Abstract:
A rotor for a gas turbine engine includes a plurality of turbine blades extending radially outwardly of a rotor body. A plurality of purge blades are positioned to rotate with the rotor body, and to drive air radially outwardly toward the turbine blades.

Description:
BACKGROUND 
       [0001]    This application relates to rotors for use in gas turbine engines, where purge blades generate a pressure to resist ingestion of hot gas. 
         [0002]    Gas turbine engines are known, and typically include a compressor compressing air, and delivering the air into a combustion section. The air is mixed with fuel and combusted in the combustion section. Products of this combustion pass downstream over turbine rotors. The turbine rotors may include a rotor with removable blades. Alternatively, integrally bladed rotors wherein the rotor and the blades are formed as one, are also known. 
         [0003]    In addition to the flow of hot combustion products across the turbine rotors, cool air is also delivered. The cool air serves to cool the rotor and blades, since they operate in a very high temperature. In addition, another purpose of the cool air is to provide a “purge” flow which resists the ingestion of the hot combustion products into the area of the rotor and blade interface. As an example, in one type removable blade, a so-called “fir-tree” includes a plurality of segments which are inserted into corresponding fir-tree grooves in the rotor. The blade is held into the disc by locking features such as a clip or rivet. 
         [0004]    However, at times, if the hot gas combustion products are ingested into the area of the fir-tree, there can be challenges raised. As one example, a concern known as both blade and disc creep occurs when there is plastic deformation of the blade and disc. This can occur if they are subjected to temperatures beyond a material creep resistance capability. 
         [0005]    Another concern is so-called “blade walking.” Blade walking typically occurs at startup of the gas turbine engine when there may be insufficient cooling air. The blade will heat more rapidly than the rotor, and thus the blade may move axially within the groves in the rotor. That occurs since the difference in thermal gradient between the disc and blade at the fir-tree gives rise to forces that overcome the strength of the locking features holding them together. As a consequence relative motion between disc and blade takes place. This is known as blade-walking. 
         [0006]    Both of the above occurrences are undesirable. 
       SUMMARY 
       [0007]    A rotor for a gas turbine engine includes a plurality of turbine blades extending radially outwardly of a rotor body. A plurality of purge blades is positioned to rotate with the rotor body, and to drive air radially outwardly toward the turbine blades. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a turbine section in a gas turbine engine. 
           [0009]      FIG. 2  shows a detail of a turbine rotor. 
           [0010]      FIGS. 3A  shows a purge blade. 
           [0011]      FIG. 3B  shows another detail of the  FIG. 3A  blade. 
           [0012]      FIG. 4  shows another embodiment. 
           [0013]      FIG. 5  is a flow chart. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  discloses a turbine section  20 . As shown, a rotor  22  is positioned adjacent to a series of vanes  24 . A turbine blade  26  has a fir-tree connection  28  received within a groove within rotor  22 . 
         [0015]    As shown, there is combustion gas flow F, which moves across the turbine blades  26 , driving them to rotate with the rotor  22 . This flow is extremely hot. To resist ingestion of this hot gas flow into the area of the fir-tree  28 , cooling air flows C are also supplied both at the upstream and downstream end of the rotor  22 . 
         [0016]    If insufficient cooling airflow is provided to purge or resist the hot gas flow, then blade creep and blade-walking can occur. On the other hand, too much cooling air flow decreases the efficiency of the overall gas turbine engine. As such, it is desirable to optimize the amount of airflow, while still ensuring it is sufficient to prevent ingestion of the hot gas. 
         [0017]      FIG. 2  shows an embodiment wherein the rotor  22  has grooves  31  interfitting with the fir-trees  28 . As also shown, purge blades  32 ,  132 ,  232  are formed on the rotor and within a radial extent of the grooves  31 . The purge blade is part of the turbine disc rim. 
         [0018]    The purge blades  32 ,  132 ,  232  are formed on the downstream end of the rotor  22 , although they may also be utilized on the upstream end if desired. 
         [0019]      FIG. 3A  shows a detail of one purge blade  32 . As shown, the purge blade  32  has an outer face that is generally trapezoidal, with angled sides  35  extending between a top  36  and a larger bottom  34 . The sides  35  provide a purge blade profile that will serve to move air when the rotor  22  is rotated. As shown, the purge blade  32  stands away from the nominal face of the rotor  22  by a distance h, and as shown at  38 . In addition, sides  40  extend for a length L. A radius R 0  from a centerline of the gas turbine engine to the top  38  of the blade is defined, while another radius R i  is defined to the bottom face  34 . 
         [0020]    As shown in  FIG. 3B , the purge blade  32  may actually extend from the rotor  22  with a radius R. For purposes of this application, the shape of the purge blade may be defined as “trapezoidal.” However, as shown in  FIG. 3B , corners C of the trapezoidal shape may be rounded. This shape would still be known as “trapezoidal” for purposes of interpreting this application. 
         [0021]    The dimensions of the blade are designed to ensure that sufficient airflow will be generated to resist ingestion of hot air into the area of the fir-tree but not utilizing an excessive amount. 
         [0022]    As is clear, the purge blade  232  can be swept forward, or in the direction of rotation ω, or may be swept rearwardly as shown at  132 , or against the direction of rotation. 
         [0023]    To prevent hot gas ingestion, the following is required to be satisfied (refer to  FIG. 1 ) 
         [0000]      P T-cavity≧P   T-flowfield-Max    
       Where 
       [0024]    P T-cavity =disc rim cavity total pressure
 
