Particle trap in a cooling system for gas turbine engines

Abstract: Means for reducing blockage of cooling apertures in a hollow gas turbine guide vane by sand or other particles entrained in cooling air, is provided by a partition within the vane for dividing the hollow region into first and second regions, the first region having a flow of high pressure cooling air, the second region being at a pressure lower than the first region. The partition is provided with a channel on the high pressure side. Apertures are located in the base of the channel providing flow between the first and second regions and directing cooling air at the inside surface of the vane. The inlet area of the channel is greater than the total cross-section area of the apertures, and the width of the channel distal to its base is less than or equal to the diameter of the apertures, thus trapping in the channel entrained particles larger than the diameter of the apertures.

Description: This invention relates to improvements to gas turbine engine airfoils. In particular, the invention concerns improvements to an internal air cooling system for a gas turbine engine airfoil and especially a nozzle guide vane by providing means to reduce the effect of blockages in certain critical passages in the cooling system, more particularly narrow cooling holes in an impingement and film cooling system, due to trapped particulate material entrained in the cooling air. In an impingement cooling system it is typical for the cooling air having passed through several passages to enter a plenum from which at least a proportion exits via a number of narrow diameter holes to provide impingement cooling jets onto another part or wall. In the event one of these holes becomes blocked, for example by air borne particles trapped in the hole itself, the associated cooling jet is lost to the detriment of overall cooling efficiency. Deterioration is progressive until eventually the airfoil suffers damage due to overheating. In the case where the impingement cooling jets subsequently feed airfoil film cooling systems, blockage of the impingement holes will reduce film cooling flows. Furthermore, any particles which pass through the impingement plate holes may subsequently block the film cooling holes leading to severe overheating and rapid deterioration of the airfoil leading edge. It is accordingly an objective of the present invention in its most general form to provide a means for trapping particulate material entrained in a fluid. It is a further objective of the present invention to provide a means for reducing blockage of cooling holes in an airfoil or guide vane of a gas turbine engine. According to one aspect of the present invention there is provided A gas turbine engine airfoil having an internal air system including air delivery holes having a certain predetermined width, wherein, in use, the air system may carry particles of sufficiently large size to block the air delivery holes, characterised in that the airfoil includes an internal partition member through which the internal air passes via at least one hole comprising an exit aperture of said predetermined width in the downstream surface of the partition member, and a non-circular entry aperture formed in the upstream surface of the partition member which has a minor dimension less than or equal to the exit aperture width whereby to exclude particles of greater size and a substantially larger major dimension such that the entry area of the entry aperture is greater than the total cross-section area of the hole. According to another aspect of the invention each air delivery hole is supplied through an individual non-circular entry aperture having said major and minor dimensions. According to a further aspect of the invention each air delivery hole is supplied from the base of a passageway or channel formed in the upstream side of the partition member and the minor dimension constituting the width of the channel is less than the width of the air delivery hole. According to a still further aspect of the present invention there is provided a means for reducing blockage of cooling apertures in a guide vane of a gas turbine engine by particulate material entrained in high-pressure cooling air directed within the vane, the vane being a hollow body externally of airfoil form, there being provided a partition within the vane for dividing the hollow region into first and second regions, the first region being adapted to receive high pressure cooling air, the second region being adapted to be at a pressure lower than that of the first region, the partition being provided with a plurality of apertures providing flow communication between the first and second regions and arranged to direct cooling air towards the inside of the guide vane, wherein the partition is provided with a channel on the high pressure side, said plurality of apertures being located in the base of the channel, the inlet area of the channel being greater than the total cross-section area of the apertures, and the width of the channel distal to its base being less than or equal to the minimum diameter of any one of said apertures, whereby an entrained particle that is larger than the diameter of the apertures is trapped by the channel. The invention will now be described by way of example only with reference to the accompanying drawings, in which, Figure 1 consisting of a section through a known nozzle guide vane in a plane at right angles to its span, shows a vane susceptible to blockage by air borne particles in the internal air cooling system; Figure 2 shows a section through a nozzle guide vane in a plane at right angles to its span, including an apertured partition incorporating the invention; Figure 3 shows a view of the apertured partition in Figure 2 taken in the direction of arrow 3; Figure 4 shows a perspective view of a portion of the apertured partition of Figure 3; Figure 