Abstract:
In a first embodiment, a system, including an exhaust duct configured to flow an exhaust gas, and an air injection system coupled to the exhaust duct, wherein the air injection system comprises a first air injector configured to inject air into the exhaust duct to assist flow of the exhaust gas through the exhaust duct.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and benefit of Chinese Patent Application No. 201320089712.6, entitled “SYSTEM AND METHOD FOR REDUCING BACK PRESSURE IN A GAS TURBINE SYSTEM”, filed Feb. 15, 2013, which is herein incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to gas turbine systems and, more specifically, a system for reducing back pressure on the turbine. 
         [0003]    Gas turbine engine systems benefit from improved efficiency. Gas turbine designs minimize inefficiencies in order to extract as much work as possible from a combustible fuel. Specifically, the gas turbine system uses the combustible fuel to create hot, pressurized exhaust gases that flow through a turbine. The turbine uses the momentum of the exhaust gases to create rotational energy for use by a load (e.g., a generator). As the exhaust gases exit the turbine into an exhaust section, they may create undesirable back pressure. The back pressure may reduce the gas turbine system&#39;s efficiency, causing the system to use more energy to move the exhaust gases out of the turbine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
         [0005]    In a first embodiment, a system, including an exhaust duct configured to flow an exhaust gas, and an air injection system coupled to the exhaust duct, wherein the air injection system comprises a first air injector configured to inject air into the exhaust duct to assist flow of the exhaust gas through the exhaust duct. 
         [0006]    In a second embodiment, a system including, a controller having instructions to control air flow through an air injection system into an exhaust duct to reduce back pressure associated with flow of the exhaust gas through the exhaust duct. 
         [0007]    In a third embodiment, a method including, receiving the air flow from a compressor of a gas turbine engine, routing the air flow through the air injection system into the exhaust duct downstream of a turbine of the gas turbine engine, and reducing the back pressure with the air flow. 
     
    
     
       BRIEF DESCRIPTION OF THE 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  is a schematic of a gas turbine system using an air injection system; 
           [0010]      FIG. 2  is a cross-sectional view of an exhaust duct along line  2 - 2  in  FIG. 1  that illustrates an air injector stage with rakes; 
           [0011]      FIG. 3  is a cross-sectional view of the exhaust duct along line  2 - 2  in  FIG. 1  that illustrates an air injector stage with rakes; 
           [0012]      FIG. 4  is a cross-sectional perspective view of the exhaust duct along line  4 - 4  in  FIG. 1  that illustrates an air injector stage with air injector nozzles; 
           [0013]      FIG. 5  is a cross-sectional perspective view of the exhaust duct along line  4 - 4  in  FIG. 1  that illustrates an air injector stage with air blades; 
           [0014]      FIG. 6  is a cross-sectional perspective view of the exhaust duct along line  6 - 6  in  FIG. 1  that illustrates an air injector stage with air injector nozzles; and 
           [0015]      FIG. 7  is a cross-sectional perspective view of the exhaust duct  18  along line  7 - 7  in  FIG. 1  that illustrates the air injector stage with air injector nozzles and air blades. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0017]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0018]    The present disclosure is generally directed towards a gas turbine system with an air injection system that reduces back pressure on a gas turbine. Specifically, the air injection system helps move back-pressure-causing-exhaust gases away from the gas turbine engine. This improves efficiency by reducing the work used by the gas turbine engine to expel exhaust gases. In certain embodiments, the air injection system includes multiple air injector stages that move the exhaust gas away from the gas turbine engine. Each injector stage may include one or more air injectors. The air injectors may include air blades or air injector nozzles. The injectors or air blades are designed to minimize air blockage and maximize air energization. In operation, the air injectors and air nozzles entrain surrounding and/or upstream air that is then energized with a small amount of compressed air. In this manner, the air injectors and air blades can move large volumes of air at high velocities. These air blades and nozzles may be modified in various ways to include changing their shape; the angle at which they inject air; sizes; quantity; and spacing between the duct and neighboring air injectors. Furthermore, the air injectors may interact in different ways with the exhaust duct. For example, some air injectors may project into the exhaust duct while others are flush or recessed with exhaust duct walls. 
