Abstract:
The present invention relates to an airfoil for a gas turbine, including an improved turbulator arrangement formed on an inner cooling channel of the airfoil. According to preferred embodiments of the invention, in order to ensure a constant angle of the cooling flow inside the channel relative to each turbulator, the angle formed between the turbulator and the vertical axis is advantageously adapted, in the curved area, for every single turbulator. Furthermore, the same principle may be applied to all the cooling channels present within the airfoil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European application 14167966.2 filed May 12, 2014, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an airfoil for a gas turbine, comprising an improved turbulators arrangement formed in an inner cooling channel of the airfoil. 
       BACKGROUND 
       [0003]    As well known, gas turbines or in general machineries configured to extract useful work by means of elaborating the flow of hot gases, include components, such as airfoils, deployed to such function. Airfoils, during operation of the machinery, need to be cooled in order to maintain the temperature of their parts within acceptable limits. For this reason, airfoils generally comprise longitudinally extending inner channels, or ducts, which are configured to receive cooling air. Usually, the airfoil comprises a plurality of inner channels connected to each other in a serpentine-type configuration. In particular, the cooling air enters in the cooling circuit in a first channel which is located in the proximity of the leading edge, which is the portion of the airfoil with high external heat load, and exits the circuit through a cooling channel which is in turn located in the proximity of the trailing edge. 
         [0004]    Inside the channels, turbulence promoters or turbulators are means commonly deployed in order to generate turbulence near the inner walls of the cooling channels, thus enhancing the thermal exchange between the cooling flow and the airfoil and therefore improving the cooling performance. 
         [0005]    Turbulators are generally provided in the form of rib-shaped elements disposed on the inner walls of the cooling channels. 
         [0006]    One of the parameter which affects the heat exchange between the airfoil and the cooling fluid is the angle formed between the direction of the cooling air flow, which follows the longitudinal axis of the airfoil, and the turbulator. For airfoils having a straight longitudinal axis, the ribs maintain a constant inclination along the vertical axis and such angle is constant throughout the cooling channel. 
         [0007]    However, in order to improve the turbine efficiency, the airfoil may be three-dimensional shaped meaning that its longitudinal axis may be curved and deviating from the vertical axis, which is the component design axis and it is generally referred to the placement of the gas turbine. Such curvature is usually mostly pronounced in the proximity of the leading edge portion of the airfoil. 
         [0008]    As a consequence, arranging the ribs along the inner wall of the cooling channel with a fixed inclination in respect to a vertical axis results in a poor cooling performance, since the areas where the longitudinal axis is curved are characterised by a reduction of the angles formed by the ribs and the direction of the cooling air flow. As indicated above, such angle is a driving parameter for the efficiency of the cooling system. 
         [0009]    In addition, still according to known airfoils, the distance between subsequent ribs, or rib-to-rib pitch, which is another driving parameter for the heat transfer, is kept constant along the vertical axis. Similarly, such known geometry leads, for airfoils having 3D curved shape, to a change in the rib-to-rib pitch, commonly known as P. In particular, this leads to even higher pitches and, as for the effective rib angle, further decreases the heat transfer in those affected regions. 
         [0010]    Therefore, the standard design approach provides inadequate cooling particularly for airfoils having curved profile. Furthermore, the standard approach leads to inhomogeneous cooling along a curved duct, since in the area with the most curved shape the heat transfer is lower compared to the straight portion. 
         [0011]    A possible counter measurement for a standard design would be to increase the cooling air consumption. However, the geometry of the turbulators deployed inside the cooling ducts would remain the same and therefore this would result in less cooling performance in the 3D shapes area compared to the straight portion and the inhomogeneous cooling along the duct will still be present. In addition, increasing the cooling air flow, such to maintain the metal temperatures in the curved portions of the airfoils within predetermined valued, would inevitably reduce turbine efficiency and power. 
       SUMMARY 
       [0012]    The object of the present invention is to solve the aforementioned technical problems by providing an innovative airfoil as substantially defined in independent claim  1 . 
         [0013]    Preferred embodiments are defined in correspondent dependent claims. 
         [0000]    According to preferred embodiments of the invention, which will be described in the following detailed description only for exemplary and non-limiting purposes, in order to ensure a constant angle of the cooling flow inside the channel relative to each turbulator, the angle formed between the turbulator and the vertical axis is advantageously adapted, in the curved area, for every single turbulator. Furthermore, the same principle may be applied to all the cooling channels present within the airfoil.
 
