Abstract:
A method of clocking a turbine is disclosed in which the leading edge of clocked downstream airfoils are bathed by either a low total pressure wake, or a cooled low total temperature wake, or both, by reshaping at least the leading edge of an airfoil along the airfoils&#39; span or radial distance. The improvement is due to the fact that gas turbine wakes tend to be non-linear, such that a straight clocked downstream airfoil will receive a benefit of low total temperature or pressure over a portion of its span, while a restacked airfoil receives a benefit over a greater portion of the airfoil span from turbine hub to casing.

Description:
[0001]    The present invention relates to turbines, and more particularly, to a method of clocking a turbine by reshaping the turbine&#39;s downstream airfoils. 
       BACKGROUND OF THE INVENTION 
       [0002]    The performance of gas turbines can be affected by thermal and pressure gradients. One major source of thermal gradients is the large circumferential and radial temperature non-uniformities (i.e., hot streaks and cooling wakes) in the flow exiting a turbine combustor. Another source of non-uniformity is wakes from upstream airfoils of the same frame of reference. It has been found that controlling the relative circumferential positions of gas turbine blades, known as clocking or indexing, can increase the efficiency of turbine stages and mitigate the effects of combustor hot streaks and upstream airfoil wakes. Thus, clocking of turbine airfoils can provide significant thermal and other performance benefits. 
         [0003]    In practice, the clocking of turbine airfoils is essentially a procedure of aligning airfoils of like count and reference frame (i.e., rotor to rotor and stator to stator) without any consideration of the optimal airfoil and wake shapes to get the best possible clocking design. 
         [0004]    For airfoils of like count, the relative position of a downstream stator to the wake emanating from an upstream stator can lead to significant swings in turbine efficiency and airfoil, platform and casing temperatures. The same applies to subsequent rotor stages. 
         [0005]    An analysis of an upstream stage, for example Stage 1, will produce a time-averaged inlet flow field to the downstream stage. This flow field will contain the upstream stator (or rotor) wake signature for stator-to-stator (or rotor-to-rotor) clocking. Design tools, such as Computational Fluid Dynamics (2D, 3D, steady, unsteady) and 2D streamtube analysis, can be used to reshape or restack the downstream to optimize the clocking for both thermal and aerodynamic performance. 
         [0006]    For highly non-linear wakes, as one would see in a low aspect ratio stage 1 of a high pressure turbine (“HPT”), it would be quite obvious that a downstream airfoil has been reshaped to make it more optimized for clocking. However, for higher aspect ratio stages, such as a low pressure turbine (“LPT”), the wakes are straighter over a larger percentage of the span. 
         [0007]    For stators of like count, the relative position of a downstream stator to the wake emanating from an upstream stator can lead to significant swings in turbine efficiency and hot gas path (“HGP”) surface temperatures. The same applies to subsequent rotor stages. The improvement is due to the fact that gas turbine wakes tend to be non-linear. A straight downstream airfoil will receive a benefit (i.e., low total temperature and pressure) over a portion of its span. Reshaping or stacking of the airfoil gives potential to a benefit over a greater portion of the span. 
         [0008]    It is nearly impossible to completely straighten wakes, particularly for low aspect ratio HPT stages, thus reshaping the downstream airfoil to optimize the thermal and performance benefit has greater potential in many applications. The present invention shows that by reshaping the leading edge of the downstream airfoil the potential hub to span benefit can be increased. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    In an exemplary embodiment of the invention, a method of clocking a turbine, in which the turbine is comprised of a plurality of airfoils, the turbine airfoils being comprised of at least a first, upstream row of airfoils in a first frame of reference, a second row of airfoils in the first frame of reference, which are downstream from the first row of airfoils, and a third row of airfoils in a second frame of reference, which are intermediate the first and second rows of airfoils, comprises the steps of changing a circumferential position of the row of downstream airfoils relative to a circumferential position of the row of upstream airfoils so that the downstream airfoils are more within the upstream airfoils&#39; wakes than before the circumferential position of the row of downstream airfoils was changed, for each upstream airfoil&#39;s wake, locating at least one portion of the wake corresponding to a lowest temperature in the wake, a lowest pressure in the wake, or a lowest temperature and pressure in the wake, for each upstream airfoil&#39;s wake, reshaping the downstream airfoil positioned within the wake so that more of at least the downstream airfoil&#39;s leading edge is within the lowest temperature portion of the wake, the lowest pressure portion of the wake or the lowest temperature and pressure portion of the wake than before the downstream airfoil was reshaped. 