         [0019]      FIG. 1  is a schematic of a gas turbine system  10  using an air injection system  12 . The gas turbine system  10  includes the air injection system  12 , a gas turbine  14 , a load  16 , and exhaust duct work  17  with an exhaust stack  18 . The air injection system  12  may advantageously improve the efficiency of the gas turbine system  10 . Specifically, the air injection system  12  may move excess compressed air from the gas turbine  14  to the exhaust duct  17  (including the exhaust stack  18 ) to reduce back pressure on the gas turbine  14 . The air injector system  12  may include one or more air injector stages or modules  11  that are mountable in or part of the exhaust duct work  17 , each having one or more air injectors  13 . Each injector  13  injects air to help flow the exhaust gases in a downstream direction to reduce back pressure. 
         [0020]    The gas turbine engine  14  includes a compressor  20 , combustor  22 , fuel nozzle  24 , and turbine  26 . In operation, the compressor  20  draws air into the gas turbine  14  and compresses it for combustion. As illustrated, the compressor includes multiple rotors or compression stages  28 ,  30 , and  32  each having a plurality of compressor blades. While only three rotors or stages are shown, a compressor  20  may include additional rotors or stages (e.g., 1, 2, 3, 4, 5, 6, 10, or more). Each stage  28 ,  30 , and  32  uses the blades to progressively compress the air to a greater pressure. After passing through the compressor  20 , the air enters the combustor  22 . In the combustor  22 , the air combines and combusts with fuel from the fuel nozzle  24 . The combustion of the air and fuel creates hot pressurized combustion gas that then travel through the turbine  26 . 
         [0021]    The turbine  26 , like the compressor  20 , includes several rotors or turbine stages  34 ,  36 , and  38 , each having a plurality of turbine blades. While only three rotors or stages are shown, a turbine  26  may include additional rotors or stages (e.g., 1, 2, 3, 4, 5, 6, 10, or more). The movement of the combustion gases through the turbine  26 , causes the turbine blades and rotors to rotate. The rotation of the rotors or turbine stages  34 ,  36 , and  38  cause shaft  40  to rotate, which then drives a load  16  (e.g., a generator). As the hot and fast moving combustion gases pass sequentially through the turbine stages  34 ,  36 , and  38 , the gases restricts exhaust gas  42  due to the stations walls  19 , turns and general flow restriction, thereby collecting back pressure on the flow of exhaust gases  42  progressively expand, cool, and slow before entering the exhaust stack  18  as slower moving exhaust gas  42 . The exhaust duct  17  generally conforms or the flow of exhaust gas  42 , and generally slows the flow of traveling through the turbine  26 . The back pressure causes the gas turbine  14  to work harder and burn more fuel to counter the back pressure. Advantageously, the gas turbine system  10  may include an air injection system  12  that reduces the back pressure and increases efficiency. In particular, the air injectors system  12  is configured to energize or add momentum to the flow of exhaust gas to counter the effects of the flow restriction. 
         [0022]    The air injection system  12  includes a controller  44 ; air injector modules or stages  46 ,  48 ,  50 , and  52 ; compressed air supply  54 ; pressure collecting valve assembly  56 ; pressure releasing valve assembly  58 ; and sensor  59 . Advantageously, the air injection system  12  may use compressed air from the gas turbine  14  to reduce the back pressure caused by the exhaust gas  42 . As explained above, the compressor  20  compresses air for combustion in the combustor  22 . The compressor  20  may create more pressurized air than the gas turbine  14  can use during combustion. Instead of wasting this excess pressurized air, the air injection system  12  uses the pressurized air in the air injector stages  46 ,  48 ,  50 , and  52  to reduce back pressure. 
