Abstract:
A turbine vane is provided which includes a radial outer platform, a radial inner platform and an airfoil extending between the outer platform and the inner platform. Each platform has a gas washed surface facing towards the respective other platform, a non gas washed surface facing away from the respective other platform and a peripheral surface extending from the gas washed surface to the non gas washed surface. The peripheral surface includes an upstream section that is designed to be directed towards the gas flow washing the gas washed surface. Cooling fluid channels each include an opening in the peripheral surface or in the gas washed surface and are located in at least a section of the outer platform and/or in at least a section of the inner platform. The respective section directly adjoins the upstream section of the peripheral surface of the respective platform.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2008/054783, filed Apr. 21, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07008697.0 EP filed Apr. 27, 2007, both of the applications are incorporated by reference erein in their entirety. 
     FIELD OF INVENTION 
     The present invention relates to a turbine vane comprising a radial outer platform, a radial inner platform and an airfoil extending between the outer platform and the inner platform. 
     BACKGROUND OF INVENTION 
     Turbine vanes are used for guiding the turbine&#39;s driving medium through the turbine so as to optimise momentum transfer from the driving medium to a rotor of the turbine. In gas turbines the driving mediums are hot and corrosive combustion gases. Therefore, the turbine vanes are usually coated with a thermal barrier coating system. However, in order to reduce gas turbine engines emissions and the specific power, one aims to achieve higher turbine entry temperatures of the combustion gas. This in turn means a higher thermal load on the turbine components, in particular on the turbine nozzle guide vanes, i.e. the first row of turbine vanes, which is facing the hot and corrosive combustion gas when it enters the turbine section of the gas turbine engine. The higher temperatures lead to increased corrosion of the nozzle guide vanes and, in particular, at the gas washed surfaces of the nozzle guide vanes&#39; platforms. 
     To reduce the thermal load on the platform, the platforms are cooled by impingement cooling, i.e. by air jets directed onto their non gas washed surfaces. Such an impingement cooling is, e.g. disclosed in DE 10 2005 013 795 A1 or in WO 2007/000409 A1. Although impingement cooling has been sufficient with the current temperatures of the combustion gas entering the turbine section, it may be insufficient with future higher turbine entry temperatures of the combustion gas. 
     In EP 0 902 164 A1 is described another alternative for platform cooling. Thereby cooling fluid channels are located in the platform through a cooling fluid guide along the platform. 
     SUMMARY OF INVENTION 
     It is therefore an object of the present invention to provide a turbine vane with an improved cooling of the gas washed surface of one or both platforms. 
     This objective is solved by a turbine vane as claimed in the claims. The depending claims define further developments of the inventive turbine vane. 
     An inventive turbine vane comprises a radial outer platform, a radial inner platform and an airfoil portion extending between the outer platform and the inner platform, the outer platform and the inner platform each having a gas washed surface showing towards the respective other platform and a non gas washed surface showing away from the respective other platform. A peripheral surface extends from the gas washed surface of a platform to the non gas washed surface of the platform. The peripheral surface comprises an upstream section that is designed to be directed towards the gas flow washing the gas washed surface when the vane is fitted to a turbine. In the inventive turbine vane cooling fluid channels with an opening in the peripheral surface or in the gas washed surface are located in at least one section of the outer platform and/or in at least one section of the inner platform. The respective section directly adjoins the upstream section of the respective platform&#39;s peripheral surface. 
     By means of the cooling fluid channels it becomes possible to provide film cooling of the platform&#39;s gas washed surface. A cooling fluid, e.g. cooling air, is directed to the upstream section of the peripheral surface from where it can enter the flow space for the hot and corrosive combustion gases entering the turbine section. Due to the fluid properties of the hot and corrosive flow the cooling fluid becomes entrained so as to form the cooling fluid film on the gas washed surface of the platform. By means of such a film cooling, the cooling efficiency for the gas washed surface can be increased so that it can withstand higher temperatures of the combustion gas. 