         [0010]    In another exemplary embodiment of the invention, a method of clocking a turbine, in which the turbine is comprised of a plurality of airfoils, the turbine airfoils being comprised of at least a first, upstream row of airfoils in a first frame of reference, a second row of airfoils in the first frame of reference, which are downstream from the first row of airfoils, and a third row of airfoils in a second frame of reference, which are intermediate the first and second rows of airfoils, each downstream airfoil being formed from a plurality of design sections which are stacked relative to one another, comprising the steps of changing a circumferential position of the row of downstream airfoils relative to a circumferential position of the row of upstream airfoils so that the downstream airfoils are more within the upstream airfoils&#39; wakes than before the circumferential position of the row of downstream airfoils was changed, for each upstream airfoil&#39;s wake, locating portions of the wake corresponding to a lowest temperature in the wake, a lowest pressure in the wake, or a lowest temperature and pressure in the wake along the downstream airfoil&#39;s span or radial height, for each upstream airfoil&#39;s wake, restacking the plurality of design sections forming the downstream airfoil positioned within the wake so that more of the downstream airfoil&#39;s leading edge and plurality of design sections are within the lowest temperature portions of the wake, the lowest pressure portions of the wake or the lowest temperature and pressure portions of the wake than before the downstream airfoil was reshaped. 
         [0011]    In a further exemplary embodiment of the invention, an clocked turbine comprises a plurality of airfoils, the turbine airfoils being comprised of at least a first, upstream row of airfoils in a first frame of reference, a second row of airfoils in the first frame of reference, which are downstream from the first row of airfoils, each downstream airfoil being formed from a plurality of design sections which are stacked relative to one another, and a third row of airfoils in a second frame of reference, which are intermediate the first and second rows of airfoils, a circumferential position of the row of downstream airfoils having been changed relative to a circumferential position of the row of upstream airfoils so that the downstream airfoils are more within the upstream airfoils&#39; wakes than before the circumferential position of the row of downstream airfoils was changed, each upstream airfoil, in operation, producing a wake including at least one portion corresponding to a lowest temperature in the wake, a lowest pressure in the wake, or a lowest temperature and pressure in the wake, each downstream airfoil within an upstream airfoil&#39;s wake being restacked so that the plurality of design sections forming the downstream airfoil cause the downstream airfoil to be positioned within the wake so that more of at least the downstream airfoil&#39;s leading edge is within the at least one lowest temperature portion, lowest pressure portion or lowest temperature and pressure portion of the wake than before the downstream airfoil was reshaped. 
         [0012]    The present invention allows a benefit (i.e., low total temperature and pressure) to be realized by allowing the leading edge or the entire outer surface of downstream airfoils to be bathed by either a low total pressure wake or a cooled low total temperature wake or both. By reshaping the leading edge of an airfoil, or the entire airfoil along its span or radial distance, the potential benefit from the leading edge or the entire outer surface of the airfoil to be bathed in either a low total pressure wake or a cooled low total temperature wake or both, can be increased. The improvement is due to the fact that gas turbine wakes tend to be non-linear. A straight downstream airfoil will receive a benefit (i.e., low total temperature and pressure) over a portion of its span. Reshaping or stacking of the airfoil gives potential to a benefit over a greater portion of the airfoil span from turbine hub to casing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a simplified schematic diagram of a multi-stage gas turbine system. 
           [0014]      FIG. 2  is a two dimensional (2D) cross-sectional view of airfoil clocking in a turbo machine, such as a turbine. 
           [0015]      FIG. 3  is a partial isometric view of a turbine airfoil showing design sections of the airfoil capable of being restacked relative to one another. 
           [0016]      FIG. 4  is a partial isometric view of a typical turbine airfoil, such as a stator or rotor blade. 
           [0017]      FIG. 5  is a partial isometric view of the turbine airfoil of  FIG. 4  with the design sections of the airfoil restacked. 
           [0018]      FIG. 6  is a two-dimensional (2D) cross sectional view of a downstream, clocked turbine airfoil before restacking. 
           [0019]      FIG. 7  is a two-dimensional (2D) cross sectional view of the downstream, clocked turbine airfoil of  FIG. 6  after restacking. 
           [0020]      FIG. 8  is a simplified isometric view of the downstream, clocked turbine airfoil of  FIG. 6  before restacking and the wake of an upstream airfoil. This wake can either be the thermal wake (total temperature) or the momentum wake (total pressure). 
           [0021]      FIG. 9  is a simplified isometric view of the downstream, clocked turbine airfoil of  FIG. 7  after restacking and the wake of an upstream airfoil, wherein the clocked airfoil is reshaped so that the wake, which can be either the thermal wake (total temperature) or the momentum wake (total pressure), is bathing the reshaped airfoil&#39;s leading edge. 
           [0022]      FIG. 10  is a graph depicting Total Pressure versus Circumferential Position at Downstream Airfoil Leading Edge at a Generic Span (i.e., Momentum Wake). 