Moreover, according to preferred embodiments of the invention, the maximum variation of relative inclination between two subsequent turbulators should not exceed 3°, because otherwise a higher variation between the pitch at the leading edge and trailing edge of the rib would result in lower cooling performance.
 
Lastly, the distance or pitch, between subsequent turbulators, is adapted such to maintain the pitch constant along the cooling flow path whilst varying along the vertical axis.
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a perspective view of an airfoil for a gas turbine according to the invention; 
           [0016]      FIG. 2  is a section view of the airfoil of  FIG. 1 ; 
           [0017]      FIGS. 3 and 4  schematically show a section view of an airfoil having a turbulator arrangement according to the present invention (on the left) versus an arrangement according to the prior art (on the right); 
           [0018]      FIG. 5  schematically highlights a preferred embodiment of the airfoil comprising a turbulator arrangement according to the present invention; and 
           [0019]      FIG. 6  shows a lateral section view of the airfoil according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    With reference to  FIG. 1 , it is showed an external view of an airfoil according to the present invention for a gas turbine, generally denoted with the numeral reference  1 . The airfoil  1  comprises an internal cooling circuit (not visible in  FIG. 1 ) having an inlet  11  for blowing inside its structure a cooling fluid, which is usually air bled from the engine&#39;s compressor, and a pressure side bleed  12  as an outlet at the end of the cooling circuit. 
         [0021]    With reference to next  FIG. 2 , it is shown a cross section of the airfoil  1 . In particular, the cross section of  FIG. 2  reveals the presence of inner cooling channels  2 ,  21 , and  22  which are connected to each other in a serpentine-type configuration. Inner channel  2  is located in the proximity of the leading edge portion of the airfoil, while inner channel  22  is located in proximity of the trailing edge of the airfoil  1 . Inner channel  21  is a middle channel positioned between channel  2  and channel  22 . It will be appreciated that the airfoil may also comprise different kind of cooling circuits, which may comprise even only one cooling channel. 
         [0022]    From now on reference will be made to leading edge inner channel  2 , but it will be appreciated that same description will apply to every channel of the cooling circuit. As visible in the figure, cooling channel  2  defines a longitudinal axis a and comprises a first lower portion (which can be attached to a rotor body of the gas turbine, not depicted) and an upper portion. In particular, the lower portion is substantially straight and the longitudinal axis a along said portion is substantially aligned with a vertical axis z, generally associated to a vertical direction associated to a design axis of the gas turbine. Differently, the upper portion is twisted in its three-dimensional development and the longitudinal axis a in said portion significantly deviates from the vertical axis z, as clearly visible in the figure. In order to generate turbulence within the cooling channel and promote the heat exchange between the cooling flow and the airfoil, a plurality of longitudinally spaced turbulators are provided along the channel. As a non-limiting example, reference will be made to subsequent turbulators  51  and  52 , as indicated in the figure. In this preferred embodiment, turbulators are provided in the form of rib-shaped elements. However, other shapes may be considered. 
         [0023]    With reference now to the following  FIG. 3  left side, it is schematically shown a typical arrangement of rib-shaped elements, or ribs, along a cooling channel according to the prior art. In particular, the ribs are disposed with a constant inclination in respect to the vertical axis z. However, the upper portion of the channel is twisted and therefore the longitudinal axis a deviates from it. Since the cooling flow bled inside the channel follows the direction defined by the longitudinal axis a of the duct, such arrangement leads to having smaller tilt angles θ between the flow and the ribs within the upper bent portion of the channel, and bigger tilt angles A in the lower portion. Such tilt angle variation