Abstract:
An object of this invention is to accelerate further mixing of a fuel and air independently of a flow rate of the fuel. 
     A gas turbine combustor comprises: a fuel nozzle for blowing out a gas fuel; an air nozzle plate with an air nozzle for jetting out the fuel and air into a combustion chamber after the blowout of the fuel from the fuel nozzle; and an obstacle formed inside the air nozzle; wherein the obstacle causes a collision of the fuel jet blown out from the fuel nozzle, and hence causes turbulence in an airflow streaming into the air nozzle. 
     According to the invention, mixing between the fuel and the air can be further accelerated independently of the flow rate of the fuel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a combustor, a method of supplying a fuel to the combustor, and a method of converting fuel nozzles in the combustor. 
         [0003]    2. Description of the Related Art 
         [0004]    Gas turbine combustors employ either diffusion burner or premix burner. In the diffusion burner, because of the high turn-down ratio from the startup of the combustor to the start of operation under rated load conditions, a fuel is injected into the combustion chamber directly to ensure the stability of combustion in a wide range. Premix burner, on the other hand, can reduce nitrogen oxides (NOx). The premix burner has had the problem that the entry of flames into the premixer causes a backfire resulting in thermal damage to the structure. 
         [0005]    JP-A-2003-148734, for example, describes a technique for arranging fuel nozzles and air nozzle plates at the upstream side of a combustion chamber and supplying fuel and air as coaxial flow to the chamber in order to avoid the above problem. 
       SUMMARY OF THE INVENTION 
       [0006]    Regulations and social demands relating to the environment have been increasing each day and further reduction of NOx has been a problem even in the combustor structure disclosed in JP-A-2003-148734. 
         [0007]    In addition, in the combustor structure of JP-A-2003-148734, a fuel jet with a momentum is blown out into each air nozzle. Accordingly, under high-fuel-flow rate conditions, in particular, the fuel jet has penetrated the turbulent flow region of an air flow formed at the fuel nozzle exit, and generated an insufficient fuel-air mixture. 
         [0008]    An object of the present invention is to accelerate further mixing of a fuel and air independently of a flow rate of the fuel. 
         [0009]    The present invention provides a gas turbine combustor comprising: a fuel nozzle for blowing out a gas fuel; an air nozzle plate with an air nozzle for jetting out the fuel and air into a combustion chamber after the blowout of the fuel from the fuel nozzle; and an obstacle formed inside the air nozzle; wherein the obstacle causes a collision of the fuel jet blown out from the fuel nozzle, and hence causes turbulence in an airflow streaming into the air nozzle. 
         [0010]    According to the present invention, further fuel-air mixing can be accelerated independently of a flow rate of the fuel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in a first embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0012]      FIG. 2  is a front view of the air nozzle as viewed from a downstream end thereof in the first embodiment; 
           [0013]      FIG. 3  is a sectional view showing the fuel nozzle, air nozzle, obstacle, and support member in the first embodiment, a relationship in position between the four members, and flows of the airflow and the fuel jet; 
           [0014]      FIG. 4  is a sectional view of the air nozzle and support member in the first embodiment; 
           [0015]      FIG. 5  is a sectional view showing a configurational example of the air nozzle, air nozzle plate, obstacle, and support member in the first embodiment; 
           [0016]      FIG. 6  is a sectional view showing another configurational example of the obstacle and support member in the first embodiment; 
           [0017]      FIGS. 7A and 7B  are a sectional view and a front view showing an example of air nozzle plate fabrication and grooving in the first embodiment, respectively; 
           [0018]      FIG. 8  is a sectional view showing yet another configurational example of the air nozzle, air nozzle plate, obstacle, and support member in the first embodiment; 
           [0019]      FIGS. 9A and 9B  are a sectional view and a rear view showing another example of air nozzle plate fabrication and grooving in the first embodiment, respectively; 
           [0020]      FIGS. 10A to 10C  are sectional views showing an example of a cross-sectional shape of the support member in the first embodiment; 
           [0021]      FIGS. 11A and 11B  are diagrams showing an example of a method of supporting the obstacle in the first embodiment and a further configurational example of the obstacle and the support member; 
           [0022]      FIG. 12  is a diagram showing another example of a method of supporting the obstacle in the first embodiment; 
           [0023]      FIGS. 13A and 13B  are diagram showing a variation on a shape of the obstacle in the first embodiment and an occurrence state of longitudinal vortices; 
           [0024]      FIGS. 14A and 14B  are diagrams showing another variation on the shape of the obstacle in the first embodiment; 
           [0025]      FIG. 15  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in a second embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0026]      FIG. 16  is an enlarged view of the fuel nozzle tip and obstacle in the second embodiment; 
