Patent Publication Number: US-11022314-B2

Title: Fuel injector, combustor, and gas turbine

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel injector. 
     BACKGROUND ART 
     When a fuel gas is supplied to a combustor, etc. in a gas turbine or the like, air and the fuel gas are previously uniformly mixed and injected in a mist form by a fuel injector. 
     As this fuel injector, a fuel injector that has a cylindrical shape to internally form a plenum and has an inner baffle disposed such that a diameter thereof enlarges toward a downstream side is disclosed in, for example, Japanese Unexamined Patent Application, First Publication No. 2011-69602. 
     The fuel injector has upstream and downstream tube supports connected by an outer wall and is provided with a fuel injector body using an inner space as a plenum. In the fuel injector body, an inner baffle spreading toward an outer side in a radial direction to transverse the inner plenum in the radial direction is disposed. Further, a fuel delivery tube is connected to the fuel injector body from an upstream side thereof. The fuel injector body is provided with a plurality of premixing tubes that penetrate and fix the upstream tube support, the inner baffle, and the downstream tube support. In the premixing tubes, fuel injection holes for introducing a fuel gas are disposed upstream from the inner baffle in the plenum. 
     In the fuel injector having such a constitution, when the fuel gas is introduced into the plenum from a fuel delivery tube, the fuel gas flows toward the outer side in the radial direction along a downstream surface of the inner baffle to reach the vicinity of the outer wall. Afterwards, the fuel gas in the plenum flows toward the inner side in the radial direction along an upstream surface of the inner baffle while being introduced from the fuel injection holes of the premixing tubes disposed at an outer side in the radial direction. A cross-sectional area of the plenum is reduced toward the inner side in the radial direction. For this reason, a flow rate of the fuel gas in the plenum is gradually reduced toward the inner side in the radial direction. Thereby, in the fuel injection holes of the premixing tubes, a flow velocity of the fuel gas is constant, and an amount of supply of the fuel gas supplied to the premixing tubes is constant. Therefore, in the fuel injector, air supplied from the upstream sides of the premixing tubes and the fuel gas supplied from the fuel introduction holes can be uniformly mixed and injected regardless of positions at which the premixing tubes are disposed. 
     Technical Problem 
     In the fuel injector described in Japanese Unexamined Patent Application, First Publication No. 2011-69602, it is important to adjust the inner baffle to a constant angle. However, because the fuel injector is disposed in the plenum that is a closed space in the fuel injector body, it is difficult to adjust to the constant angle. 
     Further, the inner baffle is formed with a plurality of through-holes for passing the premixing tubes. Welding is performed to prevent inflow of fuel from a space between the premixing tubes and the inner baffle, and unevenness occurs on a surface of the inner baffle. For this reason, it is difficult to smoothly flow the fuel gas along the surface of the inner baffle. 
     Therefore, it is difficult to adjust the flow velocity of the fuel gas to an arbitrary velocity in the vicinity of the fuel introduction holes, and to uniformly mix and inject the fuel gas in the premixing tubes. 
     SUMMARY OF INVENTION 
     The present invention provides a fuel injector, a combustor, and a gas turbine capable of easily injecting a uniformly mixed fuel gas. 
     Solution to Problem 
     A fuel injector according to a first aspect of the present invention includes an upstream support plate configured to introduce a fuel gas from a first end side into an inner side in a direction of an axis and having a shape of a tapered cylinder whose diameter gradually enlarges toward a second end side in the direction of the axis; a downstream support plate provided to intersect the axis, disposed at the second end side of the upstream support plate in the direction of the axis, and configured to define a plenum at an inner side along with the upstream support plate; and a plurality of premixing tubes provided to extend in the direction of the axis to be supported on the upstream and downstream support plates and configured to introduce air from the first end side in the direction of the axis, wherein the plurality of premixing tubes are disposed in rows shaped as a plurality of circles whose radial dimensions centered on the axis are different from each other, the neighboring premixing tubes disposed in the same row are disposed equidistantly from each other in a circumferential direction, portions of the premixing tubes which are located in the plenum are formed with fuel introduction holes penetrating the premixing tubes from outside to inside, and the fuel gas supplied from the plenum into the premixing tubes via the fuel introduction holes is mixed with the air in the premixing tubes and is injected from second end sides of the premixing tubes in the direction of the axis. 
