Patent Publication Number: US-11662095-B2

Title: Fuel nozzle and combustor and gas turbine including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. application Ser. No. 16/153,626 filed Oct. 5, 2018 which claims priority to Korean Patent Application No. 10-2017-0142545 filed on Oct. 30, 2017, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Exemplary embodiments of the present disclosure relate to a fuel nozzle and to a combustor and gas turbine including the fuel nozzle. 
     Description of the Related Art 
     A gas turbine is a power engine that mixes air compressed in a compressor with fuel for combustion and rotates a turbine using high-temperature gas produced by the combustion. The gas turbine is used to drive a generator, an aircraft, a ship, a train, and the like. 
     This gas turbine typically includes a compressor, a combustor, and a turbine. The compressor sucks and compresses outside air, and then transmits it to the combustor. The air compressed in the compressor is in a high-pressure and high-temperature state. The combustor mixes the compressed air introduced from the compressor with fuel and burns the mixture. Combustion gas produced by the combustion is discharged to the turbine. Turbine blades in the turbine are rotated by the combustion gas, thereby generating power. The generated power is used in various fields, such as generating electric power and driving machines. 
     SUMMARY OF THE INVENTION 
     An object of the present disclosure is to provide a fuel nozzle that prevents a flashback phenomenon occurring due to a reduction in pressure around a swirler and to provide a combustor and gas turbine including the fuel nozzle. 
     Other objects and advantages of the present disclosure can be understood by the following description, and become apparent with reference to the embodiments of the present disclosure. Also, it is obvious to those skilled in the art to which the present disclosure pertains that the objects and advantages of the present disclosure can be realized by the means as claimed and combinations thereof. 
     In accordance with one aspect of the present disclosure, a fuel nozzle may include a shroud; an injection cylinder surrounded by the shroud and configured to supply fuel to a combustion chamber; a swirler disposed between the injection cylinder and the shroud; and a porous disk disposed downstream of the swirler to surround an outer peripheral surface of the injection cylinder in order to prevent a flashback phenomenon occurring due to a reduction in pressure around the swirler. 
     The porous disk may include a disk body to block a flame produced in the combustion chamber, and a plurality of flow holes formed in the disk body through which the fuel flows. 
     Each flow hole may be configured as a straight through-hole aligned with a flow direction of the fuel, or as a diagonal through-hole forming a predetermined angle with a flow direction of the fuel. Each flow hole may include a curve having at least one turn. The plurality of flow holes may have different diameters which may increase from an inner peripheral surface of the disk body toward an outer peripheral surface of the disk body. 
     The disk body may have an outer peripheral surface that is spaced apart from an inner peripheral surface of the shroud by a predetermined distance which may be adjusted according to a magnitude of pressure reduction around the swirler. 
     The porous disk may consist of at least two porous disks, and each of the at least two porous disks may extend from the outer peripheral surface of the injection cylinder to an inner peripheral surface of the shroud. Further, the at least two porous disks may be arranged such that a flow direction of each of the flow holes of one of the at least two porous disks aligns with a flow direction of each of the flow holes of the other porous disks of the at least two porous disks; or arranged such that a flow direction of each of the flow holes of one of the at least two porous disks is inclined in a first direction, and a flow direction of each of the flow holes of an adjacent porous disk of the at least two porous disks may be inclined in a second direction opposing the first direction. 
     The porous disk may consist of at least two porous disks, and the at least two porous disks may include a first porous disk facing the combustion chamber that extends from the outer peripheral surface of the injection cylinder to an inner peripheral surface of the shroud; and at least one second porous disk spaced apart from the inner peripheral surface of the shroud by a predetermined distance. The first porous disk and the at least one second porous disk may have respective diameters that incrementally increase toward the combustion chamber. 
