Abstract:
A turbine blade for a gas turbine engine. An existing blade was found to exhibit bowing, or a concave configuration facing the pressure side, along its trailing edge. The invention reduces bowing by (1) changing tilt, (2) changing lean, (3) reducing the number of cooling holes, while (4) changing the diameters of the cooling holes, to maintaining the total cooling flow unchanged.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to a turbine blade having improved structural and cooling properties.  
         BACKGROUND OF THE INVENTION  
         [0002]    The turbine blades in a gas turbine engine operate in a harsh environment: a high G-field applies significant stress to the blades, and the blades operate under high-temperature conditions.  
           [0003]    The size of the G-field can be illustrated by a simple example. Centrifugal acceleration is given by the expression  
             a =( w −squared)× r,    
           [0004]    wherein  
           [0005]    a is the centrifugal acceleration,  
           [0006]    w is the rotational velocity in radians per second, and  
           [0007]    r is the radius at which the acceleration is computed.  
           [0008]    If a shaft of radius one foot rotates at 10,000 rpm, which corresponds to 167 revolutions per second, then the centrifugal acceleration a is computed as  
             a =(167×2× PI )(1 /sec )×(167×2 ×PI )(1 /sec )×1 foot,  
           [0009]    or about 1.1 million feet/second-squared. To convert this acceleration into units of G&#39;s, wherein one G is the earth&#39;s acceleration-due-to-gravity, one divides by 32.2, to obtain about 34,000 G&#39;s.  
           [0010]    Clearly, this high G-field applies significant stress to the blade: a blade which weighs one pound under static conditions will weigh 34,000 pounds in operation. In addition, the gas loading applies additional stresses to the blades, in different directions from the G-field.  
           [0011]    In addition to stresses due to G-fields, the temperature of the gas to which the turbine blade is subject is high. For example, turbine inlet temperatures of 2,500 F are common. High temperatures weaken many metals. To combat the high temperature, some turbine blades are actively cooled, as by passing cooling air through passages contained in the blades.  
           [0012]    The Inventors have developed a new structure for a turbine blade, to reduce the deleterious effects of the stresses and temperatures.  
         SUMMARY OF THE INVENTION  
         [0013]    In one form of the invention, a turbine blade is equipped with a specific tilt, specific lean, and a specific set of columns of cooling holes, wherein the hole distributions in many of the columns are non-uniform. The invention reduces a specific thermal gradient and mechanical loading in the blade, thereby reducing bowing of the blade. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates a simplified airfoil section of a turbine blade.  
         [0015]    [0015]FIG. 2 illustrates cooling passages  4 , and cooling holes  5 , in the turbine blade  3 .  
         [0016]    [0016]FIG. 3 illustrates generic temperature gradients found in the tip  6  in FIG. 1 of the blade  3 .  
         [0017]    [0017]FIG. 4 illustrates a phenomenon which the Inventors have identified.  
         [0018]    [0018]FIG. 5 illustrates a turbine blade  3 , and airfoil sections  18  superimposed thereon.  
         [0019]    [0019]FIG. 6 illustrates an exploded view of the airfoil sections  18 , arranged along a stacking axis  21 .  
         [0020]    [0020]FIGS. 7, 8,  9 , and  10  illustrate a coordinate system, used to define the terms tilt and lean.  
         [0021]    [0021]FIG. 11 illustrates a turbine blade found in the prior art.  
         [0022]    [0022]FIG. 12 illustrates one form of the invention.  
         [0023]    [0023]FIG. 13 illustrates the prior-art blade of FIG. 11, with reference stations  135  superimposed thereon.  
         [0024]    [0024]FIG. 14 illustrates the invention-blade of FIG. 12, with reference stations  145  superimposed thereon.  
         [0025]    [0025]FIG. 15 illustrates a simplified schematic of a gas turbine engine.  
         [0026]    [0026]FIG. 16 is a flow chart of processes undertaken by one form of the invention.  
         [0027]    [0027]FIG. 17 illustrates a generic pattern of cooling holes, used in a turbine blade (not shown).  
