Abstract:
A turbine section of a gas turbine engine according to an example of the present disclosure includes, among other things, a fan drive turbine section and a second turbine section. The fan drive turbine section has a first exit area at a first exit point and is configured to rotate at a first speed. The second turbine section has a second exit area at a second exit point and is configured to rotate at a second speed, which is faster than the first speed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/446,194, filed Apr. 13, 2012, which claims priority to U.S. Provisional Patent Application 61/619,116, filed Apr. 2, 2012, and is a continuation-in-part of U.S. patent application Ser. No. 13/363,154, filed on Jan. 31, 2012 and entitled “Gas Turbine Engine With High Speed Low Pressure Turbine Section.” 
     
    
     BACKGROUND 
       [0002]    This application relates to a gas turbine engine wherein the low pressure turbine section is rotating at a higher speed and centrifugal pull stress relative to the high pressure turbine section speed and centrifugal pull stress than prior art engines. 
         [0003]    Gas turbine engines are known, and typically include a fan delivering air into a low pressure compressor section. The air is compressed in the low pressure compressor section, and passed into a high pressure compressor section. From the high pressure compressor section the air is introduced into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over a high pressure turbine section, and then a low pressure turbine section. 
         [0004]    Traditionally, on many prior art engines the low pressure turbine section has driven both the low pressure compressor section and a fan directly. As fuel consumption improves with larger fan diameters relative to core diameters it has been the trend in the industry to increase fan diameters. However, as the fan diameter is increased, high fan blade tip speeds may result in a decrease in efficiency due to compressibility effects. Accordingly, the fan speed, and thus the speed of the low pressure compressor section and low pressure turbine section (both of which historically have been coupled to the fan via the low pressure spool), have been a design constraint. More recently, gear reductions have been proposed between the low pressure spool (low pressure compressor section and low pressure turbine section) and the fan. 
       SUMMARY 
       [0005]    A turbine section of a gas turbine engine according to an example of the present disclosure includes a fan drive turbine section and a second turbine section. The fan drive turbine section has a first exit area at a first exit point and is configured to rotate at a first speed. The second turbine section has a second exit area at a second exit point and is configured to rotate at a second speed, which is faster than the first speed. A first performance quantity is defined as the product of the first speed squared and the first area. A second performance quantity is defined as the product of the second speed squared and the second area. A ratio of the first performance quantity to the second performance quantity is between about 0.5 and about 1.5. The second turbine section is configured to drive a first shaft supported on a bearing, with the bearing being situated between the first exit area and the second exit area. 
         [0006]    In a further embodiment of any of the foregoing embodiments, the ratio is above or equal to about 0.8. 
         [0007]    In a further embodiment of any of the foregoing embodiments, the fan drive turbine section has between three and six stages. The second turbine section has two or fewer stages. A pressure ratio across the fan drive turbine section is greater than about 5:1. 
         [0008]    In a further embodiment of any of the foregoing embodiments, the fan drive and second turbine sections are configured to rotate in opposed directions. 
         [0009]    In a further embodiment of any of the foregoing embodiments, each of the fan drive turbine section and the second turbine section is configured to rotate in a first direction. 
         [0010]    In a further embodiment of any of the foregoing embodiments, the bearing is mounted on an outer periphery of a second shaft driven by the fan drive turbine section. 
         [0011]    In a further embodiment of any of the foregoing embodiments, the first shaft is supported by a second bearing on its outer periphery, with the second bearing mounted to a static structure. 
         [0012]    In a further embodiment of any of the foregoing embodiments, there is no bearing support structure positioned intermediate the fan drive and second turbine sections. 
         [0013]    A gas turbine engine according to an example of the present disclosure includes a compressor section including a first compressor section and a second compressor section, a gear arrangement configured to drive a fan, and a turbine section including a fan drive turbine section and a second turbine section. The fan drive turbine is configured to drive the gear arrangement. The fan drive turbine section has a first exit area at a first exit point and is configured to rotate at a first speed. The second turbine section has a second exit area at a second exit point and is configured to rotate at a second speed, which is faster than the first speed. A first performance quantity is defined as the product of the first speed squared and the first area. A second performance quantity is defined as the product of the second speed squared and the second area. A ratio of the first performance quantity to the second performance quantity is less than or equal to about 1.5. The second turbine section is configured to drive a first shaft mounted on a bearing, with the bearing situated between the first exit area and the second exit area. 