           [0027]      FIG. 17  is a sectional view showing the fuel nozzle, air nozzle, and obstacle under a misaligned state of central axes of the fuel nozzle and air nozzle, an example of a relationship in position between the three members under the misaligned state, and associated flows of the airflows and fuel jet; 
           [0028]      FIG. 18  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in a third embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0029]      FIG. 19  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in a fourth embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0030]      FIG. 20  is a front view of an air nozzle and obstacle in a fifth embodiment; 
           [0031]      FIGS. 21A and 21B  are front views showing an example of an air nozzle and obstacle in the fifth embodiment; 
           [0032]      FIG. 22  is an enlarged view showing a flow of an airflow passing through a corner of the obstacle in the fifth embodiment; 
           [0033]      FIG. 23  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in a sixth embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0034]      FIGS. 24A and 24B  are a front view of the air nozzle and obstacle in the sixth embodiment and a sectional view showing the air nozzle, a support member, and the fuel nozzle, respectively; 
           [0035]      FIG. 25  is a sectional view showing the fuel nozzle, air nozzle, obstacle, and support member in the sixth embodiment, the relationship in position between the four members, and the flows of the airflow and the fuel jet; 
           [0036]      FIG. 26  is a front view of the air nozzle and obstacle in a seventh embodiment; 
           [0037]      FIG. 27  is a sectional view showing a fuel nozzle, air nozzle, and obstacle in an eighth embodiment, a relationship in position between the three members, and flows of an airflow and a fuel jet; 
           [0038]      FIG. 28  is a front view of the air nozzle, obstacle, and support member in the eighth embodiment; 
           [0039]      FIG. 29  is a sectional view showing a fuel nozzle and air nozzle in a comparative example, and an example of flows of an airflow and a fuel jet; 
           [0040]      FIGS. 30A and 30B  show an example of fabricating the fuel nozzle, obstacle, and support member in the sixth embodiment; and 
           [0041]      FIG. 31  is a schematic diagram of an entire gas turbine combustor. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    Embodiments of the present invention are described below. 
       First Embodiment 
       [0043]      FIG. 31  shows a sectional view of an entire gas turbine combustor according to an embodiment. After being compressed by a compressor  5 , air  10  flows into the combustor  100  through a diffuser  7  and moves past between an outer casing  2  and a combustor liner  3 . Part of the air  10  flows into a chamber  1  as cooling air  11  for the combustor liner  3 . A remainder of the air  10  flows through air nozzles  21  as an airflow  12  and flows into the chamber  1 . An air nozzle plate  20  with each air nozzle  21  connected thereto is disposed between the chamber  1  and fuel nozzles  22 . 
         [0044]    Fuel supply lines  15  and  16  are divided from a fuel supply line  14  having a control valve  14   a.  Also, the fuel supply line  15  includes a control valve  15   a  and the fuel supply line  16  has a control valve  16   a,  and the two supply lines can each conduct independent control. In addition, the fuel supply lines  15  and  16  have cutoff valves  15   b  and  16   b,  downstream with respect to the respective control valves. 
         [0045]    As shown in the figure, the combustor of the present embodiment has the plurality of fuel nozzles  22 . The fuel nozzles  22  are connected to a fuel header  23  that distributes a fuel to each of the fuel nozzles. The fuel header  23  is internally segmented into a plurality of rooms to divide the fuel nozzles according to group. The fuel from the fuel supply lines  15  and  16  flows into the rooms of the fuel header  23  and is supplied to the fuel nozzle groups. Since the fuel supply lines each includes a control valve, these supply lines can control part of the multiple fuel nozzles  22  collectively. The fuel, after being blown out from each fuel nozzle  22 , flows with the airflow  12  into the chamber  1  as a coaxial flow, thus forming a homogenous and stable flame. A hot combustion gas that has thus been generated enters a turbine  6 , then performs work in the turbine  6 , and is discharged therefrom. 
         [0046]      FIGS. 1 to 4  show details of the fuel nozzle  22  and the air nozzle  21 .  FIG. 2  is a front view of the fuel and air nozzles as viewed in an upstream direction from the chamber  1  disposed downstream in an axial direction.  FIG. 1  is a sectional view taken along line A-A in  FIG. 2 .  FIG. 3  is a sectional view taken along line B-B, and  FIG. 4  is a sectional view taken along line C-C in  FIG. 2 . 
         [0047]    The fuel jet  29  blown out from a fuel hole in the fuel nozzle  22  flows in an axial direction of the fuel nozzle  22  in  FIG. 1 . Also, the airflow  12  at an upstream end of the air nozzle plate  20  flows into the air nozzle  21  along a peripheral side of the fuel nozzle  22 . A cylindrical hollow section provided in the air nozzle plate  20  constitutes the air nozzle  21 . The airflow  12  that has flown into a very narrow space of the air nozzle  21  forms an annular layer at a peripheral side of the fuel jet  29 . The fuel nozzle  22  and the air nozzle  21  are arranged so that fuel and air flow through the inside of the air nozzle  21  with the annular airflow  12  enfolding the peripheral side of the fuel jet  29  blown out from the fuel nozzle  22 . 
