Abstract:
An object is to provide a combustor that requires a small mounting space for an acoustic damper, that can achieve size reduction, and that can improve the ease of maintenance. A combustor ( 5 ) of the present invention includes a combustion cylinder ( 19 ) that defines a combustion area ( 23 ) therein and an acoustic damper ( 31 ) that has a damper cover having an acoustic-damper resonance space communicating with the combustion area ( 23 ). The damper cover is provided along the combustion cylinder ( 19 ) so as to extend in a direction intersecting an axial direction (L) of the combustion cylinder ( 19 ).

Description:
TECHNICAL FIELD 
     The present invention relates to a combustor and a gas turbine having the same. 
     BACKGROUND ART 
     A gas turbine includes a compressor, a combustor, and a turbine. The compressor takes in air, compresses the air to increase its pressure, and directs the high-pressure air to the combustor. 
     In the combustor, fuel is sprayed into the high-pressure air to combust the fuel. High-temperature combustion gas generated by the combustion of the fuel is directed to the turbine, and this high-temperature combustion gas drives the turbine. 
     Because the turbine and the compressor rotate about the same rotation shaft, this driving of the turbine drives the compressor, causing the compressor to take in and compress air, as described above. 
     The gas turbine operating as above may suffer from combustion oscillations during combustion of the fuel, and such combustion oscillations have been a cause of noise and vibration during operation of the gas turbine. 
     In particular, recent gas turbines have reduced the NOx (nitrogen oxide) level in the exhaust gas from the standpoint of the impact on the environment during operation and often employ lean combustion of fuel to reduce the NOx level. However, because lean combustion tends to cause unstable combustion, combustion oscillations are likely to occur. In order to reduce the noise and vibration caused by the combustion oscillations, combustors have been provided with an acoustic liner for absorbing relatively high-frequency noise, which is made of, for example, a porous plate and a cover that covers the outside thereof; or an acoustic damper having a large resonance space for absorbing relatively low-frequency noise. 
     Because the volume of the resonance space in the acoustic liner for relatively high-frequency noise is small, there are few space limitations in the casing during installation. 
     In contrast, because the volume of the resonance space in the acoustic damper for relatively low-frequency noise is large, there are space limitations in the casing during installation. Conventionally, as shown in, for example, PTL 1, in a combustor having a bypass flow path for allowing air in the casing to be introduced into the combustion gas, an acoustic damper that utilizes the circumference of the bypass flow path is provided. 
     Furthermore, as shown in, for example, PTL 2, a combustor having no bypass flow path has been proposed, in which the acoustic damper is connected to the acoustic liner fitted around the combustor and in which an acoustic portion forming the resonance space of the acoustic damper is provided so as to extend in the axial direction or radial direction of the combustor. 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} Japanese Unexamined Patent Application, Publication No. 2006-22966 
         {PTL 2} Japanese Unexamined Patent Application, Publication No. 2006-266671 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Meanwhile, the disclosure in PTL 1 requires a large space outside the combustor for providing the bypass flow path and the acoustic damper. Furthermore, the disclosure in PTL 2 requires a large space outside the combustor for providing the bypass flow path and the acoustic damper, because even an acoustic damper extending in the axial direction, not to mention an acoustic damper extending in the radial direction, is bent in the radial direction to ensure the volume (overall length) of the resonating space. 
     Thus, because a large casing space is required, the size of a housing is increased, which may make, for example, ground transportation of the gas turbine impossible. Thus, the manufacturing costs, including the transportation costs, increase. 
     The combustors are subjected to periodic maintenance. However, the combustors cannot be extracted unless the bypass flow path is removed in PTL 1 and the acoustic damper is removed in PTL 2. Accordingly, the maintenance involves a great deal of work. 
     The present invention has been made in view of the above-described problems, and an object thereof is to provide a combustor that requires a small mounting space for an acoustic damper, that can achieve size reduction, and that can improve the ease of maintenance, and to provide a gas turbine using such a combustor. 
     Solution to Problem 
     In order to achieve the above-described object, the present invention provides the following solutions. 
     A first aspect of the present invention is a combustor including a cylindrical body that defines a combustion area therein, and an acoustic damper that includes an acoustic portion having an acoustic-damper resonance space communicating with the combustion area. The acoustic portion is provided along the cylindrical body so as to extend in a direction intersecting an axial direction of the cylindrical body. 
     According to this aspect, because the acoustic portion having the acoustic-damper resonance space is provided along the cylindrical body so as to extend in the direction intersecting the axial direction of the cylindrical body, or the circumferential direction, the acoustic portion is disposed widely in the circumferential direction, without concentrating in a particular area of the cylindrical body in the circumferential direction. As a result, the acoustic portion is prevented from protruding toward the outer circumference of the cylindrical body, and the space needed outside the combustor can be reduced. 