5 shows an enlarged section through the partition of Figure 3 taken along line V-V; and Figure 6 shows a perspective view of another air delivery hole arrangement in which the holes are supplied individually. Figure 1 shows a section through a nozzle guide vane, at right angles to its span, which forms part of an internal air cooling system. Air tapped from a compressor section (not shown) is ducted through various parts of the cooling system to perform a cooling function. In the interior of the vane the cooling air supply is used to produce a number of impingement cooling jets for the leading edge of the vane and to supply cooling air to surface film cooling apertures. The nozzle guide vane of Figure 1 comprises a cast, airfoil shaped hollow body 10, having a leading edge 12 and a trailing edge 14 and extends across the engine gas path in a radial direction. The body 10 has an inner surface which in this particular example is generally the same shape as its external form. The inner surface is provided with a number of internal surface features including inwardly facing grooves 16 on opposite sides of the vane. The grooves 16 lie intermediate the leading edge 12 and the trailing edge 14, and extend lengthwise of the vane in a radial direction. A first metal insert 18, which essentially consists of a plate of generally U-shaped or V-shaped cross-section and extends the radial height of the vane, is located in the interior of the vane. The edges at the distal ends of the limbs of the V or U by are engaged with the grooves 16 for location. The side panels 24 of the insert 18 contact small inwardly facing projections 20 on the inner surface of the vane 10 to form air passages 22 between the insert 18 and the inner surface of the vane. The insert 18 comprises a single member having two panels 24 extending over the full spanwise length of the vane 10 which converge towards the trailing edge 14 and are in sealing engagement with the interior surface of the vane 10 at the grooves 16. Each panel 24 is provided with apertures 26 providing flow communication between a plenum region 34 bounded by the panels and the air passages 22. The air passages 22 communicate with the trailing edge 14 where they exit to ambient atmosphere. The vane 10 is further provided with a second sheet metal insert or partition member 28 comprising a panel 30 extending across the vane 10 between the grooves 16, the spanwise extending edges of which are in sealing engagement with the inside surfaces of the crimped portions of the first insert engaged with the grooves 16. Panel 30 also extends the full spanwise length of the vane 10. The panel 30 hence partitions the internal volume of the vane 10 into a leading edge plenum region 32 and a trailing edge plenum region 34. The leading edge 12 of the vane 10 is also provided with film cooling apertures 38 providing external surface cooling flow by exhausting air from the leading edge plenum region 32. The panel 30 is provided with apertures 36 which provide flow communication between the two plenum regions 32, 34. The total area of apertures 36 meters flow to surface cooling apertures 38 and individual apertures effectively create impingement cooling jets directed at the inside of the leading edge 12. Panel 30 is therefore referred to as an "impingement plate". A further panel (not shown) may be provided at the radially inner end of the vane 10 to close the otherwise open ends of the plenum region 32,24 at least between the panels 24. The radially outer end of the vane is open to a source of pressurised cooling air. Hence, cooling air under pressure entering the trailing edge plenum region 34 from the open spanwise end (ie the air entering in a direction at right angles to the plane of Figure 1) will exit via the apertures 26 in panel 24 into passages 22, and via apertures 36 in panel 30 into the leading edge plenum region 32, and thence into the gas path via cooling apertures 38 in the leading edge 12. Although described with reference to nozzle guide vanes an essentially similar arrangement could be used for internally cooled rotative blades, in which case the source of cooling air is invariably via the blade root section thereby making use of centrifugal effects. However, in the case of guide vanes it is possible to source the cooling air from a radially outward supply, flowing inwards so that the end plates may need to be located at the inner end of the vane. Another similar, but older, cooling system for a hollow nozzle guide vane for an aircraft gas turbine engine is described in British Patent 2,017,229 B (Rolls-Royce Ltd). However, in this system the partitioning panel between the leading and trailing edge plenum regions has no apertures or perforations to act as an impingement plate, or provide flow communication. United States Patent US 3,542,486 describes a structural member having an external surface exposed to a high temperature source and varied static pressures, which is provided with a common plenum chamber for containment of a cooling media, a plurality of passages of generally uniform cross section to deliver the cooling media from the chamber to the higher pressure portion of the external surface and a plurality of passageways of nonuniform cross section to deliver the cooling media to the low pressure portion of the external surface for effective and efficient film cooling of the member. The nonuniform passageways are formed with a flow rate metering portion adjacent the plenum chamber and a diffuser portion adjacent the external surface. The cooling systems for nozzle guide vanes as described above are efficient when the pressurised cooling air is clean, as for instance, in a gas turbine engine in an aircraft flying at altitude. However, there can be situations, such as an aircraft flying at low level or hovering at or near ground level, where particles (eg sand from the ground) may be entrained in the pressurised