     In accordance with another aspect of the present disclosure, a combustor may include a combustion chamber assembly comprising a combustion chamber in which fuel is burnt; and a fuel nozzle assembly including a plurality of fuel nozzles to inject the fuel into the combustion chamber, wherein each of the fuel nozzles of the fuel nozzle assembly is consistent with the above-described fuel nozzle. 
     In accordance with a further aspect of the present disclosure, a gas turbine may include a compressor to compress air; a combustor to produce combustion gas by mixing the compressed air with fuel for combustion; and a turbine to generate power using the combustion gas, wherein the combustor of the turbine is consistent with the above-described combustor. 
     It is to be understood that both the foregoing general description and the following detailed description of the present disclosure are exemplary and explanatory and are intended to provide further explanation of the disclosure as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a cutaway perspective view of a gas turbine to which may be applied a fuel nozzle according to the present disclosure; 
         FIG.  2    is a sectional view of a combustor to which may be applied a fuel nozzle according to the present disclosure; 
         FIG.  3    is a perspective view of a fuel nozzle module including a plurality of fuel nozzles according to the present disclosure; 
         FIG.  4    is a partially transparent, perspective view of a fuel nozzle according to an embodiment of the present disclosure; 
         FIG.  5    is a top view of a porous disk in the fuel nozzle according to the embodiment of the present disclosure; 
         FIGS.  6  to  9    are cross-sectional views taken along line A-A′ of  FIG.  5   , respectively illustrating modified examples of the porous disk of the present disclosure; 
         FIG.  10    is a top view of a porous disk in the fuel nozzle, illustrating another example of the porous disk of the present disclosure; 
         FIG.  11    is a partially transparent, perspective view of a fuel nozzle according to another embodiment of the present disclosure; 
         FIGS.  12  and  13    are cross-sectional side views illustrating modified examples of the porous disk shown in  FIG.  11   ; and 
         FIG.  14    is a partially transparent, perspective view of a fuel nozzle according to a further embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     A fuel nozzle and a combustor and gas turbine including the same according to exemplary embodiments of the present disclosure will be described below in more detail with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present disclosure. 
     It will be understood that when a component is referred to as “comprising or including” any component, it does not exclude other components, but can further comprise or include the other components unless otherwise specified. In addition, it will be understood that a spatially-relative term “on” used herein does not necessarily mean that an element is located on another element in the direction of gravity, but it means that the element is located on or under another element. 
       FIG.  1    illustrates the interior of a gas turbine  1000  including a combustor  1200  to which may be applied a fuel nozzle according to the present disclosure, and  FIG.  2    illustrates an example of the combustor  1200  of  FIG.  1   .  FIG.  3    illustrates a fuel nozzle module including a plurality of fuel nozzles according to the present disclosure, one fuel nozzle of which is detailed in  FIG.  4   . 
     Referring to  FIG.  1   , the gas turbine  1000  may include a compressor  1100  that compresses introduced air to a high pressure, a combustor  1200  that mixes the compressed air supplied from the compressor  1100  with fuel and burns the mixture, and a turbine  1300  that generates a rotational force by combustion gas produced in the combustor. In the present specification, upstream and downstream sides are defined based on the flow direction of fuel or air. 
     The thermodynamic cycle of the gas turbine may ideally follow a Brayton cycle. The Brayton cycle consists of four phases including isentropic compression (adiabatic compression), isobaric heat addition, isentropic expansion (adiabatic expansion), and isobaric heat dissipation. In other words, in the Brayton cycle, thermal energy is released by combustion of fuel in an isobaric environment after the atmospheric air is sucked and compressed to a high pressure, hot combustion gas is expanded to be converted into kinetic energy, and exhaust gas with residual energy is then discharged to the atmosphere. The Brayton cycle consists of four processes, i.e., compression, heating, expansion, and exhaust. The present disclosure may be widely applied to a gas turbine having the same or similar configuration as the gas turbine  1000  exemplarily illustrated in  FIG.  1   . 
