Patent Publication Number: US-6666646-B1

Title: Drag reduction for gas turbine engine components

Description:
The invention relates to a gas turbine (both air and land based) component consisting of a blade or other engine component in a gas flow. The goal is to establish an efficient interaction between the gas flow and the subject engine component. In gas turbines, the purpose of the gas flow is either to provide speed to or to rotate the subject engine components, or for the engine component to accelerate the gas flow or to give the gas flow a change in direction. Suitable components include blades, vanes, stators and rotors. The interaction between the gas flow and the subject engine component is of major importance, and at the same time the gas flow should be optimized. These engine components typically comprise a Ni, Co, Ti, Al or Fe-based alloy. 
     One of the devices that are being used for controlling of aerodynamic drag is riblets. For application of riblets in industries other than the turbine industry, these devices are used in the form of plastic or polymers. As an example, 3M aerospace commercially manufactures a product that mainly consists of polymers. A polyethylene cover or which silicon-material is applied on the inside, and on which using a adhesive (based on acrylate) a thermoplastic polyurethane is applied. On top is this layer, a rib-like fluorpolymer layer is applied. It shall be understood that although the material as is described above theoretically could be used on gas turbine engine components, practically it will not be possible to use plastic because of the high temperatures during engine operation. 
     The present invention is defined in the annexed independent claims  1 , 11  and  23 , thereof. 
     The objective of this invention is to reduce the drag of gas flow on such engine components. 
     For the engine components as described above, this objective can be realized by application of different kind of patterns on the gas flow surface of engine components, for example longitudinal riblets with a length of at least 5 mm and a height of at least 0.02 mm and a width of at least 0.01 mm. The riblets are a series of elongated projections on the surface, the projections being arranged side by side and extending lengthwise substantially in the direction of fluid flow relative to said surface for modification of a boundary layer of fluid flow on said surface. The number, height, length and width of the riblets are selected to reduce drag of the gas flow on the gas flow surface of the component. 
     It shall be understood that the length of the riblets for an array of riblets can have a constant value, but it is also possible that an array of riblets have a varying length, with the minimum length of 5 mm for individual riblets. Preferably at least 10 columns of riblets are applied to the component surface. 
     This invention describes a method known as High Velocity Oxyfuel method (HVOF) to apply a metallic and/or ceramic material as riblets on a substrate surface. Surprisingly it appears that if such a metallic/ceramic powder material is applied at high velocity and elevated temperatures it is still effective for the use on engine components to reduce the drag. Using this technique it is possible to optimize the surface of the engine components, even though the engine components are exposed to elevated temperatures, typically temperatures of greater than 500° C., for example 1000° C. and above. 
     Using the HVOF technique powder is applied at high velocities on the substrate via a nozzle. Ceramic and/or metallic powder is injected in the system as well as fuel. Generally to this fuel oxygen is added. The fuel can contain Kerosine (liquid), acetylene, propane, propene, propylene, MAPP-gas or hydrogen gases. At the exit of the nozzle, the flow is surrounded by an air shield. At very high temperatures combustion takes place. Large amounts of gasses, resulting from the combustion, accelerate the injected powders. Argon or nitrogen continues to accelerate the injected powders. With a velocity of 1.2-2.4 km/sec (4000-8000 feet) per second the powders collide with the substrate surface. 
     As is described above, various material powders that can be applied on the engine components using the above-mentioned technique. As an example of an alloy that can be applied using the HVOF method the following material can be applied: 11-12.5 wt % Cobalt; 5.0-5.5 wt % Carbon; 1.0 wt % Iron; and Tungsten base. It shall be understood that this is just an example and other alloys and ceramics can be applied using the HVOF technique, including Ni, Co, Al, W, Cr or Fe based alloys and/or carbides thereof. Advantageously the materials used for the riblets can be selected for the additional purpose of providing erosion resistance. 
     Using the HVOF technique, it is possible to manufacture coatings with an extremely high tensile bond strength. Experiments have revealed vales for the tensile bond strength as high as 844 kg/cm 2  12000 psi. The tensile bond strength can be improved by adding a second coating layer, consisting of, for example Inconel 718. Other suitable coatings can include Cr, Ni, Co, Al, W or Fe-based alloys. This coating can be applied by any technique that is commonly known and is usually applied over the entire section of the component surface which will be provided with a riblet coating in a later stage. The riblets or other pattern can be created using the HVOF thermal spraying technique utilizing a mask positioned in between the exit of the nozzle and the component substrate surface. This mask may be a sieve consisted of an array of adjacent parallel wires. As a result of the positioning of the mask in between the nozzle and the substrate, shadow masking will create the troughs and ridges of the riblets. 
     The mask dimensions can be adjusted to create an optimum riblet profile. The wire dimensions can be varied between 0.04 and 0.14 mm and the distance between the wires can be varied between 0.02 and 0.04 mm. The mask consist of a heat resistant material (eg. tungsten). 
     Another production method to create riblets can be described as follows: the first layer of the coating is applied without the mask, which results in a layer of uniform thickness (eg. 0.04-0.05 mm). On top of this layer, the mask will be positioned as is described above to create the riblet profile. 
     For producing an another type of pattern than riblets such as provision, a mask in the form of metallic plate is used with openings corresponding to the pattern being deposited. The thickness of the mask should be preferably between 0.02 and 0.03 mm, depending on the necessary height of pattern. 
     In a further method to create riblets a layer is created by the application of a coating by the HVOF technique, followed by machining/grinding a profile of riblets into the coating (eg. by electro discharge machining electro-chemical grinding or conventional grinding). 
    