cooling air supply. This is especially so in sandy desert conditions. The entrained particulate material, above a certain grain size, may block the apertures 36 in the impingement panel or plate 30 leading from the trailing edge plenum region 34. This is particularly disadvantageous because, in order to minimise any detriment to the engine cycle, the number of cooling holes in the impingement plate 30 is set to the minimum necessary to cool the vane leading edge 12 and to supply the surface cooling apertures 38. Thus the inevitable result of any blockage is an unacceptable reduction of cooling air to the inside of the leading edge 12 of the nozzle guide vane and the surface cooling apertures 38. Furthermore, any entrained particulate material which passes through the apertures 36 may subsequently block the surface cooling apertures 38 leading to overheating of the vane leading edge 12. Referring to Figures 2 - 5, where parts corresponding to similar parts in Figure 1 carry like references, there is provided, instead of the partition member 28 of Figure 1, an improved partition member or impingement plate 40 according to the invention. The partition member 40 may pass all of the cooling system flow or a part of it. Impingement plate 40 comprises a member 42 extending across the width of the plenum 34 between, and in sealing engagement with, the panel edges in grooves 16. Member 42 also extends the full spanwise length of the vane 10. The member 42 divides the internal space of the vane into leading edge plenum region 32 and trailing edge plenum region 34, similar to the plate 30 described with reference to Figure 1 above. This member 4 may be in the form of a strip or bar, or possibly made of sheet material, normally of a metal or metal alloy. Member 42 is provided, on the side facing into the trailing edge plenum region 34, with a passageway or channel 44 extending substantially the spanwise length of the panel. A plurality of impingement holes 48 are formed in the base 46 of the radially extending channel 44 through the member 42 into the leading edge plenum region 32. Hence, high pressure cooling air in plenum 34 will, subject to correct pressure differential, pass through holes 48 constituting cooling jets which impinge upon, and cool, the leading edge 12 and supply the surface cooling apertures 38. As in the prior art vane of Figure 1, the leading edge 12 is provided with film cooling exit apertures 38 whereby cooling air is allowed to flow over the external surface of the vane. The channel 44 of total length "c" is designed, according to the invention, so that the inlet area of the channel is greater than the total inlet area of the impingement holes 48. Furthermore, the distance between parallel upper edges of the channel sides, the width of the channel ("a" in Figure 3) at its inlet (ie the part remotest from the impingement holes 48) is less than or equal to the minimum inlet diameter ("b" in Figure 3) of any of the impingement holes. Hence, as shown in Figures 2-5, the channel walls 44 diverge towards the channel base 46. The length "c" (Figure 3) of the channel 44 is relatively unimportant, at least as far as concerns prevention of direct blockage of the holes 48. Obviously the total inlet area of the channel shall be sufficient to avoid choking of the holes up to a predetermined level of blocking by trapped particles. In another embodiment (not shown) the walls of the channel 44 are parallel, but the distance between the walls is no greater than the diameter of any impingement hole 48. In another version the difference between the impingement hole diameter and the channel width may be accommodated by a step change at the base 46 or by a ramp within the length of impingement hole 48. As shown in Figure 5, the depth "d" of the channel 44 is sufficient to allow air to pass around any trapped particle 50 (arrow 4) into the impingement hole 48. Thus in the described, and illustrated, embodiment only the air passing through the partition member into the leading edge plenum is effectively filtered by the channelled impingement plate. In effect only this branch of the internal cooling is filtered in this way, but if desired the plate 40 could be positioned elsewhere to pass and thus effectively filter the whole of the internal cooling system flow. Thus, in respect of a whole array of holes: a.c ≥ k.n. π ( b / 2 ) 2 where k is some constant and n is the number of holes. In use, particulate material entrained in the cooling air, eg sand particles, over a certain size equal to or larger than the width of the inlet portion of the channel 44, as illustrated in Figure 5, will be trapped between the edges of the channel. Although these trapped particles create a blockage in that they reduce the entry or inlet area of the channel the particles are trapped clear of the entry orifices of the impingement holes 36 or surface cooling apertures 38 and they do not directly block the holes. The flow of cooling air to all the impingement and film cooling holes is therefore maintained for a considerably longer period compared to the prior arrangements. The trapped particles will progressively block the channel entry but all the while the impingement and film cooling holes themselves will remain clear and operative. Eventually as the channel entry area is reduced there will be a progressive reduction in pressure within the channel and the cooling flow may become partially choked but normal cooling function will have maintained for substantially longer than is the case with the prior art arrangement. Thus not only is impingement cooling of the interior of the vane, or blade, prolonged but the external filming cooling also benefits. Smaller particles, of course, will pass straight through the channel 44 and the impingement holes 48 to exit through apertures 38 in the leading edge 12. Conveniently the member 42 may be designed to be readily removable from the