Abstract:
A gas turbine is provided that includes a rotor which is rotatable around an axis and equipped with rotor blades, and which is concentrically enclosed at a distance by a casing, which is equipped with stator blades, forming an annular hot gas passage. Rings with stator blades and rotor blades are arranged in a manner alternating in the axial direction. Between adjacent stator blades, heat shield segments are arranged, which delimit the hot gas passage on the outside in a region of the rotor blades and are cooled by impingement cooling where a cooling medium from an outer annular cavity flows into the heat shield segment.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of International Application No. PCT/EP2009/058895 filed Jul. 13, 2009, which claims priority to Swiss Patent Application No. 01146/08, filed Jul. 22, 2008, the entire contents of all of which are incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates to the field of thermal machines, in particular, gas turbines. 
       BACKGROUND 
       [0003]    Gas turbines, as are described for example in printed publication DE-A1-196 19 438, in the turbine section have a rotor which is provided with rotor blade rows and is concentrically enclosed at a distance by a casing. Rings are formed on the casing and carry stator blades which, in common with the rotor blades on the rotor, extend into the hot gas passage which is formed between rotor and casing. Stator blade rows and rotor blade rows alternate in the axial direction or in the direction of the hot gas flow. Heat shield segments, which the rotor blades move past by their blade tips, and which are supplied with cooling air or another cooling medium from an annular cavity which encompasses the heat shield segments, are arranged in a circumferentially distributed manner between adjacent stator blade rows towards the outer limit of the hot gas passage. For cooling, an impingement cooling method, for example, is used, in which the cooling medium, through repeatedly applied openings in an impingement cooling plate, impinges upon the inner side of the wall, which delimits the hot gas passage, of the heat shield segment. 
         [0004]    The heat shield segments (“heat shields”) behind the front-stage stator blades of the turbine are exposed to high heat-flow loads. In the region where the rotor blades rotate past, high heat-flow loads occur. High heat-flow loads also occur in the region of the stator blade wake. Wake pressure waves, which are associated with the wake, reduce the pressure margin (back flow margin BFM), i.e. the available pressure difference between hot gas passage and annular cavity, with regard to a hot-gas intrusion. 
         [0005]    A “failsafe design” with regard to rubbing (rubbing cracks), loss of sealing (inter heat shield feather seals), part load, ambient conditions (off-ISO design), damage as a result of impact (FOD, i.e. foreign-object damage) and manufacturing tolerances, require an appreciable margin regarding BFM, which at ISO full-load conditions has a negative effect upon the performance. 
         [0006]    The number of stator blades in the ring, in the case of conventional solutions, is independent of the number of associated heat shield segments. The number of parts is minimized as far as possible. Since the thermal and mechanical loads of the stator blades are higher, a larger number of stator blades are required in comparison to the number of heat shield segments. 
       SUMMARY 
       [0007]    The present disclosure is directed to a gas turbine including a rotor that is rotatable around an axis and equipped with rotor blades. The rotor is concentrically enclosed at a distance by a casing. The casing is equipped with stator blades, forming an annular hot gas passage. Rings including the stator blades and the rotor blades are arranged in an alternating manner in an axial direction. Between adjacent stator blades heat shield segments are arranged, which delimit the hot gas passage on its outside in a region of the rotor blades and are cooled by impingement cooling where a cooling medium from an outer annular cavity flows into the heat shield segment. The number of heat shield segments and adjacent stator blades in the rings is the same. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]    The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawing. All elements which are not essential for the direct understanding of the invention have been omitted. Like elements are provided with the same designations in the various figures. The flow direction of the media is indicated by arrows. In the drawing 
           [0009]      FIGS. 1-3  show, in a simplified view in longitudinal section, a detail from a gas turbine with heat shield segments which are arranged between the first and second stator blade row and are cooled by means of a simple impingement cooling scheme ( FIG. 1 ), a sequential impingement cooling scheme ( FIG. 2 ), and an impingement cooling scheme which operates with counterflow; 
           [0010]      FIG. 4  shows in a view comparable to  FIGS. 1-3  an impingement cooling scheme according to an exemplary embodiment of the invention; 