     Such a fuel injector can form the plenum defined inside the upstream and downstream support plates to reduce an axial distance from the center of the center toward the outer side in the radial direction. For this reason, although an amount of circulation of the fuel gas is gradually reduced in the plenum, a flow velocity of the fuel gas supplied from the fuel introduction holes into the premixing tubes provided in plural can be maintained to be constant. Therefore, a flow rate of the fuel gas supplied from the fuel introduction holes into the premixing tubes provided in plural is gradually reduced in the plenum toward the outer side in the radial direction. For this reason, although the premixing tubes are increased, the flow velocity of the fuel gas can be maintained to be constant. As a result, an amount of supply of the fuel gas supplied from the fuel introduction holes located in the plenum into the premixing tubes can be made constant regardless of positions at which the premixing tubes are disposed. Thereby, since the air and the fuel gas can be uniformly mixed by the premixing tubes, the uniformly mixed fuel gas can be easily injected. 
     In a fuel injector according to a second aspect of the present invention, a length of the plenum in each row in the direction of the axis may be set such that a flow velocity of the fuel gas circulating in circumferential spaces between the plurality of premixing tubes in a radial direction is constant. 
     In the fuel injector, an axial length of the plenum located in rows whose radius dimensions from the axis are different from each other is set such that the flow velocity of the fuel gas circulating in the circumferential space of the premixing tubes in the radial direction is constant. For this reason, the flow passage area of the fuel gas flowing in the plenum in each row can be adjusted to be small on the whole. As a result, the flow velocity in the radial direction can be made constant with high precision. Thereby, the uniformly mixed fuel gas can be easily injected. 
     In a fuel injector according to a third aspect of the present invention, when the row at an innermost side in a radial direction is set as a first row, the length of the plenum in an a-th row in the direction of the axis is defined as La, the number of premixing tubes in the a-th row is defined as Na, and a volume flow rate of the fuel gas in the a-th row is defined as Ga, the upstream support plate is configured such that the length La of the plenum in the a-th row in the direction of the axis may be represented by the following formula:
 
 La=L 1× Ga/G 1× N 1/ Na  
 
     where L1: the length of the plenum in a first row in the direction of the axis, 
     G1: the volume flow rate of the fuel gas in the first row, 
     N1: the number of premixing tubes in the first row. 
     In the fuel injector, the axial length of the plenum is determined by the number of premixing tubes and the volume flow rate of the fuel gas in each row. For this reason, the flow passage area of the fuel gas flowing in the plenum can be more accurately adjusted. Thereby, it is possible to make the flow velocity in the radial direction constant with high precision, and easily inject the fuel gas that is more uniformly mixed. 
     In a fuel injector according to a fourth aspect of the present invention, the premixing tubes may protrude toward an outer side of the plenum with respect to at least one of the upstream support plate and the downstream support plate in the direction of the axis. 
     In the fuel injector, the premixing tubes protrude toward the outer side of the plenum in the direction of the axis. For this reason, the lengths of the whole premixing tubes can be increased in the direction of the axis relative to the lengths of the premixing tubes disposed in the plenum. The plenum is formed to reduce the axial distance from the center of the axis toward the outer side in the radial direction. Thereby, the lengths of the premixing tubes disposed in the plenum are reduced toward the outer side in the radial direction. Since a loss of pressure in the premixing tubes is reduced toward the outer side in the radial direction, the premixing tubes disposed in the plenum are subjected to a difference in magnitude of the pressure loss by radial positions at which the premixing tubes are disposed from the axis, and a difference in an amount of air flowing in the premixing tubes occurs. Thus, the premixed gas cannot be uniformed and supplied. 
     In contrast, the premixing tubes extend toward the outer side of the plenum, and thereby it is possible to reduce the difference of the pressure loss of the premixing tubes in which positions disposed in the radial direction are different. For this reason, despite the positions at which the premixing tubes are disposed, the amount of supply of the fuel gas can be made uniform, and the more uniformly mixed fuel gas can be easily injected. 
     In a fuel injector according to a fifth aspect of the present invention, the fuel injector may include a fuel guide having a tapered surface whose center is the axis and whose diameter is gradually enlarged from the first end side in the direction of the axis to the second end side in the direction of the axis and fixing to a surface of the first end side of the downstream support plate in the plenum in the direction of the axis. 
