Abstract:
An improved gas turbine combustor ( 20 ) including a basket ( 26 ) and a multiplicity of micro openings ( 29 ) arrayed across an inlet wall ( 27 ) for passage of a fuel/air mixture for ignition within the combustor. The openings preferably have a diameter on the order of the quenching diameter; i.e. the port diameter for which the flame is self-extinguishing, which is a function of the fuel mixture, temperature and pressure. The basket may have a curved rectangular shape that approximates the shape of the curved rectangular shape of the intake manifolds of the turbine.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to gas turbines in general, and in particular to an improved micro-combustor for use in such gas turbines. 
       BACKGROUND OF THE INVENTION 
       [0003]    Combustor assemblies are integral components of gas turbine engines. The combustor assembly is positioned in flow communication with a compressor, a fuel injector and one or more turbines. During engine operation, pressurized air from the compressor and fuel from the fuel injector enter the combustor. The resulting fuel/air mixture is ignited to produce a high temperature combustion gas stream. The hot combustion gas then flows downstream to turbines for energy extraction. 
         [0004]    As the cost of natural gas and the adverse effects of its emissions increase, there has been a trend to use hydrogen as a fuel in stationary gas turbine engines. The flame speed and flashback tendency of hydrogen are much higher than natural gas fuels, requiring significant changes to the gas turbine combustors. There are two well-known methods to reduce flashback: 1) dilute the fuel/air mixture with gases that do not burn, such as nitrogen or steam; or 2) increase the inlet velocity of the fuel/air mixture above the flame propagation speed. Both of these methods reduce the overall turbine efficiency and have practical limitations. 
         [0005]    An example of a gas turbine that reduces undesirable nitrogen oxides (NO x ) and carbon monoxide (CO) emissions by providing a more homogeneous fuel/air mixture for main stage combustion is disclosed in U.S. Pat. No. 6,082,111, entitled ANNULAR PREMIX SECTION FOR DRY LOW—NO x  COMBUSTORS. However, the combustors disclosed in that patent will not allow the use of fuels such as hydrogen due to the flashback problem alluded to hereinabove. Flashback occurs when the flame speed of the fuel used is excessive and a flame literally flashes back to the source. This will occur when hydrogen, for example, is used in the conventional nozzle type combustors such as those disclosed in the above-cited patent. 
         [0006]    Another example of a combustor is disclosed in a technical paper entitled CONCEPT AND COMBUSTION CHARACTERISTICS OF ULTRA-MICRO COMBUSTORS WITH PREMIXED FLAME, by S. Yuasa, et al, and published in the proceedings of the Combustion Institute 30 (2005) 2455-2462. The micro-combustor disclosed in that paper is designed for use in ultra-micro gas turbines as an application of power micro-electromechanical systems (MEMS) technology. Such micro-combustors are low power laminar flow devices operating at low pressures and low temperatures, and as a result, heat loss and flame stability are significant considerations in the combustor design while flashback is of little concern. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention is explained in the following description in view of the drawings that show: 
           [0008]      FIG. 1  is a schematic diagram of a typical gas turbine that may employ a micro combustor of the present invention. 
           [0009]      FIG. 2  is a cross-sectional view of a micro combustor basket of the present invention. 
           [0010]      FIG. 3  is an end view of the micro combustor basket shown in  FIG. 2 . 
           [0011]      FIG. 3A  is an exploded view of a portion of the end wall of the micro-combustor, which view illustrates the array of micro-openings for fuel/air passage. 
           [0012]      FIGS. 4A through 4D  illustrate cross-sectional views of a variety of possible shapes for the micro openings forming jet fuel nozzles for release of the fuel/air mixture for ignition. 
           [0013]      FIG. 5  is a diagram showing use of a micro combustor of the present invention with a conventional pilot light. 
           [0014]      FIG. 6  is a diagram showing use of a micro combustor of the present invention as a pilot light with conventional combustors. 
           [0015]      FIG. 7  is a diagram illustrating a possible shape for a micro combustor basket of the present invention that approximates the shape of a conventional turbine inlet. 