         [0023]    The air injection system  12  uses the valve assemblies  56  and  58  to control the flow of the compressed air from the compressor  20  into the air injector stages  46 ,  48 ,  50 , and  52 . The controller  44  includes a processor  45 , memory  47 , and instructions stored on the memory  47  executable by the processor  45 . The controller  44  operates with and receives data from the sensor  59  (e.g., exhaust gas velocity, pressure in exhaust duct  17 ). The controller then processes this data with the processor  45  and executes instructions stored in the memory  47 . While only one sensor  59  is illustrated other embodiments may include multiple sensors measuring properties at different locations in the exhaust duct  17 . In operation the controller  44  executes instructions to open and close the valves  60 ,  62 , and  64  in the valve assembly  56  to selectively flow excess pressurized air from respective compressor stages  28 ,  30 , and  32  into the compressed air supply  54 . While only three valves are illustrated, more valves in different configurations are possible. For example, the valve assembly  56  may include (1, 2, 3, 4, 5, 10, 15 or more valves). In some embodiments, each valve may control pressurized air release from a respective compression stage in the compressor  20 . In other embodiments, a single valve may control pressurized air release from a single stage, all stages, or some of the stages. In still other embodiments, valves may only connect to some of the stages (e.g., the stages with the most pressure or suitable pressure for the exhaust duct  17 ). 
         [0024]    The compressed air supply  54  may include an air distribution manifold, storage tank, conduits, or any combination thereof. In certain embodiments, the supply  54  may simply represent, or include the source of compressed air, i.e., the compressor  20  itself. The valve assembly  56  receives the compressed air from the supply  54  and routes it to the air injector stages  46 ,  48 ,  50 , and  52 . The valve assembly  58  includes valves  66 ,  68 ,  70 , and  72 . Each valve corresponds to a respective air injector stage  46 ,  48 ,  50 , and  52 . In other embodiments there may be more air injector stages (e.g., 1, 2, 3, 4, 6, 8, 14, or more) and a corresponding number of valves (e.g., 1, 2, 3, 4, 6, 8, 14, or more). In still other embodiments, there may be fewer valves than the number of air injector stages (e.g., one valve for all of the air injectors). In operation, the controller  44  executes instructions to open and close valves  66 ,  68 ,  70 , and  72  to provide compressed air into the respective air injector stages  46 ,  48 ,  50 , and  52 . The air injector stages  46 ,  48 ,  50 , and  52  then direct the compressed air into air injectors  13 . The air injectors  13  use the compressed air to increase the speed or momentum of the exhaust gas  42  as it travels though the exhaust duct  17  (including exhaust stack  18 ), reducing back pressure on the gas turbine  14 . The controller  44  executes instructions to selectively control the valves to adjust the quantity flow rate, and distribution among the various stages and injectors  13 . For example, the controller  44  may execute instructions to progressively increase exhaust gas speed between the stages  46 ,  48 ,  50 , and  52  by increasing the amount of compressed air in each stage. In other embodiments, the controller  44  may execute instructions to increase the speed of the exhaust gas  42  in the stage closest to the turbine  26  (e.g., stage  46 ) and then progressively reduce compressed air injection into the later stages  48 ,  50 , and  52 . In each configuration the injectors  13  in each stage or module  11  helps to energize the exhaust flow to counteract the flow restriction as the exhaust gas  42  travels through the exhaust duct. Furthermore, each stage or module  11  may energize/interact with the flow in different ways. For example, the stages or modules  11  may have air injectors  13  that protrude into the flow, are flush with the exhaust duct  17 , or are angled with respect to the flow. By projecting into the flow the air injector  13  may more effectively energize the center of the flow. In contrast, the injectors  13  that are flush with the exhaust duct  17  may more effectively energize the outer portions of the flow. Furthermore, the angle of the air injectors  13  with respect to the flow may more effectively energize the flow in a direction out of the exhaust duct  17 . Thus depending on the embodiment a stage or module  11  may adjust how the air injector(s)  13  interact with the flow (i.e., energize the flow center, flow edges, or the direction of flow movement). 
         [0025]      FIG. 2  is a cross-sectional view of the exhaust duct  17  along line  2 - 2  in  FIG. 1 , illustrating an embodiment of the air injector stage  46  with rakes  90 ,  92 , and  94 . While only three rakes are shown, there may be more rakes depending on the embodiment (e.g., 1, 2, 3, 4, 5, 10, 15, or more). As illustrated, the exhaust duct  17  is rectangular with four side walls  96 ,  98 ,  100 , and  102 . In other embodiments, the exhaust duct  17  may be circular, square, oval, hexagonal, etc. In the present embodiment, the rakes  90 ,  92 , and  94  are between the side walls  96  and  98  and spaced apart by distances  104 ,  106 ,  108 , and  110 . The distances  104 ,  106 ,  108 , and  110  may change, depending on the embodiment, to achieve particular flow characteristics. For example, the distances  104  and  110  may be small in order to place the rakes  90  and  94  near the side walls  100  and  102 . In other embodiments, the rakes  90 ,  92 , and  94  may be spaced closer together. The rakes  90 ,  92 , and  94  may also have different orientations to include vertical orientations between the walls  100  and  102 . In still other embodiments, the rakes may be oriented diagonally between the walls  96 ,  98 ,  100 , and  102 . 
