure for assembling a fuel injector and swirler assembly. 
       BACKGROUND OF THE INVENTION 
       [0002]    The increase of price of natural gas has pushed the development of gas turbines in the direction of considering alternative fuels such as the so called synthesis gases. These gases typically come from gasification processes of solid feedstock such a coal, pet coke or biomass. The use of synthesis gas in gas turbines involves a much larger volumetric flow injection of fuel than standard natural gas. In order to achieve the very low NO x  values that are typical of modern gas turbines it is needed for the combustors to run in premixed mode. The fact of having such a large volumetric flow for the synthesis gas poses serious problems in firing them in a premixed mode. Another problem arising is the different reactivity of these fuels which can be different from natural gas; especially when there is a significant fraction of hydrogen the reactivity tends to be high, and this constitutes a problem as it aggravates the danger of flashback. These translate into the fact that a much larger fuel flow through the air passages leads to an increase in pressure drop, and that mixing between the air and fuel tends to be poor. The latest is mainly due to the fact that large volumetric flow rates also require large nozzles, which typically leads to bad mixing, and hence to high NO x  emissions. 
         [0003]    PCT/US2009/001336 shows an additional stage in the fuel injector and swirler assembly for injecting low calorific (LC) fuels. This stage supplies two additional rows of fuel injectors. The fuel injectors are implemented in the vanes of the swirlers and inject the fuel in a jet-in-cross flow mode. To provide the space for the two additional rows of injectors, the vanes are elongated in their upstream direction. Moreover the hub diameter has been increased to enlarge the space for the supply fuel flows. The combustor has demonstrated its functionality in terms of flashback resistance and low dynamics. However, one problem with this design is that the capacity of the LC-fuel passage is still relatively small, and that in order to keep the fuel side pressure drop as low as possible, the natural gas stages are also used for injecting the LC fuel. This makes the system relatively complicated in terms of fuel supply splits. Even when splitting the LC-fuel over all stages, pressure drops are relatively large. Moreover, the injectors for the LC fuel are relatively small. Since these fuels are typically contaminated, a clogging issue could arise. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to provide a fuel injector and swirler assembly with an improved mixing rate between air and fuel. Another object of the invention is to provide a combustion system allowing the combustor to operate in premixed mode, without increasing the pressure loss. Yet another object of the invention is an improved gas turbine. A further object of the invention is to provide an assembly method for an improved fuel injector and swirler assembly. 
         [0005]    These objects are achieved by the claims. The dependent claims describe advantageous developments and modifications of the invention. 
         [0006]    An inventive gas turbine fuel injector and swirler assembly, comprises a delivery tube structure arranged on a central axis of the fuel injector and swirler assembly, a first fuel supply channel arranged in the delivery tube structure, a shroud surrounding the delivery fuel structure, swirl vanes arranged between the delivery tube structure and the shroud, a radial passage in each swirl vane, the radial passage communicating with the first fuel supply channel, a set of apertures open between the radial passage and the exterior surface of said each swirl vane, wherein a second fuel supply channel is arranged in the delivery tube structure extending to a downstream end of the delivery tube structure and a mixer with lobes for fuel injection is arranged at the downstream end. 
         [0007]    Due to the relatively large central fuel injection the inventive fuel injector and swirler assembly reduces the pressure loss during operation with low calorific fuels. The injection of the reactive LC fuels in the centre of the fuel injector and swirler assembly also reduces the danger for flashback along the walls of the fuel injector and swirler assembly or the swirler cup compared to prior art solutions with LC fuel injection through the swirl vanes. Furthermore the susceptibility for clogging issues is reduced as a result of the relatively large second fuel supply channel. In this context it is also important that the first fuel supply channel for (for example) natural gas, does not have to be used during low calorific fuel operation. 
         [0008]    The second fuel supply channel for LC fuel having the lobed mixer arranged at its end can be sealed with an inert medium (N2 or steam), or with seal air from the mid-frame of the gas turbine to prevent any flow recirculation when this lobed mixer fuel passage is not used. 