     In the fuel injector, the fuel guide has the tapered surface whose center is the axis and whose diameter is gradually enlarged in the direction of the axis from the first end side toward the second end side. Thereby, the fuel gas in the plenum is guided toward the outer side in the radial direction by the fuel guide, and easily circulates toward the outer side in the radial direction. For this reason, the fuel gas is also easily supplied to the premixing tubes disposed at the outer side in the radial direction, and an amount of the fuel gas supplied from the fuel introduction holes can be made constant with higher precision regardless of the positions at which the premixing tubes are disposed. Thereby, the fuel gas uniformly mixed with high precision can be easily injected. 
     Advantageous Effects of Invention 
     According to the aforementioned fuel injector, the plenum is formed to reduce the axial distance from the center of the axis toward the outer side in the radial direction, and thereby the uniformly mixed fuel gas can be easily injected. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view showing a fuel injector according to a first embodiment of the present invention. 
         FIG. 2  is a transverse cross-sectional view taken along line II-II of  FIG. 1  showing the fuel injector according to the first embodiment of the present invention. 
         FIG. 3  is a longitudinal cross-sectional view showing a fuel injector according to a second embodiment of the present invention. 
         FIG. 4  is a longitudinal cross-sectional view showing a fuel injector according to a third embodiment of the present invention. 
         FIG. 5  is a longitudinal cross-sectional view showing a fuel injector according to a first modification of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a fuel injector  10  of a first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 . 
     In the fuel injector  10  of the present embodiment, a fuel gas F is introduced from a first end side in a direction of an axis O by a fuel delivery tube  1  extending along the axis O. The fuel injector  10  mixes the fuel gas F and air A, and then injects and discharges the mixture toward the second end side in the direction of the axis O. If the first end side in the direction of the axis O is defined as an upstream side (the left side of  FIG. 1 ) into which the fuel gas F is introduced, and the second end side in the direction of the axis O is defined as a downstream side (the right side of  FIG. 1 ) into which the fuel gas F is injected, the fuel gas F and the air A circulate from the upstream side toward the downstream side. 
     As illustrated in  FIG. 1 , the fuel injector  10  is provided with an upstream support plate  11  connected with the fuel delivery tube  1 , a downstream support plate  12  defining a plenum along with the upstream support plate  11 , a plurality of premixing tubes  13  supported by the upstream support plate  11  and downstream support plate  12 , and a premixing tube support  14  supporting the premixing tubes  13  at the downstream side from the downstream support plate  12 . 
     The upstream support plate  11  is connected with the fuel delivery tube  1  for introducing the fuel gas F from the upstream side. The upstream support plate  11  has the shape of a tapered tube whose diameter gradually enlarges toward the second end side in the direction of the axis O. To be specific, an interior of the upstream support plate  11  has a hollow shape. The upstream support plate  11  has an enlarged diameter part  11   a  which is connected with the fuel delivery tube  1  and whose diameter gradually enlarges toward the second end side in the direction of the axis O. 
     The enlarged diameter part  11   a  is connected with the fuel delivery tube  1 . The enlarged diameter part  11   a  has the same diameter as the fuel delivery tube  1  at a portion connected with the fuel delivery tube. The enlarged diameter part  11   a  is formed such that a diameter thereof gradually enlarges from the first end side toward the downstream side that is the second end side in the direction of the axis O. 
     The downstream support plate  12  is disposed across the axis O at the second end side of the upstream support plate  11  in the direction of the axis O. To be specific, the downstream support plate  12  has the shape of a disc whose center is the axis O. The downstream support plate  12  has a disc part  12   a  that is integrally connected with a cylindrical part  12   b  at the downstream side, and the cylindrical part  12   b  that has the shape of a cylinder connected to a first end side of the disc part  12   a  in the direction of the axis O. Along with the enlarged diameter part  11   a  of the upstream support plate  11 , the disc part  12   a  and the cylindrical part  12   b  of the downstream support plate  12  define a plenum that is a space inside these parts. 
     The disc part  12   a  has the shape of a disc whose center is the axis O. The disc part  12   a  is formed with a plurality of through-holes for inserting and supporting the plurality of premixing tubes. 
     A first end side of the cylindrical part  12   b  in the direction of the axis O is connected to the largest diameter portion of the enlarged diameter part  11   a  of the upstream support plate  11 . A second end side of the cylindrical part  12   b  in the direction of the axis O is integrally formed with an outer circumferential portion of the disc part  12   a . The cylindrical part  12   b  is fitted to the largest diameter portion of the enlarged diameter part  11   a  and extends in the direction of the axis O to have a cylindrical shape. 
