Cooling arrangement

0001The present invention relates to an arrangement of cooling holes in an aerofoil of a gas turbine engine.

0002Gas turbine blades and vanes, particularly those in the hot turbines, may require cooling and it is well known to provide a coolant flow to an internal passage of the component. Holes are provided through walls of the component so that the coolant removes heat from the wall and may form a coolant film over an external surface of the component.

0003<patcit id="pcit0001" dnum="US5062768A"><text>US5, 062, 768</text></patcit> discloses a turbine blade having a wall defining multiple angled cooling holes that intersect and have a single exit. Where the holes intersect there is a constriction. This arrangement is characterised in that the cross-sectional area of flow for the coolant is greater at the exit than at the intersection, thereby avoiding blockages at the exit.

0004<patcit id="pcit0002" dnum="US5370499A"><text>US5,370,499</text></patcit> discloses a turbine aerofoil wall having a mesh cooling hole arrangement which includes first and second pluralities of cooling holes between internal and external surfaces. The cooling holes of each plurality extend generally parallel to one another and intersect leaving internal nodes. Coolant jets interact with one another at these intersections and cause a restriction of the flow, thereby producing a pressure drop. The area of the flow inlets is substantially less than the area of the flow outlets.

0005Although these and other conventional cooling arrangements are adequate to cool aerofoils, there is a desire to reduce the amount of coolant required thereby minimising parasitic losses and improve the efficiency of the gas turbine engine. There is also a desire to tailor the amount of cooling in certain regions of the aerofoil that may be subject to a varying temperature profile.

0006Therefore it is an object of the present invention to provide an aerofoil with a more efficient cooling arrangement.

0007In accordance with the present invention an aerofoil for a gas turbine engine comprising a pressure wall and a suction wall and defining leading and trailing edges, the walls define a passage into which is supplied a cooling fluid, an array of cooling holes is provided through at least one of the walls to allow the cooling fluid to flow from an interior surface to an exterior surface; the array of holes comprise two groups, the holes of each group are angled to intersect the holes of the other group and are characterised in that the holes of at least one of the groups comprises two or more holes at different angles to one another to vary the porosity of the wall.

0008Preferably, the array of holes comprising the two groups in a single row of holes.

0009Alternatively, there are two or more row of holes, each row comprising the two groups.

0010Preferably, the array of holes comprises the two groups in two or more rows of holes.

0011Optionally, the two or more rows of holes are divergent.

0012Optionally, the two or more rows of holes are convergent.

0013Alternatively, the holes of at least one of the groups comprise all holes at different angles to one another.

0014Optionally, some of the holes of at least one of the groups comprise consecutive holes inclined at increasingly steep angles to one another.

0015Alternatively, some of the holes of both groups comprises consecutive holes inclined at increasingly steep angles to one another and together are positioned so that they both increase the porosity of the region.

0016Preferably, the porosity of the wall varies through the thickness of the wall.

0017Preferably, the porosity of the wall is greater at its exterior surface than its internal surface.

0018The present invention will be more fully described by way of example with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a schematic section of a ducted fan gas turbine engine;</li><li><figref idref="f0002">Figure 2</figref> is a view of a turbine blade;</li><li><figref idref="f0003">Figure 3</figref> is a section through a wall of the turbine blade showing first embodiment of an array of cooling holes in accordance with the present invention,</li><li><figref idref="f0003">Figure 4</figref> is a section through a wall of the turbine blade showing second embodiment of an array of cooling holes in accordance with the present invention.</li></ul>

0019Referring to <figref idref="f0001"><b>Figure 1</b></figref>, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis XX. The engine 10 comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and comprises the intake 11, two generally C-shaped ducts 20, which define bypass ducts 22, and an exhaust nozzle 23.

0020The gas turbine engine 10 works in the conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first airflow A into the intermediate pressure compressor 13 and a second airflow B which passes through the bypass ducts 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the airflow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

0021The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 16, 17, 18 respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

0022Referring now to <figref idref="f0002"><b>Figure 2</b></figref><b>,</b> a turbine blade 30, one of an annular array of blades of the high-pressure turbine 16 comprises a root portion 32, having a so-called "fir-tree" sectional shape which locates in a correspondingly shaped slot in the periphery of a turbine rotor disc (not shown); a pressure side wall 48 and a suction side wall 49 and defining therebetween leading and trailing edges 43, 45; a radially inner platform 34, which abuts the platforms of neighbouring blades to help define a gas passage inner wall for the turbine; an aerofoil 36, which extracts power from the gas flow past it; and an outer shroud portion 38 which again cooperates with its neighbours to help define the outer wall of the turbine's gas passage. Although described in relation to integrally shrouded blades, the invention is of course equally applicable to unshrouded blades as well as vanes.

0023The interior of the aerofoil 36 can contain a chordwise succession of substantially mutually parallel cooling air passages 61 (see, e.g., our <patcit id="pcit0003" dnum="US4940388A"><text>U.S. Pat. No. 4,940,388</text></patcit> for exemplary details), which passages extend spanwise of the aerofoil. One or more of the passages are connected to a cooling air entry port 40 provided in the side face of an upper root shank portion 42 just below the underside of inner platform 34. This receives low pressure cooling air, which cools the aerofoil 36 by taking heat from the internal surface of the aerofoil as it flows through the internal passage and out through holes (not shown) in the shroud 38 and also through the spanwise row of closely spaced small holes 44 in the trailing edge 46 of the aerofoil. The interior of the aerofoil 36 can also contain in combination with these passages a radial succession of substantially mutually parallel chordwise passages. Others of the internal passages are connected to another cooling air entry port (not shown) located at the base 47 of the "fir-tree" root portion 32, where high pressure cooling air enters and cools the internal surfaces of the aerofoil 36 by its circulation through the passages and eventual exit through holes (not shown) in the shroud 38. It is also utilised to film-cool the external surface of the pressure wall 48 of the aerofoil 36 by means of spanwise extending rows of film cooling holes 50 to 53.