           [0016]      FIG. 7A  is a diagram of the same micro-combustor basket shown in  FIG. 7 , but with an alternate embodiment of a wire mesh forming the micro-openings. 
           [0017]      FIG. 7B  is an exploded view of a portion of the wire mesh embodiment illustrated in  FIG. 7A . 
           [0018]      FIG. 8  is a diagram of a portion of a turbine inlet manifold. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    What is needed is a combustor for high power, high temperature, high pressure gas turbines that will reduce NO x  emissions without a loss in power output. The combustor disclosed herein employs a large number of very small fuel/air injectors or inlet openings whose respective diameters are each on the order of the quenching diameter of the fuel employed. The term quenching diameter, as used herein, is the largest cross-sectional opening size (e.g. diameter for a round hole or other corresponding limiting dimension for an opening with a non-round cross-section) that will extinguish a flame propagating through the opening. Quenching diameter is a function of the fuel/air mixture as well as the temperature and pressure conditions. For a flame to propagate through a tube, the rate of energy released by the chemical reaction must be greater than the heat loss to the tube wall. The combustor disclosed herein is less susceptible to flashback than prior art designs, thus facilitating the use of hydrogen as the fuel for the gas turbine engine. In one embodiment the limiting dimension of the openings may be no more than the quenching diameter. In other embodiments the limiting dimension may be no more than twice or thrice the quenching diameter, as examples. While these somewhat larger dimensions are greater than the quenching diameter, a combustor having such dimensions may demonstrate improved protection against flashback events when compared to prior art designs, while at the same time being less expensive to manufacture and offering less flow resistance than when the limiting dimension is no more than the quenching diameter. 
         [0020]    Referring now to the drawings and to  FIG. 1  in particular, a schematic diagram illustrates a typical gas turbine engine  10  that may employ a micro combustor of the present invention. The turbine  10  is typically cylindrical in cross section and rotates about a central shaft  12 . In a conventional well-known manner, when the turbine starts to rotate (e.g., by engagement of an electric starter engine) air is drawn into the engine as denoted by arrows  14  and then compressed by a compressor section  16 . Fuel is then injected into a fuel/air mix section  18  by means of fuel lines  20 . The compressed air and fuel mixture is then passed on to a combustion section  22 , where it is ignited. The combustion gases are then applied to a turbine section  24  for converting the energy of the ignited gases into rotation of the shaft  12  in the conventional well-known manner. 
         [0021]    Referring now to  FIGS. 2 ,  3  and  3 A, details of an embodiment of a combustor or combustor basket  26  of the present invention are shown. The basket includes a wall  27  having a plurality of openings  29  formed there through for defining an inlet  19 , and a peripheral wall  17  defining a combustion region  15 . The inlet wall  27  and optionally a portion  28  of the peripheral wall  17  of the combustor basket include a multiplicity of small openings  29 , also referred to herein as nozzles or injectors. A dimension of these openings is determined by the quenching diameter of the fuel/air mixture at gas turbine operating conditions. For a modern gas turbine engine burning a relatively lean mixture of hydrogen fuel (equivalence ratio of less than 1) at high pressure ratios (2-3 MPa) and high combustor inlet temperatures (325-525° C.) and high velocity (on the order of 100 m/sec), the quenching diameter is on the order of 3 mm±2.0 mm. The required fuel/air flow area for a hydrogen-burning combustor of such a typical land-based gas turbine engine used for a power generation application (typically about 20 MW per basket, for example) is about 0.03 m 2  To meet these conditions requires about 36 mm 2  of total area for each nozzle to allow for spacing between nozzles. This is about half the cross section area of a conventional combustor can or basket, or about half of the area of the inlet wall  27 . It should be noted that the small openings  29  may be unevenly distributed across the wall  27 . For example, all or a majority of the openings  29  may be formed near the center of the wall  27  so that the outer periphery remains cooler, thereby minimizing heat loss. 