     The premixing tubes  13  are pipe members having the shape of cylinders extending in the direction of the axis O. Air A is introduced into the premixing tubes  13  from the upstream side that is the first end side in the direction of the axis O. The premixing tubes  13  are fixed such that second end sides thereof in the direction of the axis O protrude to the downstream side that is the second end side in the direction of the axis O toward the outside of the plenum relative to the downstream support plate  12 . The premixing tubes  13  are fixed such that the first end sides thereof in the direction of the axis O are flush with the enlarged diameter part  11   a  without protruding from the enlarged diameter part  11   a  of the upstream support plate  11 . Portions of the premixing tubes  13  which protrude from the downstream support plate  12  are supported by the premixing tube support  14  to be described below. Portions of the premixing tubes  13  which are located in the plenum are formed with fuel introduction holes  13   a  that penetrate the premixing tubes  13  from outside to inside in a radial direction. 
     The plurality of premixing tubes  13  are disposed through the upstream support plate  11  and downstream support plate  12  in the direction of the axis O. The premixing tubes  13  are fixed and supported by the upstream support plate  11  and downstream support plate  12 . The plurality of premixing tubes  13  all have the same cross-sectional shape. On the other hand, the plurality of premixing tubes  13  are fixed to be flush with the upstream support plate  11  without protruding from the upstream support plate  11 . Thereby, the premixing tubes  13  have a length different from each other and are disposed in a plurality of circular rows whose radial dimensions centering on the axis O are different from each other. The premixing tubes  13  that are disposed in the same row and are adjacent to each other are disposed an equal distance t apart from each other in a circumferential direction. That is, the plurality of premixing tubes  13  are radially disposed in a plurality of rows and are separated at even intervals in the circumferential direction. As a result, the plurality of premixing tubes  13  are radially disposed around the axis O such that their number is gradually increased toward the outer side in the radial direction. For example, as illustrated in  FIG. 2 , the premixing tubes  13  of the present embodiment are disposed in five circles whose diameters gradually increase starting from the axis O. In the present embodiment, the premixing tubes  13  are configured in such a manner that 12 premixing tubes are disposed in a first row  131  that is a circle closest to the axis O, 18 premixing tubes are disposed in a second row  132 , 24 premixing tubes are disposed in a third row  133 , 30 premixing tubes are disposed in a fourth row  134 , and 36 premixing tubes are disposed in a fifth row  135  that is a circle farthest from the axis O. 
     The fuel introduction holes  13   a  are through-holes through which the fuel gas F in the plenum flows into the premixing tubes  13 . The fuel introduction holes  13   a  are formed in portions of the premixing tubes  13  which are located in the plenum. The fuel introduction holes  13   a  have circular cross-sections, and pass through the premixing tubes  13  in a radial direction. The fuel introduction holes  13   a  are disposed in the plenum at the same position in the direction of the axis O regardless of positions at which the premixing tubes  13  are disposed. 
     The upstream support plate  11  is formed such that a diameter thereof gradually increases while a length of the defined plenum in the direction of the axis O is adjusted. That is, the diameter of the upstream support plate  11  is enlarged to set lengths of the plenums, which are located in rows whose radial dimensions from the axis O are different from each other, in the direction of the axis O such that a flow velocity of the fuel gas F circulating in circumferential spaces between the premixing tubes  13  in a radial direction is constant. In the first embodiment, for example, with the increase of the radial dimensions of the rows disposed such that the flow velocity of the fuel gas F circulating in the circumferential spaces between the premixing tubes  13  disposed in the first row  131  in the radial direction is the same as the flow velocity of the fuel gas F circulating in the circumferential spaces between the premixing tubes  13  disposed in the fifth row  135  in the radial direction, the length of the plenum in the direction of the axis O is reduced. 
     To be specific, the flow velocity of the fuel gas F circulating in the circumferential spaces between the premixing tubes  13  in the radial direction is defined as v. The flow velocity v is determined by a unit flow rate G of the fuel gas F and a flow passage area S of a cross-section (e.g., see a II-II cross-section illustrated in  FIG. 2 ) that is orthogonal to the axis O at a position of each row. The flow passage area S is determined by a number N of the premixing tubes  13  that are disposed, a circumferential distance t between the premixing tubes  13  in each row, and a length L of the plenum at a position of each row in the direction of the axis O. 
