Abstract:
A thermal mechanical fatigue test rig for testing a coating, such as a thermal barrier coating, under high temperature and pressure to simulate the actual operating environment of the coating. The test rig includes a combustor to produce a hot gas flow, a hollow test specimen on which the coating is placed, and a sapphire vessel that encloses the hollow test specimen to form a hot gas flow path over the coating. The sapphire vessel is clear so that the coating can be observed by a camera during the testing. An exhaust plenum is formed around the sapphire vessel to collect the exhaust form the hot gas flow in which additional cooling air and water for quenching can be injected to reduce the temperature of the hot gas flow prior to discharge from the test rig.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit to Provisional Application 61/103,490 filed on Oct. 7, 2008 and entitled THERMAL MECHANICAL FATIGUE TEST RIG. 
    
    
     FEDERAL RESEARCH STATEMENT 
     The US Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. FA8650-08-M-2836 awarded by the USAF/AFMC DET 1 AF RESEARCH LABORATORY. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to thermal barrier coatings, and more specifically to a test rig for testing various thermal barrier coatings under operating conditions. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     Existing known techniques for testing materials to be used in a gas turbine engine are very expensive or do not properly expose the testing material to actual engine operating conditions. One known method is to use an actual gas turbine engine and place the material to be tested on a part in the engine while the engine is operating. This method requires an operating gas turbine engine that is very expensive to operate. An engine test can test a material or a coating for: spallation due to high thermal gradients; erosion due to high velocity flow; corrosion degradation due to trace elements in fuel at operating temperatures and pressures; and, includes the ability to apply axial loading in addition to thermal loading to the test specimen. However, the engine test method is very expensive to operate (about $6,000 per hour to operate), the test conditions are limited to current technologies (pressures, temperatures, stresses) used in the specific testing gas turbine engine, availability of engine hardware, engine test facility, and large staffing requirements, and limited hot time accumulated (generally less than 300 hours). 
     A less costly method of testing that does not require an operating gas turbine engine is a burner rig. Existing rigs for testing turbine coating/material combinations use a hot flame impingement onto a material/coating specimen to ascertain material/coating durability under hot conditions. While these burner tests are more easily accomplished than full engine tests, are typically of low cost, and are sometimes satisfactory as a screening method, they fail to duplicate many of the parameters leading to material/coating failures observed in actual component designs. Of the conditions described above with respect to the engine test method, a burner rig can provide for a low cost method of testing materials, the burner rig does not allow for the testing for: coating spallation due to high thermal gradients; for erosion to high velocity flow; or for the ability to apply axial loading in addition to thermal loading to the test material. 
     Realistic engine gas path conditions include high thermal gradients in the test specimen, thermal and mechanical fatigue loading, and erosion due to high velocity gas flow. In real engines, the coating/material components are subjected to cyclical mechanical loading that can affect metal and coating durability and coating adhesion. In addition, the hot gas often contains trace contaminants that can cause corrosion of the metal/coating systems. High velocity gas flows can erode the gas path materials which also reduce their durability. Burner rigs are limited in that no mechanical loading can be applied to the specimen, and that the flow is not at high velocity so that TMF and erosion mechanisms are not duplicated in the test system. 
     Other complex systems are being developed for advanced testing of gas path materials. The Westinghouse Plasma Corporation&#39;s facility in Waltz Mills, Pa. uses a plasma torch to heat material specimens to high thermal loading and also includes mechanical loading capability to simulate TMF conditions. Currently the system is limited to heat flux levels less than 1.2 MBtu/hr/ft.sup.2. The system is also not able to support investigation of erosion failure mechanisms since there is no high velocity flow. Moreover, the ability to accurately measure temperature on the front and backsides of the specimen (to determine thermal gradient) is questionable. 
     A third system is under development by NASA as part of the Ultra Efficient Engine Technology (UEET) program. This system uses a laser generated heat flux to heat the specimen to high thermal gradients. The current system can achieve approximately 1 MBtu/hr/ft.sup.2. It is unknown if mechanical loading can be applied to the specimen, however, the system is limited in its ability to duplicate erosion failure mechanisms. Further, the system is not pressurized, but does have cooling through the middle of the specimen. 
     The degradation process that require characterization include coating erosion, spallation, thermal mechanical fatigue, low cycle fatigue, hold-time effects, as well as the interaction of these failure mechanisms. With extremely high cost of developing a new engine concept, especially when operating conditions will exceed all current experience, low cost test rigs are the prudent way to screen new concepts and materials prior to committing to actual engine hardware and full engine testing. 