         [0022]    There are three mechanisms that work together to prevent flashback through the micro openings  29 . First, the fluid velocity through the holes is higher than the flame progression speed. Second, the flame front is quenched within the hole because the radicals created by the oxidation reaction are re-combined by the wall  27  surface (i.e., the surface of the wall  27  acts like a catalyst for recombining the radicals, which prevents them from causing additional reactions). Third, the temperature of the gas is cooled by the surface of the wall  27 . Additional small openings  31  may be placed downstream along the circumference of the combustor to allow for axial staging of the fuel. 
         [0023]    The small nozzles (i.e., openings  29 ,  31 ) may be produced in sections of metal, composites or laminated material. A computer controlled laser may be used to produce the small openings. Each section of nozzles may have small cooling openings, which would introduce air in the absence of fuel and could be supplied via a separate manifold. As stated hereinabove, the fuel and air is mixed in the fuel/air mix section  18  upstream of the combustor  22 , which includes a plurality of the micro-combustors  26 . As the fuel/air mixture is injected through the small nozzles (i.e., openings  29 ,  31 ) it will come into contact with the burning mixture in the combustion region  15  and will combust. The small diameter of the openings  29 ,  31  will allow the mixture to burn in a very short time. If the flame attempts to propagate upstream into the openings, it will be quenched due to the small diameter of the openings. The overall effect will be to give a very thin, flat flame sitting just downstream from the wall  27 . Accordingly, it may be seen from the above that an added benefit of the present invention is that it creates a very short flame and hence very short time to burn the fuel. This short time will reduce the amount of NO x  that is formed. This will allow moving the combustor  22  closer to the turbine inlet. Moreover, since this arrangement creates greater stability of the flame, it may be possible to eliminate the pilot, which is typically employed in prior art gas turbines for stability purposes. It is noted that the small openings should minimize low frequency combustion instability (i.e., flame flicker) and may increase high frequency stability. The multiplicity of small openings may also decrease the chance that the entire flame goes into resonance, since each individual flame will react to local conditions and it may be less likely that the entire system resonates together. 
         [0024]    Cross-sectional views of a variety of possible shapes for the micro openings  29  are illustrated in  FIGS. 4A through 4D . In accordance with one embodiment for a land-based power generating gas turbine engine, the thickness of the rear wall  27  is approximately 5 to 20 mm, thereby making the nozzles  29  about 5 to 20 mm in axial length. It is noted that openings  31  may be formed into the same shape as those illustrated herein for openings  29 . The opening  29  shown in  FIG. 4A  is the simplest to manufacture. It is a straight through or linear opening and may also have rifling scores in the walls thereof, which would impart a swirling effect on the fuel/air mixture passing there through, which may be desired since the combustion downstream of the openings  29  will be turbulent.  FIG. 4B  illustrates a slanted wall opening with the wider opening on the side opposite the air flow or leeward side, which would impart less of a pressure drop on the fuel/air mixture passing there through (assuming the flow is from left to right as depicted by arrow  35 ).  FIG. 4C  illustrates a slanted opening wall construction opposite to that shown in  FIG. 4B , with the wider opening facing into the air flow or windward side. The advantage of using this shape for the openings  29  is that the velocity of the fuel/air mixture is accelerated at the point of combustion (downwind or lee side of the wall  27 ). Moreover, the smaller diameter at the exit side affords more protection against flashback.  FIG. 4D  illustrates an opening sloped on both sides of the wall  27 , which would produce the lowest pressure drop across the wall  27  of all four of the examples shown. 
         [0025]    Alternative applications of the micro-combustor of the present invention are illustrated in  FIGS. 5 and 6 , which are views taken along the section line  5 ,  6  of  FIG. 1 . Referring now to  FIG. 5 , an end view of one of a plurality of combustion baskets  25  is shown wherein a plurality of micro-combustors  26  are disposed annularly about a conventional pilot burner  36  all within the combustion basket  25 . Each micro-combustor  26  includes a multiplicity of micro openings  29  as described hereinabove. 
