Abstract:
An injector ( 4 ) for liquid fuel ( 6   a ) and a premix burner, in particular for combustion chambers of gas turbines which burner includes an injector ( 4 ) of this type, includes a swirl nozzle ( 14 ) which is surrounded by a shielding-air passage ( 11 ) and has an enlarged cross section of flow in the region of a nozzle-internal swirl generator ( 12 ) for the liquid fuel ( 6   a ), with this enlarged cross section of flow being reduced again toward an exit opening of the swirl nozzle ( 14 ). The injector allows the premix burner to operate with a reduced admission pressure and a high atomization quality.

Description:
[0001]     This application is a Continuation of, and claims priority under 35 U.S.C. § 120 to, International application number PCT/EP2005/052563, filed 3 Jun. 2005, and claims priority therethrough under 35 U.S.C. § 119 to German application number No 10 2004 027 702.8, filed 7 Jun. 2004, the entireties of which are incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an injector for liquid fuel having a swirl nozzle for injecting the liquid fuel, which is surrounded by a shielding-air passage. The invention also relates to a staged premix burner, in particular for a combustion chamber of a gas turbine, having a swirl generator for a combustion air stream, fuel exit openings for the staged introduction of gaseous fuel into the combustion air stream, and a central carrier, which has an injector with a swirl nozzle for the injection of liquid fuel and a shielding-air passage.  
         [0004]     2. Brief Description of the Related Art  
         [0005]     Premix burners, in which a swirl is imparted to incoming combustion air and this air is mixed with the fuel as a result of fuel being injected into a premix region, are in widespread use in gas turbine installations. When used in gas turbines, the premix burners have to cover the entire operating range with sufficient reliability. This operating range also includes, for example, the starting-up of the gas turbine, as part of which, during ignition of the burner, a fuel/air mix is to be burnt at combustion pressures and preheating temperatures close to ambient conditions. To ensure stable burner operation, the burner is generally operated with a pilot stage in this operating range. For reasons of symmetry, this pilot stage is arranged centrally in the burner flow field, for example in the form of a fuel lance. The fuel is in this case added to the combustion air axially in the direction of flow at the tip of the fuel lance via an injector, in such a way that fuel-rich zones are present in the flow field of the burner, and therefore stable operation without the flame being extinguished is ensured, even at low combustion pressures and temperatures. At higher operating loads, the injection of fuel via the pilot stage is generally lowered in order to reduce pollutants, and the burner is operated in the advantageous premix mode.  
         [0006]     A further demand imposed on premix burners and all other modern gas turbine burners is that the burner should optionally be usable for gaseous and liquid fuels. This requires a further arrangement of the fuel injection, which is generally likewise positioned at the tip of the fuel lance. To avoid fuel-containing gaseous backflow into the fuel feeds to the fuel lance, the fuel injector has an annular gap at the fuel lance, with a small proportion of air, based on the total burner air flow, flowing out of the annular gap. This shielding air, as it is known, shields the two fuel nozzles for liquid and gaseous fuel with respect to undesirable backflow.  
         [0007]     One example of a configuration of a known premix burner of this type is illustrated highly schematically in  FIG. 1  in the form of a side view ( FIG. 1   a ) and plan view ( FIG. 1   b ). In this example, the swirl generator  1  is formed from two part-shells, which have been assembled to form a swirl body in the shape of a cone envelope. Between the two part-shells are air entry slots  2  for the tangential entry of combustion air, indicated in the figure by the arrows which can be seen at this location. In premix operation, gaseous fuel is introduced in the combustion air along these air entry slots via feeds (not visible) and fuel exit openings, in order to be mixed with the combustion air within the volume predetermined by the swirl generator  1 . A fuel lance  3  can be seen centrally within the burner, this fuel lance having an injector  4  arranged at its end, with nozzle openings for the injection of liquid fuel, of gaseous fuel, and for the emergence of shielding air. The nozzle opening  7  for the introduction of gaseous fuel  7   a  can be recognized in the center of the fuel lance  3  in the right-hand part of the figure. This nozzle opening  7  is surrounded by a gap  5  for the emergence of shielding air  5   a . The nozzle opening  6 , which in the present case is in the shape of a ring, for the introduction of liquid fuel  6   a  forms the outer region of this injector  4 .  
