Abstract:
An exemplary gas turbine engine includes a turbine section operative to impart rotational energy to a compressor section. The turbine section includes at least a low-pressure turbine and a high-pressure turbine, and a ratio of a number of stages in the low-pressure turbine to a number of stages in the high-pressure turbine is 2.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is continuation of U.S. patent application Ser. No. 13/163,304, which was filed on 17 Jun. 2011. U.S. patent application Ser. No. 13/163,304 is a divisional of U.S. Pat. No. 7,984,607, which was filed 6 Sep. 2007. U.S. patent application Ser. No. 13/163,304 and U.S. Pat. No. 7,984,607 are both hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosure generally relates to gas turbine engines. 
         [0004]    2. Description of the Related Art 
         [0005]    Turbine durability for high inlet temperature/high fuel-to-air ratio gas turbine engine designs can be compromised by gas path temperature and/or chemical species non-uniformities occurring at the exit of a combustor. Chemical species variation is particularly relevant to high fuel-to-air ratio turbine designs in that the combustion process may not be completed at the exit plane of the combustor section with the result that partial products of reaction (PPR&#39;s) enter the turbine section. The anticipated presence of such temperature non-uniformities contributes to the use of conservative cooling designs for the turbine in order to prevent damage that can be caused by a failure to account for such temperature/PPR&#39;s non-uniformities. 
       SUMMARY 
       [0006]    A gas turbine engine according to an exemplary embodiment of the present disclosure includes, among other possible things, a turbine section operative to impart rotational energy to a compressor section. The turbine section includes at least a low-pressure turbine and a high-pressure turbine. A ratio of a number of stages in the low-pressure turbine to a number of stages in the high-pressure turbine is 2. 
         [0007]    In a further non-limiting embodiment of the foregoing gas turbine engine embodiment, the low-pressure turbine has exactly four stages. 
         [0008]    In a further non-limiting embodiment of either of the foregoing gas turbine engine embodiments, the high-pressure turbine has exactly two stages. 
         [0009]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, the turbine section has a first set of vanes positioned adjacent a combustion section and a first set of rotatable blades positioned downstream of and adjacent to the first set of vanes. A number of vanes of the first set of vanes exceeds a number of blades of the first set of blades. 
         [0010]    In a further non-limiting embodiment of any of the foregoing gas turbine engine embodiments, a first stage of the high-pressure turbine comprises the first set of vanes and the first set of blades. 
         [0011]    A gas turbine engine according to another exemplary embodiment of the present disclosure includes, among other possible things, a turbine section operative to impart rotational energy to a compressor section. The turbine section includes at least a high-pressure turbine having exactly two stages and a low-pressure turbine having exactly four stages. 
         [0012]    In a further non-limiting embodiment of the foregoing gas turbine engine embodiment, the high-pressure turbine has a first set of vanes positioned adjacent a combustion section and a first set of rotatable blades positioned downstream of and adjacent to the first set of vanes. A number of vanes of the first set of vanes exceeds a number of blades of the first set of blades. 
         [0013]    A method of operating a gas turbine engine according to an exemplary aspect of the present disclosure includes, among other possible things, providing a gas turbine engine having a combustion section and a turbine section. The turbine section has a first set of vanes and a first set of blades. The first set of blades being located downstream from and adjacent to the first set of vanes. The turbine section includes a high-pressure turbine and a low-pressure turbine. The low-pressure turbine having four stages. 
         [0014]    In a further non-limiting embodiment of the foregoing method of operating a gas turbine engine, the method includes completing a combustion reaction along a gas flow path prior to a plane defined by downstream portions of the first set of blades. 
         [0015]    In a further non-limiting embodiment of either of the foregoing methods of operating a gas turbine engine, a vane-blade count ratio of the turbine stage is greater than unity, and completing the combustion reaction comprises mixing combustion products from the combustion section using the vanes of the first set of vanes. 
         [0016]    In a further non-limiting embodiment of any of the foregoing methods of operating a gas turbine engine, the engine is operative such that cooling air provided to the first set of vanes reacts with combustion products from the combustion section in order to complete the combustion reaction. 