     U.S. Pat. No. 7,174,797 issued to Brostmeyer et al on Feb. 13, 2007 and entitled HIGH TEMPERATURE AND PRESSURE TESTING FACILITY discloses a test facility for testing materials under high temperature, pressure, and mechanical loads. The facility provides a physically scaled system that simulates conditions in hot sections of gas turbine engines. A test article is coated with a test material and exposed to a hot combusting flow in a test section housing. The article may be a pipe or conduit member oriented at any direction to the flow. A second cooler flow of fluid is channeled through the test article to create a sharp temperature gradient in the test article and through the test material. A liquid-cooled sleeve is disposed about the test article to create an annular channel of combusting flow over the test article. The downstream end of the second cooler flow is connected to the upstream end of the main hot flow at the combustion chamber. The Brostmeyer et al test rig does not offer the capability to view the material being tested during the testing process. Also, this test rig does not offer easy access to the test material without having to disassemble the test rig. 
     There is a need in the prior art for a test rig that can provide a low cost way to test materials for use in gas turbine engines, as well as a test rig that can reproduce all the conditions such as high temperature, high pressure, erosion, corrosion, and thermal and mechanical loading, that occur in an operating gas turbine engine. 
     It is an object of the present invention to provide for an apparatus and a method that can test materials at a very low cost. 
     It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine. 
     It is another object of the present invention to provide for an apparatus and a method that can test materials at temperatures above the maximum temperature permitted by today&#39;s material limitations. 
     It is another object of the present invention to provide for an apparatus and a method that can test materials under axial and thermal loadings. 
     It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the material being tested can be visually observed during the testing. 
     It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the exhaust gas flow from the test rig has been cooled enough to prevent thermal damage to valves and conduits downstream from the test rig. 
     It is another object of the present invention to provide for an apparatus and a method to test materials under the extreme conditions operating in a gas turbine engine in which the hot gas flow passing over the material to be tested is clear so that the material being tested can be seen. 
     BRIEF SUMMARY OF THE INVENTION 
     The test rig is used to test a material such as a TBC under real conditions as would appear in a gas turbine engine. The test rig includes a test specimen tube with a coating applied on an outer surface in which cooling air flows through the inner surface. A sapphire vessel forms a hot gas flow path with the test specimen tube to channel a hot gas flow from a combustor in which the cooling air is delivered and burned with a fuel to produce the hot gas flow for testing. The sapphire vessel is clear so that the material can be observed visually during the testing. The sapphire vessel is surrounded by an annular exhaust plenum in which the hot gas flow is passed into through a hole formed on an end of the sapphire vessel. Cooling air flows around the annular exhaust plenum to provide cooling and then flows into the exhaust plenum to be mixed with the hot gas flow to lower the temperature. 
     A sapphire window is formed on the test rig casing and looks down into the annular exhaust plenum so that the material can be visually observed. The hot gas flow from the exhaust plenum is discharged through an exhaust tube that includes water injectors to inject water into the cooled hot gas flow to quench the hot gas flow and further cool it so that the exhaust will not damage valves or conduits located downstream from the test rig. The combustor burns natural gas to produce the hot gas flow, and the combustor operates at lean burning so that all of the fuel will be burned and the hot gas flow resulting will be clear such that the annular exhaust plenum is clear for viewing of the material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a cross sectional view of the test rig of the present invention. 
         FIG. 2  shows an isometric view of the sapphire vessel used in the test rig of  FIG. 1 . 
         FIG. 3   a  shows a cross section view of the sapphire vessel of  FIG. 2 . 
         FIG. 3   b  shows a front view of the sapphire vessel; of  FIG. 3   a.    
         FIG. 4  shows a cross sectional view of the test rig of the present invention with the flow of cooling air into the rear end of the exhaust plenum. 
         FIG. 5  shows a cross sectional view of the test rig of the present invention with the flow of cooling air into the forward end of the exhaust plenum and into the exhaust tube and exhaust port. 
         FIG. 6  shows a cross sectional view of the test rig of the present invention with the flow of cooling air through the test specimen tube and then into the combustor. 