         [0008]     In the lower load range of the gas turbine, this premix burner is operated exclusively with one of the two pilot stages, i.e., by the injection of liquid fuel  6   a  or of gaseous fuel  7   a  via the injector  4  of the fuel lance  3 . In the upper load range, in the case of gaseous fuels it is necessary to switch over completely to the premix stage, on account of the high level of pollutants emitted in pilot operation. In the case of combustion of liquid fuels in the pilot stage, an emulsion of water and oil is burnt as liquid fuel in the upper load range. The introduction of the water locally lowers the flame temperature in the flow field. This leads to a drop in the pollutant emissions, in particular the nitrogen oxide emissions. In addition to the pollutant emissions, pulsed combustion, which can lead to restrictions in the operating range, should also be avoided in the upper load range. Oil/water emulsion flames, which burn particularly stably, are in this case generated using swirl injectors, also known as pressurized swirl injectors. However, swirl injectors of this type cause a high pressure loss for the fuel, on account of the high throughput and the limited space in the region of the tip of the fuel lance.  
       SUMMARY OF THE INVENTION  
       [0009]     One of numerous aspect of the present invention includes providing an injector for liquid fuel and a premix burner having an injector of this type, which produce a good atomization quality at a low admission pressure for the liquid fuel.  
         [0010]     Advantageous configurations of the injector and of the premix burner can be found in the following description and the exemplary embodiments.  
         [0011]     An exemplary injector for liquid fuel includes a swirl nozzle for injecting the liquid fuel and a shielding-air passage which surrounds the swirl nozzle. In this context, the term liquid fuel is to be understood as meaning not only pure fuel, such as for example oil, but also a mixture or emulsion of this fuel with other substances, in particular an oil/water emulsion.  
         [0012]     The swirl nozzle has an internal swirl generator for the liquid fuel flowing through it, and widens out in the region of the swirl generator to an enlarged cross section of flow, which is reduced again toward the exit opening of the swirl nozzle. On account of the lance being in stepped form on the gas side, more space remains compared to conventional injectors for a larger nozzle, with the result that the pressure loss can be reduced. This allows this injector to be operated with a low admission pressure yet a good atomization quality.  
         [0013]     The nozzle-internal swirl generator is preferably designed as a swirl grate which, by way of example, can extend around a central swirl body inside the nozzle. Furthermore, it is advantageous for a swirl generator likewise to be arranged within the shielding-air passage surrounding the swirl nozzle. The two swirl generators can in this case generate swirls both in the same direction and in opposite directions. The generation of a swirl in the shielding air which is in the opposite direction to the nozzle-internal swirl generator can have a positive influence on the atomization quality of the liquid fuel which emerges. The strength and direction of the swirl can be optimized by the geometric configuration of the swirl generator, in order to generate an injection of the liquid fuel which is optimum for the particular application.  
         [0014]     In one configuration of the injector, the exit region, i.e., in particular the boundary walls of the pressure swirl nozzle and of the shielding-air passage at the injector exit, is designed in such a way that the shielding air and the liquid fuel emerge from the injector in approximately parallel directions of flow.  
         [0015]     A further configuration provides for the shielding air to be permitted to emerge from the injector at an angle with respect to the direction of flow of the liquid fuel, so that the shielding air exerts shearing forces on the liquid fuel which emerges. This can be achieved by suitably shaping the exit opening for the shielding air, in particular the outer casing which delimits the shielding-air passage. The shear rate which is generated thereby allows the atomization results, when the liquid fuel emerges, to be improved. Particularly high shear rates between the shielding air and the liquid fuel can be achieved with shielding air which emerges virtually perpendicular to the direction of flow of the liquid fuel.  
         [0016]     In a further advantageous configuration of the injector, the swirl nozzle and the outer casing which surrounds it and, together with the swirl nozzle, forms the shielding-air passage, are arranged so as to be displaceable with respect to one another in the axial direction, i.e., in the main direction of flow of the liquid fuel. This allows the geometric shape of the exit opening for the shielding air to be varied by the displacement of these components with respect to one another. When used in a premix burner of a gas turbine combustion chamber, this displacement preferably takes place as a function of the combustion air temperature and therefore the load on the gas turbine. In the upper load range, the shielding-air velocity can be reduced by suitable displacement, and consequently the atomization, in particular the spray angle and the spray quality, can be altered.  