    
     The inventions will be described in detail below, referring to drawings giving an illustration of the production method. 
     FIG. 1 gives a schematic view of the HVOF process and illustrates the application of a coating on an engine component. 
     FIG. 2 gives a schematic view of the mask that is used to create the riblet profile. 
     FIG. 3 gives a schematic view of an EDM electrode having a surface corresponding to the riblet profile. 
    
    
     In FIG. 1, the assembly is shown ( 1 ) consisting of a nozzle ( 2 ), which sprays metallic and/or ceramic powders in the molten/atomized condition at a very high velocity at the surface. The mask ( 3 ) is positioned in between the nozzle and the substrate surface. The intention of the invention is to apply riblets or other pattern such as provisions to reduce to drag onto the surface of an engine component ( 4 ). Prior to the application of the riblets, a bondcoat ( 5 ) will be applied on the component surface ( 4 ). This bondcoat ( 5 ) can consist of material that originates from nozzle ( 2 ), but can also consist of any other material applied by any other commonly known method to apply coatings. As an example, Inconel 718 material can be applied as bondcoat. 
     The nozzle is provided with several feeders for gas as well as for powders. The powder is injected into the nozzle via feeder ( 9 ). A carrier gas (argon or nitrogen) accelerates the powder particles ( 8 ). Prior to entrance into the nozzle, oxygen is added to the fuel ( 7 ). A ceramic cover ( 10 ) protects the nozzle against the high temperatures and pressure and prevents the nozzle from wear. The external part of the nozzle ( 2 ) is protected against the conditions inside of the nozzle by means of compressed air ( 6 ). This compressed air also acts as a shield over the exiting molten powder particles. The injected powder material that can exist for instance of cobalt/tungsten-carbide alloy is molten at very high temperatures (approximately 2000° C.) at the edit of the nozzle in the combusted fuel-oxygen gas mixture. The temperature at combustion can be as high as 2200-3200° C. At these elevated temperatures, shock waves are developed, shown on the figure as diamonds ( 12 ). Downstream the flow will relax slightly and atomization will be established ( 13 ). At very high velocity the particles will pass the vertical wires of the mask ( 3 ) and subsequently will collide with and adhere to the bondcoat ( 5 ). 
     FIG. 2 shows a detail of the mask in the form of sieve. This sieve exists of end faces ( 15 , 15 ), whereby heat resistant wires ( 16 ) are tightened between the end faces. The distance between the heat resistant wires and the thickness of the individual wires relates to the required pattern consisting of riblets on the substrate surface ( 4 ). 
     An example of a riblet profile with a length of 20 mm and a riblet height of 0.07 mm and a distance between the riblet-tops of 0.05 mm applied using the technique as described above, can be given. 
     In the example, the subject engine component is a high pressure compressor rotor blade. 
     In an alternate embodiment the riblets are machined into the HVOF coating by electro discharge machining. The EDM electrode will have a working surface corresponding to the surface of the engine component into which the riblets will be applied. As shown in FIG. 3 the EDM electrode  20  has a working surface  21  which corresponds to the riblet profile  22  of the HVOF coating  23  on the engine component  24 . 
     It should be understood that the invention as given above is described based on a preferred situation. Numerous adjustments on the process are possible without getting outside the range of this invention as is described in detail in attached claims.