Internal cooling of engine components

Abstract

A gas turbine engine component, especially an aerofoil-sectioned nozzle guide vane (NGV), having at least one internal cooling chamber for passage of cooling air, the chamber including leading edge portion and one inlet portion via which cooling air may enter the chamber from feed source, wherein the component includes a partitioning element, e.g. curved or scoop-shaped partitioning plate or wall, provided in the chamber inlet portion and defining within the inlet portion a sub-chamber adjacent the leading edge portion, and wherein partitioning element is configured so the cooling air velocity in the sub-chamber is less than the cooling air velocity in the remainder of inlet portion. The reduced velocity of the cooling air in the sub-chamber adjacent the leading edge serves to increase pressure therein, thereby maintaining desired backflow pressure margin between the feed pressure of the cooling air delivered to the showerhead holes and the gas-path from the combustor.

Claims

1. A nozzle guide vane or turbine blade of a gas turbine engine, the nozzle guide vane or turbine blade comprising: at least one internal cooling chamber for passage of cooling air, the at least one cooling chamber including: (i) a leading edge portion, and (ii) at least one inlet portion through which cooling air enters the at least one cooling chamber from a feed source; and a partitioning element disposed in the at least one inlet portion of the at least one cooling chamber and extending into the at least one cooling chamber to define a sub-chamber wholly or partly within the at least one cooling chamber, the partitioning element and the leading edge portion constitute boundaries of the sub-chamber, the sub-chamber having an inlet configured to receive cooling air from the feed source, the partitioning element including: an axially outer portion, relative to a longitudinal axis of the at least one cooling chamber, extending into an inlet feed passage through which cooling air enters the at least one inlet portion of the at least one cooling chamber from the feed source; and an end portion distal to the inlet feed passage in the at least one cooling chamber bending towards the leading edge portion of the at least one cooling chamber at a short axial distance from the inlet feed passage, such that the partitioning element reduces a cooling air velocity in the sub-chamber to a value smaller than a cooling air velocity in a remainder of the at least one inlet portion of the at least one cooling chamber.

2. The nozzle guide vane or turbine blade as claimed in claim 1, further comprising a forward cooling chamber and a rearward cooling chamber, and the partitioning element is provided in the forward cooling chamber.

3. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the leading edge portion includes a showerhead portion, the sub-chamber defined by the partitioning element being adjacent the showerhead portion.

4. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is mounted in at least the inlet portion of the cooling chamber by being formed integrally with side walls of the nozzle guide vane or turbine blade by casting the partitioning element as an integral internal wall portion of the nozzle guide vane or turbine blade.

5. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is mounted in the inlet portion of the cooling chamber and attached to an inner side wall of the nozzle guide vane or turbine blade.

6. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the cooling chamber contains an insert tube including impingement holes in the insert tube, the impingement holes being configured to feed cooling air to the leading edge portion, and the partitioning element is mounted and attached to an inner side wall of the insert tube.

7. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is a body, plate, wall, member or element having a cross-section or a face adjacent to the leading edge portion.

8. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is provided in the inlet portion of the at least one cooling chamber, whereby a single sub-chamber is defined adjacent the leading edge portion of the at least one cooling chamber.

9. The nozzle guide vane or turbine blade as claimed in claim 1, wherein a plurality of the partitioning elements are provided in at least the inlet portion of the at least one cooling chamber, such that a plurality of sub-chambers are defined in the at least one cooling chamber.

10. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the at least one cooling chamber has a plurality of inlet portions, and each of the plurality of inlet portions is provided with a respective one of a plurality of the partitioning elements.

11. A gas turbine engine including one or more internally cooled nozzle guide vanes or turbine blades, each being the nozzle guide vane or turbine blade as claimed in claim 1.

12. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is mounted in at least the inlet portion of the at least one cooling chamber by manufacturing the partitioning element as a discrete element and subsequently mounting the partitioning element in the nozzle guide vane or turbine blade at a desired location by welding.

13. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is a body, plate, wall, member or element having a cross-section or a face adjacent to the leading edge portion of the at least one cooling chamber, which includes two or more substantially flat sections angled with respect to each other in one or more orthogonal directions.

14. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is a body, plate, wall, member or element having a cross-section or a face adjacent to the leading edge portion of the at least one cooling chamber, which is scoop shaped.

15. The nozzle guide vane or turbine blade as claimed in claim 1, wherein the partitioning element is a body, plate, wall, member or element having a cross-section or a face adjacent to the leading edge portion of the at least one cooling chamber, which is partially cylindrical in at least one orthogonal direction.

16. A method of cooling a leading edge portion of a nozzle guide vane or turbine blade of a gas turbine engine, the nozzle guide vane or turbine blade having at least one internal cooling chamber for passage of cooling air, the least one cooling chamber including a leading edge portion and at least one inlet portion through which cooling air enters the at least one cooling chamber from a feed source, the method comprising: providing a partitioning element in the inlet portion of the at least one cooling chamber that extends to define a sub-chamber, bound by the partitioning element and the leading edge portion, wholly or partly within the at least one cooling chamber, the sub-chamber having an inlet configured to receive cooling air from the feed source, the partitioning element including: (i) an axially outer portion, relative to a longitudinal axis of the at least one cooling chamber, extending into an inlet feed passage through which cooling air enters the at least one inlet portion of the cooling chamber from the feed source; and (ii) an end portion distal to the inlet feed passage in the at least one cooling chamber bending towards the leading edge portion of the at least one cooling chamber at a short axial distance from the inlet feed passage, such that the partitioning element reduces a cooling air velocity in the sub-chamber to a value smaller than a cooling air velocity in a remainder of the at least one inlet portion of the at least one cooling chamber; and when in use, cooling the nozzle guide vane or turbine blade by feeding cooling air from the feed source into both the sub-chamber and the remainder of the at least one cooling chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is an isometric cut-away view of a typical single-stage cooled gas turbine engine showing the NGV's, rotor blades, platform structures and other components, and has already been described;

(3) FIG. 2(a) is a cross-sectional view of a typical HPT NGV aerofoil cooling scheme with forward 3F and rearward 3R cooling chambers, and FIG. 2(b) is an isometric cut-away view of the NGV aerofoil segment showing the internal cooling scheme features and coolant flows, and both have already been described;

(4) FIG. 3(a) is a cross-sectional view through another example of a known HP turbine NGV aerofoil cooling scheme, showing an arrangement of sheet metal impingement tubes inserts, and has already been described;

(5) FIG. 3(b) is a cross-sectional view through another example of a known HP turbine NGV aerofoil cooling scheme, showing an alternative arrangement employing an additional internal cast wall located in close proximity to the suction side walls of the aerofoil, and has already been described;

(6) FIGS. 4(a) and 4(b) are, respectively, an axial cross-sectional explanatory view and a transverse (top plan) sectional view of part of an inlet portion of any of the NGV cooling chambers of FIGS. 1 to 3, showing the geometry thereof and the main parameters governing its operation;

(7) FIGS. 5(a) and 5(b) are, respectively, an axial cross-sectional explanatory view and a transverse (top plan) sectional view (corresponding to the views of FIGS. 4(a) and (b)) of part of an inlet portion of an NGV cooling chamber including a partitioning element in accordance with a first embodiment of the present invention, showing the geometry thereof and the main parameters governing its operation;

(8) FIGS. 6(a), 6(b) and 6(c) are, respectively, an axial cross-sectional view, a transverse (top plan) sectional view, and a perspective orthogonal view of a dual-feed NGV including a pair of partitioning elements in accordance with a second embodiment of the present invention;

(9) FIG. 7 is an axial cross-sectional view of a further dual-feed NGV including a pair of partitioning elements in accordance with a third embodiment of the invention;

(10) FIG. 8 is a transverse (top plan) sectional view of a further dual-feed NGV including a pair of partitioning elements in accordance with a fourth embodiment of the invention; and

(11) FIGS. 9(a) and 9(b) are, respectively, an axial cross-sectional view and a transverse (top plan) sectional view of yet another dual-feed NGV including a pair of partitioning elements in accordance with a fifth embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(12) In the detailed description which follows, reference numerals referring to like or corresponding parts or features in the various embodiments are designated as such using essentially the same reference numerals but appropriately incremented by 100 going from one embodiment to the next.