         [0026]      FIG. 6  is a diagram showing use of the micro-combustor  26  of the present invention as a pilot burner with conventional pre-mix nozzles  38 . This is the reverse arrangement of  FIG. 5 . Accordingly, it may be appreciated that the micro-combustor  26  disclosed herein may be retrofitted into existing turbine combustors as a replacement for the pre-mix nozzles ( FIG. 5 ) or pilot ( FIG. 6 ) or as a single unit replacing all of the burners in a combustor can/basket. 
         [0027]    As described hereinabove, combustors are typically circular in cross section. However, the inlet to the turbine  24  is rectangular or a curved rectangle (sometimes referred to as a “smiley face” or arch-rectangular) as illustrated in  FIG. 8 . When using a circular combustor, a transition is required to connect these two different shaped components together. Such a transition increases the complexity and cost of the combustion system. Due to the high temperature of the gases inside the transition, it is often necessary to employ a combination of high temperature thermal coatings and air or steam cooling. The sealing between the combustor and transition is prone to leakage and requires periodic maintenance. The transition also increases the distance the hot gases travel, and hence, increases the time the hot gases spend at high temperatures, which increases the formation of NO x  emissions. By using a combustor that is the same shape as the turbine inlet, a transition is not required and NO x  emissions can be reduced. The combined part can be made of metal, composites or laminated material. Moreover, this will reduce the cost of the total combustion system and the associated failure modes and cooling requirements of the transition. 
         [0028]    Current transitions are 30 to 50 cm in length when using conventional fuel injectors. However, this could be reduced to 10 to 20 cm by employing a combustor arrangement of the present invention because the flame length can be greatly reduced, as described hereinabove. Accordingly, the fuel injectors (i.e., openings  29 ,  31 ) may be installed even closer to the turbine inlet, further reducing the flame time and hence production of NO x  emissions. The total length of current combustors and transitions is about 100 cm, however by employing the teachings of the micro-combustor of the present invention this can be reduced to as little as 20 cm in one embodiment. 
         [0029]    Referring now to  FIG. 7 , an alternate shape of a micro combustor basket  40  is shown, which approximates the flow path cross-sectional shape of a conventional turbine inlet  45  as illustrated in  FIG. 8 . The turbine inlet  45  is one of twelve to sixteen such inlets arranged annularly in a manifold around the outer periphery of the turbine  24 . The blades (not shown) of the turbine  24  are directly behind the manifold inlets  45 . Accordingly, the combustor  40  and transition sections coupling the combustor to an individual inlet  45  of the gas turbine may be combined into one unit of similar cross sectional shape. The openings  29  are arrayed across the rear (inlet) wall  46  of the combustor  40  as described hereinabove. The constant shape combustor  40  of this embodiment of the invention will combine the functions of the current combustor and transition into a single unit that is smaller and lighter. The openings  29  are illustrated in  FIG. 7  as being unevenly arrayed across the inlet wall  46 , with a lower concentration of openings being placed near the periphery of the basket  40  in order to reduce the heat transfer through the basket peripheral wall. 
         [0030]    In another embodiment of the present invention, as illustrated in  FIGS. 7A and 7B , a wire mesh  29 ′ is employed in lieu of a rear wall  27 . The mesh  29 ′ defines a multiplicity of openings  29  between parallel wires of the mesh. The wire mesh  29 ′ would be selected so that the spacing between the parallel wires is no greater than the quenching diameter for the fuel/air mixture being used in the turbine engine, or no greater than twice or thrice the quenching diameter. The flow passage defined between the wires of such a wire mesh may provide a cross-sectional flow geometry similar to that illustrated in  FIG. 4D . 
         [0031]    It is to be noted that the combustor disclosed herein is not limited to land-based turbines. For example, the combustor of this invention may be employed in jet engines for airplanes or in any other embodiment ranging from 1 KW per can or higher, for example. If a fuel other than hydrogen is to be used, such as natural or synthetic gases, the micro-openings  29  or  29 ′ may be increased in size by an appropriate factor, such as by a factor of 2 or greater. 
         [0032]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.