     The compressor  1100  of the gas turbine serves to suck and compress air, and mainly serves to supply cooling air to a high-temperature region required for cooling in the gas turbine while supplying combustion air to the combustor  1200 . Since the air sucked into the compressor  1100  is subject to an adiabatic compression process, the pressure and temperature of the air passing through the compressor  1100  increase. 
     The compressor  1100  of the gas turbine may be typically designed as a centrifugal compressor or an axial compressor. In general, the centrifugal compressor is applied to a small gas turbine, whereas a multistage axial compressor is applied to the large gas turbine  1000  as illustrated in  FIG.  1    because it is necessary to compress a large amount of air. 
     The compressor  1100  is driven using a portion of the power output from the turbine  1300 . To this end, the rotary shaft (not shown) of the compressor  1100  is directly connected to the rotary shaft of the turbine  1300 . 
     The combustor  1200  mixes the compressed air, which is supplied from the outlet of the compressor  1100 , with fuel for isobaric combustion to produce high-energy combustion gas. The combustor  1200  is disposed downstream of the compressor  1100  and includes a plurality of burner modules  1210  annularly arranged around the gas turbine  1000 . 
     Referring to  FIG.  2   , each of the burner modules  1210  of  FIG.  1    may include a combustion chamber assembly  1220  including a combustion chamber  1240  in which fuel is burnt, and a fuel nozzle assembly  1230  including a plurality of fuel nozzles that inject fuel to the combustion chamber  1240 . 
     The gas turbine may use gas fuel, liquid fuel, or a composite fuel of gas and liquid, and the fuel in the present disclosure includes any of these. It is important to make a combustion environment for reducing an amount of emissions such as carbon monoxide or nitrogen oxide that is subject to legal regulations. Accordingly, in spite of the relative difficulty to control such combustion, pre-mixed combustion has been increasingly used in recent years since it can achieve uniform combustion to reduce emissions by lowering a combustion temperature. 
     In the pre-mixed combustion, the compressed air supplied from the compressor  1100  is mixed with fuel in the fuel nozzle assembly  1230  and then introduced into the combustion chamber  1240 . When combustion is stable after pre-mixed gas is initially ignited by an igniter, the combustion is maintained by the supply of fuel and air. 
     The fuel nozzle assembly  1230  includes a plurality of fuel nozzles  2000  that inject fuel, and the fuel supplied from the fuel nozzles  2000  is mixed with air at an appropriate rate to be suitable for combustion. The fuel nozzles  2000  (to be described later) may be configured such that a plurality of outer fuel nozzles are radially arranged around one inner fuel nozzle, as illustrated in  FIG.  3   . 
     Referring further to  FIG.  2   , the combustion chamber assembly  1220  includes the combustion chamber  1240  as a space in which combustion is performed, and includes a liner  1250  and a transition piece  1260 . 
     The liner  1250  is disposed downstream of the fuel nozzle assembly  1230 , and may have a double structure formed by an inner liner  1251  and an outer liner  1252  surrounding the inner liner  1251 . Here, the inner liner  1251  is a hollow tubular member forming the combustion chamber  1240 . The inner liner  1251  may be cooled by the compressed air permeating an annular space inside the outer liner  1252 . 
     The transition piece  1260  is disposed downstream of the liner  1250 , and the combustion gas produced in the combustion chamber  1240  may be discharged from the transition piece  1260  to the turbine  1300 . The transition piece  1260  may have a double structure formed by an inner transition piece  1261  and an outer transition piece  1262  surrounding the inner transition piece  1261 . The inner transition piece  1261  is a hollow tubular member similar to the inner liner  1251  and may have a diameter that is gradually reduced from the liner  1250  to the turbine  1300 . In this case, the inner liner  1251  may be coupled to the inner transition piece  1261  by a plate spring seal (not shown). Since the ends of the inner liner  1251  and the inner transition piece  1261  are fixed to the combustor  1200  and the turbine  1300 , respectively, the plate spring seal must have a structure that is capable of accommodating length and diameter elongation by thermal expansion to support the inner liner  1251  and the inner transition piece  1261 . 
