Abstract:
A platform cooling system usable in a turbine engine together with an airfoil for preheating cooling fluids before the cooling fluids enter a cooling system in the airfoil in a turbine engine. The platform cooling system includes cooling channels in either the ID or OD platforms, or both, of the airfoil. The channels transfer heat to the cooling fluids flowing through the platform cooling system and thereby heat the cooling fluids. The preheated cooling fluids are particularly useful with cooling composite ceramic airfoils, which are susceptible to damage from large temperature gradients developed between combustion gases outside the airfoil and cooling fluids inside the airfoil. The platform cooling system may be combined with an airfoil cooling system to create a serial cooling system in which cooling fluids may enter the platform and flow through the platform and airfoil without being supplemented with additional cooling fluids along the flow path.

Description:
FIELD OF THE INVENTION 
     This invention is directed generally to airfoils in turbine engines, and more particularly to airfoils having a need for reduced temperature gradients within the airfoil, such as composite airfoils. 
     BACKGROUND 
     Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine vane and blade assemblies, to these high temperatures. As a result, turbine airfoils, such as turbine vanes and blades must be made of materials capable of withstanding such high temperatures. In addition, turbine airfoils often contain internal cooling systems for prolonging the life of the airfoils and reducing the likelihood of failure as a result of excessive temperatures. 
     Typically, turbine airfoils, such as turbine vanes are formed from an elongated portion having one end configured to be coupled to an outer shroud vane carrier and an opposite end configured to be movably coupled to an inner shroud. The airfoil is ordinarily composed of a leading edge, a trailing edge, a suction side, and a pressure side. The inner aspects of most turbine vanes typically contain an intricate maze of cooling circuits forming a cooling system. The cooling circuits in the vanes receive air from the compressor of the turbine engine and pass the air through the ends of the vane adapted to be coupled to the vane carrier. The cooling circuits often include multiple flow paths that are designed to remove heat from the turbine vane. At least some of the air passing through these cooling circuits is exhausted through orifices in the leading edge, trailing edge, suction side, and pressure side of the vane. 
     Composite airfoils have been developed for use in turbine engines as composite materials are typically suitable to use in higher temperature environments that conventional metals forming airfoils. Composite airfoils are often constructed as laminate layers formed from high strength fibers woven into a cloth that is saturated with a ceramic matrix material. The multiple laminate layers are stacked, compacted to the desired thickness, dried, and fired to achieve the desired structural properties. The laminates have desirable in-plane structural properties but significantly less strength in the through plane direction. The composite airfoils are often formed from an inner solid core, a laminate layer, and a FGI insulating thermal barrier coating. A ceramic bond exists between the laminate and solid core and at the interface of the laminate and thermal barrier coating. 
     The composite airfoils have been cooled in conventional composite airfoils by passing cooling air from a compressor through the airfoil. Typically, the cooling fluids are passed through a plurality of cooling channels and exhausted through the trailing edge of the composite airfoil without use of film cooling. While outer surfaces of composite airfoils are typically exposed to temperatures of about 1,600 degrees Celsius in a turbine engine, the laminate layer of the airfoil is generally kept at a temperature less than about 1,100 degrees Celsius. Typically, cooling air used in a composite airfoil cooling system is about 450 degrees Celsius. The extreme temperature gradient between the combustion gases at 1,600 degrees Celsius outside of the airfoil and the cooling gases at 450 degrees Celsius in the interior cooling channels creates thermal stress in the composite airfoil that can delaminate the laminate layer and destroy bonds between the laminate layer and the inner core and between the laminate layer and the thermal barrier coating. Such problems with thermal stress do not exist in metal airfoils because of the high thermal conductivity of the metal forming the airfoil and high strength of the metal. Thus, a need exists for reducing the thermal stresses created by cooling fluids in composite airfoils in turbine engines. 
     SUMMARY OF THE INVENTION 
     This invention relates to a preheating system for cooling fluids in an airfoil of a turbine engine. The invention also relates to use of the preheating system to form a serial cooling system for a composite turbine airfoil in which the preheating system in the platform and a cooling system in the airfoil form a continuous cooling system not supplemented with additional cooling fluids along the cooling system from a first end of the preheating system in the platform to an exhaust of the cooling system in the airfoil. By preheating the cooling fluids in the platform of an airfoil, the temperature gradient that develops in the airfoil is reduced, thereby reducing the thermal stresses in the airfoil. 