along the channel results in an inhomogeneous cooling performance along the duct. With reference to  FIG. 3  right-side, it is shown a rib arrangement along the cooling channel  2  according to the present invention. In particular, the inclination of each rib with respect to the vertical axis z is now adapted such that a tilt angle α formed between the rib and the longitudinal axis a (which is aligned with the direction of the cooling flow) is substantially constant along the inner cooling channel  2 . More preferably, angle α may vary along the duct within a range of +/−3° around a mean value. It has been evaluated that satisfying cooling performances can be achieved when the tilt angle α is selected within a range of 40°-70°, intended as a mean value if the angle varies within the above-mentioned range. More preferably, the angle is selected between a range 45°-60°. In a preferred embodiment, tilt angle α is substantially 60°. 
         [0024]    Making now reference to following  FIG. 4 , a further preferred embodiment of the present invention is schematically shown. On the left side is again depicted a rib arrangement according to the prior art. In particular, it may be appreciated that the ribs are arranged along the channel such that the distance, or pitch, between subsequent ribs is constant along the vertical axis z. On the right side it is shown a rib arrangement according to the present invention. According to the invention, advantageously, ribs are now equally spaced between each other in the direction of the longitudinal axis a, such that the pitch determined on the longitudinal axis a between subsequent ribs, indicated in the figure with the reference P, is constant along the cooling channel. 
         [0025]    With now reference to  FIG. 5 , preferably, the disposition of two subsequent ribs (in the example of the figure only ribs  51  and  52  are drawn) it is such not to have a large difference between a leading edge pitch, indicated in the figure with numerical reference  31 , and a trailing edge pitch, in turn indicated with numerical reference  32 . This is achieved maintaining the value of a second tilt angle β, which measures the variation of relative inclination between the two subsequent turbulators  51  and  52 , substantially equal or smaller than 3°. With this further preferred constraint, an optimum geometry of ribs arrangement can be achieved resulting in a best cooling performance of the airfoil during operation of the gas turbine. 
         [0026]    With now reference to the last  FIG. 6 , it is schematically shown a lateral section view of the cooling channel  2  of the airfoil according to the present invention. Cooling channel  2  is defined by a pressure side wall  41  and a suction side wall  42  opposed thereto. On the pressure side wall  41  ribs  51  and  52 , depicted as a way of example, are distanced by the pitch P as indicated in preceding  FIG. 4 . Similarly, facing wall  42  comprises in turn a sequence of ribs. In the figure ribs  51 ′ and  52 ′ are shown. In the preferred embodiment shown here as a non-limiting example, ribs distributed on the pressure side wall  41  are staggered relative to the ribs distributed on the opposed suction side wall  42 . Preferably, the distance between subsequent opposed ribs, for example 51 and 51′, is chosen equal to P/ 2 , which is half of the pitch. 
         [0000]    Still with reference to  FIG. 6 , the geometry of the ribs is further defined by parameters e and w, which are respectively the height and the width of the rib. In particular, a first parameter is defined as the ratio between the pitch and the height of the rib. The first parameter P/e is selected within the range 5-12. Preferably, the selected value of the first parameter P/e is 8.
 
Moreover, a second parameter is defined as the ratio between the width and the height of the rib. The second parameter w/e is selected between the range 0.8-1.2. Preferably, the selected value of the second parameter w/e is 1.0.
 
Lastly, the ribs preferably have, in lateral section, a trapezoidal shape, and define a draft angle γ which, according to preferred embodiments, is selected within the range of 2°-5° (in the figure the angle is increased in size only for clarity purposes).
 
         [0027]    Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering the application to be limited by these embodiments, but by the content of the following claims.