Abstract:
A gas turbine engine has a fan and a turbine having a fan drive turbine rotor. The fan drive turbine rotor drives a compressor rotor. A gear reduction effects a reduction in the speed of the fan relative to an input speed from the fan drive turbine rotor that drives the compressor rotor. The compressor rotor has a number of compressor blades in at least one of a plurality of rows of the compressor rotor. The blades operate at least some of the time at a rotational speed. The number of compressor blades in at least one row and the rotational speed are such that the following formula holds true for at least one row of the compressor rotor turbine: (number of blades×rotational speed)/60 s≧5500 Hz, and the rotational speed is in revolutions per minute. A method of designing a gas turbine engine and a compressor module are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 14/144,710, filed Dec. 31, 2013, which is a continuation of U.S. patent application Ser. No. 14/016,436, filed Sep. 3, 2013, now U.S. Pat. No. 8,714,913, issued May 6, 2014, which is a continuation of U.S. patent application Ser. No. 13/630,276, filed Sep. 28, 2012, now U.S. Pat. No. 8,632,301, issued Jan. 21, 2014. 
     
    
     BACKGROUND 
       [0002]    This application relates to the design of a gas turbine engine rotor which can be operated to produce noise that is less sensitive to human hearing. 
         [0003]    Gas turbine engines are known, and typically include a fan delivering air into a compressor. The air is compressed in the compressor and delivered downstream into a combustor section where it was mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving the turbine rotors to rotate. 
         [0004]    Typically, there is a high pressure turbine rotor, and a low pressure turbine rotor. Each of the turbine rotors include a number of rows of turbine blades which rotate with the rotor. Interspersed between the rows of turbine blades are vanes. 
         [0005]    The high pressure turbine rotor has typically driven a high pressure compressor rotor, and the low pressure turbine rotor has typically driven a low pressure compressor rotor. Each of the compressor rotors also include a number of compressor blades which rotate with the rotors. There are also vanes interspersed between the rows of compressor blades. 
         [0006]    The low pressure turbine or compressor can be a significant noise source, as noise is produced by fluid dynamic interaction between the blade rows and the vane rows. These interactions produce tones at a blade passage frequency of each of the low pressure turbine rotors, the low pressure compressor rotors, and their harmonics. 
         [0007]    The noise can often be in a frequency range that is very sensitive to humans. To mitigate this problem, in the past, a vane-to-blade ratio has been controlled to be above a certain number. As an example, a vane-to-blade ratio may be selected to be 1.5 or greater, to prevent a fundamental blade passage tone from propagating to the far field. This is known as “cut-off.” 
         [0008]    However, acoustically cut-off designs may come at the expense of increased weight and reduced aerodynamic efficiency. Stated another way, by limiting the designer to a particular vane to blade ratio, the designer may be restricted from selecting such a ratio based upon other characteristics of the intended engine. 
         [0009]    Historically, the low pressure turbine has driven both a low pressure compressor section and a fan section. More recently, a gear reduction has been provided such that the fan and low pressure compressor can be driven at distinct speeds. 
       SUMMARY OF THE INVENTION 
       [0010]    In a featured embodiment, a gas turbine engine comprises a fan and a turbine section having a fan drive turbine rotor, and a compressor rotor. A gear reduction effects a reduction in the speed of the fan relative to an input speed from the fan drive turbine rotor. The compressor rotor has a number of compressor blades in at least one of a plurality of rows of the compressor rotor. The blades operate at least some of the time at a rotational speed. The number of compressor blades in the at least one row and the rotational speed is such that the following formula holds true for the at least one row of the compressor rotor: 
         [0000]      (the number of blades×the rotational speed)/60 s≧5500 Hz; and
       the rotational speed being in revolutions per minute.       
 
         [0012]    In another embodiment according to the previous embodiment, the formula results in a number greater than or equal to about 6000 Hz. 
         [0013]    In another embodiment according to any of the previous embodiments, the gas turbine engine is rated to produce about 15,000 pounds of thrust or more. 
         [0014]    In another embodiment according to any of the previous embodiments, the formula holds true for the majority of the blade rows of the compressor rotor. 
         [0015]    In another embodiment according to any of the previous embodiments, the gas turbine engine is rated to produce about 15,000 pounds of thrust or more. 
         [0016]    In another embodiment according to any of the previous embodiments, the gear reduction has a gear ratio of greater than about 2.3. 