     In at least one embodiment, the cooling fluid preheating system is a platform cooling system positioned in one or more platforms of an airfoil for heating cooling fluids before the cooling fluids enter an internal cooling system in inner aspects of the airfoil. The platform cooling system may be formed from one or more channels in the first platform that are in communication with the cooling system in the elongated airfoil. Heat is removed from the platform and used to heat the cooling fluids passing through the channels in the platform forming the cooling fluid preheating system. The platform cooling system is particularly suited for use with composite airfoils, such as, but not limited to ceramic matrix composites, to reduce the temperature gradients between outer surfaces of an airfoil and inner cooling channels to reduce the risk of delamination and other failure of the airfoil. 
     The platform cooling system may be positioned in an inner diameter (ID) platform or in an outer diameter (OD) platform of an airfoil, or both. In at least one embodiment, the platform cooling system may include one or more cooling channels extending in close proximity to an outer surface of the platform so that cooling fluids flowing through the cooling channel may increase in temperature. In a composite airfoil, the cooling channels may be formed from a plurality of parallel spaced cooling channels extending generally along an outer surface of a platform and sealed with an outer composite layer, which may be, but is not limited to being, fiberglass. 
     The channels of the platform cooling system may be supplied with cooling fluids from a cooling fluid supply through numerous means. In at least one embodiment, the platform cooling system may include leading edge cooling fluid supply holes in the platform and proximate to the leading edge of the airfoil, and one or more trailing edge cooling fluid supply holes in the platform and proximate to the trailing edge of the airfoil. The leading and trailing edge cooling supply holes may be positioned at an angle of between about 30 degrees and about 60 degrees relative to a longitudinal axis of the airfoil. The leading and trailing edge cooling supply holes may extend between a cooling fluid supply chamber positioned between the platform and a shroud, and may include an airfoil cooling fluid supply chamber at the interface between the platform and the airfoil. The leading and trailing edge cooling fluid supply holes may have any design capable of generating the desired residency time of the cooling fluids in the platform so that the temperature of the cooling fluids may increase a desired amount. 
     The airfoil may be formed from a variety of materials and configurations. In at least one embodiment, the airfoil may be formed from an inner core encapsulated by a laminate layer, which may be, but is not limited to being, the ceramic matrix composite, and may include an outer thermal boundary coating (TBC). The airfoil may include a leading edge, a trailing edge, a pressure side, a suction side, a first platform at a first end of the airfoil, a second platform at a second end of the airfoil opposite the first end, and a cooling system formed from at least one internal cooling channel in the airfoil. In a composite airfoil, the cooling system may include, but is not limited to including, a leading edge cooling supply chamber extending generally radially within the airfoil, a plurality of pressure side channels extending generally chordwise within the inner core of the airfoil proximate to a laminate layer, and a plurality of suction side channels extending generally chordwise within the inner core of the airfoil proximate to the laminate layer. The airfoil cooling system may also include a trailing edge supply channel that extends radially within the airfoil and a plurality of trailing edge cooling channels extending generally chordwise within the airfoil for exhausting cooling fluids from the airfoil. 
     During use, the cooling fluids, which may be, but are not limited to, air, flow from a cooling fluid source, such as, but not limited to, a compressor, to a cooling fluid supply manifold proximate to the airfoil platform. The cooling fluids are then distributed to cooling channels forming the preheating system by flowing through the platform, such as the leading and trailing edge cooling fluid supply holes and the platform cooling channels. The cooling fluids increase in temperature as the cooling fluids flow through the platform. In at least one embodiment, the cooling fluids may increase between about 200 degrees Celsius and about 300 degrees Celsius. The cooling fluids collect in the airfoil cooling fluid supply chamber proximate to the airfoil and are passed into the airfoil cooling system. 
     In at least one embodiment, the cooling fluid preheating system in the platform and the airfoil cooling system form a single cooling system in which the cooling fluids are not supplemented with additional cooling fluids along the length of the cooling pathway from a first end of the cooling system in the platform to an exhaust of the cooling system in the airfoil. In this embodiment, cooling fluids flow into the leading edge cooling fluid supply chamber of an airfoil and into pressure and suction side channels. The cooling fluids increase in temperature while flowing through the airfoil, thereby causing the temperature of the airfoil to decrease. The cooling fluids are then passed from the pressure side and suction side channels into a trailing edge cooling fluid supply chamber. The cooling fluids are then passed into trailing edge cooling channels and exhausted from the airfoil through the trailing edge cooling channels. 