         [0048]    Inside the air nozzle, an obstacle  24  is disposed at an axial downstream side of the fuel nozzle  22 , relative to the fuel hole in the fuel nozzle  22 . Accordingly, the fuel jet  29  collides against the obstacle  24  and becomes diffused vertically with respect to a central axis of the fuel nozzle  22 . That is to say, the fuel jet  29 , after colliding against the disc-shaped obstacle  24 , is diffused in a radial direction of a disc plane thereof. The “axial direction” in the present embodiment is a direction in which the fluids flow along the central axis of the fuel nozzle  22 , and the “radial direction” is a radial direction relative to the disc plane of the obstacle. 
         [0049]    The obstacle  24  also obstructs the flow of the airflow  12  and generates a very significant difference in velocity at a downstream region  44  formed at an edge of the obstacle. The obstacle  24  causes a strong turbulence  26  in the flow of the airflow  12  due to the difference in velocity. 
         [0050]    At this time, since the fuel jet  29  is widely distributed outward in the radial direction, radial velocity components become small, so the fuel jet  29  is considered not to widely spread outward in the radial direction from the edge of the obstacle  24 . For this reason, the fuel jet  29  is easily entrained in the region  44  that the turbulence occurs, and the fuel jet  29  is mixed with air. 
         [0051]    A comparative example is described below using  FIG. 29 . This comparative example applies to a case in which the fuel nozzle has no obstacle at its downstream side and is ribbed at its tip. In the comparative example, the rib of the fuel nozzle tip can be utilized to generate the turbulence  26  of the airflow at the downstream side relative to the fuel nozzle tip. However, since the fuel jet  29  has a momentum and exhibits opposition to the turbulence  26  of the airflow, mixing between the fuel and the air is limited. Particularly in cases that a fuel-to-air ratio becomes too high locally under partial load conditions or that the fuel used is heavily laden with hydrogen or carbon monoxide and has a small calorific value per volume, insufficient fuel-air mixing results since the fuel jet penetrates the occurrence region of the turbulence  26 . 
         [0052]    The present embodiment can therefore attenuate the momentum of the fuel jet  29  significantly, regardless of a flow rate of the fuel, by causing a prior collision of the fuel jet  29  against the obstacle. In addition, fuel-air mixing is achievable by providing the obstacle in the air nozzle to cause the disturbance in the air flowing into the air nozzle. This, in turn, makes further fuel-air mixing achievable by introducing the momentum-attenuated fuel efficiently into the turbulence  26  of the airflow occurring at a downstream side of the obstacle. 
         [0053]    In this manner, the fuel can be supplied to the turbulence  26  of the airflow having a significant difference in velocity, compared with that attainable in the comparative example, so an even greater mixing-acceleration effect can be obtained. It is also effective to increase typical length  32  of the obstacle to a size large enough for moderate blocking of the fuel jet, that is, a size equal to or greater than a fuel hole diameter  31  of the fuel nozzle. The fuel hole diameter  31  denotes a cross-sectional area of the fuel nozzle hollow region through which the fuel flows. 
         [0054]    In addition to a natural gas consisting mainly of methane, the present embodiment is applicable to gas fuels heavily laden with hydrogen or carbon monoxide, such as a coal gasification gas and the coke oven gas (COG) occurring during purification processes at iron or steel works. Use of these fuels further enhances the above-described mixing acceleration effect, compared with that obtainable in the comparative example. Furthermore, the present embodiment is also effective for other fuels heavily laden with nitrogen or carbon dioxide and having a low calorific value per volume. 
         [0055]    As described above, the present embodiment uses a gas fuel. Compared with liquid fuels, gas fuels are small in inertial force because of their low viscosities/densities. The gas fuel that has collided against the obstacle, therefore, flows towards the downstream side of the obstacle without colliding against an inner wall of the air nozzle  21 . The fact that the gas fuel, after colliding against the obstacle, flows towards the downstream side of the obstacle without colliding against the inner wall of the air nozzle  21  means that the gas fuel flows through a very narrow region present along an outer edge of the obstacle. 
         [0056]    Accordingly, since the obstacle is disposed in the air nozzle, turbulence of the airflow occurs at the downstream side along the outer edge of the obstacle. In the present embodiment, the turbulence  26  of the airflow and the region through which the gas fuel flows are substantially equal in size, such that mixing between the gas fuel and the air can be accelerated efficiently. 