P T-flowfield-Max =Maximum total pressure of the flow field above the blade platform
 
         [0025]    It is desirable that the pressure outwardly of the purge blades be greater than the pressure in the hot gas flow F at the location of the purge blades. Thus, the mass flow rate is selected to ensure this additional pressure. In calculating the change in pressure, the following formula will apply: 
         [0000]      {dot over (m)}=ρ2πR 2 h W
 
       Where: 
       [0026]    W=velocity of the cooling air relative to the purge blade
 
ρ=air density
 
R 2  is the outer radius of the control volume (purge blade outside radius)
 
h is the purge blade height
 
L is purge blade length
 
         [0027]    The disc rim cavity pressure at the control volume exit, P T-cavity , is equal to the pressure at the control volume entrance plus the increase in pressure, Δp, due to energy transferred from the purge blade to the air. It can be simply expressed as: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               p 
             
             = 
             
               
                 
                   ρ 
                   
                     2 
                      
                     
                       g 
                       c 
                     
                   
                 
                  
                 
                   [ 
                   
                     
                       U 
                       2 
                       2 
                     
                     - 
                     
                       U 
                       1 
                       2 
                     
                   
                   ] 
                 
               
               + 
               
                 
                   ρ 
                   
                     2 
                      
                     
                       g 
                       c 
                     
                   
                 
                  
                 
                   [ 
                   
                     
                       V 
                       2 
                       2 
                     
                     - 
                     
                       V 
                       1 
                       2 
                     
                   
                   ] 
                 
               
             
           
         
       
     
       Where: 
       [0028]    U i =peripheral velocity of the turbine disc at R 2  and R 1  (purge blade outer and inner radii)
 
V 1 =peripheral velocity of the air at R 2  and R 1  (purge blade outer and inner radii)
 
         [0000]    
       
         
           
             
               
                 ρ 
                 
                   2 
                    
                   
                     g 
                     c 
                   
                 
               
                
               
                 [ 
                 
                   
                     U 
                     2 
                     2 
                   
                   - 
                   
                     U 
                     1 
                     2 
                   
                 
                 ] 
               
             
             , 
           
         
       
     
         [0000]    the first term of the above equation, represents the increase in static pressure due to the centrifugal effect acting on the air. 
         [0000]    
       
         
           
             
               
                 ρ 
                 
                   2 
                    
                   
                     g 
                     c 
                   
                 
               
                
               
                 [ 
                 
                   
                     V 
                     2 
                     2 
                   
                   - 
                   
                     V 
                     1 
                     2 
                   
                 
                 ] 
               
             
             , 
           
         
       
     
         [0000]    the second terms of the above equation represents the increase in kinetic energy due to the energy transferring from the purge blade to the air. 
         [0029]    The dimensions can thus be selected to achieve adequate pressure increase from the purge blades to resist the ingestion of the hot combustion gases. 
         [0030]      FIG. 4  shows an embodiment  60  wherein purge blade  62  is incorporated into an integrally bladed rotor. As shown, the purge blade  62  has a curved side face  64 . Stress relief slot  66  is formed from a lip of the rotor  65  downwardly to a top surface  68  of the blade. Similar calculations would be used to define the size and shape of the blades. Slot  66  may extend through the full thickness of the disc rim from the front to the back. 
         [0031]      FIG. 5  is a flow chart of a method of designing purge blade dimensions. 
         [0032]    First, at step  100 , a fluid dynamic analysis of a gas flow path for an engine which is to incorporate this blade and rotor is performed. From this analysis, which a worker of ordinary skill in the art would know how to perform, a heat transfer coefficient, and gas flow conditions including pressure, velocity and temperature are determined. Next, at step  102 , a secondary air transient analysis is made. This would include an analysis of the cooling air flow. From this secondary air transient analysis, a temperature of the metal associated with the blade and rotor, compartment air temperature, and flow rates are identified. At step  103 , a structural analysis is made of the turbine assembly, including the disc or rotor and the blades. Transient thermal and mechanical loads are analyzed. From this, at step  104 , a disc or rotor and blade life analysis is performed, including consideration of oxidation, creep, hot corrosion and low cycle fatigue (LCF). 
         [0033]    At step  105 , an amount of secondary airflow necessary to raise the rim cavity pressure to satisfy the equation set forth above at paragraph  20  is performed. Notably, after some of the steps, the flow chart returns to step  102  to re-perform that analysis, and provide better information flowing downstream. 
         [0034]    At step  106 , the dimensions of the purge blade including its various dimensions, and utilizing paragraphs  21  and  22  is performed. Finally, an envelope and structural integrity analysis is made at step  107  after the purge blade dimensions have been analyzed. 
         [0035]    Although embodiments of this invention have been disclosed, a worker of ordinary skill 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.