Patent Application
20060263613
The present invention relates generally to components having variable reflectivity and more particularly to absorptive gas turbine engine components with reflective coatings that have a transparency to incident radiation that is temperature dependent.
Gas turbine engines and other machines often operate under a variety of changing conditions and usually operate for extensive periods of time at high temperatures, resulting in the need to have effective heat transfer mechanisms. Various methods for controlling heat transfer are used. One such method includes optical tailoring of surfaces of the machines and their components. Based on known operating temperatures for a known operating cycle, surfaces of gas turbine engine components or other machines may be desired to have more reflectivity or more absorptivity to reflect or absorb incident radiation while also aiding in the control of heat transfer from or to the components.
A problem arises because many machines, including gas turbine engines, operate at different cycles and at different temperatures over the course of operation. A surface of a machine component that is ideally reflective at one temperature under a particular operating condition may have better thermal qualities if it is more absorptive at a different temperature experienced under a different operating condition.
A variety of optical coatings and surface treatments are available to modify certain surfaces to be more or less reflective and to assist with heat transfer control based on a determination of whether a more reflective or more absorptive surface is desired under a particular operating condition. These coatings include black and white paints, surface roughness treatments, and evaporated or sputtered optical coatings applied to the surfaces of machine components.
These conventional optical tailoring techniques have a fixed optical behavior. For example, a conventional coating such as a smooth layer of platinum or other noble metal applied to a component surface remains reflective regardless of the operating environment, even when it may be desirable from an engineering standpoint for that surface to exhibit absorption at a particular temperature. Thus, if a particular component surface desirably exhibits reflective behavior at one temperature and absorptive behavior at another temperature during ensuing operation, a decision must be made prior to placing the component in service to select an optical coating that provides either a reflective or an absorptive surface under all operating temperatures. Because it is not possible to re-treat internal surfaces of gas turbine engines during operation, gas turbine engine components treated by conventional optical tailoring techniques are likely to exhibit inefficient heat transfer at various points during operation.
Accordingly, it may be desirable to provide a component for use in a gas turbine engine or other machine that has a reflectivity that varies at different operating temperatures, such that at different temperatures, the component's reflectivity changes to provide a desired level of reflectivity or absorption of incident radiation.
The present invention uses a coating whose transparency is temperature dependent. The temperature-dependence of the transparency of the coating, being substantially transparent at high temperatures or temperatures above the coating glass transition temperature and substantially opaque but reflective at temperatures below the glass transition temperature, is used to provide a component that has an emissivity that is temperature dependent.
A component for use in an elevated temperature environment having temperature dependent reflectivity is disclosed. The component comprises a substrate having an optically absorptive surface and a coating disposed over and in contact with the optically absorptive surface of the substrate. The coating has a glass transition temperature and a transparency to incident radiation of greater than about 50% above the coating glass transition temperature and a reflectivity to incident radiation greater than about 50% below the coating glass transition temperature.
A method for optically tailoring a surface of a gas turbine engine component to control heat transfer is also disclosed. The method comprises the steps of providing a component of a gas turbine engine having an optically absorptive surface, coating the optically absorptive surface of the component with a material having a glass transition temperature and providing cooling to a surface of the component opposite the optically absorptive surface. The material used in coating has a higher transparency to incident radiation above the glass transition temperature than below the glass transition temperature, at which lower temperatures the coating is reflective.
An advantage of the present invention is that the component has an emissivity that is temperature dependent. As a result, the component has emissivity and heat transfer properties that vary with the environmental conditions to which it is exposed.
Another advantage of the present invention is that at high temperatures, the component absorbs incident radiation, increasing the amount of heat absorbed and thus the amount of heat transferred from the engine to the bypass stream. Increasing the amount of heat transferred to the bypass stream also decreases the likelihood that the component will absorb sufficient energy to act as a radiation emitter since the removal of heat to the bypass stream will limit the temperature rise of the component. When the component is used in a gas turbine engine in an aircraft, the ability to control the temperature of the component and thereby reduce the emittance of radiation, in particular IR radiation, reduces the ability of certain detection systems to detect the aircraft.
Other features and advantages of the present invention will be apparent from the following more detailed description of exemplary embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.