     Thus, because the casing can be made small, the housing constituting the casing can be made small. Because this enables, for example, the gas turbine to be adequately transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. 
     Furthermore, if the protrusion of the acoustic portion toward the outer circumference of the cylindrical body is reduced, the combustor can be easily extracted together with the acoustic damper. Thus, it is possible to improve the ease of maintenance of the combustor. 
     The above-described aspect may further include an acoustic liner formed by a porous plate that constitutes the cylindrical body and has a plurality of through-holes penetrating in a thickness direction and a cover member that is provided around and at a certain distance from the porous plate so as to cover the porous plate, the acoustic liner having an acoustic-liner resonance space. 
     By doing so, it is possible to attenuate oscillations in a frequency region that can be attenuated by the acoustic liner and oscillations in a frequency region that can be attenuated by the acoustic damper. Accordingly, it is possible to attenuate combustion oscillations in a wide frequency region. 
     In the above configuration, it is preferable that at least part of the acoustic portion be provided on the outer circumferential side of the acoustic liner. 
     In this configuration, because the acoustic liner and the acoustic damper are provided so as to be concentrated in a certain area of the cylindrical body in the axial direction, the other portions of the cylindrical body in the axial direction can be efficiently used. 
     In the above aspect, the acoustic-damper resonance space may be formed so as to make at least one turn. 
     This enables a sufficient volume (overall length) of the acoustic-damper resonance space to be ensured, even when, for example, the volume (overall length) of the acoustic-damper resonance space cannot be ensured by using the entire circumferential length of the cylindrical body, or, another member needs to be provided at a position of the cylindrical body in the axial direction where the acoustic damper is provided. 
     In the above aspect, at least one fluid resisting member may be provided in the acoustic-damper resonance space. 
     By doing so, it is possible to attenuate oscillations and noise caused by the combustion oscillations also with the fluid resisting member. 
     Furthermore, the frequency region of the oscillations to be attenuated can be adjusted not only by changing the volume (overall length) of the acoustic-damper resonance space, but also by changing the resistance exerted by the fluid resisting member. Accordingly, the oscillation attenuating performance of the acoustic damper can be more assuredly improved. 
     In the above aspect, a plurality of the acoustic dampers may be provided. 
     In this configuration, because the oscillations can be attenuated by a plurality of the acoustic dampers, the oscillations can be more assuredly attenuated. 
     In such a case, the volumes (overall lengths) of the acoustic-damper resonance spaces of the plurality of acoustic dampers may be different from each other. By doing so, it is possible to attenuate oscillations in different frequency regions with the respective acoustic dampers. 
     Accordingly, the oscillation attenuating performance of the acoustic dampers can be more assuredly improved. 
     A second aspect of the present invention is a gas turbine including an air compressor, the combustor according to the first aspect, and a turbine. 
     Because the gas turbine according to this aspect includes the combustor capable of reducing the size of the housing, reducing the manufacturing costs, and improving the ease of maintenance, it is possible to reduce the noise caused by the combustion during operation of the gas turbine and to improve the ease of maintenance. Furthermore, low-cost manufacturing thereof is possible. 
     Advantageous Effects of Invention 
     According to the present invention, because the acoustic portion having the acoustic-damper resonance space is provided along the cylindrical body so as to extend in a direction intersecting the axial direction of the cylindrical body, or the circumferential direction, the space needed outside the combustor can be reduced. 
     Thus, because the casing can be made small, the housing constituting the casing can be made small. Because this enables, for example, the gas turbine to be adequately transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. Furthermore, if the protrusion of the acoustic portion toward the outer circumference of the cylindrical body is reduced, the combustor can be easily extracted together with the acoustic damper. Thus, it is possible to improve the ease of maintenance of the combustor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing the overall configuration of a gas turbine according to a first embodiment of the present invention. 
         FIG. 2  is a schematic view for describing, in outline, the configuration of a combustor in  FIG. 1 . 
         FIG. 3  is a cross-sectional view taken along line X-X in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along line Y-Y in  FIG. 3 . 
         FIG. 5  is a cross-sectional view showing a first modification of an attenuating device according to the first embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of an attenuating device according to a second embodiment of the present invention, showing the same portion as in  FIG. 4 . 
         FIG. 7  is a cross-sectional view taken along line Z-Z in  FIG. 6 . 