         [0017]    In a further non-limiting embodiment of any of the foregoing methods of operating a gas turbine engine, the method includes film-cooling the vanes of the first set of vanes. 
         [0018]    In a further non-limiting embodiment of any of the foregoing methods of operating a gas turbine engine, the gas turbine engine further comprises a second set of vanes located downstream from the first set of blades, and the second set of vanes is film-cooled to a lesser extent than that provided to the first set of vanes. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0019]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0020]      FIG. 1  is a schematic diagram depicting an embodiment of a gas turbine engine. 
           [0021]      FIG. 2  is a schematic diagram depicting the embodiment of  FIG. 1 , showing detail of the first and second stages of the turbine section. 
           [0022]      FIG. 3  is a flowchart depicting an embodiment of a method for operating a gas turbine engine. 
           [0023]      FIG. 4  is a flowchart depicting an embodiment of a method for designing a gas turbine engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Gas turbine engine systems and related methods involving vane-blade count ratios greater than unity are provided, several exemplary embodiments of which will be described. In this regard, an increased vane count could increase the probability that a temperature/PPR&#39;s nonuniformity (“hot and/or fuel-rich streak”) will mix with cooler gases and, therefore, dissipate before propagating beyond the rotating blades of the first stage of the turbine. In some embodiments, the cooler gases used for dissipating such a hot streak are provided as cooling air, which is provided for film-cooling the vanes of the first turbine stage. In some embodiments, the vane and blade counts deviate from a nominal number so that overall parasitic drag and weight directly attributable to the vanes and blades are comparable to a gas turbine engine containing an equal number of vanes and blades. For example, in a first stage turbine design incorporating forty-eight (48) vanes and sixty-two (62) blades, the number of vanes could be increased to fifty six (48+8=56), whereas the number of blades could be decreased to fifty four (62−8=54). It should be noted that increasing the number of vanes can potentially enhance mixing of the gases departing the combustion section, thereby reducing the requirement for conservative cooling of downstream vanes. Thus, an overall weight reduction may be achieved by reducing the requirements of cooling air in some embodiments. Additionally, aerodynamic efficiency may be improved as an increased number of vanes can yield higher levels of unsteady flow in the downstream rotor passages. As such the formation of rotor passage secondary flow vortices and losses can be inhibited. 
         [0025]    Referring now in detail to the drawings,  FIG. 1  is a schematic diagram depicting an embodiment of a gas turbine engine  100 . As shown in  FIG. 1 , engine  100  includes a compressor section  102 , a combustion section  104  and a turbine section  106 . Notably, engine  100  is a turbofan although it should be noted that the concepts described herein should not be considered limited to use with gas turbine engines configured as turbofans. 
         [0026]    Turbine section  106  incorporates multiple stages, each of which includes a set of stationary vanes and a corresponding set of rotating blades. In this regard, a first stage  108  of the turbine section includes a first set of vanes  110  and a first set of blades  112 . The first stage of the turbine section is located immediately downstream of the combustion section and immediately upstream of a second stage  114  of the turbine, which includes a second set of stationary vanes  116 . 
         [0027]    As shown in  FIG. 2 , blades  112  are located downstream of vanes  110 , whereas vanes  116  are located downstream of blades  112 . Notably, downstream portions of the blades  112  define an exit plane  120 . Notably, interaction of gas  122  flowing along the gas path defined by the vanes and blades causes combustion products to mix and complete a combustion reaction prior to traversing the exit plane of the first set of blades  112 . This is accomplished, at least in part, by providing a greater number of vanes  110  than there are blades  112 , i.e., the vane-blade count ratio of the first turbine stage is greater than unity (1). 
         [0028]    In the embodiment of  FIGS. 1 and 2 , vanes  110  and  116  incorporate film-cooling holes that direct cooling air for film-cooling the vanes. By way of example, vane  110  includes cooling holes  130 , and vane  116  includes cooling holes  132 . Note that although the number of cooling holes in vane  110  exceeds the number of cooling holes in vane  116 , various other numbers and arrangements of cooling holes can be provided in other embodiments. 