         [0028]    [0028]FIG. 18 illustrates how a column  101  of holes in FIG. 17 can be successively modified under the invention, in pursuit of an improved hole pattern.  
         [0029]    [0029]FIG. 19 illustrates a sequence of configurations, wherein the ten holes in, for example, column  101  in FIG. 17 is reduced to nine holes, and those nine holes are distributed in a column of ten possible positions, in ten different ways. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    [0030]FIG. 1 illustrates an airfoil section of a generic turbine blade  3  used in a gas turbine engine. FIG. 2 illustrates a cross-sectional view, and shows internal passages  4 , which deliver cooling air films  5 . With such cooling, the tip  6  of the blade in FIG. 1 will attain a temperature profile such as that shown in FIG. 3, which shows isotherms  7 . The cooling holes  8  in the tip in FIG. 3 are not shown in FIG. 1.  
         [0031]    The Inventors have observed that a particular turbine blade tends to bow in operation, as indicated in FIG. 4. The bowing is indicated by the deviation of the trailing edge  11  from the normal trailing edge shape, indicated by dashed line  12 . This particular blade is found in the first stage of the high-pressure turbine of the CF6-50 gas turbine engine, which is commercially available from the General Electric Company.  
         [0032]    The Inventors suspect that a thermal gradient is partly responsible for the bowing. In order to reduce the bowing, the Inventors have examined numerous different structural configurations for this type of blade, including different patterns of cooling holes, for the blade. The examinations took the form of running computer models, and examining actual samples of blades.  
         [0033]    The Inventors have determined that, if the blade is modified to assume (1) a tilt of 3.2 degrees, (2) a lean of 2.1 degrees, and (3) a specific pattern of cooling holes on the pressure side, then the bowing is reduced substantially. In addition, a significant reduction in one particular temperature gradient is attained.  
         [0034]    Prior to explaining this reduction, the parameters of tilt and lean will be explained. FIG. 5 shows a generic turbine blade  3 . It is designed as a stacked sequence of airfoil, or foil, sections  18 . FIG. 6 shows the foil sections  18  in exploded, unstacked form. The foil sections  18  are stacked on a stacking axis  21 , as known in the art. The stacking axis  21  can assume different orientations, such as tilt and lean, which will now be explained.  
         [0035]    [0035]FIG. 7 is a schematic representation of a turbine blade  3  on a turbine disc  24 . The stacking axis  21  is shown. FIG. 8 shows the blade in simplified form, as a flat plate  27 . The flat plate  27  is shown co-planar with the geometric axial plane  30 , shown in FIG. 9. Dashed line  28  is a radial line.  
         [0036]    The term lean refers to leaning the body  27  of the blade away from the radius  28 , as indicated by leaned stacking axis  21 A. Restated, phantom blade  27 P exhibits lean, compared with flat blade  27 . However, with lean present, the stacking axis  21 A still remains within the axial plane  30 . Angle  33  represents the lean angle.  
         [0037]    The term tilt is explained in FIG. 10. It refers to tilting the body of the blade  27  away from the axial plane  30  in FIG. 9, as indicated by tilted stacking axis  21 B in FIG. 10. Restated, phantom blade  27 PP exhibits tilt, compared with blade  27 . The tilted stacking axis  21 B is moved within a radial plane  36 . Angle  39  represents the tilt angle.  
         [0038]    Now the tilt and lean angles stated above can be illustrated by reference to FIGS. 9 and 10. Under the invention, the lean angle  33  in FIG. 9 would be 2.1 degrees, and the tilt angle  39  in FIG. 10 would be 3.2 degrees.  
         [0039]    The Inventors point out that, in the prior-art blade of FIG. 4, which exhibits the bowing phenomenon, and which is shown in greater detail in FIG. 11, tilt is 2.7 degrees, and lean is 3.5 degrees. Thus, under the invention, tilt is increased by 18.5 percent, from 2.7 to 3.2 degrees, while lean is decreased by 40 percent, from 3.5 to 2.1 degrees.  