     The combustor  1200  has a structure in which the outer liner  1252  and the outer transition piece  1262  respectively surround the inner liner  1251  and the inner transition piece  1261 . Compressed air may permeate the annular space between the inner liner  1251  and the outer liner  1252  and the annular space between the inner transition piece  1261  and the outer transition piece  1262 . The inner liner  1251  and the inner transition piece  1261  may be cooled by the compressed air permeating these annular spaces. 
     The high-temperature and high-pressure combustion gas produced in the combustor  1200  is supplied to the turbine  1300  through the liner  1250  and the transition piece  1260 . In the turbine  1300 , the thermal energy of combustion gas is converted into mechanical energy to rotate a rotary shaft by applying impingement and reaction force to a plurality of blades radially arranged on the rotary shaft of the turbine  1300  through the adiabatic expansion of the combustion gas. Some of the mechanical energy obtained from the turbine  1300  is supplied as energy required for compression of air in the compressor, and the remainder is used as effective energy required for driving a generator to produce electric power or the like. 
     Hereinafter, a fuel nozzle  2000  according to the embodiment of the present disclosure will be described with reference to the accompanying drawings. 
     Referring to  FIG.  3   , a fuel nozzle module includes a plurality of fuel nozzles  2000  according to the embodiment of the present disclosure, each of which includes an injection cylinder  2100 , swirlers  2200 , a porous disk  2300 , and a shroud  2500 . 
     The injection cylinder  2100  is a means for supplying fuel and premixing fuel and air, and extends in one direction. The injection cylinder  2100  typically has a cylindrical shape, but the present disclosure is not limited thereto. The embodiment of the present disclosure is exemplified by a cylindrical injection cylinder  2100 . 
     Referring to  FIG.  4   , the injection cylinder  2100  defines a space for mixing fuel and air, and the fuel and the air may be mixed with each other while longitudinally passing through the injection cylinder  2100 . The injection cylinder  2100  may have an air introduction port  2110  through which air is introduced into the injection cylinder  2100  and a discharge port  2120  through which a mixture of air and fuel is discharged. The discharge port  2120  is formed at a downstream end of the injection cylinder  2100 , and the embodiment of the present disclosure is exemplified by a discharge port  2120  formed on the bottom of the cylindrical injection cylinder  2100 . 
     Referring again to  FIG.  2   , a head end plate  1231  is coupled to the end of a nozzle casing  1232  forming the outer wall of the fuel nozzle assembly  1230  to seal the nozzle casing  1232 , and may be coupled with a manifold, a related valve, or the like for supplying fuel to the injection cylinder  2100 . The head end plate  1231  supports the fuel nozzle  2000  disposed in the nozzle casing  1232 . The fuel nozzle  2000  is fixed to the head end plate  1231  through a nozzle flange  2400  ( FIG.  3   ) disposed at one end of the injection cylinder  2100 . 
     Fuel is introduced through a fuel injector (not shown) and the head end plate  1231 , and longitudinally flows along the injection cylinder  2100  of the fuel nozzle  2000  to be injected into the combustion chamber  1240 . 
     The shroud  2500  surrounds the injection cylinder  2100  and extends in the same longitudinal direction as the injection cylinder  2100 . In particular, the shroud  2500  is spaced apart from an outer peripheral surface of the injection cylinder  2100  to form a channel for passing fuel and air. Since the shroud  2500  is arranged on the same axis as the injection cylinder  2100  and is spaced at a certain distance from the injection cylinder  2100  so as to surround the injection cylinder  2100 , the embodiment of the present disclosure is exemplified by a cylindrical shroud  2500 . In this case, the channel formed by the injection cylinder  2100  and the shroud  2500  may have an annular cross-section. 
