Patent Application
20060263209
The present invention relates generally to components having variable reflectivity and more particularly to reflective gas turbine engine components with 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 temperatures above the coating glass transition temperature and substantially opaque at temperatures below the glass transition temperature, is used to provide a component that has a component emissivity that is temperature dependent, being a low-emissivity component at high temperatures and a high emissivity component at lower temperatures.
A component for use in an elevated temperature environment having temperature dependent emissivity is disclosed. The component comprises a substrate having an optically reflective surface and a coating disposed over and in contact with the optically reflective surface of the substrate. The coating has a glass transition temperature and a transparency to incident radiation of less than about 50% below the coating glass transition temperature and greater than about 50% above 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 reflective surface, coating the optically reflective surface of the component with a material having a glass transition temperature and providing cooling to a surface of the component opposite the optically reflective surface. The material used in coating has a higher transparency to incident radiation above the glass transition temperature than below the glass transition temperature. Thus the component has a high reflectivity, low emissivity surface whose optical properties are changed by the temperature dependent transparency of the coating. When the coating is transparent, radiation passes through said coating to be reflected off of the component. When the coating is opaque, the radiation is absorbed in the coating and no reflection takes place.
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. The component will change from low emissivity at high temperatures to high emissivity at low temperatures.
Another advantage of the present invention is that at high temperatures, the component reflects incident radiation into the exhaust gas stream, reducing the amount of heat absorbed by the engine and thus reducing or delaying thermal degradation of the component, improving its useful lifetime. Reducing the amount of heat absorbed by the component also decreases the likelihood that the component will absorb sufficient energy to act as a radiation emitter. When the component is used in a gas turbine engine in an aircraft, it permits the reduction the emittance of radiation, in particular IR radiation.
Yet another advantage of the present invention is that the coating can be applied over conventional gas turbine engine component materials in lieu of costly precious metals to provide a reflective surface that can withstand harsh operating conditions without excessive corrosion.
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. 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 a reflective surface and a coating disposed over the substrate that has a transparency/opacity that varies with temperature. As used herein, the term transparency shall be used to describe both the transparency and the opacity of the coating, which properties vary inversely. A coating having high transparency has low opacity, and a coating having low transparency has high opacity.
At ambient temperatures, the coating of the present invention has a low transparency. At low temperatures of operation, radiation incident upon the coating is absorbed, with very little of the radiation being reflected. The radiation is absorbed by the coating, converted to heat, and transferred by heat conduction mechanisms to the material underlying the coating. 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 operating temperatures increase and the material underlying the coating is heated, the internal temperature of the material increases. Unless the heat is conducted away from the material, it will become sufficiently hot so as to emit radiation (i.e., black-body radiation), typically infrared (IR) radiation and melt the superalloy. 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 coating of the present invention does not act as an absorber 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 operation of the augmentor, the coating transforms from low transparency (high opacity) 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 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 a reflective surface. The reflective surface may be a separate reflective coating applied to a substrate or it may be the substrate itself which may be produced to have a highly reflective surface. When the reflective surface is itself a reflective coating applied to the substrate, the substrate may be defined to include the reflective coating materials overlying the substrate. The reflective coating materials may be applied between the coating with the temperature-dependent transparency and the substrate and/or to a surface of the substrate opposite the coating with the temperature-dependent transparency.
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. However, rather than being absorbed, the radiation is incident upon the reflective surface, which is of a specular optical quality and is reflected back through the transparent coating into the hot gas stream. The optical quality is typically in the range of a surface roughness of about 0.5 to about 20 microinches RA, preferably less than about 8 RA, most preferably less than about 4 RA, with R>0.9. The reflected energy provides several advantages. Because the reflected energy is not absorbed by the component, the amount of cooling required on the side of the component opposite the temperature-dependent transparent coating does not substantially increase as transients are experienced. In addition, the radiation is reflected back into the gas flow, so the amount of energy in the hot gas stream is more energetic, thereby improving the efficiency of the turbine as more energy is available for extraction. The coating additionally can provide environmental protection to the reflective 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 is typically a glass having a low absorption coefficient in the range of about 0.005 to about 0.05. Exemplary materials include borosilicate glass, available as PYREX from Corning Glass Works Corporation of Corning, N.Y., lithium silicate glass, available as RE-X glass from General Electric Company of Schenectady, N.Y., potassium germinate glass, barium gallium germinate glass, and arsenic sulfide glass, by way of example only.
The glass preferably includes pigments or dopants to produce colored glass, but only in an amount that does not adversely affect the transparent characteristics of the glass at elevated temperatures (having absorption coefficients in the 0.005-0.05 range). Colored glass may enhance the amount of radiation absorbed versus clear glass at temperatures where the coating 110 exhibits absorptive optical characteristics. Exemplary pigments comprise a compound selected from the group consisting of barium fluoride, calcium fluoride, strontium fluoride, magnesium fluoride, silicon/silicon monoxide and combinations thereof.