     By preheating the cooling fluids in the platform, the temperature gradient that exists between the outer surface of the airfoil and the inner surfaces of the airfoil cooling channels is reduced when compared with conventional cooling systems. The reduction in the temperature gradient between outside surfaces of the airfoil and the cooling fluids in the cooling channels in the airfoil advantageously reduces the thermal stress encountered by the airfoil and therefore, increases the life of the airfoil. Such stress reduction increases the viability of use of composite blades in turbine engines. 
     An advantage of this invention is that the platform cooling system greatly reduces the temperature gradient across an airfoil in a turbine engine, and thus, reduces the thermal stress in the airfoil as well. The reduction in thermal stress is particularly advantageous for composite airfoils that are susceptible to damage, such as delamination between layers and destruction of bonds, resulting from temperature gradients. 
     Another advantage of this invention is that the platform cooling system makes use of airfoils formed from composite materials more viable by reducing the thermal stresses that damage composite materials. Because composite airfoils can withstand higher temperatures than conventional metals used to form airfoils, less cooling fluids are needed in composite airfoils. In fact, use of composite airfoils may reduce the total cooling fluid flow requirement by approximately 90 percent. Such a reduction in cooling fluid flow can greatly improve the efficiency of the gas turbine engine in which the platform cooling system is installed. 
     These and other embodiments are described in more detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the presently disclosed invention and, together with the description, disclose the principles of the invention. 
         FIG. 1  is a perspective view of an airfoil having features according to the instant invention with an outer layer removed revealing inner cooling channels. 
         FIG. 2  is a cross-sectional view of the airfoil shown in  FIG. 1  taken along section line  2 — 2 . 
         FIG. 3  is a perspective view of platforms of the airfoil shown in  FIG. 2  taken at detail  3 — 3 . 
         FIG. 4  is a detailed cross-sectional view of the airfoil shown in  FIG. 2 . 
         FIG. 5  is a cross-sectional view of the airfoil shown in  FIG. 1  taken along section line  5 — 5 . 
         FIG. 6  is a detailed perspective view of the airfoil shown in  FIG. 1  at detail  6 — 6 . 
         FIG. 7  is a detailed view of a trailing edge cooling channels in the airfoil shown in  FIG. 1  taken at detail  7 — 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1–7 , this invention is directed to a platform cooling system  10  usable to preheat cooling fluids in a turbine engine. The platform cooling system  10  is particularly useful in composite airfoils  12  in which temperature gradients cause layers of the airfoil  12  to delaminate and bonds between adjacent layers to break. In addition, the platform cooling system  10  may be incorporated with an internal cooling system  14  in the airfoil  12  such that the airfoil is cooled with cooling air in a serial cooling manner such that cooling fluids passed through the platform cooling system  10  flow through the airfoil  12  without being supplemented by lower temperature cooling air while in the airfoil  12 . 
     The platform cooling system  10  may receive cooling fluids from a compressor (not shown) or other source, increase the temperature of the cooling fluids, and pass the cooling fluids on to a cooling system  14  in the airfoil  12  with a temperature that is greater than a temperature of the cooling fluids entering the platform cooling system  10 . In at least one embodiment, the cooling fluids, which may be, but are not limited to, air, may be heated between about 200 degrees and 300 degrees in the platform cooling system  10 . 
     As shown in  FIG. 1 , the platform cooling system  10  may be positioned in a platform  16  of an airfoil  12 . The platform cooling system  10  may be included in either an outer diameter (OD) region  15  or an inner diameter (ID) region  17 , or both. For simplicity of discussion, the platform cooling system  10  will be discussed as being in the OD region  15 , but, as previously discussed, the platform cooling system  10  may also be in the ID region  17  alternatively or in addition to being in the OD region  15 . The platform cooling system  10  may be in communication with a cooling fluid source (not shown) and a cooling system  14  in the airfoil  12 . 