         FIG. 8  is a cross-sectional view of an attenuating device according to a third embodiment of the present invention, showing the same portion as in  FIG. 4 . 
         FIG. 9  is a cross-sectional view taken along line W-W in  FIG. 8 . 
         FIG. 10  is a partial sectional view showing a modification of the attenuating device according to the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a gas turbine of the present invention will be described below, on the basis of the drawings. 
     First Embodiment 
     Referring to  FIGS. 1 to 4 , a gas turbine  1  according to a first embodiment of the present invention will be described. 
       FIG. 1  is a schematic view for describing the configuration of the gas turbine  1  according to this embodiment.  FIG. 2  is a schematic view for describing, in outline, the configuration of combustors  5  in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , the gas turbine  1  includes a compressor  3 , the combustors  5 , a turbine unit (turbine)  7 , a rotation shaft  9 , and a housing  11  that accommodates these components in place. 
     The compressor  3  takes in and compresses the atmosphere, which is the outside air, and supplies the compressed air to the combustors  5 . 
     Note that the configuration of the compressor  3  may be any known one and is not specifically limited. 
     As shown in  FIG. 1 , the combustors  5  generate combustion gas (high-temperature gas) by mixing the air compressed by the compressor  3  and externally supplied fuel and combusting the mixed gaseous mixture. The plurality of (for example, 16) combustors  5  are disposed in the circumferential direction and are mounted to the housing  11  so as to penetrate therethrough and reach a casing  13 . 
     As shown in  FIG. 2 , each combustor  5  mainly includes air supply ports  15 , a fuel nozzle  17 , a combustion cylinder  19  (cylindrical body), and an attenuating device  21 . 
     As shown in  FIG. 2 , the air supply ports  15  are disposed around the fuel nozzle  17  in a ring-like manner and introduce the air compressed by the compressor  3  into the combustion cylinder  19 . The air supply ports  15  give a flow-velocity component in a turning direction to the air flowing into the combustion cylinder  19  and produce a circulating flow in the combustion cylinder  19 . 
     Note that the shape of the air supply ports  15  may be any known one and is not specifically limited. 
     As shown in  FIG. 2 , the fuel nozzle  17  sprays the externally supplied fuel toward the inside of the combustion cylinder  19 . The fuel sprayed from the fuel nozzle  17  is stirred by an air flow or the like created by the air supply ports  15 , forming a gaseous mixture composed of fuel and air. 
     Note that the shape of the fuel nozzle  17  may be any known one and is not specifically limited. 
     As shown in  FIG. 2 , the combustion cylinder  19  is formed in a cylindrical shape and forms a flow path extending from the air supply ports  15  and the fuel nozzle  17  to an inlet portion of the turbine unit  7 . In other words, the combustion cylinder  19  forms a combustion area  23  therein, through which the gaseous mixture composed of fuel and air, as well as the combustion gas generated by the combustion of the gaseous mixture, flow. 
     The combustion cylinder  19  is formed of a heat-resistant metal, such as a nickel-base alloy. 
     A plurality of cooling paths  25  (see  FIG. 4 ) extending in an axial direction L and disposed with spaces therebetween in the circumferential direction C are formed in a wall of the combustion cylinder  19 . 
     The cooling paths  25  are connected to, for example, a boiler (not shown) at one end so that steam, serving as coolant, flows therethrough. The cooling paths  25  are connected to a steam-discharging flow path  27  at the other end. The steam having passed through the cooling paths  25  is discharged outside the system through the steam-discharging flow path  27  or is returned to the boiler. 
     Although this embodiment shows a case where steam is used as the coolant for cooling the combustion cylinder  19 , air may also be used depending on the design conditions. In such a case, the steam-discharging flow path  27  is unnecessary. The structure of the air cooling structure may be any known one and is not specifically limited. 
       FIG. 3  is a cross-sectional view taken along line X-X in  FIG. 2 .  FIG. 4  is a cross-sectional view taken along line Y-Y in  FIG. 3 . The attenuating device  21  includes an acoustic liner  29  and an acoustic damper  31 . The acoustic liner  29  includes a liner cover (cover member)  35  and a cylindrical plate (porous plate)  33  constituting part of the combustion cylinder  19 . 
     The plate  33  has many (a plurality of) cylindrical through-holes  37  provided over substantially the entire circumference thereof. 
     Rows of the through-holes  37  are provided in the axial direction L and the circumferential direction C, so as to be spaced apart from one another. Furthermore, all the through-holes  37  may have the same shape, or the through-holes  37  in a first acoustic-damper resonance space  43  may have a shape different from those in an acoustic-liner resonance space  44  (described below); it is not specifically limited. 
