Abstract:
A method for operating a gas turbine engine includes compressing an air stream in a compressor and generating a post combustion gas by combusting a compressed air stream exiting from the compressor in a combustor. The post combustion gas is expanded in a first turbine. The expanded combustion gas exiting from the first turbine is split into a first stream and a second stream. The first stream of the expanded combustion gas is combusted in a reheat combustor. The reheat combustor is cooled using the second stream of the expanded combustion gas.

Description:
BACKGROUND 
       [0001]    The invention relates generally to gas turbines engines, and in particular, to cooling of a reheat combustor in a gas turbine engine. 
         [0002]    A conventional gas turbine engine includes a compressor for compressing air (sometime referred to as an oxidant as the air has oxidizing potential due to the presence of oxygen), which is mixed with fuel in a combustor and the mixture is combusted to generate a high pressure, high temperature gas stream, referred to as a post combustion gas. The post combustion gas is expanded in a turbine (high pressure turbine), which converts thermal energy from the post combustion gas to mechanical energy that rotates a turbine shaft. 
         [0003]    Generally, during the process of combustion in the combustor, the oxygen content in the air is not fully consumed. As a result, the hot post combustion gas, exiting from the high pressure turbine, is associated with approximately 15% to approximately 18% by mass of oxygen and therefore has the potential of oxidizing more fuel. Some gas turbine engines, therefore, deploy a reheat combustor, where the post combustion gas is re-combusted after mixing with additional fuel. The re-combusted post combustion gas is expanded in another turbine section (low pressure turbine) to generate additional power. The deployment of the reheat combustor and the low pressure turbine therefore utilizes the oxidizing potential of the post combustion gas, thereby increasing the efficiency of the engine. 
         [0004]    The reheat combustors, however, during operation, possess a high demand for cooling air, which is generally provided by extracting a stream of air from the compressor. The extraction of air reduces the engine efficiency, as the stream of extracted air is unavailable for expansion in the high pressure turbine. The extraction of compressor air for cooling the reheat combustor therefore reduces the benefits of deploying the reheat combustor. 
         [0005]    It is therefore desirable to have an alternate method to cool the reheat combustor without adversely affecting the engine efficiency. 
       BRIEF DESCRIPTION 
       [0006]    In accordance with an embodiment of the present invention, a method for operating a gas turbine engine is disclosed. The method includes compressing an air stream in a compressor and generating a post combustion gas by combusting a compressed air stream exiting from the compressor in a combustor. The post combustion gas is expanded in a first turbine. The expanded combustion gas exiting from the first turbine is split into a first stream and a second stream. The first stream of the expanded combustion gas is combusted in a reheat combustor. The reheat combustor is cooled using the second stream of the expanded combustion gas. 
         [0007]    In accordance with another embodiment of the present invention, a gas turbine engine is disclosed. The gas turbine engine includes a compressor for compressing air and a combustor for generating a post combustion gas by combusting a compressed air exiting from the compressor. The gas turbine engine also includes a first turbine for expanding the post combustion gas. The gas turbine engine further includes a splitting zone for splitting an expanded combustion gas exiting from the first turbine into a first stream and a second stream. The gas turbine engine also includes a reheat combustor for combusting the first stream of the expanded combustion gas. The reheat combustor is cooled using the second stream of the expanded combustion gas. 
     
    
     
       DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  illustrates a gas turbine engine in accordance with an embodiment of the invention. 
           [0010]      FIG. 2  illustrates a gas turbine engine with an aerodynamic coupling between a first and second turbine in accordance with an embodiment of the present invention. 
           [0011]      FIG. 3  illustrates a splitting zone and a reheat combustor of a gas turbine engine in accordance with an embodiment of  FIGS. 1 and 2 . 
           [0012]      FIG. 4  illustrates a splitting zone having flow diverters in a fully open position in accordance with an embodiment of  FIGS. 1 and 2 . 
           [0013]      FIG. 5  illustrates a splitting zone having flow diverters in a partially open position in accordance with an embodiment of  FIGS. 1 and 2 . 
           [0014]      FIG. 6  illustrates a splitting zone having flow diverters in a closed position in accordance with an embodiment of  FIGS. 1 and 2 . 