         [0017]    In another embodiment according to any of the previous embodiments, the gear reduction has a gear ratio of greater than about 2.5. 
         [0018]    In another embodiment according to any of the previous embodiments, the fan delivers air into a bypass duct, and a portion of air into the compressor rotor. A bypass ratio is defined as the volume of air delivered into the bypass duct compared to the volume of air delivered into the compressor rotor, and the bypass ratio being greater than about 6. 
         [0019]    In another embodiment according to any of the previous embodiments, the bypass ratio is greater than about 10. 
         [0020]    In another embodiment according to any of the previous embodiments, the formula results in a number greater than or equal to about 6000 Hz. 
         [0021]    In another embodiment according to any of the previous embodiments, the rotational speed being an approach speed. 
         [0022]    In another embodiment according to any of the previous embodiments, the turbine section includes a higher pressure turbine rotor and a lower pressure turbine rotor. The fan drive turbine rotor is the lower pressure turbine rotor. 
         [0023]    In another embodiment according to any of the previous embodiments, the compressor rotor is a lower pressure compressor rotor, and the higher pressure turbine rotor drives a higher pressure compressor rotor. 
         [0024]    In another embodiment according to any of the previous embodiments, there are three turbine rotors. The fan drive rotor turbine drives the fan. A second and third turbine rotor each drive respective compressor rotors of the compressor section. 
         [0025]    In another embodiment according to any of the previous embodiments, the gear reduction is positioned intermediate the fan and a compressor rotor driven by the fan drive turbine rotor. 
         [0026]    In another embodiment according to any of the previous embodiments, the gear reduction is positioned intermediate the fan drive turbine rotor and a compressor rotor driven by the fan drive turbine rotor. 
         [0027]    In another featured embodiment, a method of designing a gas turbine engine comprises the steps of including a first turbine rotor to drive a compressor rotor and a fan turbine rotor for driving a fan through a gear reduction. A number of blades is selected in at least one row of the compressor rotor, in combination with a rotational speed of the compressor rotor, such that the following formula holds true for the at least one row of the compressor rotor: 
         [0000]      (the number of blades×the rotational speed)/60 s≧5500 Hz; and
       the rotational speed being in revolutions per minute.       
 
         [0029]    In another embodiment according to the previous embodiment, the formula results in a number greater than or equal to about 6000 Hz. 
         [0030]    In another embodiment according to any of the previous embodiments, the gas turbine engine is rated to produce about 15,000 pounds of thrust or more. 
         [0031]    In another embodiment according to any of the previous embodiments, the rotational speed is an approach speed. 
         [0032]    In another embodiment according to any of the previous embodiments, the turbine section includes a higher pressure turbine rotor and a lower pressure turbine rotor. The fan drive turbine rotor is the lower pressure turbine rotor. 
         [0033]    In another embodiment according to any of the previous embodiments, the compressor rotor is a lower pressure compressor rotor, and the higher pressure turbine rotor drives a higher pressure compressor rotor. 
         [0034]    In another embodiment according to any of the previous embodiments, the first turbine rotor and the fan turbine rotor are provided by a single rotor. 
         [0035]    These and other features of this application will be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]      FIG. 1  shows a gas turbine engine. 
           [0037]      FIG. 2  shows another embodiment. 
           [0038]      FIG. 3  shows yet another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]      FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool 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), or an intermediate spool, among other systems or features. The fan section  22  drives air along a bypass flowpath B in a bypass duct defined within a nacelle  15 , while the compressor section  24  drives air along a core flowpath 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-spool architectures. 
         [0040]    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. 
         [0041]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . 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 spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . 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 axes. 
         [0042]    The core airflow is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. 
         [0043]    The terms “low” and “high” as applied to speed or pressure for the spools, compressors and turbines are of course relative to each other. That is, the low speed spool operates at a lower speed than the high speed spool, and the low pressure sections operate at lower pressure than the high pressures sections. 
         [0044]    The engine  20  in one example is a high-bypass geared aircraft engine. 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  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  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  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 and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
         [0045]    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 (‘TSFCT’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. 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 [(Tambient ° R)/(518.7)° R] 0.5 . The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. 