     When the circumferential distance t is the same between the premixing tubes  13  in each row, the number of premixing tubes  13  is increased in the plenum toward the outer side in the radial direction, and the number of flow passages between the neighboring premixing tubes  13  is also increased. On the other hand, the fuel gas F circulating in the plenum is supplied to the premixing tubes  13  that are disposed in the first row  131  located at the inner side in the radial direction. For this reason, the flow rate of the fuel gas F is reduced until the fuel gas reaches the premixing tubes  13  disposed in the fifth row  135  located at the outer side in the radial direction. 
     When a row located at the innermost side in the radial direction is set as the first row  131 , the length of the plenum in an a-th row in the direction of the axis O is defined as La, the number of premixing tubes  13  in the a-th row is defined as Na, and a volume flow rate of the fuel gas F in the a-th row is defined as Ga, a volume flow rate ratio between the a-th row and the first row  131  is expressed by Formula 1 below.
 
 Ga/G 1=( t×Na×La )/( t×N 1× L 1)  (Formula 1)
 
     L1: the length of the premixing tubes  13  in the first row in the direction of the axis O 
     G1: the volume flow rate of the fuel gas F in the first row 
     N1: the number of premixing tubes  13  in the first row 
     Therefore, the length La of the premixing tubes  13  in the a-th row in the direction of the axis O is calculated as in Formula 2 below and set.
 
 La=L 1×( Ga/G 1)×( N 1/ Na )  (Formula 2)
 
     The premixing tube support  14  has the same circular cross-section as the downstream support plate  12 , and has the shape of a column extending in the direction of the axis O. The premixing tube support  14  is formed with a plurality of through-holes into which the premixing tubes  13  are inserted. The premixing tube support  14  is fixed integrally to the downstream support plate  12 . The premixing tube support  14  extends such that a downstream end face thereof is flush with downstream ends of the premixing tubes  13 . The premixing tube support  14  fixes the premixing tubes  13  by means of the downstream end face thereof. 
     Further, the premixing tube support  14  may support the premixing tubes  13  protruding from the downstream support plate  12 . The premixing tube support  14  may, for example, be a tabular member that is disposed at a position separated from the downstream support plate  12  toward the downstream side in the shape of a disc whose center is the axis O and supports the premixing tubes  13 . 
     Next, an operation of the fuel injector  10  having the above constitution will be described. 
     In the fuel injector  10  of the present embodiment as described above, the fuel gas F is introduced from the upstream side, which is the first end side in the direction of the axis O, into the plenum via the fuel delivery tube  1 . The introduced fuel gas F flows toward the outer side in the radial direction along the shape of the upstream support plate  11  whose diameter gradually enlarges. Thus, the fuel gas F reaches the fuel introduction holes  13   a  formed in the plenums of the premixing tubes  13  disposed in the first row  131 , and flows into the premixing tubes  13 . Afterwards, the fuel gas F flows to the outer side in the radial direction toward the premixing tubes  13  disposed in the second row  132 , and flows from the fuel introduction holes  13   a  into the premixing tubes  13 . Likewise, the fuel gas F flows toward the outer side in the radial direction in the third row  133  and fourth row  134  in turn, arrives at the fuel introduction holes  13   a  of the premixing tubes  13  disposed in the fifth row  135 , and flows into the premixing tubes  13  disposed in the fifth row  135 . 
     While the fuel gas F flows towards the outer side in the radial direction from the premixing tubes  13  of the first row  131  to the premixing tubes  13  of the fifth row  135 , the fuel gas F is sequentially introduced into the premixing tubes  13  from the first row  131 . For this reason, an amount of the fuel gas F in the plenum is reduced. Further, the number of premixing tubes  13  is increased toward the outer side in the radial direction. Thereby, the number of flow passages formed in the circumferential space between the neighboring premixing tubes  13  is increased. However, the upstream support plate  11  is formed to reduce the length of the plenum in the direction of the axis O toward the outer side in the radial direction. As a result, a flow passage area of the fuel gas F directed in the radial direction is reduced in a cross-section parallel with the axis O, and a flow velocity is increased as the fuel gas F is directed to the outer side in the radial direction. For this reason, the fuel gas F flowing at the same flow velocity flows into the fuel introduction holes  13   a  of the premixing tubes  13  from the first row  131  to the fifth row  135  in which a radial distance from the axis O is gradually increased. Thereby, an amount of supply of the fuel gas F supplied into the premixing tubes  13  is constant. 
