Abstract:
A swirler including a fuel distributor is provided. Each fuel distributor includes a distribution element defining a cavity, an inlet opening arranged in the distribution element, at least one outlet opening arranged in the distribution element, and at least one third opening arranged in the distribution element, the cross-section of the at least one third opening being larger than the cross-section of the at least one outlet opening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2008/055762, filed May 9, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07009960.1 EP filed May 18, 2007, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a fuel distributor, in particular for a burner and a swirler. 
     BACKGROUND OF THE INVENTION 
     The main purpose of the burner is to mix fuel and air together to obtain stable and efficient combustion with good flame stability and the smallest possible amount of NOx emissions. Therefore, the burner design must ensure that the proper amounts of fuel and air are introduced in the right locations within the burner and that these amounts of fuel and air are thoroughly mixed, so that complete combustion takes place with a minimum amount of excess air in order to achieve optimum overall efficiency. 
     The two burner principles, which could be combined to use their respective advantages, are the premix combustion burner and the diffusion flame burner. 
     In the premix combustion burner, the air, required for combustion, is mixed with the burner fuel before delivery to the combustion zone. The better the mixing of fuel and air the less hot zones with a fuel/air ratio exceeding the stoichiometric requirements exist. Since flame temperature is the dominant factor driving NOx production it follows that the more fuel lean the mixture, the lower the NOx produced. 
     In the diffusion flame burner, the fuel is not mixed with the air ahead of the combustion zone, but delivered as pure fuel in the immediate vicinity of the combustion zone. Diffusion flame burners provide good flame stability. The NOx production is relatively high. 
     Low emission gas turbine engines often use a combustor with two operating modes including a pilot nozzle that forms a diffusion flame and a plurality of main nozzles for discharging a fuel/air mixture to form premixed flames as the main combustion around the diffusion flame. The U.S. Pat. No. 5,901,555 describes a conventional gas turbine with the main burners divided into a plurality of groups in accordance with the load. The flow rate of the pilot fuel is increased when the gas turbine load is low, to achieve stable combustion. When the gas turbine load is high, the ratio of the pilot fuel is decreased, to decrease the amount of NOx. Separately controllable fuel lines, valves, pipe work and a control logic are required to achieve the appropriate fuel flows to the pilot and main nozzles, increasing the cost of the engine. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved fuel distributor. 
     This object is achieved by the claims. The dependent claims describe advantageous developments and modifications of the invention. 
     An inventive fuel distributor uses the pressure gradient across the combustion system to control the proportion of fuel provided to different areas of the combustion system. These areas could provide pilot fuel at low loads, or better mixing of the fuel and air at high loads. 
     The system comprises a cavity with an inlet opening and at least two fuel injection openings. The fuel distributor relies on having a larger injection opening arranged in the cavity of the fuel distributor in an upstream section, relative to the flow of compressor air, and a smaller injection opening arranged in the cavity in a downstream section, relative to the flow of compressor air, and serving as feed near combustor pressure. 
     In an advantageous embodiment of the invention a restrictor is arranged at the inlet opening to balance between the fuel flows through the at least one smaller outlet opening and the at least one larger third opening, respectively. 
     In a further advantageous embodiment, the restrictor is adjustable to adapt the pressure for different fuel types. 
     Since at low fuel pressure fuel basically leaves the distributor at the outlet opening that is exposed to the lowest external air pressure, it is advantageous to use this outlet opening as pilot fuel injection opening. 
     For the same reason, it is advantageous to use the third opening with a larger cross-sectional area and exposed to higher external air pressure as main fuel injection opening. 
     In an advantageous embodiment the principle of the fuel distributor is applied to a diffusion flame burner, where the fuel distributor has a tubular form with the outlet opening at the end of the tube facing the combustion chamber and with third openings arranged upstream the tube, relative to the flow of the fuel. At low fuel flows, the majority of the fuel will enter the combustion chamber through the outlet opening. Compressor air can enter the fuel distributor through the third openings and give some premixing of the fuel and the air. As the fuel flow increases, the pressure in the cavity increases and fuel will spill out through the third openings and will mix with compressor air and enter the combustion chamber. 
     In another advantageous embodiment, the principle of the fuel distributor is applied to a swirler. The cavity of the fuel distributor is arranged in the base plate of the swirler. The fuel openings and the third openings are arranged in the mixing ducts, that is, in the passages of the swirler. The openings may be arranged in the base plate of the swirler or in the swirler vanes. If arranged in the swirler vanes, the arrangement could be at different heights to improve the fuel distribution over the swirler vane height. Smaller fuel outlet openings would be closer to the swirler exit hole with lower pressure. Larger third openings would rather be in an upstream part of the swirler passages relative to the flow of compressor air, with higher pressure. The fuel outlet openings would serve as pilot and the third openings as main fuel injection openings. 
     In yet another advantageous embodiment, the pressure drop of the air between an outlet opening and a third opening in a mixing duct or a swirler passage is controlled by making the mixing duct or swirler passage convergent or divergent. 