(13) In comparison with known designs of NGVs and other gas turbine engine components which utilise internal cooling schemes, in developing the present invention the inventors recognised that there may be substantial benefits to be had if the pressure drop across the combustor of the engine (P.sub.30-P.sub.40) were to be reduced relative to current levels.

(14) As shown in FIGS. 4(a) and 4(b), in the context of known NGVs employing showerhead cooling, the critical locations from a safe pressure margin viewpoint are the showerhead cooling holes located on either side of the aerofoil stagnation point 80 (or more correctly the stagnation region or zone, when considering unsteady flow). Typically the first row of holes to the pressure side (P/S) of the aerodynamic stagnation point 80 is where the local external pressure is at its highest value, and equal to the total pressure P.sub.t40 when considering the unsteady effects. When this peak external pressure is coupled with the local minimum pressure internally, then the lowest pressure ratio across the holes exists. The minimum internal pressure occurs at the entrance(s) to, i.e. in the inlet portion(s) 90 of, the forward cooling chamber 3F, where the local coolant flow rate is greatest. Hence the local internal velocity is at its highest value and the corresponding static pressure P.sub.s is at its minimum value. Thus, in the arrangement illustrated in FIGS. 4(a) and 4(b) the following conditions are satisfied:
P.sub.t=P.sub.s+½ρv.sup.2
and therefore:
P.sub.s=P.sub.t<½ρv.sup.2,
v.sub.total=[W.sub.c/AρC.sub.d].sub.total,
and
v↓P.sub.s↑,

(15) where: P.sub.s=static pressure, P.sub.t=total pressure, ρ=fluid (i.e. air) density, v=local velocity, C.sub.d=discharge coefficient, W.sub.c=coolant mass flow, and A (or A.sub.flow)=flow area.

(16) Since in the arrangement of FIGS. 4(a) and 4(b) there is only one (outboard) feed passage 30 supplying cooling air to the cooling chamber 3F, then additionally:
V.sub.local=[W.sub.c/AρC.sub.d].sub.total.

(17) In order to keep the local velocity v at the cooling chamber inlet portion or entrance low, the local “mass flow per unit area” [(W.sub.c/A.sub.flow).sub.local] needs to be kept at a low level.

(18) If the safe backflow pressure margin is such that:
Pressure Margin=((P.sub.s local−P.sub.40)P.sub.40)×100%,
and which is typically in the range of from 1.5 (or about 1.5) to about 2.0%, and is allowed to drop significantly below these values, then there is an increased risk of hot gas ingestion into the cooling chamber 3F, which can and often will have catastrophic consequences.

(19) Similarly, having a pressure margin significantly above about 2.0% may not be beneficial from a leakage and engine efficiency viewpoint, and may also increase the gas temperature at a design thrust level, which may therefore exacerbate the situation.

(20) In accordance with the present invention in its broadest aspects therefore, the present inventors have identified a simple change to the cooling air feed geometry that improves the local backflow pressure margin without increasing the pressure drop across the combustor or increasing the size or shape or cross-sectional area of the aerofoil locally. This is achieved by introducing the above-defined partitioning element into the inlet portion of the cooling chamber via which the cooling air is fed thereinto from the respective feed source.