grooves 16 or its engagement with the shaped insert 18 so as to allow removal of accumulated particulate material at overhaul. This may be done by arranging for the memter 42 to be a sliding insert into the grooves 16 or the corresponding portions of the insert 18 so that the panel 42 may be slid out when required. Turning now to Figure 6 the same principle is applied to individual holes, rather than to a group of adjacent holes as illustrated in Figures 3 to 5 inclusive. Maintaining the same convention and references, where applicable, as previously adopted: a plurality of air delivery holes 48 of dimension "b" are formed through a member 40, the holes are spaced apart in a linear array in the drawing but could be arranged in two-dimensional arrays or even individually. Each of the holes 48 is formed with an exit aperture 52, in this case a circular hole of diameter "b", and emerges in the far (hidden) side of the member 40. A corresponding entry aperture 54 is formed for each hole in the near side of the member 40. These entry apertures 54 are non-circular, in the drawing they are generally oval, they may be elliptical or simply a short length of channel similar to that used in the first described embodiment. As before the width or minor dimension "a" of the entry aperture is important and is always less than or at most equal to the dimension of hole 48. The major dimension "e" is less critical but again should be determined by the amount of blockage by trapped particles that may be tolerated. Up to the predetermined level of acceptable blockage sufficient of the area of the entry aperture shall remain open to avoid choking of the hole 48. Typically the total array of air passages or holes 48 is arranged to perform a metering function in an internal air system, at least as far as the downstream portions of the air system are concerned. Preferably this metering function shall continue uncomprised at levels of blocking less than the predetermined maximum blockage threshold; the present invention is designed to achieve this in all its embodiments. Thus, on the basis the entry aperture may be considered rectangular and the exit aperture circular, for each hole: a.e ≥ k.n. π ( b / 2 ) 2 where k is some constant. Various modifications of the features of the hole design are possible without departing from the spirit and concept of the invention. For example the second embodiment is illustrated as possessing an almost funnel-shaped configuration where the entry aperture is transformed into the exit aperture by roughly half hole depth. This transformation could be more progressive and occupy the whole depth of the hole. The exit aperture is not necessary circular. Preferably, however, the entry aperture is undercut having its narrowest dimension at the entry surface to best obtain the particle screening function, although a constant width hole shape could be adopted. The invention thus enables gas turbine engines to be used in dusty, sandy conditions with an appreciably reduced risk of cooling holes in vanes being blocked. The principle of the invention may be applied to other parts of the engine where cooling holes may be at risk from blockage.

Claims: A gas turbine engine airfoil 10 having an internal air system including air delivery holes 38 having a certain predetermined width, wherein, in use, the air system may carry particles 50 of sufficiently large size to block the air delivery holes, 38, characterised in that the airfoil 10 includes an internal partition member 28 through which the internal air passes via at least one hole 36 comprising an exit aperture 52 of said predetermined width in the downstream surface of the partition member, and a non-circular entry aperture 54 formed in the upstream surface of the partition member 28 which has a minor dimension less than or equal to the exit aperture 52 width whereby to exclude particles 50 of greater size and a substantially larger major dimension such that the entry area of the entry aperture 54 is greater than the total cross-section area of the hole 36. A gas turbine engine airfoil 10 as claimed in claim 1 wherein each air delivery hole 38 is supplied through an individual non-circular entry aperture 54 having said major and minor dimensions. A gas turbine engine airfoil 10 as claim in claim 1 wherein each air delivery hole 38 is supplied from the base of a passageway or channel 44 formed in the upstream side of the partition member 28 and the minor dimension constituting the width of the channel 44 is less than the width of the air delivery hole 38. A gas turbine engine airfoil 10 as claimed in claim 3 wherein the depth of the passageway or channel is greater than its width. A gas turbine engine airfoil 10 as claimed in any of preceding claims 3-4 wherein the partition member 28 has a plurality of holes 48 formed in the base of a single passageway or channel 44. A gas turbine engine airfoil 10 as claimed in any preceding claim wherein the partition member 28 passes a branch flow of the total internal air flow. A gas turbine engine airfoil 10 as claimed in any preceding claim wherein the partition member 28 is removable from the airfoil 10. A gas turbine engine airfoil 10 as claimed in claim 7 wherein it is arranged for the movable partition member 28 to be a sliding fit into the interior of the airfoil 10. A gas turbine engine airfoil 10 as claimed in any preceding claim wherein the partition member 28 divides an internal volume of the airfoil 10 into an upstream region of high pressure and a downstream region of relatively lower pressure and the passageway or channel 44 is formed in the high pressure side of the member 28.

Inventors: EMERY MARTIN ANTHONY JAMES, SELF KEVIN PAUL, WILLIAMSON ROBERT PETER ERNEST, EMERY, MARTIN ANTHONY JAMES, SELF, KEVIN PAUL, WILLIAMSON, ROBERT PETER ERNEST
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1999
Main classification: F01D5
IPC codes: F01D5