Abstract:
A gas turbine is provided which has a combustor comprising a plurality of can-type combustor elements and an annular element. Isolated combustion takes place in the can-type combustor thereby generating individual component hot gas streams. The component hot gas streams are merged in the annular part. Combustion oscillations are substantially avoided by decoupling combustion flames. Accordingly, temperature and pressure homogenization as well as combustion stabilization by cross-ignition is effected.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention is related to the field of gas turbines and, more particularly, to operating conditions of gas turbines, where combustion oscillations may occur.  
           [0002]    Gas turbines are widely used for power generation as well as in jet engine aircrafts. In a gas turbine, air and fuel are incinerated in a combustor, leading to the generation of a hot gas stream which expands in a turbine part, thereby rotating a shaft. A part of the so generated rotational energy is used for driving a compressor and providing compressed air for the combustor.  
           [0003]    Efforts for reducing emissions, in particular nitrogenmonoxide NO x , have led to the concept of lean combustion, where air and fuel are not mixed directly in the combustion flame but rather in advance, i.e. in an area different from the actual combustion zone. After mixing, the well-homogenized mixture is subsequently conducted to the combustion zone. This premixing of fuel and air has the advantage that low fuel/air ratios could be achieved, resulting in low flame temperatures and, accordingly, low NO x  emissions. On the other hand, with the lean combustion process, flame instabilities are more likely to occur, which can in turn lead to combustion oscillations. Combustion oscillations are acoustical modes that arise from a positive coupling between pressure fluctuations caused by flame instabilities and acoustical reflections from combustor walls. The combustion oscillations produce high noise emissions and may lead to extensive damage to gas turbine parts.  
           [0004]    German published patent application DE 43 39 094 A1 sets out a method of suppressing combustion oscillations in a gas turbine by injecting an inert fluid into the combustion zone, thereby redistributing the combustion zone and disturbing the mechanism that establishes the combustion oscillation. This method is applied to an annular combustor which is specifically susceptible to combustion oscillations because of the large space usually defined by this geometry. The method belongs to the category of active instability control, because combustion oscillations are monitored and suppressed with a closed loop active control cycle. However, active instability control is a complex and expensive method, introducing a significant cost increase for the equipment needed to operate the gas turbine.  
           [0005]    WO 00/12939 sets out a gas turbine with an annular combustor, to which a plurality of burners are connected. Here, the problem of combustion oscillations in the annular combustor is avoided by a passive measure: Cylindrical means are installed around the orifices of several of the burners, which leads to a shift of the flame position and therefore to a detuning of acoustical modes occurring in the combustor. Passive measures are generally less expensive than active instability control, however, only certain frequency bands are normally suppressed by the static means.  
           [0006]    U.S. Pat. No. 5,309,710 sets out a can-type combustor for a gas turbine. Can-type combustors are less prone to combustion oscillations. However, the can-type construction is more complex and expensive and not as easy to service as an annular combustor. Moreover, an annular combustor provides a more homogenous temperature and pressure distribution at the turbine inlet, which is unavailable in the can-type combustors.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In view of the foregoing, the present invention provides a gas turbine and a method of operating a gas turbine which substantially avoids combustion oscillations. The present invention also advantageously provides a gas turbine and a method of operating a gas turbine which leads to a homogenous pressure and temperature distribution of a hot gas stream at a turbine inlet. Additionally, the present invention provides a gas turbine and a method of operating a gas turbine which is easy to service. It is another advantage of the invention to provide a method which can be implemented so as to enable mass production at reasonable engineering effort and expense and with maximally replicable component characteristics. As a further advantage, the present invention provides a gas turbine and a method of operating a gas turbine which requires comparably low cooling. The present invention further advantageously provides a gas turbine and a method of operating a gas turbine which requires comparably low complexity of construction and therefore leads to lower costs. Additionally, the present invention provides a gas turbine and a method of operating a gas turbine which is comparably stable in operation. As a further advantage, the invention provides a gas turbine and a method of operating a gas turbine which lowers sealing requirements for sealing against the hot gas.  
