Patent Publication Number: US-6983600-B1

Title: Multi-venturi tube fuel injector for gas turbine combustors

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a fuel injector for gas turbine combustors and particularly relates to a multi-venturi fuel injector for catalytic and dry low-NOx applications. 
   The main components of a combustor for a gas turbine, for example, a catalytic combustor, include (1) a pre-burner which may typically constitute a diffusion style combustor that burns a small fraction of the fuel to elevate air temperature sufficiently to activate the catalyst downstream; (2) a pre-mixer which includes the main fuel injector and accomplishes fuel and air mixing; (3) a catalyst which partially converts the fuel in a flameless reaction in which no NOx is produced; and (4) a burn-out zone which includes homogeneous combustion in a post-catalyst liner of the lean fuel/air mixture flowing from the catalyst which does not generate NOx due to the relatively reduced temperature of the combustion. This type of combustor is capable of generating very low emissions. 
   A multi-venturi tube has been used in a catalytic combustor as a main fuel injector. See, for example, U.S. Pat. Nos. 4,845,952 and 4,966,001. These arrangements are intended to provide a uniform fuel/air mixture at the catalyst inlet. It will be appreciated that tight uniformity of the fuel distribution must be maintained over the large cross-sectional area at the catalyst inlet. Fuel/air mixing is accomplished by distributing the fuel among the large number of venturis that fill up the cross-section of the combustor followed by aerodynamic mixing inside the venturi tube as well as in the downstream region between the venturi exit plane and the catalyst inlet. In addition to uniform fuel/air mixture, the catalyst requires a uniform temperature and a uniform velocity across the catalyst inlet plane. Absent either one of these factors, the catalyst does not function optimally. It will also be appreciated that multiple venturi tubes produce laminar flow which suppresses large scale mixing and preconditions the flow such that only local mixing can be accomplished between the diffuser exit and the catalyst inlet. That is, mixing in that cross-sectional region is limited. For example, if a region of flow has a high temperature or velocity in comparison to the remaining flow, the thermal or velocity mal-distribution will deleteriously appear at the catalyst inlet. Accordingly, there is a need for a fuel injector for a gas turbine combustor affording improved uniform fuel/air, temperature and velocity distributions to the catalyst inlet. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In accordance with the preferred aspect of the present invention, there is provided in combination in a combustor, a flow conditioner, a venturi configuration having a diffuser with multiple sides and an improved fuel circuit. The flow conditioner may be of the type described and illustrated in co-pending U.S. patent application Ser. No. 10/648,203 filed Aug. 27, 2003, the disclosure of which is incorporated herein by reference. In addition to the flow conditioner, multiple venturi tubes having a frustum-like cross-sectional configuration are provided to enhance fuel/air mixing, to afford uniform distribution of the fuel/air, velocity and temperature at the catalyst inlet, and to eliminate flame-holding issues. The venturi configuration eliminates recirculation regions, i.e., flow gaps between the venturis in the exit planes and downstream thereof, as well as the potential for flame-holding. The venturis have a three body construction to improve fuel distribution among the various venturis and also to improve mechanical durability by thermo-shielding of the brazed joints of the construction. The venturi fuel circuit provides a secondary plenum between the main fuel plenum surrounding the venturis defined between spaced axial forward and aft walls and fuel supply inlets to the converging inlet of the venturis. By providing a secondary plenum in each venturi, the plane of fuel intake into the plenum is separated from the plane of fuel injection into the venturi by a maximum available distance. Also cold fuel flow is directed along the cold side of the fuel plenum thereby minimizing thermal stress at the front and aft plate brazed joints. 
   In accordance with a preferred aspect of the present invention, there is provided a combustor for a gas turbine comprising a combustor housing including a flow liner for receiving compressor discharge air; a main fuel injector downstream of the flow liner for receiving the compressor discharge air and mixing air and fuel; a catalytic section downstream of the main fuel injector for receiving a mix of air and fuel from the main fuel injector; the main fuel injector including (i) an array of venturis each including a convergent inlet, a throat and a diffuser for flowing a fuel/air mixture therethrough in a generally axial direction for exit from said diffuser, (ii) a front plate and (iii) an aft plate surrounded by an enclosure defining a fuel supply plenum between the plates; each plate having a plurality of openings for receiving the venturis; and each venturi inlet having at least one fuel supply hole for supplying fuel from the fuel supply plenum into the venturi inlet at a location axially upstream from the throat. 