     The swirlers  2200  are radially arranged on the outer peripheral surface of the injection cylinder  2100 , to be disposed approximately in the cylinder&#39;s longitudinal middle, thereby generating a swirl flow of fuel introduced into the space between the shroud  2500  and the injection cylinder  2100 . The swirlers  2200  may each have an internal passage communicating with the internal space of the injection cylinder  2100 . The fuel introduced into the injection cylinder  2100  may be discharged via the communication passages in the swirlers through outlets  2210  penetrating the inside and outside of the swirlers  2200 . 
       FIG.  4    illustrates the fuel nozzle according to an embodiment of the present disclosure, and  FIG.  5    illustrates a porous disk in the fuel nozzle of  FIG.  4   .  FIGS.  6  to  10    illustrate various modified examples of the porous disk. 
     In a conventional fuel nozzle, a swirl flow is generated while fuel passes through swirlers, and a pressure is reduced around the swirlers by the swirl flow. This reduced pressure causes a flashback phenomenon in which the flame produced in a combustion chamber flows backward toward the fuel nozzle. This flashback phenomenon leads to the deterioration of the fuel nozzle. 
     The fuel nozzle  2000  according to the embodiment of the present disclosure includes the porous disk  2300  installed downstream of the swirlers  2200  to prevent a flashback phenomenon occurring due to a reduction in pressure around the swirlers  2200 . 
     The porous disk  2300  includes a disk body  2310  prevents the backflow of the flame produced in the combustion chamber  1240  into the fuel nozzle. The disk body  2310  is formed as a flat, disk shape having inner and outer peripheral surfaces and includes two opposing surfaces each of which is perpendicular to the flow direction of fuel through the channel between the shroud  2500  and the injection cylinder  2100 . The porous disk  2300  may have a plurality of flow holes  2320  each having a predetermined size (diameter) and a predetermined shape (configuration) so as not to interrupt the flow of fuel. 
     The porous disk  2300  may be formed from the outer peripheral surface of the injection cylinder  2100  to the inner peripheral surface of the shroud  2500 . The porous disk  2300  may be formed to abut the inner peripheral surface of the shroud  2500 , but the present disclosure is not limited thereto. For example, the porous disk  2300  may be spaced at a predetermined distance from the inner peripheral surface of the shroud  2500 . This distance may be adjusted according to the magnitude of pressure reduction around the swirlers  2200 . That is, since there is a high possibility of flashback in the case of greater magnitudes of pressure reduction around the swirlers  2200 , the distance between the porous disk  2300  and the inner peripheral surface of the shroud  2500  may be decreased, and conversely, the distance may be increased for lesser magnitudes. 
     The flow holes  2320  may be radially and evenly arranged as illustrated in  FIG.  5   . The plurality of flow holes  2320  may extend in a pattern throughout the surface of the porous disk  2300 , that is, from the outer peripheral surface of the injection cylinder  2100  to the inner peripheral surface of the shroud  2500 . Each of flow holes  2320   a  may be configured as a straight through-hole that is perpendicular with respect to the flat surfaces of the disk body  2310  and thus aligned with the flow direction of fuel, as illustrated in  FIG.  6   . The formation of each of the flow holes  2320   a  as a straight through-hole imparts linearity to the flow of fuel introduced into the space between the shroud  2500  and the injection cylinder  2100  and thus enables a smooth flow of fuel. 
