Patent Publication Number: US-7717677-B1

Title: Multi-metering and diffusion transpiration cooled airfoil

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine airfoil with film cooling holes. 
   2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
   A gas turbine engine includes a turbine section that has a plurality of stages of stator vanes and rotor blades reacting to a high temperature gas flow passing through the turbine to convert the chemical energy from combustion into mechanical energy by rotating the turbine shaft. The efficiency of the turbine, and therefore of the engine, can be increased by increasing the hot gas flow that enters the turbine. 
   To allow for higher turbine entrance temperatures, the upper stage vanes and blades are made from exotic nickel alloys that can withstand very high temperatures and have complex internal cooling air passages to provide cooling to these airfoils. A thermal barrier coating (TBC) is also applied to the airfoil surfaces exposed to the hot gas flow in order to provide further protection from the heat. A TBC is typically made from a ceramic material. Also, the TBC is typically applied after the film cooling holes have been drilled into the airfoil surface to provide for the film cooling. These film cooling holes are limited to the diameter because of the drilling process. 
   Thicker TBC layers have been proposed to provide more protection to the airfoil substrate from the high temperature gas flow. As the TBC gets thicker, the thermal stresses developed in the TBC will tend to cause spalling. 
   It is therefore an object of the present invention to provide for an improved high temperature resistance coating applied to a turbine airfoil. 
   It is another object of the present invention to provide for a high temperature resistant coating with smaller diameter film cooling holes. 
   It is still another object of the present invention to provide for a process of forming small film cooling holes in a high temperature resistant coating on a turbine airfoil. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a turbine airfoil with a new spar airfoil cooling construction that utilizes a multi-metering diffusion compartmental cooling apparatus in conjunction with a transpiration cooling process and a thermal sprayed refractory protective coating to achieve a cooled wall for the external protective coating layer. The airfoil wall includes a plurality of diffusion chambers opening onto the outer wall surface and having cooling air supply passages opening onto the back surface. A ceramic material core having the shape of fine cooling air passages is placed in the diffusion chamber and a refractory material such as iridium or rhodium is sprayed over the airfoil to form the high temperature resistant coating. The ceramic core is then leached out, leaving in its place the fine film cooling holes. The combination of the cooling and construction process greatly reduces the airfoil coating and backing metal substrate temperature and improves the durability of the coating layer which provides for a reduction of cooling flow to improve the turbine stage performance and prolong the airfoil life. 
   In the prior art, thin refractory material is used in the turbine airfoil cooling design to provide protection for the airfoil and therefore reduce the cooling flow consumption and improve the turbine efficiency. As the turbine inlet temperature increases, the cooling flow demand for cooling the airfoil will increase and thus reduce the turbine efficiency. One prior art process for reducing the cooling air consumption while increasing the turbine inlet temperature for higher turbine efficiency is by using thicker coating on the airfoil external surface. At the same time, the cooling design becomes more reliant on the coating&#39;s endurance and the coating becomes the “prime reliance” in the cooling design. The disadvantages, associated with this approach is that the thicker the coating, the higher will be the coating surface temperature. Therefore cooling through the coating for the reduction of the external heat load onto the airfoil and special cooling flow management methods and mechanical attachment treatment for the thick coating is required. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  shows a cross sectional view of the multi-compartmented transpiration cooled airfoil of the present invention. 
       FIG. 2  shows a close-up view of a cross section of three ceramic cores placed within the diffusion chambers of the airfoil wall of the present invention. 
       FIG. 3  shows a detailed view of one of the ceramic cores used to form the fine film cooling holes in the airfoil coating of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a turbine airfoil, such as a stator vane or a rotor blade, used in a gas turbine engine in which the airfoil requires film cooling and a high temperature resistant coating to protect the airfoil from the high temperature gas flow. However, the invention is not limited to turbine airfoils. The invention could apply to any substrate material that uses a high temperature resistant coating to provide additional protection to the metal substrate. For example, the combustor liner of a gas turbine engine could also use this invention. Also, other high temperature resistant substrates that are used in an apparatus other than a gas turbine engine. 
   The airfoil  10  of the present invention is shown in  FIG. 1  in which a plurality of cooling supply compartments or channels  11  are formed by the airfoil walls and ribs  12  extending from the pressure side wall to the suction side wall. Four compartments  11  are shown in  FIG. 1 . However, more or less than four compartments can be used with the present invention. The airfoil walls include a plurality of film cooling holes  20  spaced around the airfoil to provide film cooling to a thermal resistant coating or layer that is applied over the airfoil walls.  FIG. 2  shows a more detailed view of these film cooling holes. 
   In  FIG. 2 , the airfoil wall includes an inner surface facing the cooling air supply compartment  11  and an outer surface on which a high temperature resistant coating  31  is applied. The airfoil wall includes a number of diffusion chambers that are connected to a metering hole  14 . The metering hole  14  is connected to the cooling air supply compartment  11 . The cavity formed in the airfoil wall has a “fish bowl” shape with a wide bowl portion below the top surface and a narrower throat portion connecting the bowl to the airfoil surface at the opening. The metering hole  14  connects the cooling air supply compartment  11  to the “fish bowl” cavity or compartment. A ceramic core  20  is placed within the diffusion chamber and sticks out from the chamber as shown in  FIG. 2 . The “fish bowl” compartment functions like a dove-tail slot to hold the ceramic core  20  within the cavity that forms the diffusion chamber  15 . 