A high bypass aircraft gas turbine engine 10 is shown schematically in
Operation of a low bypass gas turbine engine, shown schematically at 30 in
The combustor 18 and high pressure turbine 22 sections typical of such engines 18 as in
The hot gases of combustion then leave the combustor and enter the high pressure turbine 22, which may comprise a single stage, as shown in
The rotor turbine 70 is comprised of a plurality of blades, each having an airfoil section 72 and a platform 74, which are securely attached to the periphery of a rotating disk 78. Important associated structures to support the rotor are not shown. The blades cooperate with a stationary shroud 76 to effect a gas seal between rotor 70 and the stationary components of the engine. Blades are protected with cooling air and coatings in manner similar to the vanes 62.
Downstream of the fuel nozzle 46, the gas flow path is defined by the inner surfaces of the inner combustor liner 52 and the outer combustor liner 50, and portions of the turbine or turbines including the inner and outer bands 64 and 66, the vanes 62, which direct the flow of gas, the airfoil 72, which extracts energy from the hot gas, the shrouds 76, as well as the exhaust system 26 and/or augmentor 28 aft or downstream of the turbine section of the engine. The present invention is specifically applicable to those components which define the gas flow path downstream of the swirl nozzle 46 but upstream of the augmentor 28. Systems for providing cooling air and thermal barrier coating systems are well-known in the gas turbine engine art.
Materials employed in the combustor, turbine and exhaust system sections of aircraft gas turbines are typically high temperature superalloys based on nickel, cobalt, iron or combinations thereof. All of these superalloys are believed to be suitable substrate materials for the present invention. Also, ceramic materials may be employed in the combustor, turbine and exhaust systems sections of an aircraft gas turbine. Such ceramic materials are specifically contemplated for use in the present invention, and may have higher temperature limits than the high temperature superalloys currently used for combustors.
Even for gas turbine engines designed for commercial airliners, gas velocity through the engine may approach the speed of sound. Thus, the total gas residence time in the engine is but a small fraction of a second, during which time air entering the engine through the compressor is mixed with liquid fuel, and combustion of the mixture occurs. As the mixture is combusted to form a gas, heat, including radiant heat, is generated. Even with the most recent advances in cooling measures used in gas turbine engines, such as active cooling controls and advanced thermal barrier coating systems, which reduce the amount and/or rate of heat transferred to components due to convective and conductive heat transfer, the temperatures of the components along the flow path surface are still elevated to very high temperatures. The present invention assists in reducing the amount of heat transferred to these components by radiation transfer.
Exemplary embodiments of the invention are directed to components that may be used as gas turbine engine components and which have reflectivity that is temperature dependent. The component comprises a substrate having an absorptive surface and a coating disposed over the substrate that has a reflectivity/transparency that varies with temperature. As used herein, the term reflectivity shall be used to describe both the reflectivity and the transparency of the coating, which properties vary inversely. A coating having high reflectivity has low transparency, and a coating having low reflectivity has high transparency.
At ambient temperatures, the coating of the present invention has a low transparency, but is greater than 50% reflective, typically highly reflective, such as 90% or more reflective. At low temperatures of operation, below the glass transition temperature of the coating, radiation incident upon the coating is reflected back into the engine, with very little of the radiation being absorbed. This makes the amount of energy in the hot gas stream more energetic, thereby improving the efficiency of the turbine as more energy is available for extraction.
The coating of the present invention does not act as a reflector under all conditions. As the operating temperatures of the engine increase, such as when there is a transient, when more power is required by the pilot, such as during take-off or landing, or, for combat aircraft, when combat situations dictate maximum power, the coating transforms from low transparency (high reflectivity) to high transparency. This characteristic is related to the glass-transition temperature of the coatings of the present invention. Above the glass-transition temperature, the coating is highly transparent.
Because of the power transients discussed above, the temperatures of the hot gas stream are increased, which increases the incident radiation on engine components. Thus, the temperature of the coating also increases. Once the glass transition temperature is exceeded, the coating becomes transparent.
The transparent coating reveals the absorptive surface of the substrate below, and thus the radiation is absorbed by the component, converted to heat, and transferred by heat conduction mechanisms to the cooling bypass flow on the surfaces of the component opposing the hot gas stream. The specific chemical and physical properties of the material underlying the coating is an important aspect of the invention and its operation as discussed below.