           [0015]      FIG. 7  illustrates a splitting zone having flow diverters coupled to a servomotor controlled by a controller. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    As discussed in detail below, embodiments of the present invention provide a method for cooling a reheat combustor of a gas turbine engine. This invention may, however, be embodied in many 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 invention to those skilled in the art. 
         [0017]      FIG. 1  illustrates a gas turbine engine  10  in accordance with an embodiment of the invention. The  FIG. 1  illustrates a compressor  12 , a combustor  14 , a first turbine  16 , a splitting zone  18 , reheat combustor  20 , and a second turbine  22 . An air stream  24  comprising atmospheric air is fed into the compressor  12  for compression to the desired temperature and pressure. After compression, the air stream  24  exits the compressor  12  as a compressed air stream  26  and is mixed with a fuel stream  28  in the combustor  14 . The mixture is ignited (combusted) in the combustor  14  resulting in a high temperature, high pressure stream of a post combustion gas  30 . The post combustion gas  30  is expanded in the first turbine  16  to convert thermal energy associated with the post combustion gas  28  into mechanical energy and exits the first turbine  16  as an expanded combustion gas  32 . According to an embodiment, the first turbine  16  is coupled to the compressor  12  via a shaft  34  and drives the compressor  12 . In a specific embodiment, the first turbine  16  is a high pressure turbine. 
         [0018]    The expanded combustion gas  32  is associated with certain amount of unutilized heated oxygen (about 15% to about 18% by mass). Therefore, instead of releasing the expanded combustion gas  32  in the atmosphere, the gas turbine engine  10  deploys the reheat combustor  20  and the second turbine  22  to generate additional power. According to an embodiment, prior to entering the reheat combustor  20 , the expanded combustion gas  32  is routed through the splitting zone  18 , where the expanded combustion gas  32  is split into two streams (illustrated in subsequent figures). A first stream of the expanded combustion gas  32  is combusted in the reheat combustor  20 , whereas a second stream of the expanded combustion gas  32  is utilized for cooling the reheat combustor  20 . Details of the splitting zone  18  and the splitting of the expanded combustion gas  32  are further discussed in conjunction with subsequent figures. After utilizing for cooling, the second stream of the expanded combustion gas  32  is mixed with the combusted first stream in the reheat combustor  20  and the mixture is fed into the second turbine  22  as a flow  33 . It should be noted herein that the second stream of the expanded combustion gas  32 , after being used for cooling of the reheat combustor  20 , may partially or entirely participate in the combustion process within the reheat combustor  20 . The flow  33  is expanded in the second turbine  22  to generate power. In an embodiment, the second turbine  22  is coupled to the first turbine  16  by a shaft  36 . 
         [0019]    The  FIG. 1  also illustrates a stream of compressor air  35  and a stream of compressor air  37  drawn from various stages of the compressor  12  for cooling of the first turbine  16  and the second turbine  22  respectively. Conventionally, during operation of a gas turbine engine, air is drawn from various stages of the compressor for cooling the various components such as the combustor, the reheat combustor and the high pressure and low-pressure turbines. The use of compressor air for cooling the various components results in a loss of efficiency of the conventional gas turbine engine as the compressor air fraction is utilized for cooling is unavailable for complete acceleration and expansion in the high-pressure turbine. It should be noted herein that such loss of efficiency in the conventional gas turbine engine is greatest for the compressor air used to cool the reheat combustor and the low-pressure turbine. The present invention proposes use of the expanded combustion gas  32  for cooling the reheat combustor  20 , thereby decreasing the quantity of compressor air extracted for cooling purposes and improving the efficiency. 
         [0020]    In an embodiment of the invention, the second stream of the expanded combustion gas is mixed with a coolant  39  and the mixture is utilized for cooling the reheat combustor  20 . Coolant  39  may be introduced into the reheat combustor  20  by any suitable means. For example, coolant  39  may be introduced through a series of circumferentially spaced inlet nozzles placed downstream of the extraction location of expanded combustion gas  32 , but upstream of the reheat combustor liner coolant injection holes (not shown in  FIG. 1 ), such that expanded combustion gas  32  and coolant  39  have sufficient volume and time to mix. In a specific embodiment, the coolant  39  comprises compressor air. It should be noted that using some compressor air as coolant  39  along with a portion of the expanded combustion gas  32  for cooling still saves considerable amount of compressor air as compared to the conventional mechanism of cooling the reheat combustor solely by compressor air. In another embodiment, the coolant comprises steam. 