         [0017]     Of course, the latter embodiments can be realized both with and without a swirl generator in the shielding-air passage. If there is a swirl generator in the shielding-air passage, in this case too it is possible to additionally influence the atomization quality of the liquid fuel by means of the swirl angle and/or the swirl direction.  
         [0018]     The proposed staged premix burner has an injector of this type at the tip of a central carrier for a pilot stage. This central carrier may be designed, for example, in the form of a fuel lance. In this case, the premix burner is designed in such a way that the largest possible injector, i.e., an injector with a cross section which is widened as much as possible at the nozzle-internal swirl generator, can be used. The staged injection of fuel makes it possible to dispense with injection of gaseous fuel at the tip of the carrier as a pilot stage. Rather, pilot operation of this type with gaseous fuel is achieved by the staged fuel injection. For this purpose, the premix burner has at least two different groups of fuel exit openings with separate feeds for the staged introduction of the gaseous fuel into the combustion air stream. In one configuration of the premix burner, one of these groups of fuel exit openings can be formed in a part of the carrier which is located upstream of the injector. This group of fuel exit openings then forms the pilot stage for gaseous fuel.  
         [0019]     Of course, the different groups of fuel exit openings for the staged supply of gaseous fuel may also be arranged elsewhere. This applies, for example, to a configuration of the premix burner in which the swirl generator is formed by a plurality of part-shells which surround a premix space in the shape of a cone envelope and between which air entry slots are formed. All or at least some of the fuel exit openings for the staged supply of gaseous fuel are in this case formed in the region of the air entry openings.  
         [0020]     On account of the special injector, the present premix burner allows operation with a reduced pressure loss on the fuel side during spray formation, and in some configurations also allows additionally improved spray formation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present injector and the premix burner comprising this injector are explained in more detail below on the basis of exemplary embodiments in combination with the drawings, in which:  
         [0022]      FIG. 1  diagrammatically depicts an example of a premix burner according to the prior art, including a cross-sectional view in  FIG. 1   a  and an end view in  FIG. 1   b;    
         [0023]      FIG. 2  diagrammatically depicts an example of a configuration of a premix burner having the present injector, including a cross-sectional view in  FIG. 2   a  and an end view in  FIG. 2   b;    
         [0024]      FIG. 3  shows a first example of a configuration of the injector;  
         [0025]      FIG. 4  shows a second example of a configuration of the injector; and  
         [0026]      FIG. 5  shows a third example of a configuration of the injector. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0027]     The structure of a premix burner in accordance with the prior art, as diagrammatically depicted in  FIG. 1 , has already been explained in detail in the introduction to the description.  
         [0028]      FIGS. 2   a  and  2   b  diagrammatically depict an example of a configuration of the present staged premix burner in which the injector according to the invention is used. In a known way, this premix burner has a swirl generator  1 , which is composed of two part-shells surrounding a premix space in the shape of a cone envelope. Air entry slots  2 , which are indicated in  FIG. 2   b  (as seen in the opposite direction to the direction of flow), are formed between the two part-shells. Fuel exit openings  10  for gaseous fuel, which are supplied with this fuel via corresponding feeds, are arranged in the region of these air entry slots  2 . The introduction of the gaseous fuel into the combustion air stream, which enters tangentially through the air entry slots  2 , results in the gaseous fuel mixing with the combustion air, a process which is boosted by the swirl which has been imparted to the combustion air.  
         [0029]     In the present example, a group of fuel exit openings  10  for gaseous fuel is formed in the half of the burner close to the combustion space, forming one of in the present case two burner stages. A second group of fuel exit openings  9  for gaseous fuel is arranged in the central fuel lance  3  upstream of the tip of this lance  3 . This further stage for the supply of gaseous fuel can be used as a pilot stage during the start-up phase of the gas turbine which has this burner arranged in its combustion chamber. Of course, the first burner stage mentioned can also be divided into different stages in any desired way, and these different stages can be supplied with gaseous fuel independently of one another. Of course, these burner stages may also extend over the entire axial length of the swirl generator  1 .  
         [0030]     The present injector  4 , a plan view of which can be seen in  FIG. 2   b , is arranged at the tip of the fuel lance  3 . This figure shows the central exit nozzle  6  for the liquid fuel  6   a  and the annular exit nozzle  5  for the shielding air  5   a  surrounding it. The staged design of this premix burner avoids the need for an additional nozzle for gaseous fuel to be fitted at the lance tip, which means that more space is then available for the injector for liquid fuel. This highly advantageously allows the use of the proposed injector with the widened cross section of flow in the region of the nozzle-internal swirl generator.  