         [0057]    If, as shown in  FIG. 1 , the fuel nozzle tip is also present inside the air nozzle  21 , the airflow  12  flowing around the fuel nozzle can be brought into a direct collision against the obstacle effectively by increasing the typical length  32  of the obstacle above an outside diameter  33  of the fuel nozzle tip. If the obstacle is of a circular shape, the typical length  32  of the obstacle denotes a diameter thereof. If the obstacle has a square shape, the typical length  32  of the obstacle denotes length of one side. A stronger turbulence  26  can be generated at the downstream side of the obstacle  24  by increasing the typical obstacle length  32  above the outside diameter  33  of the fuel nozzle tip. In addition to the cross-sectional area of the fuel nozzle hollow region through which the fuel flows, the outside diameter  33  of the fuel nozzle tip includes a cross-sectional area of the fuel nozzle pipe at a thick section thereof. 
         [0058]    Conversely, if, as shown in  FIG. 19 , the tip of the fuel nozzle  22  is present outside the air nozzle  21 , the typical length  32  of the obstacle can be small, compared with the outside diameter  33  of the fuel nozzle tip. Since the tip of the fuel nozzle  22  does not narrow an entrance area of the air nozzle  21 , the airflow  12  directly collides against the obstacle  24  and can thus cause the strong turbulence  26  at the downstream side of the obstacle  24 . 
         [0059]    As shown in  FIGS. 2 and 3 , a support member  25  is provided to support the obstacle  24  and to interconnect the air nozzle  21  and the obstacle  24 . The obstacle  24  in the present embodiment is of a circular disc shape, which distributes the turbulence  26  widely in annular form at the downstream side of the obstacle  24 , allowing uniform mixing of the fuel and the air. 
         [0060]    The support member  25  has a rectangular cross section as shown in  FIG. 4 . The support member itself also acts as a turbulence generator, generating turbulence  26  to assist the mixing of the air and the fuel. Thickness, however, is desirably suppressed to a level that does not affect strength, since an increased pressure loss will otherwise result. 
         [0061]      FIGS. 5 ,  6 ,  7 A, and  7 B show an example of fabricating the present embodiment. As shown in  FIG. 5 , two split members  20 - 1  and  20 - 2  are laminated together to fabricate the air nozzle plate  20 . In addition, as shown in  FIG. 6 , the obstacle  24  and the support member  25  are created as an integrated component. As shown in  FIGS. 7A and 7B , the air nozzle plate member  20 - 1  has a groove  27  for fitting the support member  25  thereinto. Imparting this construction to the support member allows the obstacle  24  to be disposed accurately in a central section of the air nozzle and thus a face of the obstacle  24  to be opposed vertically to the airflow. In this fabricating method, since a relationship in position between the air nozzle and the obstacle becomes easy to manage accurately, the amounts of air flowing into each air nozzle can be made constant. This, in turn, suppresses spatial variation of a fuel-air ratio in the chamber  1  and hence enables NOx reduction. Furthermore, the integrated component constituting the obstacle  24  and the support member  25  can be press-machined for mass production to reduce costs as shown in  FIG. 6 . 
         [0062]      FIGS. 8 ,  9 A, and  9 B show another example of fabrication. In this example of fabrication, the obstacle  24  and the support member  25  are integrated as a single component similarly to the foregoing example of fabrication, except that since the air nozzle plate  20  is constructed using one plate, a groove  27  is formed that extends from an upstream end deeply relative to the air nozzle plate  20 . As shown in  FIG. 8 , the integrated component constituting the obstacle  24  and the support member  25  is inserted into the groove  27  and secured thereto. This example of fabrication is effective in that the air nozzle plate requires no splitting. 
         [0063]      FIGS. 10A ,  10 B, and  10 C show variations on the cross-sectional shape of the support member  25 . Referring to  FIG. 10A , the support member has a triangular cross-sectional shape and is disposed so that an apex faces upstream. The triangular support member, as with the rectangular one, causes a flow separation  45  at a downstream end of the support member and hence, turbulence in the airflow. Compared with the rectangular one, the triangular support member creates a smooth flow at the upstream side and can thus slightly reduce any pressure loss. 
         [0064]    The cross section of the support member  25  in  FIG. 10B  is rhomboid. The flow separation  45  caused at the downstream side is dimensionally suppressed in comparison with the rectangular or triangular ones and a pressure loss can be correspondingly reduced, so any pressure loss in the entire nozzle can be lessened. 
         [0065]    The cross section of the support member  25  in  FIG. 10C  is circular. The flow separation  45  caused at the downstream side is dimensionally the smallest of all three forms described above, with any pressure loss being significantly suppressible. 