         [0014]    In a further embodiment of any of the foregoing embodiments, the ratio is above or equal to about 0.5, and the fan defines a pressure ratio less than about 1.45. 
         [0015]    In a further embodiment of any of the foregoing embodiments, the fan drive turbine section and the first compressor section are configured to rotate in a first direction. The second turbine section and the second compressor section are configured to rotate in a second opposed direction. 
         [0016]    In a further embodiment of any of the foregoing embodiments, the fan rotates in the second opposed direction. 
         [0017]    In a further embodiment of any of the foregoing embodiments, the first compressor section includes three (3) stages, and the second compressor section includes (8) stages. 
         [0018]    In a further embodiment of any of the foregoing embodiments, each of the fan drive turbine section and the second turbine sections is configured to rotate in a first direction. 
         [0019]    In a further embodiment of any of the foregoing embodiments, the bearing is mounted on an outer periphery of a second shaft which is driven by the fan drive turbine section. 
         [0020]    In a further embodiment of any of the foregoing embodiments, the second shaft is supported on a second bearing on its outer periphery, with the second bearing being mounted to a static structure. A third bearing supports the second compressor section on an outer periphery of the first shaft. A fourth bearing is positioned adjacent the first compressor section, and supports an outer periphery of the second shaft. 
         [0021]    A method of designing a gas turbine engine according to an example of the present disclosure includes providing a fan section including a fan, providing a compressor section, including both a first compressor and a second compressor, and providing a turbine section, including both a fan drive turbine section and a second turbine section. The second turbine section is configured to drive a first shaft. The first shaft is mounted on a bearing driving the compressor section and the fan section via the turbine section. The fan drive turbine section has a first exit area at a first exit point and is configured to rotate at a first speed. The second turbine section has a second exit area at a second exit point and is configured to rotate at a second speed, which is faster than the first speed. A first performance quantity is defined as the product of the first speed squared and the first area at a predetermined design target. A second performance quantity is defined as the product of the second speed squared and the second area at the predetermined design target. A ratio of the first performance quantity to the second performance quantity is between about 0.5 and about 1.5. The bearing is situated between the first exit area and the second exit area. 
         [0022]    In a further embodiment of any of the foregoing embodiments, the predetermined design target corresponds to a takeoff condition. 
         [0023]    In a further embodiment of any of the foregoing embodiments, an overall pressure ratio is provided by the combination of a pressure ratio across the first compressor and a pressure ratio across the second compressor at the predetermined design target, and the overall pressure ratio is greater than or equal to about 35. 
         [0024]    In a further embodiment of any of the foregoing embodiments, the first compressor section includes fewer stages than the second compressor section. The fan drive turbine section includes between three (3) and six (6) stages, and the second turbine section includes two or fewer stages. 
         [0025]    These and other features of this disclosure will be better understood upon reading the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  shows a gas turbine engine. 
           [0027]      FIG. 2  schematically shows the arrangement of the low and high spool, along with the fan drive. 
           [0028]      FIG. 3  shows a schematic view of a mount arrangement for an engine such as shown in  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-turbine turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flow path B while the compressor section  24  drives air along a core flow path C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-turbine architectures. 
         [0030]    The engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
         [0031]    The low speed spool  30  generally includes an innermost shaft  40  that interconnects a fan  42 , a low pressure (or first) compressor section  44  and a low pressure (or first) turbine section  46 . Note turbine section  46  will also be known as a fan drive turbine section. The inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low speed fan drive turbine  46 . The high speed spool  32  includes a more outer shaft  50  that interconnects a high pressure (or second) compressor section  52  and high pressure (or second) turbine section  54 . A combustor  56  is arranged between the high pressure compressor section  52  and the high pressure turbine section  54 . As used herein, the high pressure turbine section experiences higher pressures than the low pressure turbine section. A low pressure turbine section is a section that powers a fan  42 . 