         [0009]    In an advantageous embodiment the delivery tube structure comprises coaxial cylindrical inner and outer tubes, providing a first fuel supply channel in the inner tube and forming an annular second fuel supply channel between the inner and outer tubes. 
         [0010]    In another advantageous embodiment each lobe of the mixer is located directly downstream a swirl vane. 
         [0011]    In yet another advantageous embodiment a number of lobes is equal to a number of swirl vanes. 
         [0012]    It is particularly advantageous when the lobed mixer has a twist and when the twist of the lobed mixer follows a swirl induced by the swirl vanes. The swirl flow path is then maintained and the mixer acts like an extension of the swirl vanes providing an aerodynamic application in a swirled flow. 
         [0013]    In an advantageous arrangement a twist angle of the lobed mixer is up to 45°. 
         [0014]    Preferably, a height of the lobes is up to 0.5 times an annulus height of the shroud. 
         [0015]    Also preferably, a ratio between a height and a width of the lobes is between 0 and 8, preferably being 4. 
         [0016]    In an advantageous embodiment a grain is arranged on a central axis of the lobed mixer. The grain effectively prevents that a fuel rich area remains unmixed in the centre of the lobed mixer. 
         [0017]    Advantageously a combustion system comprises at least one of the previously described inventive fuel injectors and swirler assemblies. 
         [0018]    An inventive gas turbine comprises such a combustion system. 
         [0019]    In an inventive method of assembling a fuel injector and swirler assembly with a first fuel supply channel, a second fuel supply channel, radial passages, swirl vanes, a lobed mixer and a shroud, the lobed mixer is brazed or welded to the first fuel supply channel. Then a brazing material is applied at least between the central first fuel supply channel and the radial passages, between the second fuel supply channel and the radial passages and between the second fuel supply channel and the lobed swirler. The insofar assembled fuel injector and swirler assembly is then brazed in a furnace in one cycle. 
         [0020]    It is advantageous, when the brazing material is also applied between the swirl vanes and the shroud before brazing the fuel injector and swirler assembly in the furnace. 
         [0021]    Alternatively the shroud is welded, in particular tap welded, to the swirl vanes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will now be further described with reference to the accompanying drawings in which: 
           [0023]      FIG. 1  shows a side sectional view of a prior art gas turbine combustor, 
           [0024]      FIG. 2  shows a side sectional view of a prior art fuel injector and swirler assembly using injector swirler vanes, 
           [0025]      FIG. 3  shows a side sectional view of a prior art fuel injector and swirler assembly with a dual passage fuel supply to swirler vanes, 
           [0026]      FIG. 4  shows fuel injector and swirler assembly according to the invention with a lobed mixer, 
           [0027]      FIG. 5  shows another view of the fuel injector and swirler assembly of  FIG. 4 , 
           [0028]      FIG. 6  shows a lobed mixer with twist, 
           [0029]      FIG. 7  shows a lobed mixer with twist with reduced lobe height, and 
           [0030]      FIG. 8  shows the brazing step of the assembly of the fuel injector and swirler assembly. 
       
    
    
       [0031]    In the drawings like references identify like or equivalent parts. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 1  shows an example of a prior art gas turbine combustor  1 , some aspects of which may be applied to the present invention. A housing base  2  has an attachment surface  3 . A pilot fuel delivery tube  4  has a pilot fuel diffusion nozzle  5 . Fuel inlets  6  provide a main fuel supply to main fuel delivery tube structures  7  with injection ports  8 . A main combustion zone  9  is formed within a liner  10  downstream of a pilot flame zone  11 . A pilot cone  12  has a divergent end  13  that projects from the vicinity of the pilot fuel diffusion nozzle  5  downstream of main fuel injector and swirler assemblies  14 . The pilot flame zone  11  is formed within the pilot cone  12  adjacent to and upstream of the main combustion zone  9 . 
