Patent Publication Number: US-8534991-B2

Title: Compressor with asymmetric stator and acoustic cutoff

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a compressor for a gas turbine engine, wherein the stator vanes are asymmetric, and wherein acoustic cutoff is achieved. 
     Gas turbine engines typically include a compressor which compresses air and delivers it into a combustion chamber. The compressed air is mixed with fuel and combusted in the combustion section. Products of this combustion pass downstream over turbine rotors. 
     The compressor is typically provided with rotating blades, and stator vanes adjacent to the blades. The stator vanes control the flow of the air to the compressor rotor. 
     A concept known as “cutoff” is utilized in the design of compressors, and relates the number of vanes in the stator to the number of blades in the rotor. The goal of “cutoff” is to ensure that generated noise decays in a compressor duct, instead of propagating to a far field. Compressors which have achieved cutoff in the past have equally spaced stator vanes across the entire circumference of the stator section, and equally spaced rotor blades. 
     Recently, asymmetric stator vanes have been developed, which have unequally spaced stator vanes on two halves of a circumference. The spacing of the stator vanes in a lower half is unequal from the spacing of the vanes in an upper half. The purpose of the unequal spacing is structural. 
     SUMMARY OF THE INVENTION 
     A method of manufacturing a compressor section includes the steps of defining a compressor section having a number of blades, and having at least one stator section with a number of vanes. Each stator section has at least two sections wherein the spacing between the vanes in a first of the sections is not equal to the spacing between the vanes in a second of the sections. The number of blades, and the number of vanes in all of the sections are selected to achieve acoustic cutoff. 
     A compressor section designed and manufactured by the above method is also disclosed and claimed. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a gas turbine engine. 
         FIG. 2  schematically shows a compressor stator for the  FIG. 1  gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A gas turbine engine  10 , such as a turbofan gas turbine engine, circumferentially disposed about an engine centerline, or axial centerline axis  12  is shown in  FIG. 1 . The engine  10  includes a fan  14 , compressor sections  15  and  16 , a combustion section  18  and a turbine  20 . As is well known in the art, air compressed in the compressor  15 / 16  is mixed with fuel and burned in the combustion section  18  and expanded in turbine  20 . In addition, the compressor section includes stator sections  13  having a plurality of vanes, and rotor blades  11 . The blades and vanes are shown in the low pressure compressor  15 , however, similar structure is found in the high pressure compressor section  16 . The vanes may be static vanes or variable vanes. The turbine  20  includes rotors  22  and  24 , which rotate in response to the expansion. The turbine  20  comprises alternating rows of rotary airfoils or blades  26  and static airfoils or vanes  28 . It should be understood that this view is included simply to provide a basic understanding of the sections in a gas turbine engine, and not to limit the invention. This invention extends to all types of turbine engines for all types of applications. 
     A compressor stator section  30 , such as may be employed in a gas turbine engine, is illustrated in  FIG. 2 . As shown, there is an upper half of the circumference  32  and a lower half  34 . Vanes  40  are positioned at a dividing point between the two sections  32  and  34 . The vanes  36  in the lower section are spaced by a first pitch, while the vanes  38  in the upper section are spaced by a second, greater pitch. As can be appreciated, there are more vanes on the bottom half  34  than in the top half  32  in the illustrated arrangement. 
     While  FIG. 2  shows a relatively small number of vanes, it should be understood that typically greater numbers of vanes are included. A sample calculation is provided below, however, the sample calculation is simply one example, and other numbers of blades could come within the scope of this invention. 
     One way to achieve cutoff with a compressor section having all blades equally spaced, and all stator vanes equally spaced. A formula exists that relates the number of blades, along with the number of vanes with defined when cutoff would occur. That formula is: 
                   ξ   =              nBM   t         mM     m   ⁢           ⁢   μ     *     ⁢       1   -     M   x   2                  &lt;   1             Equation   ⁢           ⁢   1               
where
 
 m=nB−kV   Equation 2
 
and
         ξ=cutoff ratio   m=nB−kV=circumferential mode order   n=Blade passing frequency harmonic order (any integer from 1 to infinity)   B=Number of compressor rotor blades   k=Vane passing frequency harmonic order (any integer from −infinity to infinity)   V=Number of compressor vanes upstream and/or downstream of the compressor rotor       

     
       
         
           
             
               M 
               t 
             
             = 
             
               
                 Local 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 tip 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 rotational 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 mach 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 number 
               
               = 
               
                 
                   Ω 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   r 
                 
                 
                   c 
                   0 
                 
               
             
           
         
       
         
         
           
             Ω=Rotor rotational speed (rad/sec) 
             r=Local tip duct radius 
             c 0 =Local speed of sound 
             M x =Mean local axial Mach number in the duct 
           
         
       
    
               M     m   ⁢           ⁢   μ     *     =         κ     m   ⁢           ⁢   μ       m     =     Cutoff   ⁢           ⁢   Mach   ⁢           ⁢     number   .               
This can be shown, such as by Equation 7.3.4 in the cited Tyler/Sofrin SAE article.
         κ mμ =Mode Eigenvalue for a given (m, μ) mode normalized by r. This can be shown such as from equation 4.7 in the cited Meyer/Envia NASA article.   μ=Radial mode order (integer from 0 to infinity) (set=0 for the purposes of this calculation)       

