Patent Publication Number: US-6340284-B1

Title: Turbine blade with actively cooled shroud-band element

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of gas turbines, and more particularly to air-cooled turbine blades 
     BACKGROUND OF THE INVENTION 
     Modern gas turbines work at extremely high temperatures. This requires intensive cooling of the turbine blades. A particular difficulty is to reliably cool the exposed regions of the blades. One of these regions is the shroud band or the -shroud-band elements of the blade. German Patent No. DE 198 13 173 A1 or U.S. Pat. No. 5,785,496 disclose turbine blades of this general type. One possibility of cooling shroud-band elements has been described in the publication DE 198 13 173 A1 mentioned at the beginning. In this publication, it is proposed (see FIGS. 3 and 4 there) to cool the shroud-band elements by a number of parallel cooling bores, which extend from the (central) moving blade through the shroud-band element to the outer edge of the shroud-band element and open there into the exterior space. 
     However, this known solution has the following disadvantages: 
     Laterally abutting joints between two shroud-band elements of adjacent blades (as can be seen, for example, from FIG. 3 of U.S. Pat. No. 5,482,435) will at least partly close the orifices of the cooling bores. This hinders cooling-air discharge and distribution. The shroud-band element is overheated in operation, 
     The known shroud-band cooling, on account of the laterally arranged orifices, does not change the conditions of flow over the shroud band; that is to say that the pressure and temperature on the top side of the shroud band remain the same. This is also not changed by the fact that, as proposed in U.S. Pat. No. 5,460,486, certain cooling bores open on the underside of the shroud-band element. 
     The cooling effect is mainly based on the mixing temperature in the shroud-band surroundings, the mixing temperature being lowered by mixing of the discharging cooling air with the hot gas. No measures are taken in the cooling bores in order to intensify the heat transfer between the cooling air and the shroud-band element. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide a turbine blade having an air-cooled shroud-band element, in which turbine blade the abovementioned disadvantages are avoided in a simple manner and which is distinguished by effective cooling of the shroud-band element, in particular on the exposed top side of the shroud-band element. 
     This object is achieved by this invention. 
     The basic idea of the invention consists, on the one hand, in directing the cooling bores through the shroud-band element in such a way that considerable heat transfer between the shroud-band element and cooling air is ensured and, on the other hand, in making these bores open into the exterior space in such a way that the cooling air is reliably admitted to the exposed regions of the shroud band and these regions are additionally cooled. This is achieved by virtue of the fact that, starting from the cooling passage in the blade, the cooling bores, in the region of the shroud-band element, run from inside to outside essentially parallel to the direction of movement of the blade tip and in each case open upstream of the outer margin of the shroud-bank element into a surface recess open toward the exterior space. 
     In this case, in a first preferred embodiment of the invention, recesses, into which the cooling bores open laterally are sunk in the shroud-band element close to the outer margin from the top side. Due to the discharging cooling air mixing with the hot combustion gases which flow over the top side of the shroud-band element, the temperature in this region is effectively reduced and thus overheating of the shroud band is avoided. In this way, uniform cooling of the shroud-band element over the entire surface is achieved. In addition to effective cooling of the top side of the shroud band, this configuration also has the advantage of very simple manufacture. The discharge of the cooling air on the top side of the shroud-band element is especially effective if, in a preferred development, spaced-apart sealing ribs running parallel to one another are provided on the top side of the shroud-band element and form a cavity in interaction with the opposite casing wall of the gas turbine, and the cooling bores open into this cavity. The discharging cooling air leads to a pressure build-up in the cavity, the result of which is that the penetration of hot gases is reduced. 
     In another embodiment of the invention, the side edges of the shroud-band elements have recessed portions, into which the cooling bores open. In this case, the recessed portions of opposite shroud-band elements form a gap. During the discharge into the gap, the cooling air splits up into two partial flows. One part flows toward the top side and has the above mentioned effect on the above mentioned cavity between the space-apart sealing ribs. The other part flows toward the underside of the shroud bank and mixes there with the hot gases while setting a mixing temperature which reduces the thermal loading in this region. The ratio of the partial quantities flowing off upward and downward can be influenced by the gap geometry. 
     In an expedient addition to the invention, it is also proposed that means for improving the heat transfer between cooling air and shroud-band element be provided in the cooling bores. The means for improving the heat transfer at the bore walls may comprise roughness features, ribs and/or turbulence points. In a manner known per se, the bores may be produced by means of the so-called STEM drilling process. Cooling bores having improved heat-transfer properties can be produced in a simple and reliable manner especially by STEM drilling, which has been described, for example, in U.S. Pat. No. 5,306,401 in connection with the production of cooling holes in turbine blades. 
     Furthermore, improved utilization of the cooling air can be achieved if, in another preferred embodiment of the invention, a choke point for limiting the cooling-air mass flow is provided in each of the cooling bores, and the choke points are each arranged on the inlet side of the cooling bores. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is to be explained in more detail below with reference to exemplary embodiments and in connection with the drawing, in which: 
     FIG. 1 shows a plan view of a preferred embodiment of the turbine blade with cooling bores discharging toward the top side of the shroud band 
     FIG. 2 shows a modified form of the embodiment of FIG. 1 with diffuser-like cooling bores 
     FIG. 3 shows a side view of the second modified form of the embodiment of FIG. 1 with cooling bores of circular cross section. 