     Thus, air A introduced from the upstream side that is the first end side in the direction of the axis O and the fuel gas F supplied into the premixing tubes  13  are mixed in the premixing tubes  13 . And air A and the fuel gas F are injected and discharged from the downstream side that is the second end side in the direction of the axis O. 
     According to the fuel injector  10  as described above, the plenum defined inward by the upstream support plate  11  having the shape of a tapered cylinder whose diameter gradually enlarges toward the downstream side that is the second end side in the direction of the axis O and by the downstream support plate  12  having the shape of a flat plate intersecting the axis O. The plenum can be formed to reduce a distance in the direction of the axis O from the center in the radial direction (the axis O) to the outer side in the radial direction. For this reason, the flow rate of the fuel gas F is gradually reduced in the plenum toward the outer side in the radial direction. The fuel gas F is supplied from the fuel introduction holes  13   a  to the plurality of premixing tubes  13  from the first row  131  to the fifth row  135  such that the radial distance from the axis O gradually increases. Thus, even if the number of remixing tubes  13  is increased, the flow velocity of the fuel gas F can be maintained to be constant. As a result, the flow rate of the fuel gas F supplied from the fuel introduction holes  13   a  positioned in the plenum into the premixing tubes  13  can be made constant regardless of the positions at which the premixing tubes  13  are disposed. For this reason, the air A and the fuel gas F can be uniformly mixed by the premixing tubes  13 . Thereby, the uniformly mixed fuel gas F can be easily injected. 
     The upstream support plate  11  having the shape of the tapered cylinder whose diameter gradually enlarges toward the downstream side that is the second end side in the direction of the axis O can be checked from the outside. As a result, the shape of the upstream support plate  11  is minutely adjusted from the outside, and the shape of the plenum can be easily changed. Thus, it is possible to change a state in which the diameter of the upstream support plate  11  is enlarged according to the disposition of the premixing tubes  13  and the number of the premixing tubes  13  disposed. For this reason, it is possible to adjust the flow velocity of the fuel gas F circulating in the plenum. Thereby, the supply amount of the fuel gas F supplied to the premixing tubes  13  can be easily made constant. 
     The lengths of the plenums located in the rows whose radial dimensions from the axis O are different from each other are set such that the flow velocity of the fuel gas F circulating in the circumferential space between the premixing tubes  13  in the radial direction is constant. For this reason, even if the number of flow passages is increased with the increase of the premixing tubes  13 , the flow passage area of the fuel gas F flowing in the plenum within a surface parallel to the axis O can be adjusted depending on the flow rate of the fuel gas F flowing in the plenum in each row. Thereby, it is possible to make the radial flow velocity constant with high precision, and easily inject the fuel gas F that is more uniformly mixed. 
     Also, the length of the plenum in the direction of the axis O is decided according to the number of premixing tubes  13  and the volume flow rate of the fuel gas F in each row. For this reason, the flow passage area of the fuel gas F flowing in the plenum within the surface parallel to the axis O in each row can be more accurately adjusted to the flow passage area of the cross-section perpendicular to the axis O. Thereby, the radial flow velocity can be made constant with high precision, and the fuel gas F that is more uniformly mixed can be injected. 
     Further, the premixing tubes  13  protrude toward the outside of the plenum in the direction of the axis O and toward the downstream side relative to the downstream support plate  12 . Thereby, the lengths of all of the premixing tubes  13  are able to extend to the second end side in the direction of the axis O relative to the lengths of the premixing tubes  13  disposed in the plenum. The plenum is formed such that the distance thereof in the direction of the axis O is reduced from the axis O toward the outer side in the radial direction. The lengths of the premixing tubes  13  disposed in the plenum are shortened toward the outer side in the radial direction. The loss of pressure generated on the premixing tubes  13  decreases in proportion to a decrease in the lengths of the premixing tubes  13  extending in the direction of the axis O, because the premixing tubes  13  are pipe members. For this reason, in the premixing tubes  13  disposed in the plenum, the loss of pressure is reduced toward the outer side in the radial direction, and a magnitude of the loss of pressure is changed due to radial positions at which the premixing tubes  13  are disposed from the axis O. For this reason, the amount of supply from the fuel introduction holes  13   a  to the premixing tubes  13  is increased toward the outer side in the radial direction, and thereby a difference occurs. A difference in an amount of the air flowing in the premixing tubes  13  occurs, and a premixed gas cannot be uniformed and supplied. 