         [0032]    In the illustrated example, the low pressure compressor  44  includes fewer stages than the high pressure compressor  52 , and more narrowly, the low pressure compressor  44  includes three (3) stages and the high (or second) pressure compressor  52  includes eight (8) stages ( FIG. 1 ). In another example, the low pressure compressor  44  includes four (4) stages and the high (or second) pressure compressor  52  includes four (4) stages ( FIG. 3 ). In the illustrated example, the high pressure turbine  54  includes fewer stages than the low pressure turbine  46 , and more narrowly, the low pressure turbine  46  includes five (5) stages, and the high pressure turbine  54  includes two (2) stages. In one example, the low pressure turbine  46  includes three (3) stages, and the high pressure turbine  54  includes two (2) stages ( FIG. 3 ). 
         [0033]    The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axis. The high and low spools can be either co-rotating or counter-rotating. 
         [0034]    The core airflow C is compressed by the low pressure compressor section  44  then the high pressure compressor section  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine section  54  and low pressure turbine section  46 . 
         [0035]    The engine  20  in one example is a high-bypass geared aircraft engine. The bypass ratio is the amount of air delivered into bypass path B divided by the amount of air into core path C. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine section  46  has a pressure ratio that is greater than about 5. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor section  44 , and the low pressure turbine section  46  has a pressure ratio that is greater than about 5:1. In some embodiments, the high pressure turbine section may have two or fewer stages. In contrast, the low pressure turbine section  46 , in some embodiments, has between 3 and 6 stages. Further the low pressure turbine section  46  pressure ratio is total pressure measured prior to inlet of low pressure turbine section  46  as related to the total pressure at the outlet of the low pressure turbine section  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine 
         [0036]    A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (“TSFC”). TSFC is the industry standard parameter of the rate of lbm of fuel being burned per hour divided by lbf of thrust the engine produces at that flight condition. “Low fan pressure ratio” is the ratio of total pressure across the fan blade alone, before the fan exit guide vanes. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Ram Air Temperature deg R)/518.7)̂0.5]. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. Further, the fan  42  may have 26 or fewer blades. 
         [0037]    An exit area  400  is shown, in  FIG. 1  and  FIG. 2 , at the exit location for the high pressure turbine section  54  is the annular area of the last blade of turbine section  54 . An exit area for the low pressure turbine section is defined at exit  401  for the low pressure turbine section is the annular area defined by the last blade of that turbine section  46 . As shown in  FIG. 2 , the turbine engine  20  may be counter-rotating. This means that the low pressure turbine section  46  and low pressure compressor section  44  rotate in one direction (“−’), while the high pressure spool  32 , including high pressure turbine section  54  and high pressure compressor section  52  rotate in an opposed direction (“+”). The gear reduction  48 , which may be, for example, an epicyclic transmission (e.g., with a sun, ring, and star gears), is selected such that the fan  42  rotates in the same direction (“+”) as the high spool  32 . With this arrangement, and with the other structure as set forth above, including the various quantities and operational ranges, a very high speed can be provided to the low pressure spool. Low pressure turbine section and high pressure turbine section operation are often evaluated looking at a performance quantity which is the exit area for the turbine section multiplied by its respective speed squared. This performance quantity (“PQ”) is defined as: 
         [0000]      PQ ltp =( A   lpt   ×V   lpt   2 )  Equation 1:
 
         [0000]      PQ hpt =( A   hpt   ×V   hpt   2 )  Equation 2:
 
         [0000]    where A lpt  is the area of the low pressure turbine section at the exit thereof (e.g., at  401 ), where V lpt  is the speed of the low pressure turbine section, where A hpt  is the area of the high pressure turbine section at the exit thereof (e.g., at  400 ), and where V hpt  is the speed of the high pressure turbine section. 