     It was originally thought that such cutoff could only occur in a compressor wherein the stator vanes were all equally spaced around the circumference. 
     However, Applicant has developed a method of identifying parameters to achieve cutoff in a compressor wherein the stator vanes are not equally spaced. In particular, in a stator section such as shown in  FIG. 2 , cutoff can still be ensured if Equation 1 is met, and wherein m now equals:
 
 m =minimum| m   1 &amp; m   2 |  Equation 3
 
where
 
 m   1   =nB− 2 kV   1   Equation 4
 
and
 
 m   2   =nB− 2 kV   2   Equation 5
 
     One calculates a new m 1  and a new m 2  and then takes the minimum absolute value of m 1  and m 2  and utilizes that in Equation 1. Notably, the m 1  and m 2  include a factor of 2× the number of vanes in each half, to account for the fact that the vanes are only across half the circumference. 
     If Equation 1 is run with this new calculation, then a compressor section designed accordingly should achieve cutoff. While two sections are shown for the stator section, it is possible that greater numbers of sections can also be utilized, each having unequal numbers of vanes. In designing such a compressor, it may be that the value 2 found in Equations 4 and 5 be increased to equal the number of sections. 
     A sample calculation is shown below:
         Set:   The blade count, B=28   Vane count upstream of the blade V=61 vanes (where V 1 =30 vanes on one half, V 2 =31 vanes on the other half)   Vane count downstream of the blade, V=61 vanes (where V 1 =30 vanes on one half, V 2 =31 vanes on the other half) (The vane counts upstream and downstream of the vane do not have to be equal, but are set equal for the purposes of this example).   M x =axial Mach number=0.5   M t =0.8 (local tip rotational Mach number)   For blade passing frequency, n=1   Thus for the upstream vane count: Use the smallest value of |m 1 | and the smallest value of |m 2 | to determine cutoff.   m=nB−kV so setting k=1 gives the smallest value of |m 1 | and also gives the smallest value of |m 2 |   |m 1 |=|1*28−2*1*30|=32   |m 2 |=|1*28−2*1*31|=34   m=minimum (32, 34)=32   For a hub/tip ratio of 0.5, and μ=0, κ mμ =34.59,       

     
       
         
           
             
               M 
               
                 m 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 μ 
               
               * 
             
             = 
             
               
                 
                   κ 
                   
                     m 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     μ 
                   
                 
                 
                    
                   m 
                    
                 
               
               = 
               
                 1.08 
                 . 
               
             
           
         
       
     
     
       
         
           
             ξ 
             = 
             
               
                  
                 
                   
                     1 
                     * 
                     28 
                     * 
                     0.8 
                   
                   
                     34.59 
                     ⁢ 
                     
                       
                         1 
                         - 
                         
                           0.5 
                           2 
                         
                       
                     
                   
                 
                  
               
               = 
               
                 0.75 
                 &lt; 
                 1 
               
             
           
         
       
         
         
           
             Repeating this calculation for the downstream vane count gives the same results. So this stage of the LPC is cutoff. As can be appreciated, the factor of “2” as found in calculating the m 1  and m 2  value is because there are two sections in the disclosed example. If there were three or more sections, that value would increase, as mentioned above. 
           
         
       
    
     One can appreciate also that the minimum absolute value of the m 1  and m 2  quantities will be found in the section having the fewest number of blades given a unit of circumferential extent. Stated another way, if all of the sections have an equal circumferential extent, would be the section with the minimum number of blades that would be used to do the calculations to insure cutoff is achieved. However, should there be unequal circumferential extents, each of the quantities would be scaled accordingly. 
     The above formulations and examples assume generally axial flow through the compressor. In fact, it may often be the case that there will be some swirl within the air. While it is likely true the above simplified calculations and formulations would still be accurate even for a compressor having swirl, another formula could be utilized wherein the following formula replaces Equation 1: 
                   ξ   =                nBM   t     -     mM   s           mM     m   ⁢           ⁢   μ     *     ⁢       1   -     M   x   2                  &lt;   1             Equation   ⁢           ⁢   6               
Generally, as the formula shows, the M s  component acts to modify the rotational speed of the mode by the swirl Mach number of the flow. M s  is a local swirl flow mach number in between two rows of vanes and/or blades, with positive being defined in the direction of rotor rotation. The M s  component can be calculated by taking two known quantities, the swirl velocity, and dividing it by the c 0 , the local speed of sound. The swirl velocity is a quantity which would be known to a worker of ordinary skill in the art, having a particular compressor design.
 
     All of the several variables would be quantities that a worker of ordinary skill in the art would be able to calculate given a particular compressor design. 
     In sum, a compressor section is disclosed which achieves cutoff even with an asymmetric stator vane section. Thus, with the inventive method, a compressor section can be designed and utilized wherein the structural benefits that may be afforded by asymmetric stators can be achieved, while still achieving the acoustic cutoff benefits which are becoming of increasing importance. 
     While the disclosed compressor has only two sections, as mentioned above, there could be more than two sections. Further, while the disclosed stator section has its two sub-sections at top and bottom, other orientations of the two distinct sections could be utilized. 
     Although an embodiment of this disclosure has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the disclosure. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.