     Alternatively, as illustrated in  FIG.  7   , each of flow holes  2320   b  may be configured as a diagonal through-hole that is inclined with respect to the flat surfaces of the disk body  2310  and thus forms a predetermined angle with the flow direction of fuel. The formation of each of the flow holes  2320   h  as a diagonal through-hole imparts a spiral swirling effect to the fuel introduced into the space between the shroud  2500  and the injection cylinder  2100  thus enables a smooth mixing of fuel. The predetermined angle of one through-hole of the flow holes  2320   b  may be different from the predetermined angle of another through-hole of the flow holes  2320   b , and the flow holes  2320  may include an arrangement of both diagonal through-holes and straight through-holes. The predetermined angle of one through-hole of the flow holes  2320   b  may be different from the predetermined angle of another through-hole of the flow holes  2320   b . The predetermined angle of the through-holes of the flow holes  2320   b  may be consistent throughout the disk body  2310  as in  FIG.  7   , or the predetermined angle of one through-hole or group of through-holes of the flow holes  2320   b  may be different from the predetermined angle of another through-hole or group of through-holes of the flow holes  2320   b . For example, as in the disk body  2310  of  FIG.  8   , each of flow holes  2320   c  may be formed such that the imaginary extension lines of one group of through-holes of the flow holes  2320   c  intersect with the imaginary extension lines of another group. Also, the flow holes  2320  may include an arrangement of both diagonal through-holes ( FIG.  7   ) and straight through-holes ( FIG.  6   ). 
     Alternatively, as illustrated in  FIG.  9   , each of flow holes  2320   d  may include a curve having at least one turn t. In this case, the overall directionality of the flow holes  2320   d  including the curve may be consistent with the configuration of any of  FIGS.  6  to  8   . That is, the relative position of the upstream and downstream sides of the flow holes  2320   d  may be aligned with each other, similar to the perpendicular configuration of the flow holes  2320   h ; or their relative positions may be offset from each other, similar to the inclined configuration of the flow holes  2320   b  or  2320   c . When the flow holes  2320   d  are configured as a curved through-hole having at least one turn t, the direction of the fuel introduced into the flow hole  2320   d  (upstream side) is switched to flow in a countering direction after the turn t. Thus, it is possible to smoothly mix fuel by imparting a spiral swirling effect to the fuel. 
     In addition, the flow holes  2320  of  FIG.  5   , configured as the flow holes of any of  FIGS.  6  to  9   , may have different diameters. For example, as illustrated in  FIG.  10   , a plurality of flow holes  2320   e  may be rendered by having diameters that increase from the inner peripheral surface of the disk body  2310  toward the outer peripheral surface of the disk body  2310 . Through such a structure, the flow of fuel may be guided to increase toward the outer peripheral surface of the porous disk  2300 . 
     Although the flow holes  2320  are illustrated as being radially arranged in  FIGS.  5  to  10    for convenience of description, they may instead be arranged in a rectilinear or arbitrary direction, or be disposed at arbitrary positions according to an amorphous pattern. 
     Next, a fuel nozzle  2000  according to another embodiment of the present disclosure will be described with reference to  FIGS.  11  to  13   , in which each of the fuel nozzles of the fuel nozzle module ( FIG.  3   ) includes an injection cylinder  2100 , swirlers  2200 , a nozzle flange  2400 , a shroud  2500 . In the embodiment of  FIGS.  11  to  13   , the structure and function of the injection cylinder  2100 , swirlers  2200 , nozzle flange  2400 , and shroud  2500  are consistent with those of the above-described embodiment, their detailed description will be omitted. 
     As illustrated in  FIG.  11   , the fuel nozzle  2000  according to another embodiment of the present disclosure includes a plurality of porous disks  2302  disposed in the space between the shroud  2500  and the injection cylinder  2100  and spaced apart by a predetermined distance or interval. These porous disks  2302  include at least one of the porous disks  2300  modified according to the above-described embodiment, namely, a porous disk according to one of  FIGS.  6  to  10   . In the present embodiment, at least two porous disks  2302  may be formed from the outer peripheral surface of the injection cylinder  2100  to the inner peripheral surface of the shroud  2500 . 
