Discreetly defined porous wall structure for transpirational cooling

This invention relates to through-wall fluid cooling of wall structures, and particularly to transpiration cooling of components in gas turbine engines.

In a gas turbine engine, hot section components, such as combustor liners, transition ducts, turbine airfoils, and turbine rings, require cooling airflows which are typically drawn from the high-pressure compressor. This air bypasses the combustion chamber, reducing the efficiency of the machine. There is a long-standing need for cooling schemes that require the use of less cooling air for improved engine efficiency and emission control.

Impingement and convection cooling have been used on interior surfaces of component walls (backside cooling). Film cooling has been used on exterior surfaces exposed to the hot combustion gas to cool the boundary layer of hot gas flow. Exterior surfaces may have thermal barrier coatings (TBC) for thermal insulation. Component walls are typically solid except for film cooling passages through the walls. Component structural walls are mostly cooled through heat conduction through the wall, interior surface coolant impingement and convection, and exterior surface film cooling.

In transpiration cooling, a coolant such as air is forced through a porous wall. It provides convection cooling inside the wall and efficient film cooling via outlet pores on the hot exterior surface of the wall. Transpiration cooling of gas turbine component walls has been implemented in various ways that generally fall into two categories: 1) holes drilled through the wall or formed by casting or molding the wall around removable pins. 2) Fibrous or other randomly porous material, including partially sintered ceramic or metal, metal felt or foam, and ceramic felt or foam. Randomly porous materials are generally not be suitable for high temperature, high load-bearing structures because they have random anisotropies due to uneven distributions of voids and solids that can cause weak points and uneven cooling. In addition, random fibrous structures are not inherently geometrically rigid.

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a sectional view of a first layer of a porous wall structure.

FIG. 2 is a sectional view of a second layer of the porous wall structure.

FIG. 3 is a sectional view of a porous wall structure formed of alternate layers of FIGS. 1 and 2 . The view is taken along line 3 - 3 of FIGS. 1 and 2 .

FIG. 4 is a perspective and sectional view of a 3-D porous wall structure according to aspects of the invention.

FIG. 5 is a perspective view of a lattice embodiment of the invention.

FIG. 6 is a perspective view of a waffle-layered embodiment of the invention.

The invention provides a porous wall structure with discretely defined cooling passages. The term “discretely defined” means individually engineered or determinate (having known geometry and location), rather than random. The structure may have a layer geometry in which voids in adjacent, layers partially overlap to form tortuous 3-D passages. Lamination layer-by-layer may be used for fabrication, or the structure may be cast as described herein or otherwise made in 3-D form, for example by direct metal laser sintering. Exemplary layer geometries are illustrated both individually and in integrated sequences.

FIG. 1 shows a first layer geometry A, with a first end 20 , a second end 22 , a solid portion 24 A, and a first void pattern 26 A. FIG. 2 shows a second layer geometry B with a first end 20 , a second end 22 , a solid portion 24 B, a second void pattern 26 B, a third void pattern 28 B, and a fourth void pattern 30 B. FIG. 3 shows a porous structure 32 formed by stacking layers A and B in an alternating sequence, and bonding them. The first ends 20 of the layers form an interior wall surface 21 that is subject to a coolant fluid 34 . The second ends 22 form an exterior wall surface 23 that is subject to a hot gas flow 36 . The coolant 34 is maintained at a higher pressure than the hot gas 36 , and it follows serpentine paths 38 through the wall structure 32 .

Some voids 28 B are open to the interior wall surface 21 , forming inlet holes 29 . Other voids 30 B are open to the exterior wall surface 23 , forming outlet holes 31 . Void shape may provide metering. For example, the void pattern 30 B shows outlets 31 constricted by tapering as seen in FIG. 2 . Metering may also be provided by limiting the degree of overlap 40 between voids of layers A and B.

FIG. 4 shows a 3-D porous wall structure 42 formed by sequences of three layer geometries A, B, and C. Layers A and B function as described for FIGS. 1-3 . Layers C provide pockets 44 in the exterior surface 23 of the wall to contain thermal barrier material 46 on a bond coat 47 . This embodiment provides transpiration and film cooling plus insulation between rows of coolant outlets 31 . It has a self-compensating flow distribution 39 , allowing flow around insulated portions 46 . This characteristic also allows flow around any blockage, such as a large contaminant particle. Coolant from adjacent layers will flow laterally within the structure to fill a pressure deficit.