0024Referring now to <figref idref="f0003"><b>Figure 3</b></figref><b>,</b> the pressure wall 48 comprises an array of cooling holes 60, in accordance with the present invention, defined by the wall 48 and extending between interior and exterior surfaces 55, 56 to allow the cooling fluid to flow therebetween. The array of holes 60 comprise a first group 62 and a second group 64 wherein the holes of each group are angled to intersect the holes of the other group and are characterised in that the holes of at least one of the groups comprises two or more holes at different angles to one another thereby creating a wall 48 of varying porosity along its radial height and therefore of preferential cooling capability.

0025In this embodiment, the arrangement of holes is such that the region 66 of wall 48 is more porous than region 68 and therefore there is a greater flow of coolant therethrough and a greater surface area over which the coolant flows, thereby region 66 is cooled more than region 68. This arrangement is particularly suitable where the blade's aerofoil 36 is subject to a varying temperature profile over its radial height, the hottest working gas temperature impinging on region 66. Alternatively, this arrangement may be used where the temperature of the coolant changes such that as the coolant increases in temperature, there is a corresponding increase in porosity to compensate for the lower temperature differential between coolant and wall.

0026In the preferred embodiment shown in <figref idref="f0003">Figure 3</figref>, both groups of holes 62, 64 have all their holes angled differently to other holes in their same group. Regarding the group of holes 62, a radially outer hole is angled at α₁ = 160° and a radially inner hole is angled at α_x = 130°. Regarding the group of holes 64, a radially outer hole is angled at β₁= 100° and a radially inner hole is angled at β_x = 135°. The consecutive holes in between these radially outer and inner holes change angle by an equal increment, although they may change by unequal amount.

0027It should be noted that the holes of each of the two (or more) groups of holes 62 and 64 have their longitudinal axes lying substantially in a single plane (i.e. the plane defined by the section A-A in <figref idref="f0001">Figure 1</figref>) and are essentially a single row of holes, for example like row 50 in <figref idref="f0002">Figure 2</figref>. As mentioned previously each group of holes 62, 64 may comprise holes at varying angles to create more and less porous regions 66, 68.

0028In an alternative embodiment of the present invention shown in <figref idref="f0003">Figure 4</figref>, the two groups of holes 62 and 64 are separate rows, for example like rows 50 and 51 in <figref idref="f0002">Figure 2</figref>. Each row has each of its holes extending in a single plane. In <figref idref="f0003">Figure 4</figref>, the group of holes 62 (e.g. equivalent to row 51 in <figref idref="f0001">Figure 1</figref>) are shown in dashed lines. The planes of each of the two (or more) rows are substantially normal to the surface 56 and therefore depending on the profile of the surface 56 the planes may be parallel, divergent or convergent with respect to the wall's inner and outer surfaces. Hence the rows may have the groups of holes 62, 64 intersecting or not intersecting. Thus not only does this embodiment vary the porosity by virtue of the angles of the holes in each of the two arrays 62 and 64, but also the porosity may be varied between internal and external surfaces 55, 56 dependant on the convergence or divergence of adjacent rows 50 and 51 (for example).

0029It should be appreciated that the planes of each of the two (or more) rows are preferably and substantially normal to the surface 56, but may be at any other suitable angle.

0030In yet another embodiment of the present invention each of the individual holes of either or both the groups of holes 62, 64, whether in one row or two or more, may be angled relative to a common plane (e.g. Section A-A in <figref idref="f0001">Figure 1</figref>) through the group of holes. In other words each of the holes 62, 64 may be further angled either into or out of the paper by varying degrees. This may be done in conjunction to the varying angle α_x, β_x of the holes 62, 64 in the plane of the paper (i.e. the plane defined by section A-A).

0031Various modifications may be made without departing from the scope of the present invention. For example, two or more adjacent holes may be parallel and angled differently than the next two or more parallel holes; consecutive holes may be inclined at increasingly steep angles relative to one another i.e. the holes are not equally incremented in their angles.

0032Although the arrangement of cooling holes has been described in relation to rows of cooling holes extending generally in a radial direction, the present invention is equally applicable to a transverse row of cooling holes, such that <figref idref="f0003">Figure 3</figref> represents a section B-B in <figref idref="f0002">Figure 2</figref>. Furthermore, the present invention is applicable to a grid of cooling holes having rows (e.g. section B-B) and columns (e.g. section A-A), thus the holes will have compound angles.

0033It should be appreciated that the porosity of the wall 48, 49 also varies through the thickness of the wall and in the example shown in <figref idref="f0003">Figure 3</figref>, in the region 66 the porosity of the wall is greater at its exterior surface 56 than its internal surface 55.

0034The Applicants have found that the present invention has produced a 50% increase in a convective cooling parameter (the product of the cooling hole convective heat transfer coefficient and the exposed holes surface area). This arrangement allows either less coolant mass flow to maintain a constant wall temperature, or a lower wall temperature for a given coolant mass flow. This arrangement has provided a significant, about 50 degrees, wall temperature reduction for the same mass flow of coolant using conventional non-intersecting cooling hole arrangements.