           [0011]      FIG. 5  shows a heat shield segment which is suitable for the arrangement according to  FIG. 4 , with the arrangement of the various cooling holes and recesses in plan view from the outside; 
           [0012]      FIG. 6  shows in a view comparable to  FIG. 4  the installed heat shield segment according to  FIG. 5 ; 
           [0013]      FIG. 7  shows the arrangement of pillars in the impingement cooling cavities of the heat shield segment, according to another exemplary embodiment of the invention; 
           [0014]      FIG. 8  shows in longitudinal section one of the possible pillars from  FIG. 7 , which is provided as a spacer for the impingement cooling plates; 
           [0015]      FIG. 9  shows in longitudinal section another of the possible pillars from  FIG. 7 , which is provided as a cooling pin with additional heat transfer surfaces; 
           [0016]      FIG. 10  shows a preferred distribution of the pillars from  FIGS. 8 and 9  in the impingement cooling cavities; 
           [0017]      FIG. 11  shows, as seen in the radial direction, the relative positioning of stator blade and heat shield segment in the circumferential direction which is important for the pressure margin, and 
           [0018]      FIG. 12  shows an example of the local reduction of wall thickness by means of a slot where the cooling holes lead into the impingement cooling cavities. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Introduction to the Embodiments 
       [0019]    The invention provides a remedy for the above-noted drawbacks. It is therefore the object of the invention to create a gas turbine with impingement-cooled heat shield segments which avoids the disadvantages of known solutions and in particular to reduce the consumption of cooling medium. 
         [0020]    The object is achieved by means of the entirety of the features of claim  1 . It is preferable that the number of heat shield segments and adjacent stator blades in the rings is the same. As a result of this, maximum occurring loads can be addressed locally, i.e. by means of local cooling. Margins and overall consumption of cooling medium can be appreciably reduced. This allows higher temperatures and a lower cooling medium requirement for a better performance and also flatter temperature profiles for lower emissions. 
         [0021]    In one embodiment, two impingement cooling cavities, into which flows the cooling medium from the annular cavity, are arranged in each case in the heat shield segment in series in the axial direction, in that the downstream-disposed impingement cooling cavity is separated from the annular cavity and both annular cavities are exposed to admission of the cooling medium at the same pressure, wherein the heat shield segments in each case have a middle, hook-like fastening element, the two impingement cooling cavities are separated from each other by means of the middle fastening element, and the downstream-disposed impingement cooling cavity is separated from the annular cavity by means of a cover plate which is arranged between the impingement cooling cavity and the annular cavity. 
         [0022]    In another embodiment, a multiplicity of pillars are arranged in a distributed manner in the impingement cooling cavities for increasing the transfer of heat, wherein the multiplicity of pillars comprise spacers for the impingement cooling plates and cooling pins for increasing the transfer of heat between cooling medium and heat shield segment, and wherein the pillars are accommodated in the impingement cooling cavities in arrangements which are regular at least in sections, and the spacers and cooling pins are arranged in a staggered manner in relation to each other. 
         [0023]    In a further embodiment, the heat shield segments have a leading edge, a trailing edge and two side sections in each case with regard to the flow of the hot gas, and in that for film cooling of the edges and side sections of the heat shield segment, provision is made for cooling holes which, extending from the impingement cooling cavities, pass through the heat shield segment to all sides and terminate in the outer space. In particular, the cooling holes which terminate on the oppositely disposed side sections of the heat shield segment are arranged in this case in a staggered manner in relation to each other so that the discharging cooling medium in adjoining heat shield segments is not mutually impeded at the outlet. 
         [0024]    Furthermore, it is advantageous if for unimpeded discharging of the cooling medium the cooling holes at the leading edge and in the side sections terminate in a set-back manner in a recess, and if the cooling holes in the region of the corners of the heat shield segment are formed in a flared manner for improved cooling of the edge regions. 
         [0025]    In another embodiment, each heat shield segment and the associated upstream-disposed stator blade are positioned relative to each other in the circumferential direction so that the wake pressure wave which is created by the stator blade can be compensated by a means of a corresponding arrangement and supply of the cooling holes in question, wherein the cooling holes lying in the region of the wake pressure wave above the impingement cooling plates lead into the impingement cooling cavities. 