         [0026]    The rakes  90 ,  92 , and  94  include one or more air injectors  13 , e.g., air injector nozzles  112 . Each rake  90 ,  92 , and  94  may include one or more nozzles  112  (e.g., 1, 2, 3, 4, 5, 10, 25, or more). In some embodiments, the number of nozzles  112  may differ between rakes  90 ,  92 , and  94 . For example, rake  94  may have twelve nozzles  112  while rakes  90  and  92  have four each. The nozzles  112  may also differ in shape and size with respect to each other. Shapes may include circular, chevron, rectangular, square, half-moon, and ellipse, among others. In other embodiments, the nozzles  112  may progressively change in size across the rake to improve flow velocity characteristics of the exhaust gas between the side walls  96 ,  98 ,  100 , and  102  of the exhaust duct  17 . For example, smaller nozzles  112  that emit pressurized air at a high velocity may be closer to the sides of the exhaust duct  17  where the flow may be slowest, while lower pressure nozzles  112  are near the center of the exhaust duct  17 . In still other embodiments, the spacing and sizes of the nozzles  112  may be equal. This may improve exhaust gas  42  flow through the exhaust duct  17 . Moreover, there are many possible combinations using the variables of nozzle size, nozzle shape, nozzle number, nozzle spacing, and rake spacing. 
         [0027]      FIG. 3  is a cross-sectional view of the exhaust duct  17  along line  2 - 2  in  FIG. 1 , illustrating the air injector stage  46  with rakes  140 ,  142 , and  144 . The rakes  140 ,  142 , and  144  include air blade slots  146 ,  148 ,  150 , and  152 . The air blades  146 ,  148 , and  150  function like the air nozzles  112  in  FIG. 2 , and provide air to energize or push exhaust gases  42  through the exhaust duct  17 . Furthermore, the air blades  146  may more uniformly energize the flow. In the present embodiment, the rakes  140 ,  142 , and  144  extend between the side walls  100  and  102  in a vertical orientation. The rakes  140 ,  142 , and  144  may change orientation (e.g., horizontal, diagonal), and change distances  154 ,  156 ,  158 , and  160  between each other and the side walls  96 ,  98 ,  100 , and  102 , depending on the embodiment. Furthermore, the rakes  140 ,  142 , and  144  may include more than one air blade. As illustrated rake  140  includes two air blades  150  and  152 , while rakes  142  and  144  have one air blade  146  and  148 , respectively. Different embodiments may include more air blades in each rake (e.g., 1, 2, 3, 4, 5, 6, or more), or different numbers of rakes (e.g., 1, 2, 3, 4, 5, 6, or more). For example, rake  140  may have two blades while rake  144  has five and rake  142  has three. Finally, the shape of the air blade may differ (e.g., straight, wave-like, zigzag, etc.). For example, the blade  148  forms a wave-like slot, while the remaining blades  146 ,  150 , and  152  form a straight rectangular slot. 
         [0028]      FIG. 4  is a cross-sectional view of the exhaust duct  17  along line  4 - 4  in  FIG. 1 , illustrating the air injector stage  48  with air injector nozzles  180 . As illustrated, the nozzles  180  are flush with the side walls  96 ,  98 ,  100 , and  102 . Accordingly, the air injector nozzles  180  may impact the portions of the flow closest to the side walls  96 ,  98 ,  100 , and  102 . The air injector stage  48  may form various configurations with the air nozzles  180  using the variables of shape, angle, size, quantity, and spacing. For example, the side walls  96 ,  98 ,  100 , and  102  may have the same or different numbers (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10) of nozzles  180 . For example, side wall  96  may have three nozzles  180 , while the remaining walls  98 ,  100 , and  102  have six, seven, and four nozzles  180 , respectively). Each of these nozzles  180  may form a variety of shapes, such as circular, chevron, rectangular, square, half-moon, and ellipse among others. Furthermore, the air injector stage  48  may place differently shaped nozzles  180  at different locations (e.g., on some or all of the side walls  96 ,  98 ,  100 , and  102 ). 