   The ceramic core  20  is shown in detail in  FIG. 3  and includes a first diffusion portion  22 , a second metering portion  23 , a second diffusion portion  24 , and a plurality of multiple film holes extending out from the second diffusion portion  24 . The size of these film holes can be in the range of 0.005 inches to 0.01 inches which is beyond the manufacturing capability by means of drilling. The number of film holes extending out can vary depending upon how many fine film cooling holes are to be supplied by the first metering hole  14 . 
   Referring to  FIG. 2 , a plurality of the ceramic cores  20  are placed within the diffusion chambers formed in the airfoil spar or wall. A refractory material such as a composition that includes Iridium and Rhodium is sprayed onto the airfoil surface with the ceramic cores held in place to form the refractory coating  31 . One reason why a refractory coating is used instead of a ceramic TBC is that the refractory material may not leach out when the ceramic core  20  that forms the multiple film cooling holes is leached out. If a ceramic TBC was applied, the ceramic core  20  and most of the TBC around the ceramic core may break down. The coating is applied such that the film holes  25  stick out through the surface of the finished coating  31 . After the coating  31  is applied, the ceramic core material is leached out from the airfoil wall, leaving the cooling air passages and fine film holes formed within the coating  31 . The ceramic cores  20  are made from a similar ceramic material as the core ties used to form the internal cooling passages within the turbine airfoils. And, the ceramic leaching process to form the cooling passages is the same as the leaching process in the present invention that is used to form the cooling passages and fine film cooling holes in the coating  31 . 
   The present invention is a turbine airfoil a multi-compartment with multi-metering and diffusion plus transpiration cooling circuit in a spar airfoil for a highly cooled and thick coating. The multi-metering and diffusion plus transpiration cooling apparatus are constructed in small individual modules spaced along the airfoil spar or wall. Individual modules are designed based on airfoil gas side pressure distribution in both chordwise and spanwise directions. In addition, each of the individual modules can be designed based on the airfoil local external heat load to achieve a desired local coating surface temperature. The individual modules can be constructed in a staggered or an inline array for the transpiration film hole pattern along the airfoil main body wall. With the cooling construction of the present invention, the usage of cooling air for a given airfoil inlet gas temperature and pressure profile is maximized. Also, the multi-metering and diffusion cooling construction utilizes the multi-hole film cooling technique for the thick coating layer cooling as well as flow metering purpose and the spent cooling air discharges onto the airfoil surface forming a multi-hole film cooling array at very high film effectiveness levels. The combination effects of multi-hole film cooling plus the multi-metering and diffusion cooling flow yields a very high cooling effectiveness and a uniform wall temperature for the airfoil wall. 
   The airfoil spar comprises several internal cooling supply channels  11 . Each individual cooling air supply channel  11  is designed at different cooling air pressure and flow rates for tailoring the airfoil external local pressure and heat load requirements. In addition, a multiple grooved structure is cast onto the spar airfoil substrate. First metering holes located in the metal substrate can be machined into the grooved structure. The metering holes can be at the same pattern as the individual transpiration film cooling modules. Mini cores made of ceramic material with second multi-metering holes and diffusion chambers are then attached into the grooved structure on the spar airfoil substrate. Refractory coating is then thermally sprayed onto the attached individual modules. The ceramic core is then leached out from the thick coating layer, leaving the cooling air passages formed in the coating. 
   As a result of the process of the present invention, a transpiration cooled turbine airfoil with built in transpiration film cooling holes and multi-metering and diffusion cooling for a thick coating layer on a spar substrate is formed. Sizes for the transpiration film cooling holes are in the range of about 0.005 to 0.01 inches which is beyond the current manufacturing capability for drilled holes. Also, drilling a large number of film cooling holes into the thick coating layer will cause spallation of the coating material. 
   The multi-compartment multi-metering and diffusion cooling holes of the present invention utilizes the multi-hole cooling technique for backside convective cooling as well as flow metering purpose. The cooling air is metered and diffused twice in each individual cooling module. Thus, diffusion cavities at various size can be used in the grooved structure to diffuse the cooling air by slowing the velocity of the cooling air and dropping the cooling side pressure before discharging the cooling air onto the thick coating layer. The additional metering and diffusion cooling arrangement allows for cooling air discharge onto the mainstream through multi-holes and produces a protective film layer for the airfoil. Since the cooling air within the thick coating is reduced in momentum, coolant penetration into the gas path is minimized, yielding good buildup of the coolant sub-boundary layer next to the airfoil surface and a better film coverage in the chordwise and spanwise directions for the airfoil. The combination affects of multi-metering and diffusion plus multi-hole near surface cooling film cooling at very high film coverage yields a very high cooling effectiveness and a uniform wall temperature for the entire airfoil. 
   In operation, the cooling air is supplied to each individual cooling flow channel as design flow rate and pressure level. Cooling air then flows through the first metering holes within the airfoil spar wall and then is diffused into the first diffusion cavity within the grooved structure. The amount of cooling air for each individual compartment is sized based on the local gas side heat load and discharge pressure, which therefore regulates local cooling performance and metal temperature. The cooling air sir then further metered through the second metering hole which is built into the thick coating layer, impinging onto the backside of the outer coating wall first and then diffusing into the second diffusion chamber formed within the coating layer. This cooling air is then bled off from the second diffusion chamber through the multi-film cooling holes which are also formed within the thick coating layer and discharged onto the coating surface forming a highly effective film layer. 
   Since the first and second multi-metering and diffusion holes are connected in series, pressure ratio and the blowing ratio across the multi-film cooling holes can be regulated by setting the cooling air pressure level in the diffusion chamber or pressure ratio across the metering holes, and thus optimizing the cooling air distribution and velocity exit from the multi-film cooling holes. With the cooling circuit of the present invention, the usage of cooling air for a given airfoil inlet gas temperature and pressure profile is maximized.