     The cooling channels are slots which are present in the non gas washed surface of the outer platform and/or in the non gas washed surface of the inner platform in at least one section adjoining the upstream section of the respective platform&#39;s peripheral surface. The slots extend to the upstream section of the peripheral surface. The cooling fluid can then be led through the slots to the upstream section of the peripheral surface. This simple design can be realised by relatively low costs. 
     If the gap between the upstream section of the peripheral surface and a neighbouring element of the gas turbine engine is too small to allow for sufficient cooling fluid flow into the flow path of the combustion gas the slots may also extend through the upstream section of the peripheral surface. By this measure, the conduit that is present in the gap for the cooling fluid can be increased. 
     It is advantageous in view of a uniform cooling fluid film if a number of slots are present in the non gas washed surface and/or the upstream section of the peripheral wall of a platform where the slots are spaced from each other in the circumferential direction of the respective platform. The distribution of the slots can be adapted to the flow paths of the hot and corrosive combustion gas along the gas washed surface of a platform. However, if the flow paths are evenly distributed, it is advantageous if the slots are also evenly distributed over the non gas washed surface and/or the upstream section of the peripheral wall of the platform. 
     The inventive turbine vane, may in particular, be a nozzle guide vane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features, properties and advantages of the present invention will become clear from the following description of embodiments in conjunction with the accompanying drawings. 
         FIG. 1  shows a gas turbine engine in a highly schematic view. 
         FIG. 2  shows the turbine entry of a gas turbine engine with two rows of guide vanes and two rows of turbine blades. 
         FIG. 3  shows an inventive nozzle guide vane in a sectional view. 
         FIG. 4  shows the guide vane of  FIG. 3  in a top view. 
         FIG. 5  shows a detail of a second embodiment of the inventive guide vane. 
         FIG. 6  shows another detail of the second embodiment. 
         FIG. 7  shows a detail of a third embodiment of the inventive guide vane. 
         FIG. 8  shows another detail of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows, in a highly schematic view, a gas turbine engine  1  comprising a compressor section  3 , a combustor section  5  and a turbine section  7 . A rotor  9  extends through all sections and carries, in the compressor section  3 , rows of compressor blades  11  and, in the turbine section  7 , rows of turbine blades  13 . Between neighbouring rows of compressor blades  11  and between neighbouring rows of turbine blades  13  rows of compressor vanes  15  and turbine vanes  17 , respectively, extend from a housing  19  of the gas turbine engine  1  radially inwards towards the rotor  9 . 
     In operation of the gas turbine engine  1  air is taken in through an air inlet  21  of the compressor section  3 . The air is compressed and led towards the combustor section  5  by the rotating compressor blades  11 . In the combustor section  5  the air is mixed with a gaseous or liquid fuel and the mixture is burnt. The hot and pressurised combustion gas resulting from burning the fuel/air mixture is fed to the turbine section  7 . On its way through the turbine section  7  the hot pressurised gas transfers momentum to the turbine blades  13  while expanding and cooling, thereby imparting a rotation movement to the rotor  9  that drives the compressor and a consumer, e.g. a generator for producing electrical power or an industrial machine. The expanded and cooled combustion gas leaves the turbine section  7  through an exhaust  23 . 
     The entrance of the turbine section  7  is shown in more detail in  FIG. 2 . The figure shows two rows of turbine blades  13  and two rows of turbine vanes  17   a ,  17   b . The turbine vanes  17   a ,  17   b  comprise radial outer platforms  25   a ,  25   b  and  27   a ,  27   b  that form walls of a flow path for the hot pressurised combustion gas together with neighbouring turbine components  31 ,  33  and with platforms of the turbine blades  13 . The combustion gas flows through the flow path in the direction indicated in  FIG. 2  by the arrow  35 . 