     The liner cover  35  is a ring-like member having a U-shaped cross-section with the inner circumferential side being open. The liner cover  35  is provided on the outer circumferential side of the plate  33  so as to surround the entire circumference thereof. 
     The length of the open portion of the liner cover  35  in the axial direction L is larger than the area where the through-holes  37  are provided. 
     The liner cover  35  is joined to the plate  33  at the open ends of the U-shaped cross-section by, for example, brazing. Note that the liner cover  35  may be mounted by welding. 
     By doing so, a space is formed between the liner cover  35  and the outer surface of the plate  33 . This space is divided by a first partition  39  and a second partition  41  in the circumferential direction C. 
     In  FIG. 3 , a space on the upper part, which extends over about one-third of the entire circumference and is surrounded by the plate  33 , the liner cover  35 , the first partition  39 , and the second partition  42 , constitutes the first acoustic-damper resonance space  43 , and an area on the lower part, which extends over about two-thirds, constitutes the acoustic-liner resonance space  44 . 
     The acoustic damper  31  includes a damper cover (acoustic portion)  45  and an opening  47  provided in the liner cover  35 . The damper cover  45  is a ring-like member having a U-shaped cross-section with the inner circumferential side being open. 
     The damper cover  45  is provided on the outer circumferential side of the liner cover  35  so as to surround substantially the entire circumference thereof. 
     As shown in  FIG. 4 , the length of the open portion of the damper cover  45  in the axial direction L is larger than the area where the steam-discharging flow path  27  and the liner cover  35  are formed. 
     Note that, as described above, when air is used as the coolant for the combustion cylinder  19 , the steam-discharging flow path  27  is unnecessary. Thus, the damper cover  45  may be formed to have a size sufficient to surround the liner cover  35 . 
     The open ends of the damper cover  45  having a U-shaped cross-section are joined to the plate  33  (combustion cylinder  19 ) by, for example, brazing. Note that the damper cover  45  may be mounted by welding. 
     By doing so, a space is formed between the damper cover  45  and the outer surface of the plate  33 . This space is divided by the second partition  41  in the circumferential direction C. 
     The space surrounded by the plate  33 , the damper cover  45 , the outer surface of the liner cover  35 , the outer surface of the steam-discharging flow path  27 , and the second partition  41  is formed as a second acoustic-damper resonance space  49 . 
     Because the second acoustic-damper resonance space  49  is formed over the entire circumference and has a large cross-sectional area, it has a much larger volume (overall length) than the acoustic-liner resonance space  44 . 
     Although the second partition  41  is a common member that divides the first acoustic-damper resonance space  43  and the acoustic-liner resonance space  44  in this embodiment, the second partition  41  may be provided as a separate member so as to ensure the necessary volumes (overall lengths) for the respective resonance spaces, if necessary. 
     The opening  47  is provided in the liner cover  35 , near the second partition  41 . The opening  47  has a substantially rectangular shape elongated in the axial direction L and penetrates through the liner cover  35 . 
     The second acoustic-damper resonance space  49  communicates with the first acoustic-damper resonance space  43  via the opening  47 . The first acoustic-damper resonance space  43  communicates with the combustion area  23  via the through-holes  37 , which consequently allows the second acoustic-damper resonance space  49  to communicate with the combustion area  23 , to serve as an integral acoustic damper  31 . 
     Because the damper cover  45  is provided along the combustion cylinder  19  so as to extend in the circumferential direction C in this manner, the damper cover  45  is disposed widely in the circumferential direction C, without concentrating in a particular area of the combustion cylinder  19  in the circumferential direction C. As a result, the damper cover  45  is prevented from protruding toward the outer circumference of the combustion cylinder  19 , and the space needed outside the combustors  5  can be reduced. Thus, because the casing  13  can be made small, the housing  11  constituting the casing  13  can be made small. Because this enables the gas turbine  1  to have such a size, for example, that it can be transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. 
     Furthermore, by forming the liner cover  35  constituting part of the acoustic liner  29  integrally with a component of the acoustic damper  31  so as to serve the function thereof, the material can be reduced compared with the case where the acoustic damper  31  is formed separately from the combustion cylinder  19 . Thus, the manufacturing costs of the acoustic damper  31  can be reduced. 
     Furthermore, if the protrusion of the damper cover  45  toward the outer circumference of the combustion cylinder  19  is reduced, the combustors  5  can be extracted together with the acoustic damper  31 , by, for example, slightly enlarging the mounting portion of the combustors  5 , or even without changing anything. Because this facilitates extraction of the combustors  5 , the ease of maintenance of the combustors  5  can be improved. 