         [0033]    Compressed air  15  from a compressor (not shown) flows between support ribs  16  through the swirler assemblies  14 . Within each main swirler assembly  14 , a plurality of swirler vanes  17  generate air turbulence upstream of main fuel injection ports  8  to mix compressed air  15  with fuel  18  to form a fuel/air mixture  19 . The fuel/air mixture  19  flows into the main combustion zone  9  where it combusts. A portion of the compressed air  20  enters the pilot flame zone  11  through a set of vanes  21  located inside a pilot swirler assembly  22 . The compressed air  20  mixes with the pilot fuel  23  within pilot cone  12  and flows into pilot flame zone  11  where it combusts. The pilot fuel  23  may diffuse into the air supply  20  at a pilot flame front, thus providing a richer mixture at the pilot flame front than the main fuel/air mixture  19 . This maintains a stable pilot flame under all operating conditions. 
         [0034]    The main fuel  18  and the pilot fuel  23  may be the same type of fuel or different types. 
         [0035]      FIG. 2  illustrates basic aspects of a compared to the gas turbine combustor of  FIG. 1  refined prior art main fuel injector and swirler assembly  14  such as found in U.S. patent application Ser. No. 12/356,131 of the present assignee. A fuel supply channel  24  supplies fuel  18  to radial passages  25  in vanes  26  that extend radially from a fuel delivery tube structure  7  to the shroud  57 . Combustion intake air  15  flows over the vanes  26 . The fuel  18  is injected into the air  15  from apertures  27  open between the radial passages  25  and an exterior surface  28  of the vane. The vanes  26  are shaped to produce turbulence or swirling in the fuel/air mixture  19 . 
         [0036]    The prior design of  FIG. 2  could use alternate fuels with similar viscosities and energy densities, but would not work as well for alternate fuels of highly dissimilar viscosities or energy densities. Synthesis gas has less than half the energy density of natural gas, so the injector flow rate for synthesis gas must be at least twice that of natural gas. This results in widely different injector design criteria for these two fuels. 
         [0037]    Existing swirler assemblies have been refined over the years to achieve ever-increasing standards of performance. Altering a proven swirler design could impair its performance. For example, increasing the thickness of the vanes  26  to accommodate a wider radial passage for a lower-energy-density fuel would increase pressure losses through the swirler assemblies, since there would be less open area through them. To overcome this problem, higher fuel pressure could be provided for the low-energy-density fuel instead of wider passages. However, this causes other complexities and expenses. Accordingly, it is desirable to maintain current design aspects of the swirler assembly with respect to a first fuel such as natural gas as much as possible, while adding a capability to alternately use a lower-energy-density fuel such as synthetic gas. 
         [0038]      FIG. 3  illustrates aspects of another, improved, prior art design. A first fuel supply channel  29  provides a first fuel  30  to a first radial passage  31  in vanes  32  that extend radially from a fuel delivery tube structure  33 . Alternately, a second fuel supply channel  34  provides a second fuel  35  to second and third radial passages  36 ,  37  in the vanes  32 . The fuel delivery tube structure  33  may be formed as concentric tubes as shown, or in another configuration of tubes. Combustion intake air  15  flows over the vanes  32 . The first fuel  30  is injected into the air  15  from first apertures  38  formed between the first radial passages  31  and an exterior surface  28  of the vane. Selectably, the second fuel  35  is injected into the air  15  from second and third sets of apertures  39 ,  40  formed between the respective second and third radial passages  36 ,  37  and the exterior surface  28  of the vane. The vanes  32  may be shaped to produce turbulence in the fuel/air mixture  19 , such as by swirling or other means, and may have pressure and suction sides. 
         [0039]    The first fuel delivery pathway  29 ,  31 ,  38  provides a first flow rate at a given backpressure. In order to accommodate fuels with dissimilar energy densities, the second fuel delivery pathway  34 ,  36 ,  37 ,  39 ,  40  provides a second flow rate at the given backpressure. The first and second flow rates may differ by at least a factor of two. This difference may be achieved by providing different cross-sectional areas of one or more respective portions of the first and second fuel delivery pathways, and may be enhanced by differences in the shapes of the two pathways. It was found that contouring the transition area  41  between the fuel supply channel  34  and the second and third radial passages  36 , 37  increases the fuel flow rate at a given backpressure, due to reduction of fuel turbulence. A more equal fuel pressure between the radial passages  36  and  37  was achieved by providing an equalization area or plenum  41  in the transition area, as shown. This equalization area  41  is an enlarged and rounded or graduated common volume of the proximal ends of the radial passages  36  and  37 . A partition  42  between the radial passages  36  and  37  may start radially outwardly of the second fuel supply channel  34 . This creates a small plenum  41  that reduces or eliminates an upstream/downstream pressure differential at the proximal ends of the respective radial passages  37 ,  36 . 