     A turbine vane  17   a  of the first row of turbine vanes is shown in more detail in  FIG. 3 . The figure shows a sectional view in a cut through the platforms  25   a ,  27   a  but not through the airfoil  37  of the vane  17   a.    
     The airfoil  37  extends radially with respect to the turbine&#39;s rotor from the inner platform  27   a  to the outer platform  25   a . It is usually hollow to allow a cooling fluid to flow through the vane. It may comprise film cooling openings (not shown) to discharge cooling fluid into the flow path of the combustion gas so as to provide film cooling for the surface of the airfoil  37 . 
     Each platform comprises a gas washed surface  39 ,  41  which forms part of the wall of the flow channel for the combustion gas. The gas washed surfaces  39 ,  41  of the outer platform  25   a  and the inner platform  27   a  therefore face each other. Each platform further comprises a non gas washed surface  43 ,  45 . The non gas washed surfaces form the opposite side of the respective platform so that the non gas washed surfaces of the inner and outer platform face away from each other. The non gas washed surfaces  43 ,  45  show towards cooling air supply chambers  47 ,  49  through which cooling air is supplied as a cooling fluid to the airfoil  37  and the non gas washed surfaces  43 ,  45  of the platforms  25   a ,  27   a.    
     In the non gas washed surfaces  43 ,  45  fixing elements  51 ,  53  are present which are used to fix the turbine vane  17   a  to the casing  19  of the gas turbine engine. By fixing the turbine vane  17   a  to the casing  19  the turbine vane  17   a  is also fixed with respect to neighbouring turbine components, for example the turbine components  31 ,  33  neighbouring the turbine vane  17   a  on the upstream side. Usually a sealing contact is present between the turbine components  31 ,  33  and the respective platform  25   a ,  27   a . Therefore, cooling air flow from the cooling air supply chambers  47 ,  49  to a gap  61  between the turbine component  31  and the radial outer platform  25   a  and to a gap  65  between the turbine component  33  and the radial inner platform  27   a  is rather small, if at all present. Therefore, slots  55  are cut into a section of the outer platform&#39;s non gas washed surface  43  that directly adjoins the upstream section  59  of the platform&#39;s peripheral surface  58 . In the present embodiment of the invention the slots  55  are evenly distributed over the whole length of the non gas washed surface  43  that adjoins the upstream section  59  (see  FIG. 4 ). These slots allow cooling air to flow into the gap  61  that is present between the upstream section  59  of the peripheral surface  58  and the surface of the neighbouring turbine component  31 . The cooling air supplied through the slots  55  can then, through the gap  61 , enter the flow path of the hot pressurised gas flowing through the turbine. The hot pressurised gas entrains the cooling air leaving the gap  61  towards the flow path of the combustion gas so that a cooling air film is formed above the gas washed surface  39  of the radial outer platform  25   a . This cooling air film enhances the cooling of the gas washed surface  39  and thereby reduces oxidation and/or corrosion caused by the hot pressurised combustion gas. As a further cooling measure, the non gas washed surface  43  may be cooled by impingement cooling, as it is known from the state of the art. 
     Like the radial outer platform  25   a  the radial inner platform  27   a  is cooled by film cooling. To achieve film cooling of the gas washed surface  41  of the inner platform  27   a  slots  57  are cut into its non gas washed surface  45  in a section directly adjoining the upstream section  63  of the platform&#39;s peripheral surface  62 . As described with respect to the upper platform  25   a , cooling air can enter a gap  65  between the upstream section  63  of the peripheral surface  62  and the surface of the neighbouring turbine component  33 . The cooling air can then enter the flow path of the combustion gas through this gap  65  and form a cooling air film over the gas washed surface  41  of the inner platform  27   a . Like the outer platform  25   a , the inner platform  27   a  may also be cooled by impingement cooling, as it is known from the state of the art. 