(21) In its simplest form, a first embodiment of the invention is shown in FIGS. 5(a) and 5(b), which show diagrammatically the geometry of the new arrangement according to this embodiment. Mounted within the inlet portion of the chamber 103F is a partitioning element 100, which is in the form of a curved, arcuate or scoop-shaped plate or sheet (e.g. of the same or a compatible metal or alloy as used to cast the NGV side walls). The new partitioning element 100 divides the inlet portion of the cooling chamber 103F into two sub-chambers: a primary sub-chamber 150 which is located forwardly immediately adjacent the leading edge portion of the NGV (and thus adjacent the stagnation zone 80) and a secondary, rearward located, sub-chamber 160 which carries the remainder of the cooling air feed into the cooling chamber 3F. A sheet metal (e.g. planar) baffle plate 170 is located slightly below mid-span within the cooling chamber 3F. As shown schematically in FIG. 5(a), the partitioning element 100 extends a short distance, e.g. from about 10 or 20 or 30 up to about 50% of its height, axially upwardly and partially into the feed passage itself. It also extends axially downwardly into the main body of the cooling chamber 103F itself by a like or similar short distance. The partitioning element 100, e.g. in the form of a sheet, plate or wall, preferably of the same metal or alloy as is used to cast the walls of the component itself, has a generally concave shape/configuration in an axial section (as shown in FIG. 5(a)) and an arcuate or convoluted shape/configuration in a transverse section (as shown in FIG. 5(b)). The convoluted shape may for example be derived from a mid-portion 102 (relative to a width direction of the element) having a generally arcuate or concave shape, which is bounded on at least one lateral side by a or a respective connection portion 104 via which the element is attached to the component side wall. The element 100 is mounted in the chamber inlet portion by virtue of being united integrally with the component side cast walls, e.g. by being cast integrally therewith or being attached thereto by post-production welding or suchlike.

(22) In the arrangement of this first embodiment as illustrated in FIGS. 5(a) and 5(b), the local feed pressure of the forward sub-chamber 150 is therefore governed by the following equations:
P.sub.s1 feed=P.sub.t−½ρv.sub.c1.sup.2
and
v.sub.c1=[W.sub.c1/A.sub.1ρC.sub.d1].
Hence if:
W.sub.c1/A.sub.1 is less than W.sub.c total/A.sub.total,
then:
v.sub.total>V.sub.c1
and therefore:
P.sub.s1 feed>P.sub.s total feed
and
v.sub.1=[W.sub.c/AρC.sub.d].sub.1<v.sub.total=[W.sub.c/AρC.sub.d].sub.total,

(23) where the various subscripts represent the following: .sub.1=forward sub-chamber 150, .sub.2=rear sub-chamber 160, .sub.c=coolant (air), .sub.stag=stagnation value, .sub.feed=at entrance to cooling chamber (i.e. in inlet portion thereof), .sub.t=total value.

(24) In general if the value of [feed area of the sub-chamber 150/number of holes it supplies] is greater than the value of [total feed area/number of hole it supplies], then the static feed pressure will rise above that with a single feed chamber into the NGV interior chamber.

(25) As shown in FIG. 5(a), the partitioning element 100 locally creates a pair of sub-chambers 150, 160 which are both smaller in transverse section than the original cooling chamber 103F at its entrance. However, although the forward sub-chamber 150 adjacent the aerofoil leading edge has a smaller flow area than the overall inlet feed passage at the chamber's entrance, it only supplies cooling air to a reduced number of showerhead cooling holes. Therefore the flow per unit area is decreased relative to the value for the original geometry, i.e. in the absence of the partitioning element 100 but with the otherwise same arrangement and feed flow parameters. Thus the local air flow velocity in the forward sub-chamber 150 is reduced and the corresponding local static pressure therein is increased above the value for the original geometry, i.e. for the same arrangement and flow parameters but in the absence of the partitioning element 100.

(26) The local backflow pressure margin across the showerhead portion of the NGV will thus be increased as a consequence of the change in geometry arising from the presence of the partitioning element 100. This increase can be beneficial in several ways. For example, it may enable there to be a reduced pressure drop across the combustor. Alternatively or additionally it may be used to increase the pressure drop across an impingement baffle plate or wall located or mounted within the cooling chamber 103F to improve the “back-face” impingement heat transfer. Further alternatively or additionally, it may be used simply to achieve the desired pressure margin across the showerhead portion without the need for increasing the local aerofoil shape. Any or all of these benefits may lead to improved efficiency of the gas turbine (or other) engine, which ultimately may manifest itself in improved specific fuel consumption (SFC), among possibly other benefits.