     In at least one embodiment, as shown in  FIG. 5 , the airfoil  12  may be formed from a composite airfoil  12 . The composite airfoil  12  may be formed from a monolithic structure or a multi-component structure. In at least one embodiment, the composite airfoil  12  may be formed from an inner core  18 , a laminate layer  20 , and a thermal barrier coating  22 . The laminate layer  20  may be, but is not limited to being, a ceramic matrix composite material having an outer surface  24  defining the airfoil  12 . The ceramic matrix composite may be any fiber reinforced ceramic matrix material or other appropriate material. The fibers and matrix material surrounding the fibers may be oxide ceramics or non-oxide ceramics, or any combination thereof. The ceramic matrix composite may combine a matrix material with a reinforcing phase of a different composition, such as, but not limited to, mullite/alumina, or of the same composition, such as, but not limited to, alumina/alumina, mullite/mullite or silicon carbide/silicon carbide. The ceramic matrix composite may also be reinforced with plies of adjacent layers being directionally oriented to obtain the desired strength. In at least one embodiment, the laminate layer  20  may be formed from A-N720, which is available from COI Ceramics, San Diego, Calif. with mullite-alumina Nextel 720 reinforcing fibers in an alumina matrix. The thermal barrier coating  22  may be formed from the composition described in U.S. Pat. No. 6,197,424 or other appropriate material. As shown in  FIG. 5 , the thermal barrier coating  22  may have a larger thickness near the leading edge  28  than at the trailing edge  32  as the heating load on the leading edge  28  is greater than the heating load on the trailing edge  32 . The inner core  18  may be, but is not limited to being, AN-191, which is available from Saint-Gobain, Worcester, Mass. 
     In at least one embodiment, as shown in  FIGS. 2 and 4 , the platform cooling system  10  may be formed from one or more cooling channels  31  for preheating cooling fluids in the platform  16 . The cooling channel  31  may be in communication with a cooling fluid source and with the cooling system  14  of the airfoil  12 . The leading and trailing edge cooling fluid supply holes  26 ,  30  may be sized with cross-sectional areas to accommodate the amount of cooling fluids needed by the cooling system  14  in the airfoil  12 . The leading and trailing edge cooling fluid supply holes  26 ,  30  may also have any configuration necessary to give cooling fluids the necessary resident time in the platform  16  to sufficiently increase in temperature. 
     In at least one embodiment, the cooling channel  31  may include one or more leading edge cooling fluid supply holes  26  in the platform  16  positioned proximate to a leading edge  28  of the airfoil  12 , and may also include one or more trailing edge cooling fluid supply holes  30  in the platform  16  positioned proximate to the trailing edge  32  of the airfoil  12 . As shown in  FIG. 4 , the leading and trailing edge cooling fluid supply holes  26 ,  30  may be positioned at angles  36 ,  38 , respectively, which may be between about 30 and 60 degrees relative to a longitudinal axis  40  of the airfoil  12 . The leading and trailing edge cooling fluid supply holes  26 ,  30  may be positioned at the same angle or at different angles. In an alternative configuration, a central cooling channel may extend through the platform  16  in a central region not near the leading or trailing edges  28 ,  32  of the airfoil  12 . 
     The leading and trailing edge cooling fluid supply holes  26 ,  30  supply cooling fluids to the plurality of platform cooling channels  31 . The platform cooling channels  31  pass cooling fluids through the platform  16 , thereby increasing the temperature of the cooling fluids, and supply cooling fluids to an airfoil cooling fluid supply chamber  34  positioned at the interface between the platform  16  and the airfoil  12 . As shown in  FIG. 6 , the platform cooling channels  31  may extend along an outer surface of the platform. The platform cooling channels  31  may have any shape and configuration capable of sufficiently preheating the cooling fluids flowing through the platform  16 . In at least one embodiment, as shown in  FIG. 6 , the platform cooling channels  31  are positioned generally parallel to each other and extend along the platform  16 . The platform cooling channels  31  may be formed on an outer surface of the platform  16  and covered with an outer layer  33 , as shown in  FIG. 2 . In at least one embodiment, the outer layer  33  may be formed from a fiberglass cloth or other appropriate material. 
     The airfoil cooling fluid supply chamber  34  may have any configuration appropriate for receiving cooling fluids from the platform cooling channels  31  and passing the cooling fluids to the cooling system  14  in the airfoil  12 . As shown in  FIG. 4 , the airfoil cooling fluid supply chamber  34  may be an elongated chamber positioned between the interface of the platform  16  and the airfoil  12 . The airfoil cooling fluid supply chamber  34  may be in communication with the cooling system  14  through one or more airfoil supply holes  42 . 