         [0066]      FIGS. 11A and 11B  show a variation on the method of supporting the obstacle  24 . Although the supporting methods hitherto described use two points to support the obstacle, this variation employs three-point support. This variation also assumes that as shown in  FIG. 11B , the obstacle  24  and the support member  25  are constructed as an integrated component. Because of the three-point support of the support member in  FIG. 11B , when the integrated component is mounted in the air nozzle  21  using any one of the fabricating methods shown in  FIGS. 5 to 9A  and  9 B, the plane of the obstacle  24  is easy to dispose vertically with respect to the axial direction of the fuel nozzle. 
         [0067]      FIG. 12  shows another variation on the method of supporting the obstacle  24 . In this variation, the number of support points is further increased to four. As with three-point support, four-point support makes it easy to dispose the plane of the obstacle  24  vertically with respect to the axial direction of the fuel nozzle, and increases strength as well. 
         [0068]      FIGS. 13A and 13B  show a variation on the shape of the obstacle  24 . The obstacle  24  in this variation has a triangular shape. In this shape, since a corner  54  protrudes towards the region through which air moves past, a longitudinal vortex  41  directed downstream from the corner  54  of the obstacle  24  is generated with the occurrence of turbulence due to the flow separation caused at the downstream side of the obstacle  24 . The longitudinal vortex  41  causes a further turbulence, allowing the acceleration of fuel-air mixing. In general, however, longitudinal vortices have the characteristics that they are long in life and that they elude attenuation. Therefore, a triangular obstacle  24  is desirably applied to the air nozzle disposed at a distant position from a flame surface. 
         [0069]      FIG. 14A  shows another variation on the shape of the obstacle  24 , as with  FIGS. 13A and 13B . The obstacle  24  in this variation is of a square shape, having more corners  54  than in the variation of  FIGS. 13A and 13B . This obstacle can therefore generate a longitudinal vortex at a larger number of positions. In addition, since an angle of the corners  54  is small, each longitudinal vortex generated is considered to weaken. Accordingly, if the longitudinal vortex can be attenuated prior to leaving the air nozzle, turbulences can be generated uniformly over the entire air nozzle interior. 
         [0070]    Furthermore, a multi-cornered polygonal or starlike shape or any other shape having protrusions with respect to a flow channel for air also yields a similar effect. The shape shown in  FIG. 14B , for example, is useable for the obstacle  24 . 
         [0071]    In the gas turbine combustor including plural combinations of such the fuel nozzle, air nozzle, and obstacle as described above, a fuel and air can be mixed at a very short distance and then supplied to the entire chamber  1  uniformly and homogenously. This allows combustion at a very low NOx emission level. Also, the combustor has stable mixing performance because of the fuel-air mixing state not depending upon the flow rate of the fuel. When the fuel-air ratio is high or a low-calorie fuel is used, therefore, deterioration of mixing characteristics can be suppressed, even if the flow rate of the fuel increases. In addition, when the fuel-air ratio is high or a low-calorie fuel is used, the fuel increases in blowout velocity and is distributed in a wide range upon collision against the obstacle. Accordingly, a boundary area between the fuel and the airflow is ensured sufficiently. Additionally, sufficient mixing occurs and NOx emissions can be reduced. 
         [0072]    Since the present invention allows two fluids to be mixed at a very short distance, the invention can be used not only as a gas turbine combustor, but also as a mixer for mixing two fluids at a short distance or as other combustors. 
         [0073]    The existing combustor described in JP-A-2003-148734 is convertible by replacing the combustor with that which employs the air nozzle plate of the present embodiment. 
       Second Embodiment 
       [0074]    A second embodiment is shown in  FIG. 15 .  FIG. 16  is an enlarged view of the fuel nozzle tip and an obstacle. The shape of the obstacle in the present embodiment is changed from the shape shown and described in the first embodiment. The second embodiment is substantially the same as the first embodiment in that the obstacle  24  is disposed at the downstream side of the fuel nozzle, inside the air nozzle  21 . A face of the obstacle  24 , formed at the upstream side, is formed into a conical shape and has a recess  56 . Forming this shape assigns to the fuel jet  29  a velocity vector of an inverse-directional component with respect to the blowout direction of the fuel jet  29  upon collision against the obstacle  24 , and generates vortices  43  in the flow of the fuel. In addition, since the fuel jet blown out from the fuel nozzle  22  becomes significantly recessed along the recess  56  in the obstacle  24 , a flow of air into the recess of the fuel jet generates vortices  42  at the airflow side as well. These vortices interfere with and strengthen one another, resulting in stronger turbulences, and mixing the fuel and the air. While maintaining the vortex components, the fuel jet  29  is acquired into a strong-turbulence generating region arising from an edge of the obstacle  24 , and the air and the fuel are further mixed. 
         [0075]    In this way, the present embodiment conducts a first mixing phase at the upstream side of the obstacle and can preassign turbulent components. Additionally, the embodiment conducts a second mixing phase at the downstream side of the obstacle and provides a further mixing acceleration effect. 