     As illustrated in  FIG.  12   , a plurality of porous disks  2302  are formed and each have flow holes  2320 . In this case, the arrangement of the porous disks  2302  from one disk to the next is repetitive. When the porous disks  2302  are arranged such that each of the flow holes  2320  is aligned with the flow direction of fuel, it is possible to render a smooth flow of air by imparting a repeated linearity to the fuel introduced into the space between the shroud  2500  and the injection cylinder  2100 . 
     As illustrated in  FIG.  13   , a plurality of porous disks  2302  are formed and each have flow holes  2320 . In this case, the arrangement of the porous disks  2302  from one disk to the next is alternated. When the porous disks  2302  are arranged such that each of the flow holes  2320  is alternately inclined to form a series of predetermined opposing angles with the flow direction of fuel, it is possible to more smoothly mix fuel by imparting a repeated swirling effect to the fuel introduced into the space between the shroud  2500  and the injection cylinder  2100 . 
     The fuel nozzles  2000  of  FIGS.  11  to  13    each includes the plurality of porous disks  2302 . Therefore, even when a porous disk  2302   a  facing the combustion chamber  1240  is damaged due to a flashback phenomenon, the flashback phenomenon can be prevented by a next porous disk  2302   h.    
     Next, a fuel nozzle  2000  according to a further embodiment of the present disclosure will be described with reference to  FIG.  14   , in which each of the fuel nozzles of the fuel nozzle module ( FIG.  3   ) includes an injection cylinder  2100 , swirlers  2200 , a nozzle flange  2400 , a shroud  2500 . In the embodiment of  FIG.  14    the structure and function of the injection cylinder  2100 , swirlers  2200 , nozzle flange  2400 , and shroud  2500  are consistent with those of the above-described embodiment, their detailed description will be omitted. 
     As illustrated in  FIG.  14   , the fuel nozzle  2000  according to the further embodiment of the present disclosure includes a plurality of porous disks  2303  and  2304  respectively provided in successive stages along the longitudinal length of the injection cylinder  2100 . These porous disks  2303  and  2304  include at least one of the porous disks  2300  modified according to the above-described embodiment, namely, a porous disk according to one of  FIGS.  6  to  11   . In the present embodiment, a first porous disk  2303  facing the combustion chamber  1240  extends from the outer peripheral surface of the injection cylinder  2100  to the inner peripheral surface of the shroud  2500 , and second porous disks  2304  disposed behind the first porous disk  2303  are each spaced at a predetermined distance from the inner peripheral surface of the shroud  2500 . In general, the second porous disks  2304  have respective diameters that incrementally increase toward the combustion chamber  1240 . Moreover, since the first porous disk  2303  may have the largest diameter, the incremental increase in diameters toward the combustion chamber  1240  is effected for all of the first and second porous disks  2303  and  2304 . 
     Since the first and second porous disks  2303  and  2304  have diameters that gradually increase toward the combustion chamber  1240 , it is possible to render a smooth flow of air by securing a flow passage for the fuel introduced into the space between the shroud  2500  and the injection cylinder  2100 . Therefore, even when the first porous disk  2303  facing the combustion chamber  1240  is damaged due to a flashback phenomenon, it is possible to prevent the flashback phenomenon using one or more the subsequent (second) porous disks  2304 . That is, flashback may be partially prevented by the first porous disk  2303  assuming its function is not effectively destroyed, in which case the second porous disks  2304  can prevent the flashback that may traverse a damaged portion of the first porous disk  2303  from reaching the fuel nozzle. In this case, one or more of the second porous disks  2304  may not fully span the distance between the shroud  2500  and the injection cylinder  2100 . 
     As is apparent from the above description, in accordance with the exemplary embodiments of the present disclosure, it is possible to prevent a flashback phenomenon occurring due to a reduction in pressure around the swirler by installing a porous disk downstream of the swirler. In addition, by forming variously configured flow holes in the porous disk, it is possible to impart linearity or a swirling effect to the flow of fuel passing through the fuel nozzle. 
     While the present disclosure has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure as defined in the following claims.