       DETAILED DESCRIPTION  
       [0026]    In  FIGS. 1 to 3 , in a simplified view, different impingement-cooling schemes in a gas turbine  10  are exemplified, based on the heat shield segments  11  which are arranged opposite the first rotor blades B 1  between the first stator blades V 1  and the second stator blades V 2 . In the hot gas passage  29 , hot gas flows from right to left with a mass flow density {dot over (m)} HG , wherein at the leading edge (LE) of the rotor blade B 1 , a pressure P s,LE  prevails, and at the trailing edge (TE), a pressure P s,TE  prevails. The hot gas passage  29  is delimited in the region of the rotor blade B 1  on the outside by the heat shield segment  11  which is fastened on a casing (not shown) by means of hook-like fastening elements  12 ,  13 ,  14 . The heat shield segment  11  is encompassed on the outside by an annular cavity  30  from which a cooling medium, as a rule cooling air, under pressure P 1  or P 2 , flows into two corresponding impingement cooling cavities  17 ,  18  via perforated impingement cooling plates  15 ,  16 , cools the heat shield segment there by means of impingement cooling and then discharges through cooling holes  19 ,  20  into the hot gas passage  29 . 
         [0027]    In the simple case of  FIG. 1 , P 1 =P 2 , so that the cooling medium flows into the two impingement cooling cavities with the same mass flow density {dot over (m)} c . In order to maintain the necessary pressure margin in the case of different pressures in the hot gas passage, operation must be carried out with a very large pressure difference over the entire length of the heat shield segment  11 . The leakage losses are therefore high. 
         [0028]    In the case of the sequential impingement cooling scheme of  FIG. 2 , this disadvantage is corrected by P 1 &gt;P 2  being selected. However, as a result of possible crossflows between the impingement cooling cavities  15 ,  16  (upper broader arrow in  FIG. 2 ), the system is sensitive to the seals (not shown) which are provided on the end face of the fastening element  13  for sealing the gaps between adjacent heat shield segments. 
         [0029]    In the case of the counterflow-impingement cooling scheme of  FIG. 3 , this is corrected by P 1 &lt;P 2  being selected. However, in this case setting the pressure margin in relation to the wake maximum of the pressure proves to be critical. 
         [0030]    In  FIG. 4 , in a view which is comparable to  FIGS. 1 to 3 , an exemplary embodiment of the invention is reproduced. In this case, the same number of parts in the ring for the stator blades V 1  and the heat shield segments  11  is assumed. The heat shield segment  11  has two impingement cooling cavities  17  and  18  which are separated from each other by means of the middle hook-like fastening element  13  and are operated with the same pressure P 1 . The second, downstream-positioned impingement cooling cavity  17  is isolated from the annular cavity  30  by means of a cover plate  21 . The pressure margin for the impingement cooling and pressure margin for the spring seals between adjacent segments can be set independently of each other. A loss of sealing no longer leads to lowering of the cooling medium pressure. The margin of the cooling medium pressure can be reduced. The pressure above the cover plate  21  (P 2 ) can be set so that the moving past of the rotor blade B 1  does not create oscillation of the seal and therefore sealing failures also do not occur. 
         [0031]    For improving the cooling of the heat shield segment  11 , provision is preferably made for film cooling for the leading edge LE, the trailing edge TE and the side sections SW according to  FIGS. 5 and 6 . For this purpose, cooling holes  19 ,  19 ′,  20 ,  20 ′,  25  and  26  lead outwards from the impingement cooling cavities  17 ,  18  and lead into the outer space. The cooling holes  25  and  26  in the side sections SW (as seen in the circumferential direction) are arranged in a staggered manner in relation to each other so that the discharging air in the adjoining heat shield segments  11  is not mutually impeded at the outlet. 