         [0040]    As stated above, the invention also includes a specific pattern of cooling holes. FIG. 11 illustrates a first-stage high-pressure turbine blade  3 , as in the prior art, which encountered the bowing problem described above. Cooling holes  42  are shown. FIG. 12 illustrates a cooling hole pattern according to one form of the invention.  
         [0041]    The Inventors have found that, for the blade which exhibits bowing, the temperature differential  41  of FIG. 11 is a specific temperature difference, which will be called T herein. This differential is between the pressure side and the suction side, at the trailing edge, measured at mid-span, that is, near region  43 .  
         [0042]    The corresponding differential  44  in FIG. 12, under the invention, is found to be lower, at 68 percent of T. This reduction in temperature differential, together with the change in tilt and lean, reduces the bowing described above.  
         [0043]    A detailed discussion of some characterizations of the hole patterns of FIGS. 11 and 12 will be given. The hole patterns can be divided into groups: (1) a single row and (2) multiple columns. The prior art blade  3  of FIG. 11 contains (1) a row  48  of ten holes at the blade tip  51  and (2) ten columns of holes, labeled  59 - 68 .  
         [0044]    The invention-blade of FIG. 12 contains (1) a row  70  of eight holes at the tip, as opposed to ten holes in FIG. 11, and (2) nine columns  71 - 79  of holes, rather than ten columns, as in FIG. 11.  
         [0045]    Hole  78 A in FIG. 12 is considered a member of column  79 . Hole  67 A in FIG. 11 is considered a member of column  67 .  
         [0046]    The populations of these row and columns are indicated by Tables 1 and 2, below.  
                             TABLE 1                           PRIOR ART BLADE (FIG. 11)                COLUMN   NUMBER OF HOLES                       59   14           60   15           61    3           62   12           63   12           64    2           65   19           66   20           67   20           68   19                      
 
         [0047]    [0047]                             TABLE 2                           INVENTION (FIG. 12)                COLUMN   NUMBER OF HOLES                       71    9           72    9           73    3           74   11           75   10           76   25           77   15           78   15           79   21                        
         [0048]    Under the invention of FIG. 12, the diameters of the holes are given in Table 3. All holes in a given column are of the same diameter, with the exception of column  71 , which contains two groups of holes. The holes in each group are the same diameter. The holes in row  70  are all of the same diameter, which is 0.012 inches.  
                             TABLE 3                           INVENTION (FIG. 12)                COLUMN   HOLE DIAMETER                       71   lower 3 holes - 16 mils           71   upper 6 holes - 17 mils           72   17 mils           73   17           74   15           75   15           76   15           77   15           78   15           79   13                      
 
         [0049]    Several similarities and differences between these hole patterns are the following.  
         [0050]    One, the row  48  in FIG. 11 contains ten holes. Row  70  in FIG. 12 contains 8 holes. One definition of row is a discrete chain of holes at the topmost position, that is, nearest the blade tip, on the pressure side of the blade, excluding any holes such as  78 A which are member of the leading edge columns  77 - 79 .  
         [0051]    Two, the column  64  of two holes in FIG. 11 has been deleted in FIG. 12.  
         [0052]    There, the column  65  in FIG. 11 has been replaced by a column  76  of twenty-five staggered holes. The stagger was imposed to attain a sufficiently large total area of holes, to attain a large airflow, while retaining high structural strength. That is, if the holes were placed in a single column, the distance separating adjacent holes would be small, and thus the material spanning that distance would be weak.  
         [0053]    As a specific example of relative distances, holes A, B, C, and D are labeled in FIG. 14. Vertical distances AB, BC, and CD are substantially equal, within 5 percent. The term AB refers to the distance between holes (A, B), and this convention applies to other pairs, such as BC and AC. Stagger distances AC and BD are substantially equal, within 5 percent. These relationships of vertical distance and stagger distance apply to all holes in column  76 .  
         [0054]    From another perspective, column  76  is divided into two sub-columns, spaced 20 mils, or 0.020 inch, apart, with the horizontal distance, or projection, between A and B representing the spacing.  