           [0008]    More particularly, the present invention provides a gas turbine, comprising a combustor for combustion of an air/fuel-mixture, thereby generating a hot gas stream, and a turbine with a turbine inlet through which the hot gas stream is conducted, wherein the combustor comprises an annular part, located next to the turbine inlet and a can-type part, comprising a plurality of can-type combustors, each connected to the annular part and opened to the annular part through a respective can outlet. The combination of a can-type combustor with an annular combustor combines advantages of the respective single concepts, thereby avoiding disadvantages of the respective single concepts. In particular, separate combustion within the can-type part avoids coupling of flame instabilities, thereby avoiding combustion oscillations. Moreover, resulting from the annular part, the temperature and pressure at the turbine inlet is homogenized. Additionally, cross-ignition within the annular combustor assures stable operation with all operating conditions. Furthermore, the overall surface of the combustor that needs to be cooled is smaller compared to a can-type combustor or element thereby resulting in decreased cooling requirements.  
           [0009]    Preferably, the can-type combustors each have a head-part and a sidewall extending from said head-part to said can outlet. Through the head part fuel and air is introduced into the can-type combustors. A main combustion zone is defined as being the zone of a visible flame. The extension of the sidewall of each can-type combustor is preferably sized such that each can-type combustor encloses essentially a complete main combustion zone. In another preferred embodiment, the extension of the sidewall of each can-type combustor is sized such that it does not significantly exceed the main combustion zone. By enclosing essentially all of the main combustion zones, a high degree of decoupling between the different flames is achieved. In that the sidewalls do not significantly exceed the main combustion zones, unnecessary enlargement of the combustor is avoided, thereby keeping cooling requirements and construction costs low.  
           [0010]    The annular part has an annular part axial extension, measured along the direction of said hot gas stream and the can-type part has a can-type part axial extension, measured along the direction of the hot gas stream and the annular part axial extension is preferably at least a half of the can-type part axial extension. This assures a sufficient merger of the individual hot gas streams from the can-type combustors, resulting in a high degree of homogenization of temperature and pressure.  
           [0011]    The turbine inlet is at least partially formed by a vane support structure, carrying vanes located in the turbine and the annular part is preferably connected to and thereby supported by the vane support structure. As a main advantage, sealing of clearances between the combustor and the turbine come out to be comparably easy because of lack of relative movement of these parts against one another.  
           [0012]    Each of the can-type combustors is preferably connected to the annular part and thereby supported by the annular part. Accordingly, sealing of clearances between the can-type combustors and the annular part come out to be comparably easy because of lack of relative movement of these parts against one another. Moreover, the support of the can-type combustors is less complex compared to combustors which have only cans, because those usually require support from an outer casing. Accordingly, costs are reduced. In a further preferred embodiment, each of the connections of the can-type combustors to the annular part is designed in such a manner that each of the can-type combustors is individually easily removable from the annular part, thereby leading to a construction that is easy to service by simply exchanging single can-type combustors.  
           [0013]    The can-type combustors are preferably equally distributed along the circumference of the annular part, thereby defining hollow spaces between respective sidewalls of adjacent can-type combustors. The hollow spaces can be used for introducing a cooling fluid for cooling of the can-type combustors.  
           [0014]    The annular part is preferably designed with a radically outward located outer shell and a radially inward located inner shell, wherein ribs, extending from the outer shell to the inner shell, are located next to the can-type part and wherein each of said can-type combustors is connected with its side wall to at least one of said ribs.  
           [0015]    Preferably, the sidewalls are diffuser-shaped; thereby diffusing the hot gas stream discharged from the can-type combustors into the annular part.  