     A porous metal member (fluid resisting member)  51  is provided in the second acoustic-damper resonance space  49 . This porous metal member  51  is composed of a porous metal, i.e., a metal having multiple small holes. The porous metal member  51  is provided in the second acoustic-damper resonance space  49 , at part of the damper cover  45 , such that the porous metal member  51  has substantially the same shape as the internal space of the damper cover  45 . 
     Note that the porous metal member  51  is used depending on necessity and, thus, it may be omitted. 
     As shown in  FIG. 1 , the turbine unit  7  generates a rotational driving force by receiving a supply of high-temperature gas produced by the combustors  5  and transmits the generated rotational driving force to the rotation shaft  9 . 
     As shown in  FIG. 1 , the rotation shaft  9  is a cylindrical member supported so as to be rotatable about the rotation axis and transmits the rotational driving force generated by the turbine unit  7  to the compressor  3 . 
     Note that the configurations of the turbine unit  7  and rotation shaft  9  may be any known ones and are not specifically limited. 
     Next, the effects and advantages of the gas turbine  1  having the above-described configuration will be described. 
     As shown in  FIG. 1 , the gas turbine  1  takes in the atmosphere (air) as the compressor  3  is rotationally driven. The intake atmosphere is compressed by the compressor  3  and is directed to the combustors  5 . 
     The compressed air flowing into the combustors  5  is mixed with externally supplied fuel in the combustors  5 . The gaseous mixture composed of fuel and air is combusted in the combustors  5 , and the combustion heat produces high-temperature combustion gas. 
     The combustion gas produced in the combustors  5  is supplied from the combustors  5  to the downstream turbine unit  7 . The turbine unit  7  is rotationally driven by high-temperature gas, and the rotational driving force thereof is transmitted to the rotation shaft  9 . The rotation shaft  9  transmits the rotational driving force extracted in the turbine unit  7  to the compressor  3  and the like. 
     When the fuel is combusted in the combustors  5 , the combustion may generate combustion oscillations. 
     In particular, because lean combustion of fuel for reducing the NOx level in the exhaust gas tends to cause unstable combustion, combustion oscillations are likely to occur. 
     When such combustion oscillations are generated, air oscillations (pressure wave) caused by the combustion oscillations enter the through-holes  37  in the plate  33 . 
     The air in the acoustic-liner resonance space  44  and the air in the through-holes  37  in the acoustic liner  29  constitute a resonator system because the air in the acoustic-liner resonance space  44  serves as a spring. Accordingly, because the air in the through-holes  37  is severely oscillated and resonated with respect to the noise in the frequency region corresponding to the volume (overall length) of the acoustic-liner resonance space  44  and the overall length of the through-holes  37  among the air oscillations and noise caused by the combustion oscillations generated inside the plate  33 , the noise at this resonant frequency is absorbed by the friction between the air and the surfaces of the through-holes  37 . Thus, the amplitude of the combustion oscillations is attenuated and the noise caused by the combustion oscillations is reduced. 
     The first acoustic-damper resonance space  43  and the second acoustic-damper resonance space  49  are connected via the opening  47 . Therefore, the combustion oscillations generated in the combustion area  23  are transmitted to the second acoustic-damper resonance space  49  via the first acoustic-damper resonance space  43 , and these acoustic-damper resonance spaces serve as the integral acoustic damper  31 . 
     The volume (overall length) of this acoustic damper  31  is larger than that of the acoustic-liner resonance space  44 . Therefore, the resonance space of the acoustic damper  31  (the first acoustic-damper resonance space  43  and the second acoustic-damper resonance space  49 ) can attenuate oscillations with a longer wavelength than oscillations attenuated in the acoustic-liner resonance space  44 , in other words, oscillations in a lower frequency region than the frequency region of the oscillations that can be attenuated in the acoustic-liner resonance space  44 . 
     Although the acoustic liner  29  and the acoustic damper  31  both attenuate oscillations as described above, the acoustic liner  29  attenuates oscillations in a relatively high frequency region, whereas the acoustic damper  31  attenuates oscillations in a relatively low frequency region. 
     By providing both the acoustic liner  29  and the acoustic damper  31 , it is possible to attenuate oscillations in several frequency regions or oscillations in a wide frequency region. 
     Accordingly, noise generated during combustion in the combustors  5  can be effectively reduced. 
     The steam from the boiler is supplied to the cooling paths  25  and is exhausted outside the system from the steam-discharging flow path  27 . The steam exchanges heat with the combustion cylinder  19  (plate  33 ) while flowing through the cooling paths  25 , whereby the combustion cylinder  19  is cooled. Thus, the combustion cylinder  19  is cooled during the operation of gas turbine  1 . 