The layers herein may be non-parallel to a plane of the wall. Herein, “a plane of the wall” means the plane of the interior wall surface 21 or the plane of the exterior wall surface 23 . In the case of curved walls, it means a plane tangent to the interior wall surface 21 or tangent to the exterior wall surface 23 . The layers may or may not be normal to a plane of the wall, and they may or may not all have the same thickness.

Known processes may be used to create the structures herein. Various additive manufacturing technologies are known to create 3-D objects by successive layering of materials under computer control. They copy successive slices of a numeric solid model into the material. For example, direct metal laser sintering can directly produce complex 3-D forms in metals such as the superalloy Inconel 625 and others, and can directly form a production component. Selective laser sintering can produce 3-D forms in plastic, ceramic, or glass that can be used for tools, dies, molds, or cores to produce structures of the invention. 3-D printing may also be used to produce such elements in plastic or plaster.

FIG. 5 shows a 3-D lattice embodiment 70 of the invention, composed of nodes 72 and links 74 . It may have geometry like that of a crystal lattice model, and may have a scale of 0.1-5.0 mm spacing per unit cell in one embodiment (a node and its directly attached links). This forms a meta-material with characteristics determined by the lattice geometry, scale, and thicknesses of the nodes and links. Rigidity can be controlled in part by the lattice geometry. For example, in FIG. 5 rigidity and flow restriction can be increased by adding vertical links between nodes lacking them (exemplified by nodes 76 , 77 , 78 ) and/or by adding horizontal links between nodes lacking them (exemplified by nodes 77 and 79 ) and/or by adding additional nodes such as halfway between nodes 77 and 79 . Such additional vertical links, horizontal links and additional nodes will impact rigidity and flow restriction in the respective directions to a different degree, thereby providing the designer with flexibility to customize a particular porous structure to a particular application. Herein, orientation terminology such as “vertical”, “horizontal”, and “lateral”, is used for descriptive purposes relative to a drawing viewpoint. A layer of this embodiment may be defined as a plane of interconnected lattice unit cells. The scale, geometry, and complexity of this structure may be engineered such that the flow passages allow passage of particles at least up to a selected size, such as 10 microns in diameter, thus defining a minimum scale of the lattice by particle size.

FIG. 6 shows an embodiment 80 formed by layers of waffle structures D, E. Each layer D and E has opposed open-ended cells 82 , especially rectangular cells. Square cells are shown. Each cell has a bottom wall 84 , which may or may not have a pass-through hole 86 . Two adjacent layers D and E may have the same geometry. They may be offset from each other vertically or diagonally by up to half a cell spacing. In FIG. 6 , the layers D and E are offset from each other by half a cell diagonal length 88 . Using this offset, adjacent layers D, E contact and connect to each other only at crossing areas 83 on each side wall of each cell. These crossing areas 83 may be enlarged relative to a thickness of the cell side wall as shown.

This vertical and/or diagonal offset allows air to flow both vertically and horizontally between the overlapping voids of the cells 82 . Three exemplary flow paths 38 - 38 A, 38 - 38 B, and 38 - 38 C are shown. An air flow 38 enters through inlets in the layer in front of layer E, not shown. An exemplary inlet 29 in layer D and an outlet 31 in layer E are shown. If pass-through holes 86 are provided, air can flow between the layers D, E for additional routing 38 C as shown. If the layers D, E are offset vertically instead of diagonally, and they do not have pass-through holes 86 , air can only flow vertically in a serpentine path between alternating layers.

This invention provides superior cooling effectiveness and lower cooling flow requirement. It provides serpentine cooling passages and is suitable for load-bearing elements. A characteristic of the invention is a repeating pattern of interconnected voids and interconnected solids that is determinate, not random, thus forming a structure that can be fully engineered to predictably handle cyclic thermal and mechanical loads. “Determinate” does not mean that the pattern of voids and solids cannot vary along a wall. It means that the dimensions and locations of the solid and voids forming the serpentine paths in the wall are specified and known. For this reason, the geometry and distribution of the solid and the porosity may be fully engineered. Methods herein describe fabrication of airfoils and other components with structural walls having determinate and complex coolant passages. Porosity can be varied from the cold side to the hot side to effectively distribute cooling air through the structure.

This structure offers strength and predictability to carry mechanical and thermal loads for specific applications, such as in component walls in the hot gas flow path of gas turbines. For example, the layers described herein may form sequential cross sections of a gas turbine airfoil, which may be built-up from one end to the other by lamination or additive manufacture, or may be cast as described herein to form either a full airfoil or sections that are bonded together to form a full airfoil.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.