         [0029]    The air nozzles  180  may also form an angle  182  with respect to the side walls  96 ,  98 ,  100 , and  102  in the direction of the exhaust gas flow. The angle of the air nozzles  180  may change how they energize the flow (i.e., smaller angles may energize flow in a direction parallel to the exhaust duct  17  while a large angle will increasingly energize the flow in a direction perpendicular to the exhaust duct  17 ). The angle  182  may be any angle between approximately 0 and 90 degrees (e.g., approximately 10-30, 20-70, 45-65 degrees). For example, the angle  182  may be approximately 18, 20, or 30 degrees. In some embodiments, the air nozzles  180  on the side wall  96  may form an angle of approximately 90 degrees, while the air nozzles  180  on side wall  100  are at approximately 45 degrees. In still other embodiments, each of the air nozzles  180  may form an angle  182  that differs from the others. 
         [0030]    As discussed above, the air nozzles  180  may form different sizes and be spaced differently with respect to each other. As illustrated, the side wall  102  includes different sizes of air nozzles  180 . The different sizes of the air nozzles  180  may increase or decrease air flow in portions of the air injector stage that optimize the flow of the exhaust gas  42 . The air nozzles  180  on the side wall  102  are spaced apart by distances  184 ,  186 ,  188 ,  190 , and  192 . The spacing between the air nozzles  180  may change the profile of the exhaust gas  42  flow through the air injector stage  48 . For example, the air injectors  180  may provide greater air flow near the side walls  96  and  98  by decreasing the distances  184 ,  186 ,  190 , and  192  and increasing the distance  188 , thereby providing greater energization of the exhaust gas  42  flow along the side walls  96  and  98 . In other embodiments, the opposite may occur by decreasing distance  188  and increasing distances  184 ,  186 ,  190 , and  192 . 
         [0031]      FIG. 5  is a cross-sectional perspective view of the exhaust duct  17  along line  4 - 4  in  FIG. 1 , illustrating the air injector stage  48  with air blades  210 . The blades  210  like the nozzles in  FIG. 4  move exhaust gas  42  through the exhaust duct  17 . The air blades  210  like the nozzles in  FIG. 4  are flush with the exhaust duct  17  and will therefore impact the portions of the flow closest to the side walls  96 ,  98 ,  100 , and  102 . The air blades  210  may form various configurations by changing the shape, angle, and quantity. The air blades  210  may form different shapes, including wave-like, zigzag, and straight rectangular slots. The air blades  210  may project from side walls  96 ,  98 ,  100 , and  102 . This angle  212  may be any angle between approximately 0 and 90 degrees (e.g., approximately 10-30, 20-70, or 45-65 degrees). For example, the angle  212  may be approximately 10, 20, or 30 degrees. In certain embodiments, one of the air blades  210  may have an angle  212  of approximately 90 degrees with the side wall  96 , while the other air blades  210  have an angle  212  of approximately 30 degrees with respective side walls  98 ,  100 , and  102 . In still other embodiments, each of the air blades  210  may form an angle  212  that differs from the others. Furthermore, each side wall  96 ,  98 ,  100 , and  102  may include more than one air blade  210  or some walls may have no air blades  210 . 
         [0032]      FIG. 6  is a cross-sectional perspective view of the exhaust duct  17  along line  6 - 6  in  FIG. 1 , illustrating the air injector stage  50  with air injector nozzles  240 . As illustrated, the nozzles  240  project from the side walls  96 ,  98 ,  100 , and  102 . In other embodiments, air blades instead of the nozzles  240  may project from the side walls  96 ,  98 ,  100 , and  102 . By projecting into the flow the air nozzles  240  (or air blades) may more effectively energize the center of the flow. 