   In accordance with another aspect of the present invention, there is provided a combustor for a gas turbine comprising a combustor housing including a flow liner for receiving compressor discharge air; a main fuel injector downstream of the flow liner for receiving the compressor discharge air; a catalytic section downstream of the main fuel injector for receiving a mix of air and fuel from the main fuel injector; the main fuel injector including an array of venturis about a combustor axis, each venturi including a converging inlet, a throat and a diffuser for flowing the fuel/air mixture, each venturi including a fuel supply hole for flowing fuel into the venturi, said diffuser having multiple discrete angularly related side walls therealong, the array of venturis being arranged in circumferential side-by-side relation to one another about the axis and spaced radially from one another. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a fragmentary perspective view with parts broken out and in cross section illustrating a portion of a catalytic combustor for use in a gas turbine incorporating a multi-venturi tube arrangement according to a preferred aspect of the present invention; 
       FIG. 2  is a perspective view of the multi-venturi tube arrangement; 
       FIG. 3  is a cross-sectional view thereof; 
       FIG. 4  is a cross-sectional view thereof taken generally about on line  4 — 4  in  FIG. 3 ; 
       FIG. 5  is an enlarged fragmentary view with parts in cross-section illustrating a venturi and the fuel plenums; 
       FIG. 6  is a fragmentary perspective view of a portion of the diverging tube of the venturi; and 
       FIG. 7  is an enlarged fragmentary end view of the diverging sections of the multi-venturi tubes as viewed in an upstream direction. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As will be appreciated a typical gas turbine has an array of circumferentially spaced combustors about the axis of the turbine for burning a fuel/air mixture and flowing the products of combustion through a transition piece for flow along the hot gas path of the turbine stages whereby the energetic flow is converted to mechanical energy to rotate the turbine rotor. The compressor for the turbine supplies part of its compressed air to each of the combustors for mixing with the fuel. A portion of one of the combustors for the turbine is illustrated in  FIG. 1  and it will be appreciated that the remaining combustors for the turbine are similarly configured. Smaller gas turbines can be configured with only one combustor having the configuration illustrated in  FIG. 1 . 
   Referring to  FIG. 1  a combustor, generally designated  10 , includes a preburner section  12  having an interior flow liner  14 . Liner  14  has a plurality of holes  16  for receiving compressor discharge air for flow in the preburner section  12 . Preburner section  12  also includes a preburner fuel nozzle  18  for supplying fuel to the preburner section. The flow of combustion products, from the preburner section has a center peaked flow distribution, i.e., both flow velocity and temperature, which does not result in the desired uniform flow to the additional fuel injectors, e.g., the venturi fuel type injectors described and illustrated in U.S. Pat. No. 4,845,952. The main fuel injector is designated  20  in  FIG. 1  and forms part of a multi-venturi tube arrangement of which certain aspects are in accordance with a preferred embodiment of the present invention. The air and products of combustion from the preburner section  12  and the fuel from the fuel injector  20  flow to a catalyst or catalytic section  22 . As a consequence there is a lack of uniformity of the flow at the inlet to the catalytic section  22 . One effort to provide such uniformity, has resulted in the design of a flow controller generally designated  24  between the preburner section  12  and the fuel injector  20 . Details of the flow conditioner  24  may be found in U.S. patent application Ser. No. 10/648,203 filed Aug. 27, 2003 for Flow Controller For Gas Turbine Combustors, the subject matter of which is incorporated herein by reference. 
   At the inlet to the multi-venturi tube arrangement  21  (hereinafter MVT) forming part of the main fuel injector  20 , there is provided a perforated plate  24  to assist in conditioning the flow of fuel/air to obtain optimum mixing and uniform distribution of the flows and temperature at the inlet to catalytic section  22 . 
   The main fuel injector  20  includes a pair of axially spaced perforated plates, i.e. a front plate  30  and an aft plate  32  ( FIGS. 1 ,  3  and  5 ). Plates  30  and  32  are perforated and form axially aligned annular arrays of openings, e.g., openings  34  in  FIG. 4  of plate  30 . A casing  36  defining a plenum  38  surrounds and is secured to the outer margins of the front and aft plates  30  and  32  respectively. As illustrated in  FIGS. 2 and 4 , a plurality of fuel inlets  40 , four being shown, are equally spaced about the periphery of the casing  36  for supplying fuel to the plenum  38 . 
   The openings through the plates  30  and  32  are closed by venturis generally designated  42  and forming part of the MVT  21 . Thus each pair of axially aligned openings  34  through the plates  30  and  32  receive a venturi  42 . Each venturi includes a converging inlet section  44 , a throat  46  and a diverging section or diffuser  48 . Each venturi is a three part construction; a first part including the inlet converging portion  44 , a second part comprising the throat and diffuser  46  and  48 , and a third part comprising an annular venturi member or body  50 . Body  50  extends between each of the axially aligned openings in the front and aft plates  30  and  32  and is secured thereto for example by brazing. The converging inlet section  44  of the venturi  42  includes an inlet flange  52  which is screw threaded to a projection  54  of the body  50 . The integral throat and diffuser  46  and  48 , respectively, has an enlarged diameter  56  at its forward end which surrounds the aft end of the inlet  44  and is secured, preferably brazed, thereto. 