         [0038]    Thus, a ratio of the performance quantity for the low pressure turbine section compared to the performance quantify for the high pressure turbine section is: 
         [0000]      ( A   lpt   ×V   lpt   2 )/( A   hpt   ×V   hpt   2 )=PQ ltp/ PQ hpt   Equation 3:
 
         [0000]    In one turbine embodiment made according to the above design, the areas of the low and high pressure turbine sections are 557.9 in 2  and 90.67 in 2 , respectively. Further, the speeds of the low and high pressure turbine sections are 10179 rpm and 24346 rpm, respectively. Thus, using Equations 1 and 2 above, the performance quantities for the low and high pressure turbine sections are: 
         [0000]      PQ ltp =( A   lpt   ×V   lpt   2 )=(557.9 in 2 )(10179 rpm) 2 =57805157673.9 in 2  rpm 2   Equation 1:
 
         [0000]      PQ hpt =( A   hpt   ×V   hpt   2 )=(90.67 in 2 )(24346 rpm) 2 =53742622009.72 in 2  rpm 2   Equation 2:
       and using Equation 3 above, the ratio for the low pressure turbine section to the high pressure turbine section is:       
 
         [0000]      Ratio=PQ ltp/ PQ hpt =57805157673.9 in 2  rpm 2 /53742622009.72 in 2  rpm 2 =1.075 
         [0040]    In another embodiment, the ratio was about 0.5 and in another embodiment the ratio was about 1.5. With PQ ltp /PQ hpt  ratios in the 0.5 to 1.5 range, a very efficient overall gas turbine engine is achieved. More narrowly, PQ ltp/ PQ hpt  ratios of above or equal to about 0.8 are more efficient. Even more narrowly, PQ ltp/ PQ hpt  ratios above or equal to 1.0 are even more efficient. As a result of these PQ ltp/ PQ hpt  ratios, in particular, the turbine section can be made much smaller than in the prior art, both in diameter and axial length. In addition, the efficiency of the overall engine is greatly increased. 
         [0041]    The low pressure compressor section is also improved with this arrangement, and behaves more like a high pressure compressor section than a traditional low pressure compressor section. It is more efficient than the prior art, and can provide more compression in fewer stages. The low pressure compressor section may be made smaller in radius and shorter in length while contributing more toward achieving an overall pressure ratio design target of the engine. In some examples, engine  20  is designed at a predetermined design target defined by performance quantities for the low and high pressure turbine sections  46 ,  54 . In further examples, the predetermined design target is defined by pressure ratios of the low pressure and high pressure compressors  44 ,  52 . 
         [0042]    In some examples, the overall pressure ratio corresponding to the predetermined design target is greater than or equal to about 35:1. That is, after accounting for a pressure rise of the fan  42  in front of the low pressure compressor  44 , the pressure of the air entering the low (or first) compressor section  44  should be compressed as much or over 35 times by the time it reaches an outlet of the high (or second) compressor section  52 . In other examples, an overall pressure ratio corresponding to the predetermined design target is greater than or equal to about 40:1, or greater than or equal to about 50:1. In some examples, the predetermined design target is defined at sea level and at a static, full-rated takeoff power condition. In other examples, the predetermined design target is defined at a cruise condition. 
         [0043]    As shown in  FIG. 3 , the engine as shown in  FIG. 2  may be mounted such that the high pressure turbine  54  is “piggy-back” bearing mounted. As shown, the high spool and shaft  32  includes a bearing  112  which supports the high pressure turbine  54  and the high spool  32  on an outer periphery of the low spool shaft  30 . The forward end of the high spool  32  is supported by a bearing  110  at an outer periphery of the shaft  32 . The bearing  110  is supported on static structure  108  associated with the overall engine casings arranged to form the core of the engine as is shown in  FIG. 1 . In addition, the shaft  30  is supported on a bearing  100  at a forward end. The bearing  100  is supported on static structure  102 . A rear end of the shaft  30  is supported on a bearing  106  which is attached to static structure  104 . 
         [0044]    With this arrangement, there is no bearing support struts or other structure in the path of hot products of combustion passing downstream of the combustion section, and into the high pressure turbine  54 , and no bearing compartment support struts in the path of the products of combustion as they flow across to the low pressure turbine  46 . Stated otherwise, the bearing  112 , and its associated mount, are positioned radially inwardly of the hubs  106  and  108  associated with the turbine sections  54  and  46 . 
         [0045]    As shown, there is no mid-turbine frame mounted in the area  402  between the turbine sections  54  and  46 . 
         [0046]    While this invention has been disclosed with reference to one embodiment, it should be understood that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.