     However, the premixing tubes  13  extend toward the outside of the plenum, and thereby a percentage of the difference of the pressure loss of the premixing tubes  13  disposed in different radial positions can be reduced. For this reason, regardless of the positions at which the premixing tubes  13  are disposed, the amount of supply of the fuel gas F can be made uniform, and the more uniformly mixed fuel gas F can be easily injected. 
     Next, a fuel injector  10  of a second embodiment will be described with reference to  FIG. 3 . 
     In the second embodiment, the same components as the first embodiment are given the same signs and symbols, and detailed description thereof will be omitted. The fuel injector  10  of the second embodiment is different from that of the first embodiment in that a plurality of disposed premixing tubes  23  have the same length. 
     That is, as illustrated in  FIG. 3 , in the second embodiment, the fuel injector  10  has premixing tubes  23  that protrude to a first end side in the direction of the axis O toward the outside of a plenum with the same lengths, an upstream premixing tube support  24  that supports the premixing tubes  23  at the upstream side of an upstream support plate  11 , and the same upstream and downstream support plates  11  and  12  as the first embodiment. 
     The premixing tubes  23  are pipe members that have the same cross-sectional shape as in the first embodiment, extend in the direction of the axis O, and have cylindrical shapes. The premixing tubes  23  is formed fuel introduction holes  13   a  penetrating the premixing tubes  23  from outside to inside at a portion located in a plenum. The premixing tubes  23  are fixed in a state in which the first end side in the direction of the axis O protrudes from the upstream support plate  11  toward an upstream side which is the first end side in the direction of the axis O and the outside of the plenum. The premixing tubes  23  are fixed such that the second end sides thereof in the direction of the axis O are flush with the downstream support plate  12  without protruding from the downstream support plate  12 . The plurality of premixing tubes  23  have the same length, and are separated into concentric circles whose center is the axis O. Similarly to the first embodiment, the premixing tubes  23  are disposed in numerous rows in a radial direction, and thereby the number of the premixing tubes  23  is radially increased around the axis O. Similarly to the first embodiment, the premixing tubes  23  are also disposed in five rows in the second embodiment. 
     An upstream premixing tube support  24  has a cylindrical shape with an interior that is recessed to correspond to a shape of the enlarged diameter part  11   a  of the upstream support plate  11 . The upstream premixing tube support  24  is disposed to cover the upstream support plate  11  from the upstream side that is the first end side in the direction of the axis O. That is, the upstream premixing tube support  24  is fixed integrally to the upstream support plate  11 , and thereby an external shape is a columnar shape along the upstream and downstream support plates  11  and  12  defining the plenum. The upstream premixing tube support  24  has a plurality of through-holes extending in the direction of the axis O. The upstream premixing tube support  24  fixes the premixing tubes  23  by means of an upstream end face to cause the premixing tubes  23  to be inserted into the through-holes and to be flush with upstream ends of the premixing tubes  23 . Similarly to the premixing tube support  14 , the upstream premixing tube support  24  may support the premixing tubes  23  protruding from the upstream support plate  11 . The upstream premixing tube support  24  may be, for example, a tabular member that is separated from the upstream support plate  11  at the upstream side and is disposed in the shape of a disc whose center is the axis O. 
     According to the fuel injector  10  of the second embodiment as described above, the lengths of the premixing tubes  23  in the direction of the axis O are made equal regardless of the positions at which they are disposed, and thereby the lengths of the premixing tubes  23  in the direction of the axis O are made equal regardless of the positions at which they are disposed. For this reason, a loss of pressure in the premixing tubes  23  disposed at different radial positions can be constant. As a result, an amount of supply of a fuel gas F from the fuel introduction holes  13   a  into the premixing tubes  23  can be made constant regardless of the radial positions at which they are disposed. Thereby, it is possible to easily inject the more uniformly mixed fuel gas F. 
     Next, a fuel injector  10  of a third embodiment will be described with reference to  FIG. 4 . 
