Abstract:
Cooling with regard to high-pressure turbine platforms is important in order to maintain gas turbine engine efficiency. Cottage Roof dampers located below junction gaps between adjacent platforms have been used but tend to present spent coolant flow at a high angle relative to hot gas flows about the aerofoil blades. The present arrangement has the junction gap angled such that the emergent coolant flow remains adjacent to the suction side to create a coolant film lingering above that suction side of the platform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to GB 0523106.3, filed 12 Nov. 2005. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a cooling arrangement and, more particularly, to a cooling arrangement utilised in a gas turbine engine with regard to inter-blade platforms. 
     It will be understood that the efficiency and output of a gas turbine engine is related directly to turbine gas temperature. In such circumstances it is desirable to operate a gas turbine engine at the highest temperature possible. At such temperatures it is necessary to provide cooling of components within the gas turbine engine in order to remain within acceptable temperature ranges for the materials from which various components are formed. 
     One of the most difficult locations to cool in a gas turbine engine is the inter-blade platform structure of the high-pressure turbine stage. In the past, embedded convective holes have been used, along with various film cooling configurations. However, these cooling schemes have proved problematic from a stress concentration point of view. The platform gas washed surfaces are highly-stressed both mechanically, due to the centrifugal loading, and thermally, due to the temperature gradients present. Drilling cooling holes has been successful in reducing the metal temperature level associated thermal gradients but these holes have significantly increased the local three-dimensional stress levels in the component and so have been counter-productive in terms of a desire for improved extension of component life. 
     More recently, as described in U.K. Patent application number 0304329.6, a cooling arrangement has been proposed which utilises a damper below a junction between platforms in order to release coolant. It will be understood that cooling air taken from the compressor used to cool the hot turbines is not used to extract work from the turbine. Extracting coolant, therefore, has an adverse effect on engine operating efficiency and it is, therefore, necessary to utilise cooling air as effectively as possible in order to reduce the amount of cooling air extracted. The controlled leakage of coolant through a series of staggered slots machined or cast into contact surfaces of a “Cottage Roof” damper is used to provide cooling about the platform. The coolant air is used initially to cool the disc post or zone between two disc fir tree mounting root serrations and this is bled from the cavity beneath the blade platform surfaces through the slots in the damper surface in order to cool the surfaces of the damper and the platform edges and then the coolant emerges into the gap or junction between two neighbouring platforms. The spent coolant then impingement cools the adjacent platform edge before escaping radially into the gas path and becoming entrained with the strong hot gas flows about the platform. 
     Although the “Slotted Cottage Roof Damper” arrangement described in the UK Patent application 0304329.6 provides distinct improvements and advantages over the previous approach, it will be appreciated that improvements can still be made. A primary disadvantage relates to the angle and, to a lesser extent, the velocity of the spent coolant emerging from the junction gap between neighbouring platform edges. In short, the spent coolant rapidly mixes with the hot gas-flows and, therefore, does not provide any significant cooling effect. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an annular array of aerofoils for a gas turbine engine, the array defining a cooling arrangement, the arrangement comprising a junction gap between two overlapping platforms of adjacent aerofoils and a damper radially inwardly of the junction gap, a damper surface and a platform surface arranged to have a coolant flow passing between them in use, the arrangement characterised in that the junction gap is at an angle relative to a radial line to angularly present a coolant flow in use adjacent to an exit of the junction gap. 
     Preferably, the junction gap is angled ø at 60 degrees, but may be angled between 30 and 75 degrees. The angle of the junction gap varies along the length of the platforms. 
     Preferably, the damper surface has a ridge with surfaces either side and the angle of the junction gap is substantially aligned with one of the surfaces. 
     Normally, the junction gap forms a slot which is continuous along the length of the platforms. 
     Normally, the ridge is directly radially inward the slot. 
     Preferably, the surfaces are arranged such that respective coolant flows over both surfaces merge at the ridge to form the coolant flow presented adjacent to the exit of the junction gap. 
     Normally, the slot has an exit configured to present the coolant flow adjacent to the junction gap. 
     Preferably, the exit is arranged to present the coolant flow at a substantially consistent angle to gas flows over the platforms in use. 
     Typically, the exit comprises edges of each platform and one edge is displaced relative to the other edge. 
     Typically, one edge is displaced above the other edge such that the coolant flow is presented adjacent to the junction gap downstream of the raised component edge. 