         [0021]    In some embodiments, the temperature of the expanded combustion gas  32  is in a range of about 1500 degrees Fahrenheit to about 1600 degrees Fahrenheit. In a specific embodiment, the expanded combustion gas  32  is utilized for cooling the reheat combustor  20  such that the temperature of any metallic material temperature of the reheat combustor  20  stays below 1700 degrees Fahrenheit or lower, for example. A reheat combustor gas  29  (shown in  FIG. 3 ) may have temperature in the range of 2200 to 3200 degrees Fahrenheit depending on the engine design and operating point. The amount and effectiveness of the cooling mechanisms will dictate the resulting material temperatures. 
         [0022]      FIG. 2  shows an alternate embodiment wherein the second turbine  22  is aerodynamically coupled to the first turbine  16  but on an independent shaft  31 . In this embodiment, the first turbine  16  drives the compressor  12  and the second turbine  22  provides shaft power, for example to drive an electric power generator  27 . 
         [0023]      FIG. 3  illustrates a blown up view of the splitting zone  18  and the reheat combustor  20 . In the splitting zone  18 , the expanded combustion gas  32  is split into a first stream  34  and a second stream  36  using a diverter  38  and a diverter  40 . It should be noted the diverters  38 ,  40  are exemplary embodiments for splitting the expanded combustion gas  32 . Various other means can be deployed for splitting the expanded combustion gas  32 . Also, in other exemplary embodiments, the diverter system may not be limited to two diverters. In other words, there may be one or more such diverters, or a diverter system, deployed about a periphery of the reheat combustor  20 . According to an embodiment, the diverter  38  and the diverter  40  are positioned upstream of the reheat combustor  20 . In a specific embodiment, the diverter  38  and the diverter  40  are coupled to the body of the reheat combustor  20  at a location  42  and a location  44  respectively through hinge joints. The diverter  38  and the diverter  40  can rotate about the hinge joints at the location  42  and the location  44  and control the splitting of the flow of the expanded combustion gas  32  into the first stream  34  and the second stream  36  as will be discussed in subsequent figures. The first stream  34  constitutes the main flow to the reheat combustor  20  and undergoes combustion in a main chamber  46  of the reheat combustor  20 . 
         [0024]    In an embodiment, the reheat combustor  20  comprises a casing  41  and an outer liner  43 . The diverter  38  and the diverter  40  are configured to split the expanded combustion gas  32  in such a way that the second stream  36  of the expanded combustion gas  32  flows through passage  48  between the casing  41  and the outer liner  43  of the reheat combustor  20 , and passage  51  between an inner liner  47  and an engine center line  53 . The second stream  36  is used to cool the inner and outer liners  43 ,  47  of the reheat combustor  20 . The second stream  36  is used to cool the reheat combustor  20  through various mechanisms. In an embodiment, impingement cooling is employed, wherein the second stream  36  is impinged on the cold surface of the reheat combustor  20 , that is the surface in contact with the second stream  36 . In another embodiment, effusion cooling or film cooling is employed, wherein the second stream  36  is injected through injection holes  49  of the liners  43 ,  47  to form a thin film cooling layer over the surface of the reheat combustor  20  that is bounded by the reheat combustion gases. It is to be noted that a combination of two or more mechanisms can also be employed to cool the reheat combustor  20  using the second stream  36 . 
         [0025]    After being utilized for cooling, the second stream  36  enters the main chamber  46  of the reheat combustor  20  as illustrated in the figure. The outer liner  43  of the reheat combustor  20  may include the injection holes  49 , which facilitate the entry of the second stream  36  in the main chamber  46 . The injection holes  49  may be used for dilution or film cooling purposes. In some embodiments, the inner liner  47  may include the injection holes  55 . After entering the main chamber  46 , the second stream  36  gets mixed with the first stream  34  (undergoing combustion) and in the process a fraction of the second stream  36  may also undergo combustion in the main chamber  46 . The mixture of the combusted first stream  34  and the second stream  36  (a part of which may have undergone combustion) leaves the reheat combustor  20  as the flow  33 . The flow  33  is expanded in the second turbine  22  (illustrated in  FIG. 1 ). 