   It will be appreciated that the space between the front and aft plates  30  and  32  and about the annular bodies  50  of each venturi constitutes a main fuel plenum  60  which lies in communication with the fuel inlets  40 . The main fuel plenum  60  lies in communication with each inlet section  44  via an aperture  62  through the annular body  50 , a mini fuel plenum  64  formed between the body  50  and the inlet  44  and supply holes  66  formed adjacent the leading edge of the inlet section  44 . The fuel supply holes  66  are spaced circumferentially one from the other about the inlet  44  and preferably are four in number. It will be appreciated that the fuel inlet holes  66  to the venturi are located upstream of the throat  46  and in the converging section of the inlet section  44 . Significantly improved mixing of the fuel/air is achieved by locating the fuel injection holes  66  in the converging inlet section of the venturi without flow separation or deleterious flame holding events. 
   Fuel from the fuel inlet plenum  38  circulates between the front and aft plates  30  and  32  and about the annular bodies  50  for flow into the venturis  42  via the fuel apertures  62 , the mini plenums  64  between the inlet sections  44  and annular bodies  50  and the fuel inlet holes  66 . With the fuel inlet holes located adjacent the inlets to the converging sections of the venturis, the fuel is injected in a region where the air side pressure is higher, e.g., compared to static pressure at the throat. It will be appreciated that the magnitude of the fuel/air mixing taking place in each venturi is directly related to the jet penetration which in turn depends on the pressure ratio across the fuel injection holes  66  and the jet momentum ratio, i.e., between the jets and the main flow stream. To increase the pressure ratio and decouple the fuel injection from airflow distribution, the fuel holes are located upstream of the throat. The fuel is therefore injected in a region where the air-side pressure is higher compared to the static pressure at the throat and therefore, for the same fuel side effective area, the pressure ratio is increased. An optimum pressure ratio-circumferential coverage is achieved. Air velocity is also lower than at the throat and therefore the jets of fuel adjacent the venturi inlet sections  44  develop under better conditions from a momentum ratio standpoint. Further, improved air fuel mixing due to this fuel inlet location is achieved also by the increased mixing length, i.e., the actual travel distance inside the venturi for the same overall length of tube. Additionally, the venturis  42  are fixed between the two plates  30  and  32  to form the main fuel plenum  60  between the plates and the outside surfaces of the venturis. Fuel is introduced into plenum  60  from the outside diameter. A general flow of fuel with some axial symmetry occurs from the outside diameter of the plenum toward the center of the MVT as the venturis are fed with fuel. Thus, a potential imbalance in fuel flow around the tubes and among the tubes with a penalty in mixing performance which occurs with fuel injection at the venturi throats is avoided since the fuel injection holes into the venturis are spatially displaced from a plane in which the general plenum flow occurs. Finally, because the fuel inlet injection holes  66  are located adjacent the venturi inlet section  44 , the potential for fuel jet induced flow separation inside the venturis is greatly reduced. 
   Referring now to  FIGS. 2 ,  6  and  7 , each diffuser  48  transitions from a circular shape at the throat  46  to a generally frustum shape at the exit. That is, the diffuser  48  transitions from a circular shape at the throat into multiple discrete angularly related sides  70  ( FIG. 7 ). Sides  70  terminate in circumferentially spaced radially extending side walls  72  as well as radially spaced circumferentially extending arcuate side walls  74  opposite one another. As illustrated, the diffusers  48  are arranged in circular patterns to achieve an axisymmetric geometry by transitioning from circular throat areas to generally frustum areas at their exits. Any gaps between the adjacent venturis both in a radial and circumferential directions are substantially eliminated as can be seen in  FIGS. 2 and 7 . Thus, as illustrated in  FIG. 7 , the radial extending walls  72  of each diffuser at each venturi exit lie in contact with and are secured to the corresponding wall  72  of the circumferentially adjacent diffusers. Similarly, the arcuate walls  74  of each diffuser exit lie in contact with adjacent walls  74  of the next radially adjacent diffuser exit. Also, the venturis are arranged in a pattern of circular arrays at different radii about the axis. Thus, gaps between the radially and circumferentially adjacent diffuser exit walls are minimized or eliminated at the exit plane. Previously, for example, as illustrated in U.S. Pat. No. 4,845,952, the exit plane of the venturi diffusers had large gaps between the circular exits. Those interventuri gaps produced large recirculation regions downstream of the exit plane which are filled in by the exit flow from the circular venturis. By transitioning from the circular cross-section at the throat of the venturis to generally frustums at the exit plane of the venturis with minimized or eliminated gaps between circumferentially and radially adjacent venturi exits, these prior large recirculation regions formed downstream of the venturi exits and the risk for flame holding are greatly reduced or eliminated. It will also be appreciated that by providing each venturi in a multi part construction, i.e., an inlet  44  and a combined throat and diffuser section  46 ,  48 , the inlet  44  can be removed for tuning, refurbishing or testing flexibility purposes. 
   Further, from a review of  FIG. 3 , the venturi exits are stepped towards the outside diameter and in an upstream direction. That is, the venturi exits are spaced axially increasing distances from a plane normal to the flow through the combustor in a radial outward upstream direction. This enables any gap between adjacent venturis to be further reduced. Also, by making the radial outer venturis shorter, the angle of the exit diffuser is reduced, e.g. to about 7.8° thereby reducing the potential for flow separation in the exit diffuser. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.