         [0076]    Constructing a gas turbine combustor that includes a number of fuel nozzles and air nozzles according to the present embodiment, as in the first embodiment, makes combustion achievable at a very low NOx emission level, since a fuel and air can be mixed at a very short distance and since the fuel-air mixture can be supplied to the entire chamber  1  uniformly and homogenously. 
       Third Embodiment 
       [0077]    A third embodiment is shown in  FIG. 18 . The shapes of the air nozzle and fuel nozzle in the present embodiment are changed from the shapes shown and described in the first embodiment. As in the first embodiment, the obstacle  24  is disposed downstream with respect to the fuel nozzle  22 , inside the air nozzle  21 , and is positioned so that the fuel jet  29  collides against the obstacle  24 . 
         [0078]      FIG. 17  shows a case in which the central axis of the fuel nozzle  22  in the first embodiment is shifted from central axes of the air nozzle  21  and the obstacle  24  significantly (decentered downward in a Y-direction. The flow of the airflow  12  into the air nozzle  21  is biased in such a case. Since the airflow  12  flows in great quantities into a wide-open end of the flow channel, a greater amount of air flows into an upper position of the Y-direction. This results in a significant flow separation  45  occurring near the tip of the fuel nozzle  22 , at the upper position of the Y-direction. 
         [0079]    Meanwhile, the fuel jet  29  blown out from the fuel nozzle  22  flows into a position that permits the jet to flow more easily and readily, such that a greater quantity of jet flows in an inverse direction relative to that of the strong flow separation  45  (i.e., downward in the Y-direction). This results in the distribution of the fuel being biased at the downstream side of the obstacle  24 . In addition, the bias in the distribution of the fuel is liable to remain at an exit of the air nozzle  21 . Continued combustion with the bias remaining unremoved causes a hot-flame region to occur locally, and resultingly increase NOx. 
         [0080]    In the present embodiment, therefore, the air nozzle  21  has a taper  50  at its entrance, and the fuel nozzle  22  also has a taper  51  at its tip. Constructing the embodiment smoothens the flow of the airflow  12  existing at a time up to an arrival at the obstacle  24 , and prevents the flow separation  45  in  FIG. 17  from occurring at the tip of the fuel nozzle  22 . As a result, any biases of the fuel distribution can be minimized, even if deviations occur between the central axes of the fuel nozzle  22 , the air nozzle  21 , and the obstacle  24 . Increases in NOx emissions due to biases of the fuel distribution can therefore be suppressed. 
         [0081]    To match the central axes of the fuel nozzle, the air nozzle, and the obstacle, machining accuracy of these members requires management during fabrication. Increases in NOx emissions due to mismatching between these central axes, however, can be minimized in the present embodiment. In addition, even if the machining accuracy of each member is lowered, costs can be reduced since NOx emissions can be suppressed with fuel-air mixing performance maintained. 
       Fourth Embodiment 
       [0082]    A fourth embodiment is shown in  FIG. 19 . The present embodiment with changes and conversions to the fuel nozzle and air nozzle shapes and fuel nozzle tip position in the first embodiment is effective for combustion, particularly of a fuel lower in calories and higher in flow rate. 
         [0083]    A higher fuel flow rate increases the velocity in the fuel nozzle, and hence, a pressure loss. Accordingly, a need arises, for example, to increase an initial pressure of the fuel and introduce changes in valve specifications, and conducting these changes and conversions is liable to increase a total plant cost. To avoid increases in the cost, an inside diameter of the fuel nozzle needs to be increased for reduced velocity inside the nozzle. In the configuration of  FIG. 1 , thickening the fuel nozzle  22  results in the internal flow channel of the air nozzle  21  being blocked significantly. This, in turn, increases any pressure drops at the airflow side and reduces total gas turbine efficiency. 
         [0084]    In addition, in a combination of the fuel nozzle and air nozzle according to the comparative example shown in  FIG. 29 , a rib  52  provided at the fuel nozzle tip generates turbulence in the airflow, thus prompting fuel-air mixing. However, if the tip of the fuel nozzle  22  is disposed upstream with respect to the entrance of the air nozzle  21  in order to avoid air nozzle blocking, periphery of the rib faces a wide space and reduces the air velocity at the periphery. Accordingly, the turbulence  26  stemming from the rib  52  is weakened to degrade the mixing acceleration effect. 
         [0085]    In the present embodiment, therefore, a taper  50  is provided at the entrance of the air nozzle  21  and the tip of the fuel nozzle  22  is disposed upstream relative to the entrance of the air nozzle  21 . The air nozzle plate  20  has the taper  50  at the entrance of the air nozzle  21  so that the cross-sectional area of the air flow channel gradually diminishes from the entrance, towards the downstream side. Thickening the fuel nozzle  22  does not block the flow channel of the air nozzle significantly. 