At temperatures below the glass transition temperature, the coating 110 is a diffuse absorber, providing an absorbing surface on the component 100 for heat transfer into and through the component 100. The coating 110 absorbs at least about 50%, typically at least about 60%, of incident radiation and preferably absorbs at least about 80%, most preferably at least about 85%, of incident radiation at ambient temperature. The amount of absorption and/or transmission may vary slightly depending on the indices of the refraction of the glass and pigment, as well as the size and shape of pigment particles. Because the coating 110 is opaque and absorbs the radiation, the underlying reflective surface 122 cannot reflect the heat or the radiation and becomes a conductor of heat across the material from the hot surface to a cooler surface according to standard thermodynamic principles. 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 absorption, exposing the optically reflective surface 122 of the substrate 120 to incident radiation. Preferably, the coating 110 becomes highly transparent, reflecting up to 90% or more of the incident radiation away from the component 100 by the optically reflective surface 122 back into the gas 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 electromagnetic radiation, including infrared, visible and ultraviolet radiation. By reflecting radiation away from machinery components, the internal temperature of the components is 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, as well as reducing their detection signature.
Absent reflection of incident radiation, radiation is absorbed by the component and its internal temperature increases. 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 electro-magnetic radiation, making the gas turbine engine and any aircraft or other vehicle attached thereto more readily visible to infrared devices. By reflecting incident radiation away from engine components and into the gas stream, the components are less likely to have sufficient energy to be significant sources of radiation emission, 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. In some cases, these materials may already have, or be polished to have, reflective surfaces for heat transfer. However, these materials are also subject to oxidation and corrosion, which reduces the effective reflectivity of the surface over the components' service life. When the gas turbine engine is a CMC, an optically reflective surface must be applied to it.
As is known in the art, precious metals resistant to oxidation and corrosion, such as platinum, palladium, gold and rhodium may be applied over gas turbine engine components to protect the components from corrosion and still provide a reflective surface. In addition to their high cost, precious metals provide a fixed level of reflectivity that does not vary with temperature. Of course, these materials may be applied over the component to provide the optically reflective surface if desired.
By using a coating 110 with a variable transparency in accordance with exemplary embodiments of the invention, costs may be reduced by avoiding precious metals. The optically reflective surface 122 of the substrate 120 is still protected from corrosion and oxidation and at high temperatures, the transparency of the coating 110 permits incident radiation to be reflected away from the component 100 without the use of expensive precious metals.
It should be appreciated that in some cases, the substrate 120 may not have a surface with a sufficient level of reflectivity to produce the desired heat transfer characteristics, such as a CMC matrix. Thus, the use of an optical coating that is transparent at high temperatures will not result in a major increase in reflectivity to incident radiation if the underlying substrate does not have a reflective surface, for example, when ceramic materials are used to form the substrate. To overcome this, the substrate may first receive a layer of reflective 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
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, to achieve a level of optical reflection in a component using ceramic materials, it may be necessary to polish or otherwise finish a rough surface prior to applying a layer of reflective material. This in turn reduces the thickness of the reflective material needed to provide an optically reflective surface versus a surface that is reflective in a diffuse manner.
Components having variable reflectivity according to exemplary embodiments of the invention may advantageously be used as components of gas turbine engines and especially those portions of the gas turbine engine aft of the combustion chamber. 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, the exhaust nozzle, the nozzle liner, the combustion liner, exhaust strut, seals, and flaps.
It should be appreciated that different locations of the gas turbine engine may operate at different temperatures. For example, in a gas turbine engine that includes an augmentor, a component forward of the augmentor may have a surface that should transition from absorptive to reflective at a different temperature than a component aft of the augmentor, where aft of the augmentor designates the portion of the engine closer to the exhaust and further from the turbine portions of the engine. 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 invention.
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 an aluminum mirror and covering it with a 20 mil layer of sodium silicate glass doped with iron. At ambient temperature, the glass had approximately 80% absorptivity, resulting in a large amount of incident radiation being absorbed by the sample component. The sample size was 2 inches square. A Nicolet FTIR spectrometer having a diffuse reflection attachment was used to measure the changes in absorption and reflectivity. When the temperature was elevated to about 425° C., above the glass's Tg, the glass became 95% transparent to incident radiation revealing the reflective surface of the aluminum mirror beneath the glass. It was determined that at this level of transparency, the sample component had a reflectivity of about 92% of incident radiation, while the overall absorption was about 8%. This demonstrated an order of magnitude decrease in the total amount of radiation absorbed by the structure at ambient temperature.
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 No. ______, Attorney Docket No. 120699 (07783-0169), entitled “Temperature Dependent Transparent Optical Coatings for High Temperature Absorption” 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.
