Fluid reaction surfaces (i.e. – impellers) – With heating – cooling or thermal insulation means – Changing state mass within or fluid flow through working...
Reexamination Certificate
2001-03-27
2002-10-08
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With heating, cooling or thermal insulation means
Changing state mass within or fluid flow through working...
C416S24100B, C029S889100
Reexamination Certificate
active
06461108
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to gas turbine engines, and in particular, to a cooled flow path surface region of a turbine blade tip.
BACKGROUND OF THE INVENTION
In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary-type compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor and fan. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
During operation of gas turbine engines, the temperatures of combustion gases may exceed 3,000° F., considerably higher than the melting temperatures of the metal parts of the engine, which are in contact with these gases. Operation of these engines at gas temperatures that are above the melting temperatures of the metal components is a well established art, and depends in part on supplying a cooling air to the outer surfaces of the metal parts through various methods. The metal parts of these engines that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are metal parts forming combustors and parts located aft of the combustor including turbine blades, turbine vanes and exhaust nozzles.
The hotter the turbine inlet gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the turbine inlet gas temperature. However, the maximum temperature of the turbine inlet gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at metal surface temperatures of up to 2100°-2200° F.
The metal temperatures can be maintained below melting levels with current cooling techniques by using a combination of improved cooling designs and insulating thermal barrier coatings (TBCs). For example, with regard to the metal blades and vanes employed in aircraft engines, some cooling is achieved through convection by providing passages for flow of cooling air internally within the blades so that heat may be removed from the metal structure of the blade by the cooling air. Such blades essentially have intricate serpentine passageways within structural metal forming the cooling circuits of the blade.
Small internal orifices have also been devised to direct this circulating cooling air directly against certain inner surfaces of the airfoil to obtain cooling of the inner surface by impingement of the cooling air against the surface, a process known as impingement cooling. In addition, an array of small holes extending from the hollow core through the blade shell can provide for bleeding cooling air through the blade shell to the outer surface where a film of such air can protect the blade from direct contact with the hot gases passing through the engines, a process known as film cooling.
In another approach, a TBC is applied to the turbine blade component, which forms an interface between the metallic component and the hot gases of combustion. The TBC includes a ceramic coating that is applied to the external surface of metal parts within engines to impede the transfer of heat from hot combustion gases to the metal parts, thus insulating the component from the hot combustion gas. This permits the combustion gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. TBCs have also been used in combination with film cooling techniques wherein an array of fine holes extends from the hollow core through the TBC to provide cooling air onto the outer surface of the TBC.
Certain designs of airfoil tips utilize film cooling techniques. Film cooling is achieved by passing cooling air through discrete film cooling holes, typically ranging from 0.015″ to about 0.030″ in hole diameters. The film cooling holes are typically drilled with laser or EDM or ES machining. Due to mechanical limitations, each film hole has an angle ranging from 20° to 90° relative to the external surface. Therefore, each film jet exits from the hole with a velocity component perpendicular to the surface. Because of this vertical velocity component and a complex aerodynamic flow circulation near the tip of a turbine blade, commonly referred to as the “squealer tip”, each film jet will have a tendency to lift or blow off from the external surface and mix with the hot exhaust gases, resulting in poor film cooling effectiveness in the area surrounding the squealer tip.
TBCs are well-known ceramic coatings, for example, yttrium-stabilized zirconia (YSZ). Ceramic TBCs usually do not adhere well directly to the superalloys used in the substrates. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the thermal barrier coating. The bond coat may be made of a nickel-containing overlay alloy, such as a MCrAlX, or other compositions more resistant to environmental damage than the substrate, or alternatively, the bond coat may be a diffusion nickel aluminide or platinum aluminide, whose surface oxidizes to a protective aluminum oxide scale that provides improved adherence to the ceramic top coatings. The bond coat and the overlying TBC are frequently referred to as a thermal barrier coating system.
Multi layer coatings are known in the art. For example, U.S. Pat. No. 5,846,605 to Rickerby et al. is directed to a coating having a plurality of alternate layers having different structures that produce a plurality of interfaces. The interfaces provide paths of increased resistance to heat transfer to reduce thermal conductivity. A bond coat overlying a metallic substrate is bonded to a TBC. The TBC comprises a plurality of layers, each layer having columnar grains, the columnar grains in each layer extending substantially perpendicular to the interface between the bond coat and metallic substrate. The structure is columnar to ensure that the strain tolerance of the ceramic TBC is not impaired. The difference in structure of the layers is the result of variations in the microstructure and/or density/coarseness of the columnar grains of the ceramic.
U.S. Pat. No. 5,705,231 to Nissley et al. is directed to a segmented abradable ceramic coating system having enhanced abradability and erosion resistance. A segmented abradable ceramic coating is applied to a bond coat comprising three ceramic layers that are individually applied. There is a base coat foundation layer, a graded interlayer, and an abradable top layer. The coating is characterized by a plurality of vertical microcracks.
U.S. Pat. No. 4,503,130 to Bosshart et al. is directed to coatings having a low stress to strength ratio across the depth of the coating. Graded layers of metal/ceramic material having increasing ceramic composition are sequentially applied to the metal substrate under conditions of varied substrate temperature. Excessive stresses induced by differential strains between the layers is avoided. The effect of substrate temperature control and the differing coefficients of thermal expansion between materials of successive layers are matched to achieve the desired result.
U.S. Pat. No. 6,045,928 to Tsantrizos et al. is directed to a TBC comprising an MCrAlY bond coat and a dual constituent ceramic topcoat. The topcoat comprises a monolithic zirconia layer adjacent to the bond coat, a monolithic layer of calcia-silica representing the outer surface of the TBC and a graded interface between the two to achieve good adhesion between the two constituents to achieve an increased thickness of the topcoat, thereby, providing for a greater temperature drop across the TBC system. As used by Tsantrizos et al., monolithic refers to a uniform composition of a layer, while a graded interface refers to a layer having a changing composition from one monolithic composition to the other monolithic composition.
Darolia Ramgopal
Lee Ching-Pang
Schafrik Robert Edward
Maria Carmen Santa
McCoy Kimya N
McNess Wallace & Nurick
Narciso David
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