         [0086]    Additionally, the obstacle  24  is disposed inside the air nozzle  21 , air flows through a peripheral region of the obstacle  24  at high velocity, and thus a strong turbulence  26  occurs downstream with respect to the obstacle  24 . For this reason, fuel-air mixing can be accelerated. 
         [0087]    The fuel jet  29  collides against the obstacle  24  one time and loses the momentum. This prevents the mixing acceleration effect from being significantly limited by increases in the flow rate of the fuel. As described above, for a fuel having a low calorific value and increasing in flow rate, such as a hydrogen-rich fuel, the present embodiment can mix the fuel and air while at the same time suppressing any increases in the pressure loss of the fuel-air mixture. 
         [0088]    The present embodiment has the taper  50  at the entrance of the air nozzle. However, provided that there is a margin on total combustor pressure loss and that a sufficient flow channel area is ensured between the fuel nozzle tip and the entrance of the air nozzle, there is no problem, even if the taper is not provided. 
         [0089]    The present embodiment is effective for hydrogen-rich fuels, in particular. Hydrogen-rich fuels are very high in combustion rate and in a potential risk rate of backfire. For these reasons, diffusion combustors are used in gas turbines since use of a hydrogen-rich fuel in a gas turbine equipped with a premix combustor is liable to cause a backfire because of a long premixing distance. In the former case, the necessity of lowering the flame temperature by supplying a jet of nitrogen or water vapors to the chamber to suppress NOx emissions in the diffusion combustor could result in reduced total plant efficiency. 
         [0090]    The potential risk rate of backfire in the configuration of the present embodiment is low since fuel and air can be mixed at a very short distance. In addition, NOx emissions can be suppressed without supplying a jet of nitrogen or water vapors to the chamber, such that highly reliable and highly efficient total plant operation can be implemented. 
       Fifth Embodiment 
       [0091]    A fifth embodiment is shown in  FIG. 20 . The shape of the obstacle in the first embodiment is changed in the fifth embodiment. In the present embodiment, the obstacle  24  is an elongated plate and the obstacle itself has a support function. As in the first embodiment, the obstacle  24  is disposed in the air nozzle  21 , downstream relative to the fuel nozzle  22 , to establish the relationship in position that makes the fuel jet  29  collide against the obstacle  24 . Since the obstacle  24  also functions to block the fuel jet  29  moderately and attenuate the momentum of the fuel, typical length  32  of the obstacle is preferably greater than the fuel hole diameter  31  of the fuel nozzle  22 . The typical length  32  of the obstacle in the present embodiment is equivalent to plate width of the obstacle. 
         [0092]    The turbulence  26  in the airflow occurs at the downstream side of the obstacle  24 , and this turbulence accelerates fuel-air mixing. Simplifying the shape of the obstacle  24  in this way makes cost reduction achievable. 
         [0093]      FIGS. 21A and 21B  show further variations on the obstacle  24 . These variations, unlike that of  FIG. 20 , include corners  53 . As shown in the enlarged corner view of  FIG. 22 , intersection between an airflow  46  that collides against the obstacle  24  and changes in flow direction, and an airflow  47  that passes through intact, is considered to occur at the corner  53 , thus cause a number of airflows of different flow directions to collide, and result in a strong turbulence. The fuel jet that has collided against the obstacle flows into the turbulence of the airflows, so the fuel and the air are mixed. In addition, an increase in the number of corners uniformizes the distribution of the fuel at the downstream side of the obstacle, and the uniformization is advantageous for fuel-air mixing. 
       Sixth Embodiment 
       [0094]    A sixth embodiment is shown in  FIGS. 23 to 25 .  FIG. 23  is a sectional view of section D-D,  FIGS. 24A and 24B  are a front view and an sectional view taken in the direction of arrows F-F, and  FIG. 25  is a sectional view of section E-E. In the present embodiment, as in the first embodiment, the obstacle  24  is disposed in the air nozzle  21 , downstream relative to the fuel nozzle  22 , to establish the relationship in position that makes the fuel jet  29  collide against the obstacle  24 . In the first embodiment, the obstacle  24  is fixed to the air nozzle  21  by the support member  25 . In the present embodiment, however, the obstacle  24  is fixed to the fuel nozzle  22  by the support member  25 , as shown in  FIG. 25 . 
         [0095]    The present embodiment has an advantage in that since the support member  25  does not block the flow channel within the air nozzle  21 , increases in a pressure loss rate of the airflow side can be suppressed. The embodiment is also advantageous in that since the obstacle  24  is fixed to the fuel nozzle  22 , it is easy to align both, that is, to match the central axes of the obstacle  24  and the fuel nozzle  22 . 