         [0029]    Because of work extraction in a first turbine stage, the temperature of gas is reduced at exit plane of that stage relative to the temperature at the entrance of the first vanes. As a result, for conventional turbine designs, cooling requirements for the downstream vanes are usually much lower than the cooling requirements for the first vanes. However, for high-fuel-to-air ratio turbine designs (i.e., designs that exhibit significant PPR concentrations at the inlet to the first stage), the oxygen included in the cooling air provided in the first stage  108  completes the combustion reaction and can significantly increase the temperature of gas  122  temperature at exit plane  120 . The degree of circumferential and radial uniformity of this hot and/or fuel-rich streak temperature increase is a factor that should be considered for the cooling design of vanes  116 . If the hot and/or fuel-rich streaks are highly concentrated (non-uniform temperature at station  120 ), then all of the vanes  116  should be designed to accommodate the hottest possible streak. 
         [0030]    However, if the hot and/or fuel-rich streaks are diffused by the first stage  108  (a highly uniform temperature at station  120 ), then the vanes  116  can be designed to accommodate a lower peak temperature. This can result in a weight reduction of the gas turbine engine as lighter and/or fewer components associated with routing of the cooling air may be provided. 
         [0031]    In this regard,  FIG. 3  is a flowchart depicting an embodiment of a method for operating a gas turbine engine. As shown in  FIG. 3 , the method may be construed as beginning at block  302 , in which a gas turbine engine having a combustion section and a turbine stage is provided. 
         [0032]    Notably, the turbine stage includes a first set of vanes and a first set of blades, with the first set of blades being located downstream from and adjacent to the first set of vanes. In block  304 , a combustion reaction is completed along a gas flow path prior to a plane defined by downstream portions of the first set of blades. In some embodiments, design of such a first stage incorporates, through the use of both CFD analysis and empirical correlations, any combination of (a) Vane(N)/Blade(N) ratio, (b) vane and/or blade film-cooling schemes or (c) vane and/or blade aerodynamic designs such that temperature and PPR&#39;s nonuniformities at the exit plane of the turbine stage are reduced, e.g., minimized or eliminated. 
         [0033]      FIG. 4  is a flowchart depicting another embodiment of a method. Specifically, the flowchart of  FIG. 4  involves a method for designing a gas turbine engine. That method may be construed as beginning at block  402 , in which a number (N) of vanes and a corresponding number (N) of blades for a turbine stage of a gas turbine engine are selected. In block  404 , the number of the vanes is increased by M. In block  406 , the number of blades is decreased by at least M such that the number of vanes of the turbine stage exceeds the number of blades of the turbine stage. In some embodiments, the turbine stage is a first turbine stage downstream of a combustor. 
         [0034]    A set of numerical experiments were conducted to quantify the impact of change in the number of first vanes on the temperature non-uniformity at the exit of a representative turbine stage. These simulations were conducted by changing the number of airfoils in the vane row while holding the airfoil count for the rotor row constant. The ratio of vanes to blades in these studies were 2/3, 111 and 3/2. The temperature at inlet to the stage was held constant at a typical combustor exit temperature value while the metal temperature for the vane was maintained at a constant value consistent with the airfoil durability requirements. The rotor airfoils and endwalls, however, were maintained at adiabatic wall temperatures. An unsteady 3-D ReynoldsAveraged-Navier-Stokes CFD code was used to conduct these simulations. Results from these simulations indicated that the absolute temperature distortion at the exit of the rotor was about 60%, 30% and 16% of the inlet distortion to the rotor, which was constant for the above three numerical experiments. These simulations clearly indicate that increasing the number of vanes relative to the blades enhances mixing between the hot and cold stream in the rotor passages. 
         [0035]    Interrogation of numerical data from these simulations also indicated that the loss levels in the rotor passages were also reduced as the number of vanes was increased. In addition, increasing the number of vanes was also found to reduce the hot spot temperature on the rotor airfoil pressure side indicating that the cooling air in the rotor passages may also be favorably impacted by increased vane count. 
         [0036]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.