As the material underlying the coating is heated, the internal temperature of the material increases. Unless the heat is conducted away from the material, the temperature will continue to rise until it becomes sufficiently hot so as to emit radiation (i.e., black-body radiation), typically infrared (IR) radiation. Cooling is provided to the component below the coating and the incident radiation. Typically the flow is across the back-side of the component opposite the coating, but it is not so limited and may include, for example, cooling channels extending through the component. The flow of air across this back-side surface conducts heat away from the material and prevents the temperature from continuing to increase.
The component comprises a layered material system with the coating having a temperature-dependent transparency as the layer exposed to hot gases of combustion, or high temperatures. Immediately underlying this coating is an absorptive surface. The absorptive surface may be an absorptive coating applied to a substrate or it may be the substrate itself which may be produced to have a highly absorptive surface. When the absorptive surface is itself an absorptive coating applied to the substrate, the substrate may be defined to include the absorptive coating materials overlying the substrate.
When the temperature dependent coating is exposed to hot gases that result in temperatures that raise the coating above the glass transition temperature, as previously noted, the coating becomes transparent to incident radiation, typically IR. The radiation is transmitted through the coating to the component and converted into heat, which heat is transferred away by the cooling bypass flow. The coating additionally can provide environmental protection to the absorptive surface.
Referring now to
The coating 110 of the present invention is any material that has a transparency to incident radiation that is different at different temperatures and that has a high reflectivity at temperatures below its glass transition temperature, and selection may further depend on the location where the coating to be applied. For example, where the coating 110 is used in an aircraft engine, engine inlets and other areas exposed to relatively lower temperatures may be coated with various fluoropolymers, preferably polytetrafluorethylene. For higher temperature applications, lithium silicate glass, may be used, such as RE-X glass available from General Electric Company of Schenectady, N.Y. for example. However, it will be appreciated that the material of the coating 110 may be any non-absorptive crystallizing glass, as long as the desired material is serviceable at the expected range of temperatures to which the coating will be exposed.
At temperatures below the glass transition temperature, the coating 110 is a diffuse reflector, providing a reflective surface on the component 100 for reflecting heat transfer away from the component 100. The coating 110 reflects at least 50% of incident radiation and preferably reflects at least about 90%, most preferably at least about 95% of incident radiation at temperatures below the glass transition temperature in the frequency region of interest. Because the coating 110 is opaque and reflects the radiation, the underlying absorptive surface 122 absorbs only small amounts of the heat or the radiation. Because the operating temperatures are low, the components are not sufficiently elevated to pose a problem with black-body radiation. Of course, the operating temperatures increase and the temperatures of the components also increase in a corresponding manner. As temperatures increase, at temperatures above the glass transition temperature of the coating 110, the optical properties of the coating 110 change and the coating 110 becomes at least about 50%, typically at least about 60%, transparent, with a low degree of reflection, exposing the optically absorptive surface 122 of the substrate 120 to incident radiation. Preferably the coating 110 becomes highly transparent, permitting up to 90% or more of the incident radiation to pass into and be absorbed by the component 100. When heated above the glass transition temperature, the component 100 becomes a better conductor of heat across the material from the hot surface to a cooler surface according to standard thermodynamic principles, the cooler surface provided by the cooling bypass flow.
The coating 110 may be of any thickness sufficient to provide the desired change in optical properties and thus affect the component's 100 heat transfer properties. Typically, the coating 110 is about 10 mils (0.254 mm) or greater in thickness and is more typically about 20 mils (0.508 mm) thick. Coatings 110 may be applied using any conventional techniques for coating turbine engine parts, such as slurry spraying or plasma spraying.
Incident radiation reflected from the component 100 or absorbed by the component 100 includes heat and electro-magnetic radiation, including infrared, visible and ultraviolet radiation. By absorbing radiation, converting it to heat and transferring the heat away from machinery components by heat transfer, the internal temperatures of the components are lowered, improving the lifetime of the component, particularly in high stress, high temperature environments such as gas turbine engines where components may be subjected to temperatures approaching or above their melting point.