     A second embodiment of the inventive turbine vane will now be described with respect to  FIGS. 5 and 6 . While  FIG. 5  shows a detail of the turbine vane&#39;s outer platform  25   a ,  FIG. 6  shows a detail of the turbine vane&#39;s inner platform  27   a . Also shown in these figures are parts of the neighbouring turbine components  31 ,  33 . Elements of this embodiment which do not differ from the respective elements in the first embodiment are denoted with the same reference numerals as in  FIGS. 3 and 4  and will not be described again to avoid repetition. 
     The second embodiment differs from the first embodiment in that no slots are present in the non gas washed surfaces  43 ,  45  of the radial outer platform  25   a  and the radial inner platform  27   a , respectively. Instead, bores  67  are present in a section of the outer platform which adjoins the upstream section  59  of the outer platform&#39;s peripheral surface  58  and bores  69  are present in a section of the inner platform  27   a  which adjoins the upstream section  63  of the inner platform&#39;s peripheral surface  62 . These bores form through holes extending from the non gas washed surface  43  of the outer platform  25   a  to the upstream section  59  of the outer platform&#39;s peripheral surface  58  and from the non gas washed surface  45  of the inner platform  27   a  to the upstream section  63  of the inner platform&#39;s peripheral surface  62 , respectively. Hence, cooling air can be supplied through the bores  67 ,  69  into the gaps  61 ,  65  between the outer platform  25   a  and the neighbouring turbine component  31  and between the inner platform  27   a  and the neighbouring turbine component  33 , respectively. 
     A third embodiment of the inventive turbine vane will now be described with respect to  FIGS. 7 and 8 . While  FIG. 7  shows a detail of the vane&#39;s outer platform  25   a ,  FIG. 8  shows a detail of the vane&#39;s inner platform  27   a . Elements that do not differ from the respective elements of the first embodiment are designated by the same reference numerals as in the first embodiment and will not be described again to avoid repetition. 
       FIG. 7  shows, in a sectional view, a part of the radial outer platform  25   a  of the vane  17   a  and a part of the neighbouring turbine component  31 .  FIG. 8  shows a part of the inner platform  27   a  of the turbine vane  17   a  and a part of the neighbouring turbine component  33 . As in the second embodiment, bores  71 ,  73  are present in sections of the outer platform  25   a  and the inner platform  27   a  that adjoin the upstream sections  59 ,  63  of the respective platform&#39;s peripheral surface  58 ,  62 . However, in contrast to the first and second embodiments, no gaps are present between the platform&#39;s upstream section  59 ,  63  and the respective neighbouring turbine component  31 ,  33 . In this context, no gap means that no gap is present which allows a sufficient cooling air flow into the flow path of the hot pressurised combustion gas, such as to allow for film cooling of the gas washed surfaces  39 ,  41 . Therefore, the bores  71 ,  73  in the third embodiment extend from the non gas washed surface  43  of the outer platform  25   a  to its gas washed surface  39  and from the non gas washed surface  45  of the inner platform to its gas washed surface  41 , respectively. 
     The exits  75 ,  77  of the through holes formed by the respective bores,  71 ,  73  are open towards the flow channel through which the hot pressurised gas flows and are located as close as possible to the upstream sections  59 ,  63  of the peripheral walls  58 ,  62  so that areas not cooled by film cooling can be minimised. However, the remaining areas that are not film cooled in the outer platform&#39;s and the lower platform&#39;s gas washed surfaces  39 ,  41  can be cooled by impingement of the cooling air flow on the insides  79 ,  81  of the upstream sections of the peripheral surfaces  58 ,  62 . 
     As an alternative to providing bores with openings in the gas washed surfaces it would be possible to extend the slots present in the first embodiment over the upstream section of the peripheral surface so as to provide channels extending from the non gas washed surface to the gas washed surface. 
     Like the slots in the first embodiment the bores in the second and third embodiments may be evenly distributed over the upstream section of the platform&#39;s peripheral surfaces.