           [0016]    Also, the present invention provides a gas turbine with a combustor for combustion of an air/fuel-mixture, thereby generating a main hot gas stream and a turbine with a turbine inlet through which the main hot gas stream is conducted, wherein the combustor comprises a first stage in which individual hot gas streams are generated in distinct main combustion zones, which are defined to be the region of a visible flame, each combustion zone being separated from another, wherein the first region of the combustor is followed by a second region in which the individual hot gas streams are brought together to form the main hot gas stream. The second region is preferably designed to assure appropriate intermixing of the individual hot gas streams, thereby homogenizing the temperature distribution in the main hot gas stream.  
           [0017]    Also, the present invention provides a method for reducing combustion oscillations in a gas turbine, the method comprising the following steps: conducting air and fuel to a plurality of distinct combustion zones, incinerating the air and fuel in the distinct combustion zones which isolated from one another, thereby generating individual hot gas streams and thereby substantially avoiding acoustical coupling of flame instabilities occurring in the distinct combustion zones, merging the individual hot gas streams to a main hot gas stream thereby substantially homogenizing the temperature and pressure distribution and feeding the main hot gas stream through a turbine inlet. The advantages of this method correspond to the foregoing described advantages of the gas turbine. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0018]    Some of the features, advantages, and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is a schematic view on a gas turbine,  
         [0020]    [0020]FIG. 2 illustrates combustion in an annular combustor,  
         [0021]    [0021]FIG. 3 illustrates schematically an improvement of the present invention over that shown in FIG. 2,  
         [0022]    [0022]FIG. 4 illustrates a plurality of can-type combustors connected to an annular combustor  
         [0023]    [0023]FIG. 5 illustrates in a longitudinal section a gas turbine with a can-type combustor connected to an annular combustor.  
         [0024]    [0024]FIGS. 6 and 7 illustrate in a three dimensional view can-type combustors connected to an annular combustor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
         [0026]    [0026]FIG. 1 illustrates schematically a gas turbine  1 . The gas turbine  1  comprises of a compressor  3 , a combustor  5  and a turbine  7 . The turbine  7  is mounted on a shaft  9  together with the compressor  3 . Within the compressor  3 , air  11  is compressed and conducted to the combustor  5 . Also, fuel  13  is conducted to the compressor  5 , being mixed with the air  11  to give an air/fuel mixture  15 . The air/fuel mixture  15  is incinerated, thereby generating a hot gas stream  17 . The hot gas stream  17  is conducted through a turbine inlet  19  into the turbine  7 , where through an arrangement of vanes and blades (not shown) energy is transferred to the shaft  9  which rotates. The rotational energy is partly used for driving the compressor  3 . Another part of the rotational energy could be used, for instance, for driving an electric generator.  
         [0027]    [0027]FIG. 2 illustrates an annular combustor  5 . Burners  21  function as suppliers of air/fuel mixtures  15  which are incinerated in a visible flame  25 , defining a main combustion zone  23 . Flame instabilities can occur, resulting in pressure pulses which in turn can induce more flame instabilities. In particular, adjacent burners  21  can have mutually acoustic coupling, thereby mutually inducing flame instabilities indicated by arrows  27 . By reflection of pressure pulses from the combustor wall, stable combustion oscillations can arise from the flame instabilities.  
         [0028]    [0028]FIG. 3 illustrates a novel concept for the combustor  5 . The combustor  5  comprises a can-type part  33  and an annular part  35 . The can-type part  33  comprises of a plurality of can-type combustors  31 , each connected to the annular part  35  and opened to the annular part  35  through respective can outlets  37 . Through respective head-parts  39  of each can-type combustor  31 , air  11  and fuel  13  is introduced, thereby forming an air/fuel mixture  15 . A sidewall  41  extends from the head part of each can-type combustor  31  to the respective can outlet  37 . The air/fuel mixture  15  is incinerated in the main combustion zones  23 , defined by visible flames  25 , in each of the can-type combustors  31 , thereby each generating an individual hot gas stream  42 . The individual or component hot gas streams  42  are merged in the annular part  35  to form the main hot gas stream  17 . As a consequence, the temperature and pressure of the main hot gas stream  17  turns out to be well homogenized before entering the turbine  7 . Additionally, cross-ignition between the different flames  23  is established which results in improved stability of operation, even under extreme lean combustion conditions. Preferably, the air  11  and fuel  13  undergo at least in part a premixing to form the air/fuel mixture  15 , thereby achieving a lean combustion which is, if appropriate, stabilized by a diffusion flame.  