         [0046]    The use of the gear reduction between the low speed spool and the fan allows an increase of speed to the low pressure compressor. In the past, the speed of the low pressure turbine and compressor has been somewhat limited in that the fan speed cannot be unduly large. The maximum fan speed is at its outer tip, and in larger engines, the fan diameter is much larger than it may be in smaller power engines. However, the use of the gear reduction has freed the designer from limitation on the low pressure turbine and compressor speeds caused by a desire to not have unduly high fan speeds. 
         [0047]    It has been discovered that a careful design between the number of rotating blades, and the rotational speed of the low pressure turbine can be selected to result in noise frequencies that are less sensitive to human hearing. The same is true for the low pressure compressor  44 . 
         [0048]    A formula has been developed as follows: 
         [0000]      (blade count×rotational speed)/60 s≧5500 Hz.
 
         [0049]    That is, the number of rotating blades in any low pressure turbine stage, multiplied by the rotational speed of the low pressure turbine  46  (in revolutions per minute), divided by 60 s should be greater than or equal to 5500 Hz. The same holds true for the low pressure compressor stages. More narrowly, the amounts should be above 6000 Hz. A worker of ordinary skill in the art would recognize that the 60 s factor is to change revolutions per minute to Hertz, or revolutions per one second. 
         [0050]    The operational speed of the low pressure turbine  46  and low pressure compressor  44  as utilized in the formula should correspond to the engine operating conditions at each noise certification point defined in Part 36 or the Federal Airworthiness Regulations. More particularly, the rotational speed may be taken as an approach certification point as defined in Part 36 of the Federal Airworthiness Regulations. For purposes of this application and its claims, the term “approach speed” equates to this certification point. 
         [0051]    It is envisioned that all of the rows in the low pressure turbine  46  meet the above formula. However, this application may also extend to low pressure turbines wherein the majority of the blade rows in the low pressure turbine meet the above formula, but perhaps some may not. The same is true for low pressure compressors, wherein all of the rows in the low pressure compressor  44  would meet the above formula. However, the application may extend to low pressure compressors wherein only the majority of the blade rows in the low pressure compressor meet the above formula, but some perhaps may not. 
         [0052]    This will result in operational noise that would be less sensitive to human hearing. 
         [0053]    In embodiments, it may be that the formula can result in a range of greater than or equal to 5500 Hz, and moving higher. Thus, by carefully designing the number of blades and controlling the operational speed of the low pressure turbine  46  (and a worker of ordinary skill in the art would recognize how to control this speed) one can assure that the noise frequencies produced by the low pressure turbine are of less concern to humans. 
         [0054]    The same holds true for designing the number of blades and controlling the speed of the low pressure compressor  44 . Again, a worker of ordinary skill in the art would recognize how to control the speed. 
         [0055]    In embodiments, it may be only the low pressure turbine rotor  46 , or the low pressure compressor rotor  44  which is designed to meet the meet the above formula. On the other hand, it is also possible to ensure that both the low pressure turbine  46  and low pressure compressor  44  meet the above formula. 
         [0056]    This invention is most applicable to jet engines rated to produce 15,000 pounds of thrust or more. In this thrust range, prior art jet engines have typically had frequency ranges of about 4000 hertz. Thus, the noise problems as mentioned above have existed. 
         [0057]    Lower thrust engines (&lt;15,000 pounds) may have operated under conditions that sometimes passed above the 4000 Hz number, and even approached 6000 Hz, however, this has not been in combination with the geared architecture, nor in the higher powered engines which have the larger fans, and thus the greater limitations on low pressure turbine or low pressure compressor speed. 
         [0058]      FIG. 2  shows an embodiment  200 , wherein there is a fan drive turbine  208  driving a shaft  206  to in turn drive a fan rotor  202 . A gear reduction  204  may be positioned between the fan drive turbine  208  and the fan rotor  202 . This gear reduction  204  may be structured and operate like the gear reduction disclosed above. A compressor rotor  210  is driven by an intermediate pressure turbine  212 , and a second stage compressor rotor  214  is driven by a turbine rotor  216 . A combustion section  218  is positioned intermediate the compressor rotor  214  and the turbine section  216 . 
         [0059]      FIG. 3  shows yet another embodiment  300  wherein a fan rotor  302  and a first stage compressor  304  rotate at a common speed. The gear reduction  306  (which may be structured as disclosed above) is intermediate the compressor rotor  304  and a shaft  308  which is driven by a low pressure turbine section. 
         [0060]    The  FIGS. 2 and 3  engines may be utilized with the speed and blade features disclosed above. 
         [0061]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize 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.