     The cooling system  14  may be any cooling system  14  capable of adequately cooling the airfoil  12 . In at least one embodiment, as shown in  FIGS. 2 ,  5 , and  6 , the cooling system  14  may be a serial cooling system in which the cooling fluids are not supplemented with reduced temperature cooling fluids as the cooling fluids flow through the airfoil  12 . Instead, the cooling fluids flow throughout the entire airfoil cooling system  14  without having cooling fluids added. As shown in  FIGS. 2 ,  5 , and  6 , the cooling system  14  may be formed from a leading edge cooling fluid supply chamber  44  that extends radially along the leading edge of the airfoil  12  and a trailing edge supply channel  54 , as shown in  FIGS. 1 ,  2 , and  7 , positioned in the trailing edge for receiving cooling fluids from pressure side and suction side channels  48 ,  52 . The cooling system  14  may also include a plurality of pressure side channels  46  positioned in the inner core  18  proximate to the laminate layer  20  on the pressure side  48  of the airfoil  12  and a plurality of suction side channels  50  positioned in the inner core  18  proximate to the laminate layer  20  on the suction side  52  of the airfoil  12 . The pressure and suction side channels  46 ,  50  extend generally in the chordwise direction from the leading edge cooling fluid supply chamber  44  to the trailing edge supply channel  54 . The cooling system  14  may also include a plurality of trailing edge cooling channels  56  in the trailing edge  32  of the airfoil  12  extending generally chordwise from the trailing edge supply channel  54  to the trailing edge  32  of the airfoil  12 . 
     The platform cooling system  10  may also include a cooling supply manifold  58  positioned between a shroud  60  and the platform  16 . The shroud  60  may include one or more cooling fluid supply holes  62  providing a pathway for cooling fluids through the shroud  60 . As shown in one embodiment in  FIG. 3 , the shroud  60  may include four cooling fluid supply holes  62 . 
     During operation, cooling fluids, which may be but are not limited to, air, may be channeled from a compressor, or other source, to a cooling supply manifold  58  through cooling fluid supply holes  62 . The cooling fluid collects in the cooling supply manifold  58  and flows through the one or more leading and trailing edge cooling fluid supply holes  26 ,  30  to the platform cooling channels  31 . As the cooling fluids flow through the cooling fluid supply holes  26 ,  30  and the platform cooling channels  31 , the cooling fluids increase in temperature. In at least one embodiment, the airfoil  12  may be exposed to gases having temperatures of about 1,600 degrees Celsius, and the cooling fluid entering the platform cooling system  10  may be about 450 degrees Celsius. After flowing through the platform cooling system  10 , the cooling fluids may increase in temperature between about 200 degrees Celsius to about 300 degrees Celsius. Thus, the cooling fluids may be about 650 degrees Celsius to about 750 degrees Celsius upon entering the cooling system  14  in the airfoil  12 . The cooling fluids collect in the airfoil cooling fluid supply chamber  34  and are passed through airfoil supply holes  42  into the cooling system  14  of the airfoil  12 . The cooling fluids then flow through the leading edge cooling fluid supply chamber  44  and into the pressure and suction side channels  46 ,  50 , where the cooling fluids increase in temperature. The cooling fluids collect in the trailing edge supply channel  54  with a temperature of about 750 degrees Celsius and are distributed into the trailing edge cooling channels  56 . The cooling fluids are expelled from the airfoil  12  through the trailing edge  32  of the airfoil  12 . 
     In at least one embodiment, the cooling system  14  of the airfoil  12  may be configured such that the cooling fluids received from the platform cooling system  10  are not further supplemented with cooling fluids in the airfoil  12  along the route to the exhaust holes in the trailing edge  32 , as described in detail above. Rather, the airfoil cooling system  14  receives cooling fluids from the platform cooling system  10  and passes those cooling fluids through the airfoil  12  without receiving cooling fluid supplements. The cooling fluids may be injected into the leading edge cooling fluid supply channel  44  from either the platform cooling system  10  in the OD region  15  or the platform cooling system  10  in the ID region  17 , or from both. Thus, cooling fluids may flow through an ID platform cooling system  10 , through the airfoil cooling system  14 , and be expelled out of the airfoil  12  without being supplemented by additional cooling fluids, or cooling fluids may flow through an OD platform cooling system  10 , through the airfoil cooling system  14 , and be expelled out of the airfoil  12  without being supplemented by additional cooling fluids, or cooling fluids may flow from OD and ID platform cooling systems  10 , through the airfoil cooling system  14 , and be expelled out of the airfoil  12  without being supplemented by additional cooling fluids. Such a cooling configuration is practical in composite airfoils  12  as composite airfoils  12  as capable of being operated at a higher temperature than conventional metal airfoils and, as a result, require less cooling fluids to prevent damage from thermal stress. 
     The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.