         FIG. 7  shows a cross sectional view of the test rig of the present invention with the flow of hot gas from the combustor, over the coating on the test specimen tube and then into the exhaust plenum and out the exhaust port. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The thermal mechanical fatigue (TMF) test rig  10  of the present invention is shown in  FIG. 1 . The test rig is used for testing one or more coatings at a time under the operating conditions (high temperature, high pressure) simulating the conditions in a turbine of a gas turbine engine in which the coating would be exposed to but at significant lower cost. The test rig  10  includes a specimen port  11  for easy access to the test rig. A centering rod  12  having a bolt head on the outside end and a three pronged end on the inner end fits within a forward specimen grip  13  that has a hollow inner portion and a narrow opened portion on the inner end. A test specimen tube  14  is mounted within the test rig  10  and includes one end with a radial outward extending flange that is secured between a narrow opened portion of the forward specimen grip  13  and the three prongs of the centering rod  12 . The test specimen tube  14  is a hollow tube with an outer surface on which the material to be tested is mounted and an inner surface that forms a flow path for cooling air. The outer end of the test specimen tube  14  is secured into a hole formed within a rear specimen grip  15 . The rear specimen grip  15  includes a radial flange  16  that forms an abutment surface on the inside for a bellows seal  19  that is secured to a rear housing  17  opposite to a forward housing  18 . An air inlet port  20  is formed in the rear specimen grip  15  to supply cooling air to the combustor and the inner surface of the hollow test specimen tube  14 . 
     An igniter  21  is located in the forward end of the test rig and is used to ignite the fuel within the combustor  23 . A plurality of fuel injectors  22  is arranged around the combustor  23  in a radial inward direction to inject fuel circumferentially around the combustor  23 . The combustor  23  is a high swirl combustor in order to produce a longer burn zone and to produce a full combustion of the fuel. A combustor outer liner  24  is formed around the combustor  23  to form a cooling air supply passage for the air that is eventually supplied to the combustor. The combustor  23  includes air supply holes  25  spaced around to allow for the cooling air to flow into the combustor. 
     An annular shaped exhaust plenum  35  is formed by an outer cooling liner  26  secured to an inner cooling liner  27  with a sapphire vessel  31  on the inner side to enclose the exhaust plenum  35 . The sapphire vessel  31  forms a hot gas flow path with the outer surface of the hollow test specimen tube  14 . The purpose of the sapphire vessel  31  is to withstand the extremely high temperatures of the hot gas flow while allowing for the test material to be viewed from the outside the test rig  10 . Sapphire offers both this type of high temperature resistance and visibility.  FIGS. 2 and 3  shows the sapphire vessel  31  in more detail which includes an aft end with four slots  33  spaced around the end and two opening  32  on opposite sides to discharge the hot gas flow passing through the inner portion of the vessel and exhaust into the exhaust plenum  35 . The slots  33  fit within outwardly extending fingers formed on the test specimen tube  14 .  FIG. 3   a  shows a cross section side view of the sapphire vessel  31  which is made of a very high temperature resistant clear material that can withstand the high temperature combustion gas and allow for the coating on the test specimen tube  14  to be seen visually.  FIG. 3   b  shows a front view of the sapphire vessel  31  of  FIG. 3   a . The sapphire vessel  31  and the test specimen tube  14  form a closed path between them so that the hot gas flow from the combustor  23  will flow through the openings  32  and into the exhaust plenum  35 . The main function of the annular shaped exhaust plenum  35  with the cooling air injection is not to cool the plenum but to cool the hot gas flow from the combustor  23  prior to it being discharged from the test rig  10 . Without a cool down, the hot gas flow passing over the coating will be exhausted from the test rig at far too high a temperature. 
     The test rig casing includes an opening  36  covered with a sapphire window  41  that allows for the coating on the test specimen tube  14  to be viewed from outside the test rig  10 . The sapphire window  41  is secured in the opening  36  by an annular ring bolted to the casing as seen in  FIG. 1 . 
     The annular shaped exhaust plenum  35  is enclosed within the casing  52  and the aft end piece  17  to form cooling air flow paths between the casing  52  and the outer surface of the exhaust plenum  35 . Cooling air supply ports  41   a  and  41   b  allow for pressurized cooling air to flow through passages in the casing  52  and into a space  42  on the aft end of the exhaust plenum  35  and in the space formed between the outer surface of the outer cooling liner  26  and the inner surface of the housing or casing  52 . The aft end of the inner cooling liner  27  includes holes (arrows in  FIG. 4 ) to allow for the cooling air to flow from the space  42  and into the exhaust plenum  35 . Air supply port  41   a  supplies cooling air to the aft space between the outer cooling liner  26  and the casing  52 , air supply port  41   b  supplies cooling air to the space  42 , and air supply port  41   c  supplies cooling air to the forward space between the outer cooling liner  26  and the casing  52 . The cooling air supply ports  41   a - c  each are annularly spaced around the casing to inject the cooling air into the space in an annular manner to cool the annular exhaust plenum. 