         [0096]      FIGS. 30A and 30B  show an example of fabricating the present embodiment. For the fuel nozzle in the comparative example of  FIG. 29 , the internal flow channel  57  of the fuel jet extends through to the fuel nozzle tip. In this example of fabrication, however, as shown in  FIG. 30 , the internal flow channel  57  of the fuel jet does not extend through to the fuel nozzle tip  59 . Only a portion of a region  58  is chipped off to serve as a support. The configuration with the obstacle disposed at the downstream side of the fuel nozzle tip can thus be obtained. The fuel nozzle tip  59  plays a role of the obstacle, and the fuel stream  48  flowing through the fuel nozzle  22  collides once at the fuel nozzle tip before becoming diffused widely over a surrounding downstream region. Adopting this fabricating method allows the fuel nozzle, the obstacle, and the support member to be integrally fabricated. In addition, machining such as aligning the fuel nozzle and the obstacle is easy to improve in accuracy, and the number of components required can be reduced. 
         [0097]    The existing combustor described in JP-A-2003-148734 can be converted by replacing the combustor with that which employs the fuel nozzle of the present embodiment. More specifically, the conversion includes two steps. Firstly, the existing fuel nozzle is replaced with an obstacle-equipped fuel nozzle (equivalent to the fuel nozzle  22  in  FIG. 23 ) that includes an obstacle for causing turbulence in the airflow flowing into the air nozzle, as well as for causing a collision of the fuel jet blown out from the fuel nozzle. Secondly, the relationship in position between the fuel nozzle and the air nozzle plate is adjusted so that the obstacle is positioned inside the air nozzle. Using this procedure makes even the existing combustor easily convertible and fuel-air mixing further accelerable without relying upon the flow rate of the 
       Seventh Embodiment 
       [0098]    A seventh embodiment is shown in  FIG. 26 .  FIG. 26  shows a front view of the air nozzle and the obstacle. In the present embodiment, as in the sixth embodiment, the obstacle  24  is set up downstream with respect to the fuel nozzle, the obstacle being disposed inside the air nozzle. Also, the obstacle  24  is fixed to the fuel nozzle by the support member. Whereas the obstacle in the sixth embodiment is a mere circular disc, the obstacle  24  in the present embodiment is a circular disc with a number of cuts  55 . 
         [0099]    In the present embodiment, as in the sixth embodiment, the fuel jet blown out from the fuel nozzle collides against the obstacle  24  and then spreads outward in the radial direction of the obstacle  24 . Since an airflow that passes through the cuts  55 , and an airflow that flows in after colliding against the obstacle  24  and changing in flow direction meet similarly to the event shown in  FIG. 22 , a vortex occurs at a boundary surface of the flows whose directions greatly differ from each other, and the vortex generates a strong turbulence. The fuel flows in there, so the fuel and the air can be mixed rapidly. 
         [0100]    The shape of the obstacle  24  in the present embodiment is also effective for fixing the obstacle to the air nozzle side. Also, the shapes shown in  FIGS. 13 and 14A ,  14 B are likewise effective for fixing the obstacle to the fuel nozzle. 
       Eighth Embodiment 
       [0101]    An eighth embodiment is shown in  FIGS. 27 and 28 .  FIG. 27  is a sectional view showing the air nozzle, the fuel nozzle, and the obstacle.  FIG. 28  is a front view of the air nozzle  21  as viewed from the combustion chamber side. In the present embodiment, as in the first embodiment, the obstacle is disposed downstream relative to the fuel nozzle and has the relationship in position that makes the fuel jet  29  collide against the obstacle. However, the present embodiment differs from the first embodiment in that the air nozzle  21  has a taper  50  and in that the obstacle  24  is disposed such that an upstream wall thereof is in proximity to a section of the entrance of the air nozzle  21 . That is to say, the obstacle  24  is disposed at the entrance section having the largest airflow channel area of the air nozzle  21 . Since the air nozzle  21  has the taper  50 , an aperture area at the entrance section of the air nozzle  21  correspondingly increases. Because of this, even if the typical length  32  of the obstacle is increased, a sufficient airflow channel aperture area can be obtained and a pressure loss at the airflow side can be prevented from increasing. In addition, a fuel-air boundary area can be increased by dimensionally increasing the obstacle  24 . This effect can be utilized to accelerate fuel-air mixing. 
         [0102]    The groove  27  can be shallowed by fabricating the present embodiment using the method shown in  FIGS. 8 and 9 . This offers an advantage in that the obstacle and the support member can be easily connected to the air nozzle plate. 
         [0103]    The air nozzle  21  has a wide sectional flow channel not only at the entrance of the air nozzle, but also anywhere else in a range of the taper  50 . Accordingly, the obstacle  24  may be provided at an air nozzle spatial interval including the taper  50 .