         [0031]     When the gas turbine is starting up, a large part of the fuel is added via the fuel lance  3 . Only at relatively high loads is the burner operated with lower levels of fuel via the lance stage, allowing the pulsation and pollutant emission properties to be optimized.  
         [0032]     FIGS.  3  to  5  show exemplary configurations of injectors  4  as can be used in a premix burner in accordance with  FIGS. 2   a  and  2   b . The geometric shape of the swirl nozzle  14  is clearly apparent from the configuration shown in  FIG. 3 . This swirl nozzle  14  is considerably locally widened in terms of its cross section of flow in the region of the nozzle-internal swirl grate  12 . The swirl grate is in this case formed around a central swirl body  15 . On account of the more favorable space conditions in the tip of the lance compared to conventional injectors, it is possible to install a larger nozzle, with the result that the pressure loss is considerably reduced, so that this injector can be operated with a reduced admission pressure at a high atomization capacity.  
         [0033]     The shielding-air passage  11  surrounds the swirl nozzle  14 . In this exemplary embodiment, the shielding air  5   a  emerges from the lance tip in the axial direction. This is achieved by virtue of the geometric design of the boundaries of the shielding-air passage  11  at the exit end, which run parallel to the axial direction. An additional swirl grate  13 , which imparts a swirl in the same direction or the opposite direction with respect to the nozzle-internal swirl grate  12 , may optionally be arranged in the shielding-air passage  11 . The quality of atomization of the liquid fuel when it emerges from the injector can be influenced by the setting of the swirl angle.  
         [0034]      FIG. 4  shows a further example of the present injector  4 , in which the injector is of similar structure to that shown in  FIG. 3 . However, the injector  4  illustrated in  FIG. 4  has different exit directions for the shielding air  5   a  and the liquid fuel  6   a . On account of the inwardly directed design of the exit of the shielding-air passage  11 , the shielding-air flow is directed onto the direction of flow of the liquid fuel  6   a  at the exit from the injector. In the present example, the configuration illustrated imposes a flow field on the shielding air which is virtually perpendicular to the direction of flow of the liquid fuel, so as to produce a high shear rate between the shielding air and the liquid fuel. This high shear rate promotes the atomization effect. In this case too, a swirl grate  13  may be arranged in the shielding-air passage, which can boost the effect of atomization still further, in particular if an oppositely directed swirl is generated.  
         [0035]     Finally,  FIG. 5  shows a further example of an injector  4 , which is of similar design to the injector shown in  FIG. 4 . In this example, however, the shielding-air passage  11  has a variable geometry, which is achieved by making the outer casing  16  displaceable in the axial direction with respect to the swirl nozzle  14 . This displaceability is indicated by the double arrows in the figure. If the swirl nozzle  14  is displaced with respect to the outer casing  16 , the exit gap for the shielding air opens or closes up. The displacement is preferably carried out as a function of the combustion temperature and therefore the load on the gas turbine. By way of example, in the upper load range, this injector can reduce the shielding air velocity by increasing the shielding-air gap, and can thereby alter the spray angle and the spray quality.  
         [0036]     Configurations of the injector or premix burner adhering to principles of the present invention allow low-pollutant, pulse-free operation during the combustion of liquid fuels or fuel emulsions in a gas turbine combustion chamber. This is made possible in particular by the combination of a staged-fuel gas turbine burner on the gas side, allowing the installation of the injector, which is larger by virtue of local widening of the swirl nozzle, in the lance tip. The different configurations of the injector can be used to influence the atomization and/or spray characteristics in order to optimize the particular applications.  
         [0037]     List of Designations 
     1  Swirl generator      2 ,  2   a  Air entry slots      3  Fuel lance      4  Injector for liquid fuel      5  Exit nozzle for shielding air      5   a  Shielding air      6  Exit nozzle for liquid fuel      6   a  Liquid fuel      7  Nozzle opening      7   a  Gaseous fuel      8  Direction of flow or axial direction      9 ,  10 ,  10   a  Exit openings for gaseous fuel      11  Shielding-air passage      12  Nozzle-internal swirl grate      13  Swirl grate in the shielding-air passage      14  Swirl nozzle      15  Nozzle-internal swirl body      16  Outer casing    
 
         [0056]     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.