     The combustion gas sometimes enters the through-holes  37  during the operation of the gas turbine  1 . The through-holes  37  are heated by the combustion gas that has entered therein, whereby the thermal stress due to the temperature difference with respect to the peripheral portions increases. 
     Because the plate  33  is cooled by the steam passing through the cooling paths  25 , the peripheral portions of the through-holes  37  are sufficiently cooled. Thus, an increase in this thermal stress can be prevented. 
       FIG. 5  is a cross-sectional view showing the relevant part of the attenuating device  21  according to a first modification of this embodiment. As shown in  FIG. 5 , the attenuating device  21  according to this modification has two acoustic dampers  31 A and  31 B spaced apart in the axial direction L. Two damper covers,  45 A and  45 B, are each joined to the outer surface of the liner cover  35  at one end in the axial direction L. The liner cover  35  has openings  47 A and  47 B provided at portions covered by the damper covers  45 A and  45 B, respectively. 
     The frequency of oscillations that can be absorbed may be changed by changing the length of the damper covers  45 A and  45 B in the circumferential direction C (the overall length of the resonance space), by changing the mounting position of the porous metal member  51  in the circumferential direction C, or by doing both. 
     Because the oscillations can be attenuated by the plurality of acoustic dampers  31 A and  31 B, the oscillations can be more assuredly attenuated. Furthermore, because the two acoustic dampers  31 A and  31 B attenuate different frequency regions, it is possible to attenuate oscillations in several frequency regions in a relatively low frequency region or oscillations in a wide frequency region. 
     Accordingly, the oscillation attenuating performance of the acoustic dampers  31 A and  31 B can be more assuredly improved. 
     Although the second acoustic-damper resonance space  49  is formed over substantially the entire circumference in this embodiment, it is not limited thereto. The second acoustic-damper resonance space  49  does not need to be formed over the entire circumference but may be formed over a certain portion, as long as it has a volume (overall length) set according to the target frequency region. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 6 and 7 . 
     Although the basic configuration of the gas turbine according to this embodiment is the same as that according to the first embodiment, the configuration of the attenuating device  21  is different from that according to the first embodiment. Accordingly, in this embodiment, the attenuating device  21 , which is different from that according to the first embodiment, will be mainly described, and overlapping descriptions of the other components will be omitted. 
       FIG. 6  is a cross-sectional view for describing the configuration of the relevant part of the attenuating device  21  in the combustor  5  of the gas turbine  1  according to this embodiment.  FIG. 7  is a cross-sectional view taken along line Z-Z in  FIG. 6 . 
     Note that the components the same as those in the first embodiment will be denoted by the same reference numerals, and the descriptions thereof will be omitted. 
     In this embodiment, a damper cover (acoustic portion)  53  is a box that has a substantially rectangular cross-section and is curved so as to constitute part of a ring. As shown in  FIG. 6 , the damper cover  53  is provided on the outer circumferential side of the liner cover  35  so as to cover the circumference thereof. 
     Although a portion of the damper cover  53  in the circumferential direction C is removed, at least a portion of this removed portion overlaps the position where the first acoustic-damper resonance space  43  is provided. 
     A damper groove  55  extending in the circumferential direction C is formed in the inner circumferential surface of the damper cover  53 . The damper groove  55  is provided over substantially the overall length of the damper cover  53 . The outer circumference of the damper groove  55  is formed of an outwardly protruding wall. 
     The length of the damper cover  53  in the axial direction L, i.e., the width, is much larger than that of the liner cover  35 . As shown in  FIG. 7 , the length of the damper groove  55  in the axial direction L is smaller than that of the liner cover  35 . 
     The wall of the damper groove  55  in the damper cover  53  is joined to the liner cover  35  by, for example, brazing. Note that the damper cover  53  may be mounted by welding. 
     As shown in  FIG. 7 , the damper cover  53  is fitted so as to be placed away from the plate  33  (combustion cylinder  19 ) so as not to touch the plate  33 . 
     By doing so, a space is formed between the damper cover  53  and the outer surface of the liner cover  35 . This space is formed as a second acoustic-damper resonance space  57 . 
     Because the second acoustic-damper resonance space  57  is provided over substantially the entire circumference and has a large cross-sectional area, it has a much larger volume (overall length) than the acoustic-liner resonance space  44 . 
     The length of the damper cover  53  in the circumferential direction C is determined so as to ensure the volume (overall length) set according to the target frequency region. 