     FIG. 4 shows a side view of a third modified form of the embodiment of FIG. 1 with cooling bores of oval cross section 
     FIG. 5 shows a partial sectional view of a shroud-band element along the line  5 — 5  in FIG. 1 
     FIG. 6 shows a plan view of a fourth modified form of the embodiment of FIG. 1 having two shroud-band elements in with cooling bores discharging toward the side edge 
     FIG. 7 shows a partial sectional view of a fifth modified form of the shroud-band element along the line  7 — 7  in FIG. 6 
     FIG. 8 shows a partial sectional view of a sixth modified form of the embodiment as in FIG. 7 of a shroud-band element with cooling air discharge toward the underside of the shroud-band element 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of a turbine blade according to the invention is shown in plan view in FIG.  1 . The turbine blade  10  comprises the actual blade profile  23  and a shroud-band element  11 , which is arranged at the blade tip transversely to the blade profile  23  and, together with the shroud-band elements of the other blades (not shown), produces a continuous, mechanically stabilizing shroud band. The blade profile  23  is partly hollow in the interior, and passing through the blade: profile  23  are one or more cooling-air passages  18 , which direct cooling air from the blade root up to the blade tip. On its top side, the shroud-band element  11  has two sealing ribs  12  and  13 , which run in parallel in the direction of movement of the blade tip and, together with the opposite casing wall  20  of the gas turbine, form a cavity  21  connected to the surrounding environment by gaps. In the interior of the shroud-band element  11 , a plurality of cooling bores  17  run from the center outward between and essentially parallel to the ribs  12 ,  13 . On the inlet side, the cooling bores  17  are connected to the cooling-air passage  18  and are supplied with cooling air from the latter. As can be seen from FIG. 1, the cooling bores  17  do not extend quite up to the lateral end or margin of the shroud-band element  11  but open in each case from the side into an elongated recess  14  sunk in the shroud-band element  11  on the top side  22 . It goes without saying that, instead of a continuous recess  14 , each of the cooling bores  17  taken by itself may also be connected to a separate recess. Furthermore, it is also conceivable for the cooling bores  17  to run at a slight angle and to deviate from an arrangement in parallel with one another, if this is necessary in order to optimize the cooling over the entire surface of the shroud-band element  11 . 
     Furthermore, blow-out of the cooling air upward leads to an “inflation” of the cavity  21  between shroud-band and casing  20 . This leads to an increase in the pressure in the cavity and thus helps to reduce the penetrating mass flow of hot gas  24 . Furthermore, the mixing temperature in this region is of course also lowered, as a result of which the thermal loading of the shroud-band element  11  from the top side  22  is reduced. 
     The cooling bores  17  in the cooling arrangement shown are preferably produced by the so-called STEM drilling method, which is fully described in detail in U.S. Pat. No. 5,306,401. As a result, it is possible (by varying the feed) to provide the surface of the cooling bores  17  with roughness features, ribs or turbulence points. This leads to markedly more efficient cooling, since the shape of the cooling bore can be optimized. Furthermore, it is advantageous to provide the cooling bores  17  with a choke point  19  in each case, preferably on the inlet side, i.e. in the region of the cooling-air supply at the profile  23 . As a result, it becomes possible to deliberately limit the cooling-air mass flow and obtain markedly more efficient cooling. The embodiment according to FIG. 2 differs from that according to FIG. 1 in that the cooling bores  17 , starting from the choke point  19 , which is arranged in each case on the inlet side of each cooling bore, are designed as a diffuser or in a diffuser-like manner. In a further embodiment—shown in FIG.  4 —the cooling bores have an oval configuration. Like the surface provided with inner roughness features or like the diffuser-like widening, this increases the effective surface available for the heat transfer. In addition or alternatively, the cooling bores  17  may have configurations different from those described above. Such configurations may be, for example, regularly or irregularly maintained recesses or undulations. 
     In a further favorable refinement of the invention according to FIGS. 6 and 7, the cooling bores  17  discharge at the side edge  25  of the shroud-band element  11 . To avoid the disadvantages of the prior art, however, the side edges  25  of, the shroud-band elements  11  are designed such that adjacent elements  11  are only in contact zonally, whereas the region of the discharging cooling bores is set back in a recess  15 . Between the adjacent elements, the opposite recesses  15  form gaps  26 , into which the cooling air enters. This embodiment reliably prevents closure of the orifices by adjacent shroud-band elements. It ensures that the cooling air can always pass through the cooling bores  17 , even if two adjacent shroud-band elements  11  are in mechanical contact. The cooling air entering the gap  26  from both adjacent elements  11  splits up into two partial flows. One partial flow flows upward and leads to inflation of the cavity  21  above the shroud band, whereas the other partial flow passes to the underside of the shroud band and mixes there with the hot gases. The mixing temperature which occurs reduces the thermal loading in this region. The volumetric ratio of the two partial flows can be influenced by the design of the gap. Thus the top side and under side may have a different gap width or the boundary walls may be inclined or designed differently from the fluidic point of view. 
     FIG. 8 shows an embodiment with cooling-medium discharge on the underside of the shroud-band element. The cooling bores  17  open laterally into the recess  16 . In this variant, the mixing temperature in the region of the underside of the shroud band is lowered and thus the thermal loading is reduced.