         [0055]    Four, the holes in trailing column  59  in FIG. 11 are uniformly spaced. That is, the distances between neighboring holes are identical. Trailing column  59  is that closest to the trailing edge. However, in FIG. 12, the holes in trailing column  71  are not uniformly spaced. They are arranged in two groups  85  and  89 .  
         [0056]    The distance  91  between group  85  and  89  is greater than the spacing between neighboring holes in either group  85  or  89 . That is, distance  91  is greater than the hole-to-hole spacing in group  89 , and is greater than the hole-to-hole spacing in group  85 .  
         [0057]    Five, FIG. 13 repeats the blade  3  of FIG. 11, and shows parallel lines, which divide the height  125  of the trailing edge  137  into ten equal parts, or stations. Each station represents ten percent of the blade height  125 . In the prior art blade  3 , cooling holes in both the aft-most two columns  59  and  60  are present below the 20-percent station, labeled  135 .  
         [0058]    Under the invention-blade of FIG. 14, no holes are present in the columns  72 , below the 30-percent station  145 , labeled 30%. Further, no holes are present in the last column  71  between the 30 percent station and the 50 percent station.  
         [0059]    The Inventors point out that the blade of FIG. 14 is drawn to actual scale. In the actual blade, overall height, from the very bottom of the root to the tip, is 4.2 inches.  
         [0060]    Six, the total number of holes in columns  59 - 68  in FIG. 11 is  136 . The total number of holes in columns  71 - 79  in FIG. 12 is  116 , or a reduction to 85 percent of the previous number. Equivalently, the reduction is by 15 percent.  
         [0061]    While the holes in question are distributed among different numbers of columns (10 columns  59 - 68  in FIG. 11 and 9 columns  71 - 79  in FIG. 12), both these sets of columns are located aft of corresponding reference points, such as point marked X in FIG. 14, and labeled  81 . To locate point  81 , for example, on the blades of FIGS. 11 and 12, one would find the point on one blade which is forward of all columns, and then locate the corresponding point on the other blade by measurement.  
         [0062]    Last column  71  in FIG. 14 should not be confused with the column of holes  87  in the trailing edge.  
         [0063]    Some of the columns  71 - 79  in FIG. 12 are supplied by a separate internal passage (not shown), of the type shown in FIG. 2, although the internal passages in FIG. 12 can be connected to each other by manifolds. Thus, holes in a column such as column  75  in FIG. 12 need not be exactly aligned in a straight line, and, in blades having twist, probably will not be.  
         [0064]    Therefore, the configuration shown in FIG. 12, together with the stated twist and lean, reduce the temperature differential 44 to 68 percent of the parameter T identified above, when measured in degrees F, as opposed to absolute temperature. Stated another way, temperature T is reduced by 32 percent. This reduction, and the structural modifications described above, reduce the bowing illustrated in FIG. 4.  
         [0065]    In another form of the invention, existing turbine blades on an existing gas turbine engine are replaced with blades modified according to the invention. Total cooling flow through the replacement blades remains the same as in the replaced blades. Cooling flow is measured either in pounds of air per second, or percentage of compressor flow. FIG. 15 illustrates one context in which this replacement occurs.  
         [0066]    The turbine blades in question are located in dashed circle  80 . Hot gases  83  from combustor  85  are ducted onto these turbine blades. The turbine inlet temperature, at point  88 , lies in the range of 2,500 degrees F. As stated above, under these conditions, the temperature differential  44  in FIG. 12 is reduced to 68 percent of the corresponding differential in FIG. 11. This differential is measured under full power, hot day conditions.  
         [0067]    A generalized procedure for attaining a similar differential, for a generalized blade, will now be given.  
         [0068]    [0068]FIG. 16 illustrate a flow chart. One overall goal is to first eliminate one cooling hole in, for example, the column  101  of ten holes in FIG. 17, thereby leaving nine holes. Next, the diameters of the nine holes is computed which will give the equivalent flow as in column  101 . Then, the nine holes are distributed over the ten positions of column  101 , as in FIG. 19. A parameter of interest is computed for each distribution in FIG. 19, such as average blade temperature. After all ten distributions have been computed, the distribution providing the best value of the parameter is selected. The process is repeated for the other columns in FIG. 17.  