     With such a design of the fuel distribution system emissions of NOx are reduced. The inventive fuel distributor provides an increasing level of premix as the fuel flow increases. The inventive fuel distributor even provides some premixing of fuel and air at low flows, thus further reducing NOx emissions. Furthermore, the fuel/air mixing within a premix duct like e.g. a swirler passage can be varied as the fuel flow changes without the use of control valves, thus reducing costs and increasing reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be further described with reference to the accompanying drawings in which: 
         FIG. 1  represents an inventive diffusion flame burner at low fuel flow, 
         FIG. 2  represents an inventive diffusion flame burner at high fuel flow, 
         FIG. 3  represents a swirler, 
         FIG. 4  represents a fuel distributor arranged in a swirler base plate with openings in the swirler base plate at low fuel flow, 
         FIG. 5  represents a fuel distributor arranged in a swirler base plate with openings in the swirler base plate at high fuel flow, 
         FIG. 6  represents a fuel distributor arranged in a swirler base plate with openings in the side face of a swirler vane, 
         FIG. 7  shows a swirler vane corresponding to the fuel distributor of  FIG. 7 , 
         FIG. 8  shows the percentage of mass flow through the fuel injection openings as a function of the fuel mass flow, and 
         FIG. 9  represents a fuel distributor arranged in a swirler base plate with openings in the side face of a swirler vane and converging swirler passage. 
     
    
    
     In the drawings like references identify like or equivalent parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows the scheme of the inventive fuel distributor  1  applied in a diffusion flame burner  2 . The fuel distributor  1  comprises a distribution element  18  defining a cavity  3  with an inlet opening  4 , an outlet opening  5 , opposing the inlet opening  4 , and two third openings  6 . The third openings  6  are larger than the outlet opening  5 . A restrictor  7  is arranged upstream the inlet opening  4  relative to the fuel flow  8 , and sized to give the correct pressure to balance the fuel flows  8  between the outlet opening  5  and the third openings  6 . Pressure P 1  at the third openings  6  is greater than pressure P 2  at the outlet opening  5 . At low fuel flows  8 , the majority of the fuel  8  will enter the combustion chamber  9  through the outlet opening  5 . If the fuel flow  8  is low enough, air  10  may enter the cavity  3  through third openings  6  and give some premixing of the fuel  8  and air  10 . As the fuel flow  8  increases, the pressure in the cavity  3  increases. When the pressure in the cavity  3  is higher than P 1 , fuel  8  will spill out of the third openings  6 , as shown in  FIG. 2 , and mix with air  10 . The fuel/air premix will then enter the combustion chamber  9 . 
     Referring to  FIG. 3  a swirler  11  for a gas turbine engine is shown. The swirler  11  comprises swirler vanes  12  arranged on a swirler vane support  13 . The swirler vanes  12  can be fixed to a burner head (not shown) with their sides showing away from the swirler vane support  13 . Between neighbouring swirler vanes  12  swirler passages  14  are formed. The swirler passages  14  extend between a swirler passage inlet opening  15  and a swirler passage outlet opening  16 . The swirler passages  14  are delimited by opposing side faces  16  of swirler vanes  12 , by the surface of the swirler vane support  13  which shows to the burner head (not shown) and by a surface of the burner head to which the swirler vanes  12  are fixed. Outlet openings  5  and third openings  6  are arranged in the swirler passages  14  in the swirler vane support  13 . 
     Referring to  FIGS. 4 and 5  a cross-sectional view of an inventive fuel distributor  1  arranged in a swirler vane support  13  is shown. The outlet opening  5  and the third opening  6  open out into a swirler passage  14 . Compressor air  10  is entering the swirler passage  14  from the left by the swirler passage inlet opening  15 , where the pressure P 1  exceeds the pressure P 2  at the swirler passage outlet opening  16 .  FIG. 4  shows the fuel distributor at low loads. Fuel  8  enters the cavity  3  of the fuel distributor  1  by the inlet opening  4  through the restrictor  7 . A predominant proportion of the fuel  8  enters the swirler passage  14  through the outlet opening  5 . Only a small amount of fuel  8  enters the swirler passage  14  through the third opening  6 . This is beneficial for providing pilot fuel to the outlet opening  5 . At very low load a part of the compressor air  10  entering the swirler passage  14  flows into the cavity  3  through the third opening  6 , leading to some premixing in the cavity  3 . 
     At high loads the proportion of fuel  8  entering the swirler passage  14  through the third opening  6  is increased, as shown in  FIG. 5 . The fuel pressure in the cavity  3  overcomes the pressure at the swirler passage inlet opening  15  and fuel  8  spills out into the swirler passage  14  mainly through the third opening  6  with the larger cross-sectional area. 
       FIGS. 6 and 7  show an alternative arrangement where the distribution of the fuel flow  8  across the height of the swirler passage  14  could be varied. The cavity  3  of the fuel distributor  1  is again arranged in the swirler vane support  13 . The outlet opening  5  and the third opening  6  are arranged at different heights of the swirler vane  12  in the swirler passage  14 . Again, the outlet opening  5  has a smaller cross-sectional area and is arranged close to the swirler passage outlet opening  16 , where the pressure is low and the third opening  6  has a larger cross-sectional area and is arranged close to the swirler passage inlet opening  15 , where the pressure is higher than at the swirler passage outlet opening  16 . 
     Referring to  FIG. 8  the percentage of fuel mass flow through the outlet opening  5  and the third opening  6  as a function of the total fuel mass flow is shown. At low load, i.e. at low mass flow, fuel mainly flows through the outlet opening  5 . The higher the fuel mass flow the higher the percentage of fuel flowing through the third opening  6 . 
       FIG. 9  shows a further embodiment of the inventive fuel distributor  1 . The general layout is similar to the embodiment described in  FIG. 6 . The pressure drop of the air  10  in the swirler passage  14  between the outlet opening  5  and the third opening  6  is varied by making the swirler passage  14  convergent (as shown in  FIG. 9 ) or divergent (not shown).