Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gas turbine engines and, more particularly, to an air/fuel mixer for a combustor. The type of gas turbine engine may be used in power plant applications. 
     2. Description of the Prior Art 
     Low NOx emissions from a turbine engine of below 10 volume parts per million (ppmv) are becoming an important criterion in the selection of turbine engines for power plant or aircraft applications. The current technology for achieving low NOx emissions involves a combination of a combustor with a fuel/air premixer. This technology is known as Dry-Low-Emissions (DLE) and offers the best prospect for clean emissions combined with high engine efficiency. The technology relies on a higher air content in the fuel/air mixture. 
     An air/fuel mixer is described in copending U.S. patent application Ser. No. 09/742,009, filed on Dec. 22, 2000, and assigned to the present applicant, which is herewith incorporated by reference. As described in that patent application, it is important to provide a uniform fuel/air mixture in the burn zone of a combustion chamber. The challenge is to achieve low emissions over different load conditions, yet obtain low cost of operation. 
     Although the above-mentioned application describes a particular fuel manifold assembly for a DLE system, it does not teach the environment in which the assembly would be used in a combustion chamber. For one thing, the burn zone should be located in a location within the chamber where the flame can be stabilized and to avoid coming into contact with the walls of the combustor can forming the chamber. It is also important to prevent cooling air from entering the burn zone formed in the combustion chamber. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide an improved fuel/air mix in a burn zone formed within the combustion chamber. 
     It is a further embodiment of the present invention to provide an air/fuel mixer using a fuel manifold instead of a nozzle. 
     It is a further aim of the present invention to provide a combustion chamber with a low power ignition stage and a second stage for full load combustion. 
     A combustion system in accordance with the present invention comprises a gas turbine engine having an annular cylindrical combustion casing with an inner wall and a radially spaced outer wall defining a combustion chamber, an annular air/fuel inlet at an end of the combustion casing, concentric with the inner and outer walls, a combustion chamber outlet downstream of the combustion chamber, the air/fuel inlet including a diffuser passageway formed between diffuser portions of the inner and outer walls respectively wherein each inner and outer diffuser wall portion has an upstream and a downstream portion relative to the air flow; the diffuser passageway formed by the adjacent inner and outer diffuser wall portions includes a converging cross-sectional section at the upstream portion of the inner and outer diffuser wall portions and a diverging cross-section at the downstream portion of the diffuser inner and outer wall portions and a throat is defined at the narrowest part of the passageway formed by the diffuser inner and outer wall portions; a concentric fuel manifold ring is provided upstream of the diffuser passageway whereby the manifold ring is located in axial alignment upstream of the diffuser passageway whereby air flows around the manifold ring and through the diffuser passageway mixing with fuel from the manifold ring and directed to a burn zone in the combustion chamber. 
     In a more specific embodiment of the present invention, the angle of the downstream portions of the diffuser inner and outer wall portions is selected to define the location of a burn zone in the combustion chamber. 
     Furthermore, in a yet more specific embodiment, the inlet may be offset relative to the inner and outer walls of the combustion casing in order to better locate the burn zone within the combustion chamber. 
     In a further embodiment of the present invention, a pair of annular air/fuel inlets is provided at the end of a combustion casing concentric with each other and with the inner and outer walls of the casing. The pair of annular air/fuel inlets includes an inner inlet adjacent the inner wall and an outer inlet adjacent the outer wall and an intermediate annular wall concentric with the inner and outer walls and located between the inner and outer inlets such that inner and outer combustion chambers are formed; each inner and outer air/fuel inlet including an inner and outer diffuser passageway respectively, wherein the outer passageway is formed between inner and intermediate diffuser portions of the outer and intermediate walls and wherein each outer and intermediate diffuser wall portion has an upstream and a downstream portion relative to the air flow; the inner passageway is formed between inner and intermediate diffuser portions of the inner and intermediate walls wherein each inner and intermediate diffuser wall portion has an upstream and a downstream portion relative to the air flow; the inner and outer diffuser passageways each include a converging cross-sectional section at the upstream portion of the diffuser wall portions and a diverging cross-section at the downstream portion of the diffuser wall portions and a throat is defined at the narrowest part of the passageway; and an inner and an outer concentric fuel manifold ring is provided upstream of each inner and outer diffuser passageway respectively whereby each inner and outer fuel manifold ring is located in axial alignment with the respective inner and outer diffuser passageway whereby the air flow flows around each manifold ring mixing with fuel from the respective inner and outer manifolds and through the respective inner and outer diffuser passageway and into the inner and outer combustion chamber respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration, a preferred embodiment thereof, and in which: 
     FIG. 1 is a schematic fragmentary axial cross-section showing the combustion section of a gas turbine engine in accordance with the present invention; and 
     FIG. 2 is a fragmentary axial cross-section, similar to FIG. 1, but showing another embodiment thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIG. 1 shows an embodiment of a gas turbine engine used for a power plant application. An engine casing  10  is illustrated. The casing is cylindrical and surrounds an annular combustion can  12 . The combustion can  12  has an inlet  14 , and the combustion chamber  15  defined by the can  12  exhausts in a reverse direction through the turbine section  16  which includes a typical turbine wheel  18 . 
     The combustion can  12  includes an outer cylindrical wall  20  and an inner concentric cylindrical wall  22 . The annular combustion can  12  is surrounded by a cooling air space  24 . 
     The inlet  14  is located axially at one end of the combustion can  12 . The inlet is made up of a pair of spaced-apart inner and outer inlet wall portions  32  and  30  respectively. These inlet and outlet wall portions  32 ,  30  are extensions of the inner cylindrical wall  22  and outer cylindrical wall  20 . An annular fuel manifold ring  50  is located in the annular space defined by the outer inlet wall  30  and inner inlet wall  32 . Air flow space is provided around the fuel manifold ring  50 , as will be described later. 
     The fuel manifold  50  is better described in copending U.S. patent application Ser. No. 09/742,009 and includes a fuel line  48  which communicates with an annular chamber within the manifold  50 . A slotted axial opening is provided downstream of the ring, and typically fuel will pass through openings in the so-formed slot to migrate towards the downstream end of the manifold ring where it will be picked up by the shearing action of the air flow passing around the manifold  50  and heading downstream towards the passageway  34  formed between the outer inlet wall  30  and the inner inlet wall  32 . The passageway  34  includes a throat  44  which is defined by upstream converging wall portions  36  and  38  and down stream diverging diffuser outer and inner wall portions  40  and  42  respectively. To define the throat area, the following formula should be followed: 
     