     An exhaust port  53  is formed on the casing  52  to discharge the hot gas from the exhaust plenum  35  after the hot exhaust gas from the combustor has been diluted with cooling air. An exhaust tube  43  is secured to the outer cooling liner  26  to direct the exhaust into the exhaust port  53  and to form a cooling air flow path between the inner surface of the exhaust port  53  and the outer surface of the exhaust tube  43 . Cooling air flowing over the outer cooling liner  26  will eventually flow around the exhaust tube  43  and then into the exhaust port  53  to be merged with the hot gas exhaust from the exhaust plenum  35 . 
     The operation of the test rig is described below. A coating material to be tested under high pressure and temperature conditions is applied to the outer surface of the test specimen tube  14  in an area that can be viewed through the clear sapphire vessel  31 . The coating could be a TBC that will be used on a turbine blade or vane in the turbine section of a gas turbine engine.  FIG. 4  shows the path that the cooling air supplied to port  41   b  will take. The cooling air flows through the passage in the casing and into the space  42  and then into the holes formed in the back of aft end of the inner cooling liner  27  (as seen by the arrows) and into the exhaust plenum  35 . A number of cooling air ports  41   b  are annularly spaced around the casing so that enough cooling air flows around the inner cooling liner  27 . 
       FIG. 5  shows the cooling air supplied to ports  41   c  and  41   a  where the ports are spaced around the casing to form an annular array of cooling air injection ports just like ports  41   b . The cooling air from ports  41  and  41   c  flow around and over the outer cooling liner  26  and then between the space formed between the exhaust port  53  and the exhaust tube  43  (see the arrows in  FIG. 5 ) to join the hot gas flow from the exhaust plenum  35  and flow out from the exhaust port  53 . 
       FIG. 6  shows the cooling air flow through the test specimen tube  14  that eventually enters the combustion chamber to be burned with the fuel to produce the hot gas flow used in the testing rig. The cooling air enters through the cooling air inlet port  20  and flows through the hollow test specimen tube  14  from the rear or aft end, enters the space formed between the outer surface of the centering rod  12  and the inner surface of the forward specimen grip  13 , flows through radial ports  61  in the forward specimen grip  13  and into the space  51 , over the combustor outer liner  24  toward the aft end, through a space formed between the combustor  23  and the combustor liner  24  in an opposite direction, and then through two rows of annular openings  25  and into the combustor  23  to be burned with the fuel from the injectors  22 . This flow provides cooling for the combustor prior to being burned with the fuel in the combustor. The cooling air also provides cooling for the hollow test specimen tube  14  which is exposed to the hot gas flow from the combustor  23 . 
       FIG. 7  shows the flow of the hot gas that is produced in the combustor  23  with the fuel injected by the injectors  22 . The hot gas flows from the combustor  23  and through a passage formed between the sapphire vessel  31  and the hollow test specimen tube  14  and over the coating or coatings that are to be tested. The hot gas then flows through the two openings  32  formed near the aft end of the sapphire vessel  31  and into the exhaust plenum  35 , where the hot gas flow is merged with the cooling air that flows through the holes in the aft end of the inner cooling liner  27 . This reduces the overall temperature of the hot gas flow prior to e discharged from the test rig  10 . The mixture of hot gas flow and cooling air then flows out through the exhaust tube  43 , where the other cooling air is merged with the hot gas exhaust and flows into the exhaust port  53 . Quench water injection ports  44  spaced around the outlet of the exhaust port  53  injects water into the cooled hot gas flow to further cool the gas to a low enough temperature to prevent thermal damage to the valves and conduits downstream (in the hot gas flow path) from the test rig. 
     On the rear or aft end of the rear specimen grip  15  and connected to the radial flange  16 , a double acting hydraulic cylinder can be connected in order to apply a load to the hollow test specimen tube  14  in order to test the coating under high pressure and temperature while a tensile load is being applied to the coating. 
     Because the sapphire vessel  31  and the sapphire window  41  are both clear see-through materials that can be exposed to these high temperatures, the coating can be observed during testing with a camera or other video equipment. The fuel used in the combustor is natural gas and is burned lean with extra cooling air so that most of the natural gas is combusted. This leaves a clean exhaust that enters the exhaust plenum  35 , and when combined with the cooling air, does not block the view of the coating on the outer surface of the test specimen tube  14 .