           [0023]      FIG. 11  is a graph depicting Total Temperature versus Circumferential Position at Downstream Airfoil Leading Edge at a Generic Span (i.e., Thermal Wake). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 1  is a simplified schematic diagram of a multi-stage gas turbine system  10 . The gas turbine system  10  shown in  FIG. 1  includes a compressor  12 , which compresses incoming air  11  to a high pressure, a combustor  14 , which burns fuel  13  so as to produce a high-pressure, high-velocity hot gas  17 , and a turbine  16 , which extracts energy from the high-pressure, high-velocity hot gas  17  entering the turbine  16  from the combustor  14  using turbine blades (not shown in  FIG. 1 ) that are rotated by the hot gas  17  passing through them. As the turbine  16  is rotated, a shaft  18  connected to the turbine  16  is caused to be rotated as well. As shown in  FIG. 1 , turbine  16  is a multi-stage turbine with the first and second stages shown and designated as  16 A and  16 B, respectively. To maximize turbine efficiency, the hot gas  17 / 17 A is expanded (and thereby reduced in pressure) as it flows from the first stage  16 A of turbine  16  to the second stage  16 B of turbine  16 , generating work in the different stages of turbine  16  as the hot gas  17  passes through. In a gas turbine engine, a single turbine section is made up of either a disk that holds many turbine stator blades or a rotating hub that holds many turbine rotor blades. The turbine blades are responsible for extracting energy from the high temperature, high pressure gas produced by the combustor that flows through the turbine blades. Eventually, exhaust gas  19  exits the last stage of turbine  16 , which is shown in  FIG. 1  as the second stage  16 B. 
         [0025]      FIG. 2  is a two dimensional (2D) cross-sectional view  20  of “airfoil clocking” in a turbo-machine, such as a turbine  16 . Turbo-machinery airfoil clocking involves three blade rows. Two blade rows are in the same frame of reference; that is, two blade rows are both either stators or rotors. One of the two blade rows is an upstream airfoil. The other of the two blade rows is a downstream airfoil. The third blade row, which is intermediate the two blade rows, rotates relative to the other two blade rows. The downstream airfoil is “clocked”, i.e., circumferentially positioned, relative to the wake of the upstream airfoil. 
         [0026]    The clocked airfoil count needs to be an integral multiple of the upstream blade row, such that typically a ratio of 1:1 would be used. But, it should be noted that other ratios, such as 2:1, etc., could also be used, because they could see some benefit, as well, to the clocking of downstream airfoils relative to upstream airfoils. 
         [0027]      FIG. 2  shows a series of turbine rotors and stators, which include an upstream stator  24 , an upstream rotor  25 , a downstream, clocked stator  26  and a downstream, clocked rotor  27 . The upstream rotor  25  and the downstream, clocked rotor  27  are each rotating in a direction indicated by an arrow  21 . The upstream stator  24  produces a wake  22 . Similarly, the upstream rotor  25  produces a wake  23 . The downstream airfoil, i.e., downstream stator  26  is clocked relative to the upstream stator  24 . The downstream airfoil, i.e., rotor  27 , is clocked relative to the upstream rotor  25 . 
         [0028]      FIG. 3  is a partial perspective, elevational view of a three dimensional (3D) turbine airfoil  30  showing design sections  31 - 35  of the airfoil  30  capable of being restacked relative to one another. A three-dimensional airfoil, such as airfoil  30 , is created by “stacking” design sections relative to one another, both circumferentially and/or axially. Airfoil  30  includes, as shown in  FIG. 3 , an outer diameter design section  31 , an 80% radial span design section  32 , a 50% radial span design section  33 , a 20% radial span design section  34 , and an inner diameter or hub design section  35 . The relative stacking of these design sections can produce different shaped airfoils. 
         [0029]      FIG. 4  is a partial perspective, elevational view of one example of a turbine airfoil  40 A, such as a rotor or stator blade, in which the design sections have not been restacked. Turbine airfoil  40 A includes a leading edge  42 A. In contrast,  FIG. 5  is a partial perspective view of one example of a turbine airfoil  40 B, which is the turbine airfoil  40 A in which the design sections have been restacked. Turbine airfoil  40 B includes a reshaped leading edge  42 B. 
         [0030]      FIG. 6  is a two-dimensional (2D) cross sectional view of a downstream, clocked turbine airfoil  40 A before restacking, while  FIG. 7  is a two-dimensional (2D) cross sectional view of the downstream, clocked turbine airfoil  40 B after restacking.  FIG. 6  shows the downstream, clocked turbine airfoil  40 A before restacking as including an 80% radial span design section  54 , a 50% radial span design section  55  and a 20% radial span design section  56 .  FIG. 7  shows the downstream, clocked turbine airfoil  40 B after restacking as including an 80% radial span design section  57 , the 50% radial span design section  55  and a 20% radial span design section  58 . 