         [0033]    The air injector stage  50  may form various configurations with the air nozzles  240  using the variables of shapes, angles, sizing, quantity, and spacing. For example, the side walls  96 ,  98 ,  100 , and  102  may have the same or different numbers (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10) of nozzles  240  on each wall. For example, side wall  96  may have three nozzles  240 , while the remaining walls  98 ,  100 , and  102  have four, five, and six nozzles  240  respectively. In some embodiments, some walls may exclude nozzles  240 . Each of these nozzles  240  may form a variety of shapes to include circular, chevron, rectangular, square, half-moon, and ellipse shaped nozzles, among others. Furthermore, the air injector stage  50  may place differently shaped nozzles  240  at different locations (e.g., on different side walls  96 ,  98 ,  100 , and  102 ). 
         [0034]    The air nozzles  240  may also form an angle  242  with respect to the side walls  96 ,  98 ,  100 , and  102  in the downstream direction of the exhaust gas flow. The angle of the air nozzles  240  may change how they energize the flow (i.e., smaller angles may energize flow in a direction parallel to the exhaust duct  17  while a large angle will increasingly energize the flow in a direction perpendicular to the exhaust duct  17 ). The angle  242  may be any angle between approximately 0 and 90 degrees (e.g., approximately 10-30, 20-70, or 45-65 degrees). For example, each nozzle  240  may have an angle  242  of approximately 10, 20, or 30 degrees. In some embodiments, the air nozzles  240  that connect to side wall  96  may form an angle of approximately 90 degrees, while the air nozzles  240  that connect to side wall  98  are at approximately 45 degrees. In still other embodiments, each of the air nozzles  240  may form an angle  242  that differs from the others. 
         [0035]    As discussed above, the air injector stage  50  may change spacing and sizing between nozzles  240 . The different sizing of air nozzles  240  may increase or decrease air flow in portions of the air injector stage  50  to optimize the flow of the exhaust gas  42 . The air nozzles  240  may also change spacing with respect to each other. For example, the nozzles  240  are spaced from one another by distances  244 ,  246 ,  248 , and  250 . The spacing between the air nozzles  240 , like the size of the air nozzles  240 , may change how the exhaust gas  42  accelerates through the air injector stage  50 . For example, changing the distances  244 ,  246 ,  248 , and  250  may move the nozzles  240  closer to side walls  96  and  98 , accelerating the exhaust gas near the opposite edges of side wall  102 . In other embodiments, the opposite may occur by decreasing distances  244 ,  246 ,  248 , and  250  the nozzles  240  may accelerate the exhaust gas  42  flow near the exhaust duct  18  center. 
         [0036]      FIG. 7  is a cross-sectional perspective view of the exhaust duct  18  along line  7 - 7  in  FIG. 1 , illustrating the air injector stage  52  with air injector nozzles  270  and  280  and air blades  300  and  310 . The embodiment shown in  FIG. 7  combines the different air nozzles and air blades from the previous embodiments in  FIGS. 2-6  into the air injector stage  52 . Specifically, the air injector stage  52  includes nozzles  270  that are flush with the wall  96 , nozzles  280  that project from the side wall  98  into the duct  17 , air blade  300  that projects from the wall  102  into the duct  17 , and the air blade  310  that is flush with the wall  100 . While  FIG. 7  illustrates one possible configuration, many others are possible. For example, some walls may include combinations of air blades and air nozzles that are flush recessed, or projecting relative to the exhaust duct  17 . In still other embodiments, different walls may combine projecting air nozzles  280  with flush nozzles  270  on all the walls  96 ,  98 ,  100 , and  102 , or an embodiment that combines projecting air blades  300  and flush air blades  310 . Furthermore, the air injector stage  52  may further modify the air nozzles  270  and  280  and air blades  300  and  310  in  FIG. 7  using the variables discussed above in  FIGS. 2-6 , including changing shapes, angle  312 , sizing, quantity, and spacing. 
         [0037]    Technical effects of the invention include the ability to reduce back pressure on a gas turbine system using excess compressed air from the compressor. Specifically, the disclosed embodiments reduce back pressure on a gas turbine engine with air injector stages along an exhaust duct. The air injector stages include air injectors that use the excess compressed air to accelerate the exhaust gases out of the system. In this manner, the system reduces back pressure on the gas turbine engine improving its efficiency. 
         [0038]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.