     The liner cover  35  has an opening  59  near one circumferential end of the damper cover  53 . The opening  59  has a substantially rectangular shape elongated in the axial direction L and penetrates through the liner cover  35 . 
     The second acoustic-damper resonance space  57  communicates with the first acoustic-damper resonance space  43  via the opening  59 . The first acoustic-damper resonance space  43  communicates with the combustion area  23  through the through-holes  37 , which consequently allows the second acoustic-damper resonance space  57  to communicate with the combustion area  23 , to serve as the integral acoustic damper  31 . 
     Because the damper cover  53  is provided along the liner cover  35 , i.e., the combustion cylinder  19 , so as to extend in the circumferential direction C in this manner, the damper cover  53  is disposed widely in the circumferential direction C, without concentrating in a particular area of the combustion cylinder  19  in the circumferential direction C. 
     As a result, the damper cover  53  is prevented from protruding toward the outer circumference of the combustion cylinder  19 , and the space needed outside the combustors  5  can be reduced. Thus, because the casing  13  can be made small, the housing  11  constituting the casing  13  can be made small. Because this enables the gas turbine  1  to have such a size, for example, that it can be adequately transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. 
     If the protrusion of the damper cover  53  toward the outer circumference of the combustion cylinder  19  is reduced, the combustors  5  can be extracted together with the acoustic damper  31 , by, for example, slightly enlarging the mounting portion of the combustors  5 , or even without changing anything. Because this facilitates extraction of the combustors  5 , the ease of maintenance of the combustors  5  can be improved. 
     Because the damper cover  53  is fitted so as to be placed away from the plate  33  (combustion cylinder  19 ) heated by the operation of the combustors  5  in this embodiment, the thermal stress can be reduced compared with the damper cover  45  according to the first embodiment. Because the damper cover  53  is mounted so as not to cover the entire liner cover  35 , it is easy to supply purge air to the acoustic-liner resonance space  44  in the liner cover  35 . 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIGS. 8 and 9 . Although the basic configuration of the gas turbine according to this embodiment is the same as that according to the first embodiment, the configuration of the attenuating device  21  is different from that according to the first embodiment. Accordingly, in this embodiment, the attenuating device  21 , which is different from that according to the first embodiment, will be mainly described, and overlapping descriptions of the other components will be omitted. 
       FIG. 8  is a cross-sectional view for describing the configuration of the relevant part of the attenuating device  21  in the combustor  5  of the gas turbine  1  according to this embodiment.  FIG. 9  is a cross-sectional view taken along line W-W in  FIG. 8 . Note that the components the same as those in the first embodiment will be denoted by the same reference numerals, and the descriptions thereof will be omitted. 
     The acoustic damper  31  has a damper cover (acoustic portion)  61  and an opening  63  provided in the liner cover  35 . 
     As shown in  FIG. 9 , the damper cover  61  has a rectangular cross-section with the inner circumferential side being open and is curved so as to constitute part of a ring (for example, an area of substantially 160 degrees). As shown in  FIG. 8 , the damper cover  61  has a small-diameter portion  65  and a large-diameter portion  67 , which are different in height and extend in the direction along the curve. Both ends of the large-diameter portion  67  are closed by end plates  69  and  71 . The end of the small-diameter portion  65  is closed by an end plate  73 . 
     The end of the small-diameter portion  65  on the large-diameter portion  67  side extends beyond the end plate  71  into the large-diameter portion  67  up to near the end plate  69 . 
     The large-diameter portion  67  has a partition  75  that extends in the circumferential direction and divides the space outside the small-diameter portion  65 . An end of the partition  75  extending in the circumferential direction is fixed to the end plate  69 , and the other end thereof extends up to near the end plate  71 . 
     As shown in  FIG. 9 , the length of the open portion in the damper cover  61  in the axial direction L is smaller than that of the liner cover  35 . 
     The open ends of the damper cover  61  having a U-shaped cross-section are joined to the liner cover  35  by, for example, brazing. Note that the damper cover  61  may be mounted by welding. 
     By doing so, a space is formed between the damper cover  61  and the outer surface of the liner cover  35 . This space is formed as a second acoustic-damper resonance space  77 . 
     The second acoustic-damper resonance space  77  includes a first space defined inside the small-diameter portion  65 , a second space defined outside the small-diameter portion  65  and inside the partition  75  extending in the circumferential direction, and a third space defined outside the partition  75  extending in the circumferential direction and inside the large-diameter portion  67 . 
     The first space communicates with the second space near the end plate  69 . The second space communicates with the third space near the end plate  69 . Accordingly, the second acoustic-damper resonance space  77  is formed to have two turns. 