         [0032]    In the leading edge section LE and in the side section SW, the cooling holes  20 ,  20 ′ and  25 ,  26  are arranged on the end faces in a set-back manner by means of corresponding recesses  22 ,  23  and  24  so that when the component makes contact with the adjacent component the air can still discharge without being impeded. The cooling holes  19 ′,  20 ′ are flared in the region of the corners of the heat shield segment  11  (flared cooling holes) in order to optimally cool the edge regions. 
         [0033]    The impingement cooling can be further improved if according to  FIG. 7  provision is made in the impingement cooling cavities  17 ,  18  for additional conical pillars  28  which, staggered with the holes  27 , are arranged in a distributed manner in the impingement cooling plates. The combination of impingement cooling with two types of conical pillars  28  ( FIGS. 8-10 ) is especially advantageous. One type of pillar ( FIG. 8 ) is formed as a spacer  28   a  for the impingement cooling plates  15 ,  16 . The other type of pillar ( FIG. 9 ) serves as a cooling pin  28   b  for increasing the turbulence, the heat flow and the heat transfer surface. Both types of pillars, that is to say the spacers  28   a  and the cooling pins  28   b,  can be arranged in a staggered manner according to  FIG. 10  for increasing the transfer of heat. 
         [0034]    In the region behind the previous stator blade V 1 , where the wake in the form of a wake pressure wave  31  moves over the heat shield segment  11 , specifically over the leading edge LE and the side edge SW ( FIG. 11 ), the corresponding cooling holes  20 ″ (dotted in  FIGS. 4 ,  11 ) are fed with cooling medium (air) of higher pressure from above the impingement cooling plate  16  in order to increase the pressure margin. Since the pressure margin of all the cooling holes does not have to be increased, a significant performance advantage results. 
         [0035]    In particular, by projecting or setting back the components  11 , V 1  in the parting plane in relation to each other, the wake pressure wave  31  is positioned on the heat shield segment  11  (displacement arrows in  FIG. 11 ) so that the pressure margin of the cooling holes in the leading edges and in the side section, and of the annular gap and also the consumption of cooling air, are altogether optimally set. 
         [0036]    The size of the impingement cooling cavities  17 ,  18  is selected so that optimum cooling occurs. The heat shield segment  11  is preferably provided with a ceramic thermal barrier coating (TBC), wherein different thicknesses and tolerances are selected in the regions upstream of the rotating-past of the rotor blade B 1  and at the place where the rotor blade B 1  moves past. For the region upstream of the rotating-past of the rotor blade B 1 , large thicknesses of the thermal barrier coating are selected in order to reduce the wake effect, and for the region where the rotor blade B 1  moves past, however, small manufacturing tolerances are selected in order to minimize performance losses. 
         [0037]    The cooling holes  19 ,  19 ′  20 ,  20 ′,  25 ,  26  are positioned as close as possible to the hot gas in the hot gas passage  29 . Manufacturing tolerances and global wall thicknesses are subject to minimum criteria for rubbing and oxidation. Therefore, locally, where the cooling holes lead into the impingement cooling cavities, the wall thickness is preferably reduced by means of a slot  32  ( FIG. 12 ). 
       LIST OF DESIGNATIONS 
       [0000]    
       
           10  Gas turbine 
           11  Heat shield segment 
           12 ,  13 ,  14  Fastening element 
           15 ,  16  Impingement cooling plate 
           17 ,  18  Impingement cooling cavity 
           19 ,  19 ′ Cooling hole 
           20 ,  20 ′,  20 ″ Cooling hole 
           21  Cover plate 
           22 ,  23 ,  24  Slot 
           25 ,  26  Cooling hole 
           27  Hole 
           28  Pillar 
           28   a  Spacer 
           28   b  Cooling pin 
           29  Hot gas passage 
           30  Annular cavity 
           31  Wake pressure wave 
           32  Slot 
         B 1  Rotor blade 
         LE Leading edge 
         TE Trailing edge 
         SW Side section 
         {dot over (m)} c  Mass flow density (cooling air) 
         {dot over (m)} HG  Mass flow density (hot gas) 
         P 1 , P 2  Pressure (cooling air) 
         P S,TE  Pressure (trailing edge) 
         P S,LE  Pressure (leading edge) 
         V 1 , V 2  Stator blade