       
         
           M=ACd{square root over (2 ρΔP )} 
         
       
     
     wherein 
     M=mass flow 
     ACD=effective flow area 
     ρ=density of the air 
     ΔP=pressure drop 
     It is possible to relax the tolerance with respect to throat  44  by including airholes between inlet  14  and manifold  50 . 
     Thus, the air, which represents 97% of the fluid passing through the passageway  34  and the fuel being mixed with the air presents a homogeneously mixed air/fuel fluid in the burn zone  46  defined centrally within the combustion chamber  15 . The burn zone  46  is located in an area spaced from the inner and outer combustor walls  20  and  22 . This is accomplished by specifically selecting the angle of the diffuser walls  40  and  42  as well as locating the inlet  14  offset from the center line of the combustion chamber  15 . Thus, the inlet will be selected by locating the inlet and by arranging the angle of walls  40  and  42  to arrive at the best location for the burn zone  46  in a given engine. 
     The burn zone  46  in the combustion chamber is kept cool by providing impingement liners  26  on the exterior of the outer and inner walls  20  and  22  of the combustion can  12 . This enables the combustion process to be controlled and to avoid wall quenching. 
     Referring now to the embodiment shown in FIG. 2, a double combustion chamber  112  is illustrated as being within an engine casing  110 . In this case, there is an outer burn zone  146  and an inner burn zone  246  which is created and separated by intermediate walls  123  and  223 . Thus, the outer wall of the combustion chamber is illustrated at  120 , and the inner combustor wall is illustrated at  222 . 
     Likewise, there are two inlets  114  and  214  which are concentric to each other as well as to the combustion chamber walls  120  and  222 . Impingement liners  126  and  226  are also strategically located to surround the intermediate walls  123  and  223  as well as the inner wall  120  and outer wall  222 . The air space  124  and  224  surrounds the two combustion chamber sections. 
     The outer inlet  114  includes outer inlet wall segment  130  and intermediate inlet wall portion  132  defining a passageway  134  with converging inlet wall portions  136  and  138 . Similarly, there are diverging diffuser inlet wall portions  136  and  138 . Finally, the fuel manifold ring  150  is fed by fuel line  148  and is set upstream of passageway  134 . 
     The main inlet  214  has a similar construction with inner inlet wall segment  232  and intermediate inlet wall segment  230  defining passageway  234 . The fuel manifold ring  250  is located upstream of inlet  234 . 
     The provision of two annular combustion chambers, such as in the embodiment of FIG. 2, operates as follows. The outer combustion chamber  115  includes fuel manifold  150  and is used to light and operate the engine below approximately 60% load capacity. To accelerate the engine to full load, the inner combustion chamber  215  includes fuel manifold  250  which is then supplied by fuel, and the fuel/air mixture so formed will ignite, due to the burning process in the outer combustion chamber  115 . This allows the combustor to operate with literally no quenching effects and providing low CO emissions at low power. The ignition and mainstage might be reversed depending on the operating requirements of the combustor.

Technology Category: 2