         [0069]    Explaining this in greater detail, it is first assumed, for simplicity, that the blade in question contains four columns  101 - 104  of holes, as shown in FIG. 17. This number four is not critical, because the procedure outlined applies to any number of columns.  
         [0070]    In block  110  of FIG. 16, one column is selected, such as column  101 . The column contains ten holes, with 10 corresponding to N in block  110 .  
         [0071]    In block  115  in FIG. 16, the required diameter needed for (N−1) holes to deliver the same airflow as N holes is computed. That is, one hole is eliminated, and then the diameter of the remaining holes, of equal diameter, is computed which will give the same airflow.  
         [0072]    Block  120  indicates the beginning of a loop  125  in which a parameter of interest, such as average blade temperature, is computed for different configurations of the nine holes in question. A configuration of the nine holes is selected, and then temperature is computed.  
         [0073]    In block  120 , a dummy variable X is set to unity. In block  130 , a blank, or absence of a hole, is set to the Xth position. FIG. 18 illustrates the blank: with dummy variable X set to 1, iteration 1 is occurring, and the blank in FIG. 18 is set to position number 1, as indicated in the column for iteration 1.  
         [0074]    Block  135  in FIG. 16 indicates that a computer simulation is run. Block  140  indicates that a parameter of interest, computed in the simulation, is stored for that simulation. Average blade temperature is indicated as that parameter. However, the temperature differential  44  of FIG. 12 can be selected as the parameter of interest, as can other temperatures of the blade, or other combinations of parameters.  
         [0075]    Thus, at this time, a computation has been made for temperature, with (1) column  101  in FIG. 17 being replaced by the column labeled iteration 1 in FIG. 18 and (2) columns  102 ,  103 , and  104  in FIG. 17 being unchanged.  
         [0076]    In block  145 , the dummy variable X is incremented. In decision block  150 , inquiry is made as to whether X equals (N+1). If not, indicating that a blank has not been placed at all hole positions, the NO branch is taken, and the process returns to block  130 .  
         [0077]    Repeated excursions along the NO branch, to thereby repeatedly cause executions of loop  125 , cause successive simulations to be undertaken, with the blank, or absent hole, to be successively positioned as indicated in FIG. 19.  
         [0078]    If, in decision block in FIG. 16, it is determined that X does equal (N+1), that fact indicates that a blank has been placed at all hole positions. The YES branch is taken, and the logic reaches block.  
         [0079]    In that block, the iteration providing the lowest value of the parameter of interest is identified. Thus, the position of the hole providing the lowest value of the parameter of interest is identified.  
         [0080]    Block  160  in FIG. 16 indicates that the process is repeated. The process can be repeated for all columns of holes in FIG. 17. Numerous options arise at this point. For example, the preceding process can be repeated for each column  102 ,  103 , and  104  in FIG. 17. In each repetition, the other columns are left in their original state, containing ten holes. Then, when the optimal configuration in each column is found, those four optimals are combined into four new columns, of nine holes each.  
         [0081]    As another example, when an optimal configuration is found for a column, that column can be replaced by the optimal, and the replaced column is used in the computations for finding the optimals in other columns.  
         [0082]    In addition, elimination of a single hole in each computation was discussed above. In another approach, two, or more, holes can be eliminated, and the remaining holes distributed over the original positions.  
         [0083]    In the general case, a matrix of all possible positions for holes is generated. Different combinations of column-configurations are tested, and the optimal configuration is selected.  
         [0084]    In one mode of operation, the overall hole pattern, or that of an individual column, in FIG. 12 is selected. That is, the pattern used by the invention is selected as a starting point. Then modifications of that pattern are made, and the temperature behavior is examined.  
         [0085]    The temperatures described herein are measured under conditions of full power applied on a hot day, as those terms are defined in the gas turbine aircraft industry.  
         [0086]    One result provided by the invention is that the trailing edge  90  in FIG. 14 is constrained to lie along a radial line of the engine, both during operation, and in static, non-operational conditions.  
         [0087]    Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.