     Preferably, a gas turbine engine includes an annular array of aerofoils as described in the above paragraphs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:— 
         FIG. 1  is a schematic cross-section of a prior cooling arrangement; 
         FIG. 2  is a schematic plan view of a cooling arrangement; and 
         FIG. 3  is a schematic cross-section of a cooling arrangement in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated above, a recent improved cooling arrangement for platform structures and particularly in an annular array of aerofoils in a gas turbine engine utilises a damper with a sloped ridge surface incorporating grooves through which coolant flows in order to cool the platform as well as the damper. This configuration is commonly referred to as a “Cottage Roof”.  FIG. 1  is a schematic cross-section of a prior cooling arrangement  1 , generally described in U.K. Patent application number 0304329.6. Thus, the arrangement  1  has a first platform  2  and a second platform  3 , secured upon the mounting  4 , with a gap  5  between them. As indicated above, generally, in use these platforms and associated blades will be subject to high temperatures. Blade aerofoil coolant  6  will pass through conduits  7  in those aerofoils. The present cooling arrangement particularly relates to mounting disc and under-platform coolant flows  8 . 
     As described previously, these coolant flows  8  are utilised to cool the platforms  2 ,  3 . A damper  10  is presented and generally is in contact with opposed platform cavity surfaces  12 ,  13 . It will be noted that the damper  10  has a roof-like cross-section with a ridge  11  and diverging slopes either side which engage the surfaces  12 ,  13 . Grooves are provided between the damper  10  and the surfaces  12 ,  13  so that coolant flow can pass between these surfaces  12 ,  13  and the damper  10  to exit through a slot  14  into a space  15  above the platforms  2 ,  3 . This ejected and spent coolant flow  16  mixes with hot gas flows  17  as a result of operation of the blade aerofoils. In such circumstances, the platform section  2  will generally be considered a pressure surface whilst the platform section  3  will generally be considered a suction surface. As the coolant flow  16  rapidly and turbulently mixes with the hot gas flow  17 , it will be understood that some cooling effectiveness with regard to that flow  16  is lost, particularly with regard to potential in suction surface marked with XXXXX on the platform  3 . Ideally, so-called film cooling where a coolant gas lingers about a surface could be utilised in order to protect the platform  3  from hot gas impingement. 
       FIG. 2  provides a schematic plan view of the cooling arrangement depicted in  FIG. 1 . As can be seen, the damper  10  incorporates slots  20  in order to present coolant flow  16 . This flow  16  as indicated mixes with hot gas flow  17  about aerofoils  21  and so normally provides little cooling effect. 
     It will be appreciated that the limitations with the prior cooling arrangement depicted in  FIGS. 1 and 2  concerns the loss of coolant effect upon particularly the suction surface XXXXX of the platform  3 . This is generally due to the angle and, to a lesser extent, the velocity of the spent coolant which emerges from the exit of the slot  14  in the junction gap between the juxtaposed platforms  2 ,  3 . As a direct result of the fact that the coolant emerges across the gas path  17 , that is to say perpendicular to the gas platform washed surfaces, there is no film cooling protection felt on the platform  3  suction surface XXXXX. It will be understood that this is due to the emerging stream of coolant from the slot being at a very different angle to the hot gas flow  17  direction and, consequently, the coolant  16  does not linger or “stick” by forced laminar flow to the platform  3  surface but rather becomes entrained and vigorously mixed with the hot gas flow  17 , so destroying any potential film cooling effect. It will also be understood that the aerodynamic mixing losses associated with the emerging coolant are substantial and this may have a detrimental effect on turbine efficiency and so the specific fuel consumption of the gas turbine engine overall. Further problems with this prior arrangement relate to the possibility that there may be an unpredictable positive or negative step between juxtaposed platform edges as a result of component dimensional tolerance stack-up. Such steps between the edges of the opposed platforms may again prove detrimental to aerodynamic component and turbine efficiency. 
     Finally, with regard to the prior cooling arrangement depicted in  FIGS. 1 and 2 , it will be understood that the junction gap which creates the slot may change during engine cycling as a result of more expansion or less relative expansion between the components. Although there may not be an actual ‘pinch point’ where the platforms effectively engage and lock up with each other, there will be a point normally at the highest gas temperature condition experienced when the junction gap has a minimum dimension. During this period of minimum dimensions, the velocity of the emerging coolant  16  will reach a maximum so that if the cold or start-up gap has been set too narrowly then the coolant flow rate may be affected. 