         [0026]    In some embodiments, the second stream  36  is mixed with the coolant  39  in the passage  48  and the mixture is used to cool the reheat combustor  20 . In a specific embodiment, the coolant  39  is air drawn from a stage of the compressor  12  ( FIG. 1 ). In another embodiment, the coolant  39  is steam. 
         [0027]      FIG. 4  illustrates a further blown up view of the splitting zone  18 . The splitting zone  18  comprises the diverter  38  and the diverter  40  positioned upstream of the reheat combustor  20  ( FIG. 1 ). In the illustrated embodiment, the diverter  38  and the diverter  40  are coupled to the body of the reheat combustor  20  (illustrated in  FIGS. 1 and 2 ) via hinge joints at the location  42  and the location  44  respectively. According to an embodiment, each of the diverters  38 ,  40  have an aerodynamic shape to minimize flow separations and associated pressure losses. The diverters  38 ,  40  split the flow of the expanded combustion gas  32  into the first stream  34  and the second stream  36 . The rotations of the diverters  38 ,  40  about respective hinge joints regulate the amount of second stream  36  to be split from the post combustion gas  32  for cooling the reheat combustor  20  (illustrated in FIG.  1 , 2 ). The  FIG. 4  illustrates the diverters  38 ,  40  in a fully open position, which enables drawing of maximum mass of second stream  36  via the passages  48 ,  51  from the expanded combustion gas  32 . The greater the firing temperature of the reheat combustor  20 , the greater is the cooling requirement for the reheat combustor  20 . Therefore with increasing firing temperature of the reheat combustor  20 , the opening of the passages  48 ,  51  is increased through rotation of the diverters  38 ,  40  so that an increasing amount of the second stream  36  can be drawn from the post combustion gas  32  for the cooling of the reheat combustor  20 . 
         [0028]      FIG. 5  illustrates the splitting zone  18  with the diverters  38 ,  40  in a partially open position. As compared with the fully open position of the diverters  38 ,  40  in  FIG. 4 , the partially opened position reduces the opening of the passages  48 ,  51  for the flow of the second stream  36 , thereby reducing the mass of second stream  36  extracted from the expanded combustion gas  32 . As the load on the turbine reduces, the cooling demand for the reheat combustor ( FIG. 1 ) reduces and the diverters are rotated from a fully open position to the partially open position. 
         [0029]      FIG. 6  illustrates the splitting zone  18  with the diverters  38 ,  40  in a closed position. As compared with the fully open position of the diverters  38 ,  40  illustrated in  FIG. 4  and the partially open position illustrated in  FIG. 5 , the closed position allows only a small leakage flow of the second stream  36  and almost all of the post combustion gas  32  enters the reheat combustor  20  ( FIG. 1 ) as the first stream  34 . The diverters  38 ,  40  are usually kept in a closed position when there is no requirement for the reheat combustor  20  ( FIG. 1 ) to combust the expanded combustion gas  32 . In such a scenario there is no requirement for cooling of the reheat combustor  20  ( FIG. 1 ) and hence no expanded combustion gas  32  is diverted as second stream  36  (FIG.  4 , 5 ) for cooling of the reheat combustor  20  ( FIG. 1 ). 
         [0030]      FIG. 7  illustrates the splitting zone  18  with the diverter  38  and the diverter  40  coupled to a servomotor  52 , which is controlled by a controller  54 . The controller  54  controls the rotation of the diverter  38  and the diverter  40  via the servomotor  52 , thereby regulating the opening of the passages  48 ,  51 . The aerodynamically shaped flow diverters  38 ,  40  are configured to split the expanded combustion gas based on an operating point of the gas turbine engine. The operating point can be a function of load demand, inlet air temperature, fuel type, or the like. In an embodiment, the controller  54  controls splitting of the expanded combustion gas  32  based on the load on the gas turbine engine  10  ( FIG. 1 ), or the firing temperature of the reheat combustor  20 , causing the diverter  38  and the diverter  40  to be in a fully open, partially open, or closed positions as discussed in conjunction with  FIGS. 4 ,  5  and  6 . In a specific embodiment, the opening of the passages  48 ,  51  is adjusted by rotation of the diverters  38 ,  40  such that the second stream  36  is about 20% to about 45% by mass of the flow of the post combustion gas  32 . 
         [0031]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.