In the absence of effective transfer of incident radiation, radiation is absorbed by the component and converted to heat, causing its internal temperature to increase. If the heat absorbed outpaces heat removed, the components may become a source of radiation emission. For example, the components may emit radiation in the form of infrared electromagnetic radiation, making the gas turbine engine and any aircraft or other vehicle attached thereto more readily visible to infrared devices. By converting radiation into heat and carrying it away from engine components and into the cooler bypass stream which exits to the atmosphere, the components are less likely to have sufficient energy to be significant sources of radiation emission, reducing its detection signature and decreasing visibility to infrared devices.
In addition to modifying the optical characteristics to provide a change in the reflectivity of the component 100, the coating 110 also serves as an environmental coating over the substrate 120. As previously discussed, gas turbine engine components are typically constructed of high temperature superalloys based on nickel, cobalt, iron or combinations thereof.
It should be appreciated that in some cases, the substrate 120 may not have a surface with a sufficient level of absorption to produce the desired heat transfer characteristics. Thus, the use of an optical coating that is transparent at high temperatures may not result in a major increase in absorption of incident radiation if the underlying substrate does not have an absorptive surface. To overcome this, the substrate may first receive a layer of absorptive material on its surface to produce a compound substrate as shown in more detail with respect to
In
A coating 210 of a material having an elevated glass transition temperature is applied over the compound substrate 220 to produce the component 200. As discussed with respect to
It should further be appreciated that the layer of absorptive material 215 may be a material having temperature dependent transparency as described in the related and previously mentioned U.S. application Ser. No. ______. In this manner, the coating 210 is reflective until its glass transition temperature, at which point the coating 210 becomes primarily transparent and incident radiation passes through the coating 210 and is absorbed by the component 200 via the layer of absorptive material 215. As the absorption of radiation causes the temperature to further increase, the glass transition temperature of the layer of absorptive material 215 is also reached. Thus, the layer of absorptive material 215 becomes transparent revealing the reflective surface of the base substrate 205 and reflecting the heat back into the gas stream. In this manner, the invention may be used in a manner to absorb and transfer heat to the bypass flow only over a specified range of temperatures, which may be advantageous in certain applications.
When exemplary embodiments of the invention are used as components of a machine in a high temperature environment, such as may be experienced in a gas turbine engine, for example, it may be desirable to also apply a thermal barrier coating (TBC) to the components as shown in
It will be appreciated by those of ordinary skill in the art that ceramic materials, such as those found in the substrate or TBC coatings according to exemplary embodiments of the invention, often have rough or irregular surfaces. Accordingly, these materials may serve the dual purpose of a TBC and an absorptive coating.
Components having variable reflectivity according to exemplary embodiments of the invention may advantageously be used as components of gas turbine engines. Components of a gas turbine engine which may advantageously use embodiments of the present invention include any of those components previously discussed and specifically include, by way of example only, combustor casings and linings, flaps and seals, ejectors and any other part cooled by backside airflow in a bypass engine.
It should be appreciated that different locations of the gas turbine engine may operate at different temperatures. Accordingly, the coating may be selected so that the material used has a glass transition temperature that matches the desired transition temperature. Coatings having different glass transition temperatures may be used in different sections of the engine.
Exemplary embodiments of the invention may further be appreciated by way of the following, non-limiting investigation conducted to demonstrate a component having variable reflectivity.
A sample component according to an exemplary embodiment of the invention was produced by obtaining anodized aluminum and covering it with a 20 mil layer of polytetrafluoroethylene (PTFE). At ambient temperature, the glass had approximately 98% reflectivity, resulting in a large amount of incident radiation being reflected by the sample component. The sample size was 2 inches square. A Nicolet FTIR spectrometer was used to measure the changes in absorption and reflectivity. The temperature was elevated from ambient temperature to about 326° C., above the polymer's Tg. It was determined that above the Tg, the PTFE transitioned to about 3% reflective, resulting in an absorption of about 95% of incident radiation by the sample.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This application is related to application Ser. No. ______, Attorney Docket No. 120700 (07783-0170), entitled “Temperature Dependent Transparent Optical Coatings for High Temperature Reflection” filed contemporaneously with this Application and which is also assigned to the assignee of the present invention and which is hereby incorporated by reference in its entirety.