     In the third embodiment, the same components as the first embodiment are given the same signs and symbols, and detailed description thereof will be omitted. The fuel injector  10  of the third embodiment is different from that of the first embodiment in that it has a fuel guide  3  for guiding a fuel gas F into a plenum. 
     That is, as illustrated in  FIG. 4 , in the third embodiment, the fuel injector  10  further includes the fuel guide  3  whose diameter gradually enlarges around the axis O from the first end side in the direction of the axis O to the second end side in the direction of the axis O. 
     The fuel guide  3  has a conical bottom fixed to a first end face of a downstream support plate  12  in a plenum in the direction of the axis O. The fuel guide  3  has a conical shape with a tapered surface  3   a  whose diameter gradually enlarges around the axis O from the upstream side that is the first end side in the direction of the axis O to the downstream side that is the second end side in the direction of the axis O. 
     According to the fuel injector  10  of the third embodiment as described above, the fuel guide  3  whose center is the axis O has the conical shape with the tapered surface  3   a  whose diameter gradually enlarges from the upstream side that is the first end side in the direction of the axis O to the downstream side that is the second end side in the direction of the axis O. For this reason, a fuel gas F introduced into a plenum via a fuel delivery tube  1  flow to an outer side in a radial direction along a shape of the fuel guide  3 . That is, the fuel gas F introduced into the plenum is guided toward the outer side in the radial direction by the fuel guide  3 , and circulation toward the outer side in the radial direction becomes easier. For this reason, the fuel gas F is also easily supplied to premixing tubes  13  disposed at the outer side in the radial direction. As a result, regardless of positions at which the premixing tubes  13  are disposed, the amount of the fuel gas F supplied from the fuel introduction holes  13   a  can be made constant with higher precision. Thereby, it is possible to easily inject the fuel gas F that is uniformly mixed with high precision. 
     The present invention is not limited to the aforementioned embodiments, and various modifications are possible without departing from the gist thereof. For example, a modification of the present embodiment may include a fuel injector  10  having both the second embodiment and the third embodiment. 
     That is, as illustrated in  FIG. 5 , in the modification, the fuel injector  10  of the second embodiment may have a fuel guide  3 . 
     While embodiments of the present invention have been described in detail with reference to the drawings, the constitutions of the embodiments and combinations thereof are examples, and additions, omissions, substitutions, and other variations of the constitutions are possible without departing from the spirit and scope of the present invention. Also, the present invention is not limited to the embodiments, but is limited only by the scope of the claims. 
     In the present embodiment, the premixing tubes  13  protrude from the upstream side that is the first end side in the direction of the axis O or the downstream side that is the second end side in the direction of the axis O, but the protruding direction is not limited to that of the present embodiment. The premixing tubes  13  may protrude in a different direction or in opposite directions. For example, as in the second embodiment, the premixing tubes  23  having the same length may protrude toward the downstream side. 
     Further, the plurality of premixing tubes  13  are disposed around the axis O in the five rows, but are not limited to the five rows. The rows may be appropriately selected according to performance of the required fuel injector  10 . 
     Also, the premixing tube support  14  is preferably provided to maintain the premixing tubes  13  in a posture parallel with the axis O, but it may not be provided. In this case, the premixing tubes  13  preferably support themselves with their own strength and maintain a posture parallel with the axis O. 
     Further, the present invention is not limited to the constitution in which the length of the plenum in the direction of the axis O is set to make the radial flow velocity constant. For example, the length of the plenum in the direction of the axis O may be set to make the flow velocity of the fuel gas F in a flow direction of the fuel gas F having a component in not only the radial direction but also the direction of the axis O constant. 
     INDUSTRIAL APPLICABILITY 
     According to the aforementioned fuel injector, the plenum is formed to reduce an axial distance from the center of the axis to the outer side in the radial direction. Thereby, it is possible to easily inject the uniformly mixed fuel gas. 
     REFERENCE SIGNS LIST 
     
         
         O axis 
         F fuel gas 
         A air 
           1  fuel delivery tube 
           10  fuel injector 
           11  upstream support plate 
           11   a  enlarged diameter part 
           12  downstream support plate 
           12   a  disc part 
           12   b  cylindrical part 
           13 ,  23  premixing tube 
           13   a  fuel introduction hole 
           131  first row 
           132  second row 
           133  third row 
           134  fourth row 
           135  fifth row 
           14  premixing tube support 
           24  upstream premixing tube support 
           3  fuel guide