         [0029]    The sidewalls  41  are sized such that they fully enclose the respective combustion zones  23 . This results in an acoustical decoupling of the flames  25  and, accordingly, an avoidance of combustion oscillation. However, the sidewalls  41  do not exceed substantially the extensions of the combustion zones  23 , thereby keeping the can-type part  33  reasonable small.  
         [0030]    [0030]FIG. 4 illustrates in a three-dimensional view more particularly the assembly of the annular part  35 . It is formed of an outer shell  51  and an inner shell  53  which enclose an annular shaped space  54 . The inner surface of the outer shell  51  as well as the inner shell  53  is covered by heat shield elements  55  for protection against hot gas. The can-type combustors  31  of the can-type part  33  may be distributed substantially equally along a circumferential direction  57  of the annular part  35 .  
         [0031]    [0031]FIG. 5 illustrates a longitudinal section of the combustor  5  supported in the gas turbine  1 . The compressor  3  has blades  61  and vanes  63  which, from a rotation of the shaft  9 , compress and conduct air  11  through a compressor diffuser  65  to a space  67  that is formed from an outer casing  69 . The compressed air  11  is mainly conducted to the combustor  5  but a part of the air  11  is used for cooling the combustor  5  and turbine  7 . The outer casing  69  also encloses a vane support structure  71  of the turbine  7 . This vane support structure  71  supports vanes  73 . Between two respective rows of vanes  73 , blades  75  are mounted on the shaft  9 . The vane support structure  71  also supports the annular part  35  of the combustor  5  which is attached to the vane support structure  71 . Accordingly, very little relative movement between the annular part  35  and the turbine  7  occurs which leads to an easy sealing against discharge of hot gas out of clearances between the annular part  35  and the turbine  7 . Moreover, the can-type combustors  31  are attached to the annular part  35  which again leads to very little relative movement between the annular part  35  and the can-type combustors  31  and, accordingly, to an easy sealing against discharge of hot gas out of clearances. Furthermore, without the need of supporting the combustor  5  from the outer casing  69 , a less complex and less expensive construction is achieved. In a direction of the hot gas stream, the can-type part  33  has an can-type part extension C and the annular part  35  has an annular part extension A which is at least one half of the can-type part extension C.  
         [0032]    FIGS.  6  and FIG. 7 illustrate in a three-dimensional view, the connection of the can-type combustors  31  to the annular part  35 . The annular part  35  has ribs  91 , extending from the outer shell  51  to the inner shell  53 . The can-type combustors  31  are connected to these ribs  91  with their sidewalls  41 . The sidewalls  41  are diffuser shaped and have flanges  85  with which they are connected to the annular part  35 . Every single can-type combustor  31  is easily removable for service. The head parts  39  have openings  83  in which a burner can be mounted.  
         [0033]    [0033]FIG. 6 illustrates furthermore the can-type combustors  31  having a double wall structure which comprises an outer wall  101  and an inner wall  103 , enclosed by the outer wall  101 . Between the outer wall  101  and the inner wall  103  hollow space or opening  105  is formed which provides for conduction of a cooling fluid. In particular, steam could be used as a cooling fluid, separated in a closed cooling cycle from the hot gas inside the combustor. The inner wall  103  may be manufactured as a cylinder  103 a with an on welded flange  103   b . The inner wall  103  is mounted within the outer wall  101  by guidance elements  107  and fixation in a fork-like support  109 . Between adjacent can-type combustors  31 , the space  105  for cooling fluid is narrowed. Accordingly, an impingement cooling is provided in this area  111  in order to improve cooling effectiveness. The low cost design and easy mounting of the inner wall  103  allows complete exchange of this inner wall  103  during a service cycle.  
         [0034]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.