         [0040]      FIGS. 4 and 5  illustrate aspects of a fuel injector and swirler assembly  58  according to the invention. A delivery tube structure  44  comprises coaxial cylindrical inner and outer tubes, providing a first fuel supply channel  43  in the inner tube and forming an annular second fuel supply channel  50  between the inner and outer tubes. Radial passages  45  in the swirl vanes  46  communicate with the first fuel supply channel  43 . A set of apertures  47  open between the radial passage  45  and the exterior surface  48  of the swirl vanes  46 . A first fuel, for example natural gas  49 , is supplied to the radial passages  45  and the apertures  47  via the first fuel supply channel  43  which is the inner tube of the delivery tube structure  44 . A second fuel supply channel  50  is arranged in the delivery tube structure  44  and extends to a downstream end  51  of the delivery tube structure  44 , where a lobed mixer  52  injects a second fuel, for example a synthesis gas fuel  53 , into the air  15  or fuel/air mixture  19  respectively in a co-flow arrangement. Due to the folded edges (=lobes  55 ) of the lobed mixer  52 , the contact surface between second fuel  53  and air  15  or first fuel/air mixture  19  is large. Moreover, secondary flow effects will be reduced. Both these aspects lead to a very good mixing performance of the injectors. Furthermore, a grain  54  is introduced in the middle of the lobed mixer  52 , effectively preventing that a fuel rich area remains unmixed in the centre. To maintain the swirl flow path, the lobes  55  of the mixer  52  are given a twist. Also the number of lobes  55  is preferably equal to the number of vanes  46 , and each lobe  55  is located directly downstream a vane  46 .  FIGS. 4 and 5  represent a design in which the LC-fuel capacity is large enough to inject LC fuels with a Wobbe number &gt;10MJ/Nm3. The larger passage for the LC fuel supply also reduces the risk for clogging. 
         [0041]      FIGS. 6 and 7  show two versions of the lobed mixer  52 , both versions have a twist incorporated to follow the swirl induced by the upstream swirl vanes  46 . Adding a twist to the lobes of the mixer allows for a better follow-up of the stream line of the air in the swirler cups. As a variation to this, however, an untwisted lobed mixer may also be implemented. The twist angle of the lobed mixer may be between 0° and 45°. The height of the lobes may lie between 0 and 0.5 times the annulus height of the shroud  57 . Another important parameter of the lobes is the ratio between their height and their width. This ratio should lie between 0 and 8, with a preferred value of 4. A ratio of 0 represents the situation in which the lobes are flat, and effectively a jet-in-cross flow injection is used. This more or less corresponds with the design shown in  FIG. 7 . 
         [0042]    When the combustion system is operating on the backup fuel (fuel oil or natural gas) the lobed mixer  52  may be purged with an inert medium (N 2  or steam) or with seal air from the mid-frame of the gas turbine. 
         [0043]    For assembling this fuel injector and swirler assembly, the lobed mixer  52  is first welded or brazed to the central first fuel (natural gas) supply channel  43 .  FIG. 8  illustrates the brazing locations of the next assembling steps. After having jointed the lobed mixer  52  and the first fuel supply channel  43  a brazing material is applied at least between central first fuel supply channel  43  and the radial passages  45  (see reference sign  59 ), between the second fuel supply channel  50  and the radial passages  45  (see reference sign  60 ) and between the second fuel supply channel  50  and the lobed swirler  52  (see reference sign  61 ). The assembled component is then brazed in a high temperature vacuum furnace in one cycle. 
         [0044]    Either, in the same breath, brazing material is also applied between the swirl vanes  46  and the shroud  57  (see reference sign  62 ) or, after the component left the furnace, the shroud  57  is welded, in particular tap welded, to the swirl vanes  46 .