     Although the second acoustic-damper resonance space  77  is simply provided over an area of substantially 160 degrees in the circumferential direction C, it has two turns. Accordingly, it is possible to ensure a sufficient volume (overall length) for the second acoustic-damper resonance space  77 . 
     Because the second acoustic-damper resonance space  77  has a large cross-sectional area, it has a much larger volume (overall length) than the acoustic-liner resonance space  44 . 
     The opening  63  is provided in the liner cover  35 , near the end plate  73 . In other words, the opening  63  is located at one end of the second acoustic-damper resonance space  77 . 
     The opening  63  has a substantially rectangular shape elongated in the axial direction L and penetrates through the liner cover  35 . 
     The second acoustic-damper resonance space  77  communicates with the first acoustic-damper resonance space  43  via the opening  63 . The first acoustic-damper resonance space  43  communicates with the combustion area  23  via the through-holes  37 , which consequently allows the second acoustic-damper resonance space  77  to communicate with the combustion area  23 , to serve as the integral acoustic damper  31 . 
     Because the damper cover  61  is provided along the combustion cylinder  19  so as to extend in the circumferential direction C in this manner, the damper cover  61  is disposed relatively widely in the circumferential direction C of the combustion cylinder  19 . 
     As a result, the damper cover  61  is prevented from protruding toward the outer circumference of the combustion cylinder  19 , and the space needed outside the combustors  5  can be reduced. 
     Thus, because the casing  13  can be made small, the housing  11  constituting the casing  13  can be made small. Because this enables the gas turbine  1  to have such a size, for example, that it can be adequately transported on the ground, it is possible to reduce the manufacturing costs, including the transportation costs. 
     Furthermore, if the protrusion of the damper cover  61  toward the outer circumference of the combustion cylinder  19  is reduced, the combustors  5  can be extracted together with the acoustic damper  31 , by, for example, slightly enlarging the mounting portion of the combustors  5 , or even without changing anything. Because this facilitates extraction of the combustors  5 , the ease of maintenance of the combustors  5  can be improved. 
     Because the damper cover  61  simply covers less than substantially half of the circumference in the circumferential direction C, it is possible to provide another member in the remaining part, which is more than half of the circumference. 
     In such a case, as shown in  FIG. 10 , the two acoustic dampers  31 A and  31 B may be provided. The two acoustic dampers  31 A and  31 B are provided such that small-diameter portions  65 A and  65 B of damper covers  61 A and  61 B face each other. The small-diameter portions  65 A and  65 B are each joined to the outer surface of the liner cover  35 . The liner cover  35  has openings  63 A and  63 B provided in portions covered by the damper covers  61 A and  61 B, respectively. 
     Because the oscillations can be attenuated by the plurality of acoustic dampers  31 A and  31 B, the oscillations can be more assuredly attenuated. 
     Accordingly, the oscillation attenuating performance of the acoustic dampers  31 A and  31 B can be more assuredly improved. 
     Furthermore, the volumes (lengths in the circumferential direction C, i.e., overall lengths of the resonance spaces) of the two acoustic dampers  77 A and  77 B may be differentiated, and the mounting positions of porous metal members  51 A and  51 B may be changed. By doing so, two acoustic dampers  31 A and  31 B having different attenuation frequency regions are created. Thus, it is possible to attenuate oscillations in several frequency regions in a relatively low frequency region or oscillations in a wide frequency region. 
     Note that the present invention is not limited to the above-described embodiments, but may be appropriately modified within a scope not departing from the spirit thereof. 
     For example, although the acoustic damper  31  and the acoustic liner  29  are integrally formed in the above-described embodiments, they may be independent and both mounted on the combustion cylinder  19 . This can further reduce the amount of protrusion of the acoustic damper  31  toward the outer circumference. 
     In such a case, the acoustic-damper resonance spaces  49 ,  57 , and  77  each directly communicate with the combustion area  23 . 
     REFERENCE SIGNS LIST 
     
         
           1 : gas turbine 
           3 : compressor 
           7 : turbine 
           19 : combustion cylinder 
           23 : combustion area 
           29 : acoustic liner 
           31 ,  31 A,  31 B: acoustic damper 
           33 : plate 
           35 : cover 
           37 : through-hole 
           43 : first acoustic-damper resonance space 
           44 : acoustic-liner resonance space 
           45 ,  53 ,  61 : damper cover 
           49 ,  57 ,  77 : second acoustic-damper resonance space 
           51 ,  51 A,  51 B: porous metal member (fluid resisting member) 
           53 ,  55 : groove portion 
         L: axial direction