       FIG. 3  provides a schematic cross-section of a cooling arrangement  31  for an annular array of aerofoils  52  in accordance with the present invention. Thus, two neighbouring blade platforms  32 ,  33  are damped and cooled using a “cottage roof” damper  34  as described previously with regard to  FIG. 1 . However, in the present cooling arrangement  31 , pressure surface  35  of the platform  32  has been slightly extended circumferential and a corresponding platform suction surface  36  has been shortened to form a partially overlapping seal arrangement. Coolant  37  leaks from the under platform cavity  38  through the damper surfaces in grooves upon surface  39  on either side of the roof ridge  40  and convectively cools the damper  34  and platform  32 ,  33  edges. Coolant air  29  in the cavity  38  is taken from the usual compressor stages and coolant network. There is a junction gap  30  between platforms  32 ,  33 . An emergent coolant flow  41  then cools by impingement the neighbouring platform edges  43 ,  44 . 
     The coolant flow  29  meets in a continuous stream and flows between the juxtaposed neighbouring platform edges  32 ,  33  in a continuous slot formed between the adjacent platform edges as a junction gap to emerge as coolant flow  41 . The coolant flow  41  emerges as a continuous film onto the platform suction surface XX before becoming entrained by hot gas secondary flows  42  that are a characteristic of a rotating aerofoil endwall geometry. The gentle mixing of the coolant  41  within the secondary flow hot gas  42  is achieved by consistently directing the film in substantially the same direction as the secondary flows  42 . In addition, a platform pressure surface YY and the suction surface XX are designed with a negative step at an exit  45  with respect to the hot gas secondary flow  42  direction. This step is effectively filled in with the emergent spent cooled flow  41  through the junction gap between the adjacent platforms  32 ,  33 . As a consequence, the arrangement  31  is less sensitive to gas path discontinuities due to dimensional geometries. In short, the arrangement  31  is made such that there will always be a negative step between surface YY and surface XX. Similarly, the circumferential gap between neighbouring blade platforms  32 ,  33  which effectively controls the exit Mach number of the flow  41  will be less important from an aerodynamic loss point of view as the coolant  41  is being directed in substantially the same direction as the hot mainstream secondary flow. 
     In view of the above it will be appreciated that the present cooling arrangement  31  utilises a “Cottage Roof” damper including slots for projection of coolant flow whereby there is a proportion of coolant passing over each sloped surface until combined to pass through the slot between the platforms. This slot, as indicated, is at the junction gap between the platforms and is at an angle ø relative to a radial line  50 . Although any angling of the junction gap is beneficial, a preferred range of angles ø is between 30 and 75 degrees and as shown in  FIG. 3  the angle is approximately 60 degrees. The angle is preferably aligned with one of the slopes of the damper. In such circumstances the coolant flow emerges from the slot for appropriate film retention against the suction surface XX of the platform  33  for cooling effect and less turbulent loss with the hot gas flow  42 . 
     Furthermore, angling the junction gap may be more complex where either different flow pattern occurs within the space between aerofoils or where the platform edges are curved in the axial direction. In either of these circumstances, the angle of the junction gap may vary along the length or edge of the platforms. 
     In the above circumstances it will be appreciated that the damper  34  utilised in accordance with the present arrangement will be similar to that utilised with regard to  FIGS. 1 and 2 . However, in an area immediately above the ridge  40  of the damper  34 , rather than as described previously with respect to  FIG. 1 , the coolant flow components  39  passing over the respective slopes of the damper  34  to merge and project vertically upwards, it will be understood that one flow component  39   a  will be generally aligned with the gap between the platform  32 ,  33  whilst the other flow component  39   b  will normally be presented across that flow component  39   a . In such circumstance there may be some coolant flow turbulence created directly above the ridge  40 . In such circumstances, generally, as illustrated in  FIG. 3 , a mixing zone may be created to utilise or diminish the effects of such turbulence upon cooling within the arrangement  31 . 
     The junction gap  30  is a slot which is normally continuous along the length of the platforms  32 ,  33  between the blades. In such circumstances a uniform film will be created upon the suction surface XX of the platform  33  to achieve efficient coolant effects. 
     It will be noted that in the cooling arrangement  31  there is now a lack of symmetry between the respective coolant flow components on either sloped side of the roof ridge  40 , namely flow components  39   a  or  39   b  of the damper  34 . It will be noted that the coolant flow component  39   a  on the pressure side of the damper wets a greater surface than the coolant flow component  39   b  on the suction side XX. In order to address this disparity the grooves on one side of the damper  34  may be increased or decreased in relative cross-section and the number and angular presentation of the grooves may be altered to achieve best cooling performance.