         [0031]    The 80% radial span design section  54  is shown in  FIG. 6  as being near a portion  51  of an upstream airfoil wake  50 . Likewise, the 50% radial span design section  55  is shown in  FIG. 6  as being near a portion  52  of the upstream airfoil wake  50 . Finally, the 20% radial span design section  56  is shown in  FIG. 6  as being near a portion  53  of the upstream airfoil wake  50 . 
         [0032]      FIGS. 5A and 5B  are intended to depict differences that occur in airfoil  40 A when it is restacked as airfoil  40 B. In essence,  FIGS. 5A and 5B  show tangential restacking of the 80% radial span design section  54  and the 20% radial span design section  56  of the downstream airfoil  40 A, although it should be noted that airfoil  40 A could be restacked both circumferentially and/or axially. The 80% radial span design section and the 20% radial span design section of airfoil  40 A are shown in  FIG. 7  as being shifted, in airfoil  40 B, to be placed in line of the wake portions  51  and  53 , respectively. Here, the restacked 80% radial span design section and the restacked 20% radial span design section are designated with the references numeral  57  and  58 , respectively, to show the outer and inner design sections as being shifted to be placed in line with the upstream airfoil wake portions  51  and  53 . 
         [0033]      FIGS. 5A and 5B  also show the 50% radial span design section  55  of the downstream airfoil  40 A as not being restacked because the leading edge of section  55  already is already in line with the upstream airfoil wake portion  52 . The result of what is depicted in  FIGS. 5A and 5B  generally corresponds to the airfoils  40 A and  40 B, respectively, depicted in  FIGS. 4A and 4B . 
         [0034]      FIG. 8  is a simplified isometric of a downstream, clocked turbine airfoil  40 A like the airfoil  40 A of  FIG. 4 , before restacking, showing a wake  50  of an upstream airfoil bathing the downstream airfoil  50 A in its best clocking position. This wake  50  can either be the thermal wake (total temperature) or the momentum wake (total pressure). 
         [0035]    For the restacked stacked airfoil  40 B shown in  FIG. 9  in the best clocking position, the leading edge  42 B of the airfoil  40 B is bathed more by the wake  50  due to the upstream airfoil along the entire radial height of airfoil  40 B than is the leading edge  42 A of the airfoil  40 A before restacking. 
         [0036]      FIG. 10  shows the total pressure as a function of circumferential position at a selected one of the leading edge sections  54 ,  55  or  56  of the downstream airfoil  40 A at a specific radial height or span corresponding to the selected one ( 54 ,  55  or  56 ) of the leading edge portions. The wake due to the upstream airfoil is represented by the low total pressure region. 
         [0037]      FIG. 11  shows the total temperature as a function of circumferential position at one of the leading edge sections  57 ,  55  or  58  of the downstream airfoil  40 E at a specific radial height or span corresponding to the selected one ( 57 ,  55  or  58 ) of the leading edge portions. The thermal wake due to the upstream airfoil is represented by the low total temperature region. 
         [0038]    The criteria used to decide how to restack downstream airfoils would include an area of low total pressure or low total temperature in the wake of the upstream airfoil corresponding to a given downstream airfoil. A one-dimensional plot of pressure or temperature versus circumferential position (theta) along a given downstream airfoil&#39;s span or radial height would result in a series of low spots (deficits) or valleys corresponding to several portions of the wake of the upstream airfoil at the several leading edge sections of the airfoil. These wake “valleys” would have some width. Each valley width would correspond, for example, to the left to right distance of one of the portions of an upstream airfoil wake, such as the portions  51 ,  52  or  53  of the upstream airfoil wake  50 . Ideally, the restacking of the downstream airfoil leading edge portions, such as the leading edge sections  57 ,  55  or  58  of the downstream airfoil  40 B, would correspond to the bottom spots (i.e., the lowest temperatures or the lowest pressures), recognizing that there would be some margin of adjustment in the restacking of the downstream airfoil. The result would be a restacked airfoil, like airfoil  40 B, that was aligned using a criteria of the lowest temperature or the lowest pressure at each of the leading edge sections of the airfoil, plus some percentage of the pitch, that is, the circumferential distance between two airfoils. 
         [0039]    An example of how this can be done as shown in  FIG. 10 , which shows the total pressure as a function of circumferential position at one of the leading edge sections along the radial height or span of a downstream airfoil. The location of minimum total pressure is the momentum wake. To restack the downstream airfoil, the design section at this point of the radial height or span would be shifted to be aligned with the location of the minimum total pressure. This could also apply for the thermal wake by evaluating total temperature instead of total pressure. This is shown in  FIG. 7B . It should be noted that, for a given airfoil, it is possible that there could be several graphs like those of  FIG. 7A  or  7 B corresponding to the several leading edge sections of the airfoil. 
         [0040]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.