(27) FIGS. 6(a), 6(b) and 6(c) show a second embodiment of the invention, in which a HPT NGV aerofoil cooling arrangement comprises a dual-end cooling air feed supplying cooling air from both outboard 230 and inboard 232, 234 sources to the forward cooling chamber 203F. Mounted generally centrally within and across the width of the chamber 203F is a planar baffle plate 270. Mounted within the rear cooling chamber 203R is impingement plate 253R, adjacent the rear suction-side side wall of the rear chamber 203R. A first partitioning element 200a is provided in the outboard inlet portion of the forward chamber 203F (i.e. that fed from the outboard cooling air feed source 230) and a second partitioning element 200b is provided in the inboard inlet portion of the forward chamber 203F (i.e. that fed from the inboard cooling air feed source 232). Each partitioning element 200a, 200b thus defines a respective forward sub-chamber 250a, 250b, which are fed from, respectively, outboard 230a and inboard 232a sources, and a respective rearward sub-chamber 260a, 260b, which are fed from, respectively, outboard 230b and inboard 232b sources.

(28) Because the forward sub-chambers 250a, 250b each have a respective partitioning element 200a, 200b incorporated into both the inboard and outboard feed systems, the showerhead cooling holes 220 close to the entrances to the chamber 203F are supplied by air that passes through the respective forward sub-chambers 250a, 250b adjacent the leading edge of the aerofoil. Similarly, the showerhead cooling holes 220 closer to the mid-span locations are fed from the respective rearward sub-chambers 260a, 260b also defined within the forward cooling chamber 203F by the respective partitioning elements 200a, 200b.

(29) In order to optimise the geometry of the arrangement it may be necessary or advantageous to locate each respective partitioning element 200a, 200b in a position where the local feed pressure in each respective forward sub-chamber 250a, 250b is identical at the first cooling hole 220 supplied by these sub-chambers 250a, 250b.

(30) Note also that the curved shape of the respective partitioning elements 200a, 200b is designed to reduce the inlet C.sub.d (discharge coefficient) or losses at the respective entrances to the respective forward sub-chambers 250a, 250b. However, this may not be mandatory.

(31) FIG. 7 shows a third embodiment, which is a HPT NGV aerofoil cooling arrangement with a dual-end feed to a sheet metal insert tube 385a fitted into the forward cooling chamber 303F. There are provided an array of impingement holes in the leading edge wall of the insert tube 385a. The insert tube 385a also has a sheet metal (e.g. planar) baffle plate 370 located slightly below mid-span. The insert tube 385a is mounted in the forward cooling chamber 303F and supported therein by pin-fins 381 (like those labelled 481 in the alternative embodiment of FIG. 8) cast onto the internal walls of the NGV casting. Cooling air enters the insert tube 385a from one or both ends (inboard and/or outboard) and is then bled through an array or series of rows of holes in the insert tube 385a, impinging onto the internal walls of the casting, where the pin-fins 481 provide additional turbulent mixing of the cooling air. The cooling air then passes out of the various internal cooling sub- and mini-chambers onto the external surface of the aerofoil through a series of rows of film cooling holes.

(32) Located at the outboard and inboard entrances to the insert tube 385a adjacent the leading edge are respective ones of a pair of partitioning elements 300a, 300b. These partitioning elements 300a, 300b take the form of curved, scoop-shaped sheet metal elements which are attached to the forward insert tube 385a by laser welded joints 390, which may help to reduce separation of the critical air flows and may improve any inlet pressure losses. In operation a proportion of coolant air passes into the respective forward sub-chambers 350a, 350b adjacent the leading edge, each of which feeds a respective proportion of the showerhead cooling holes 320 close to the insert tube entrance, while the remainder of the cooling air flows 330b, 332b feeds the larger rearward sub-chamber 360a, 360b within the insert tube 385a. By balancing the flow per unit area of each forward sub-chamber 350a, 350b with each other and with that of the rearward sub-chambers 360a, 360b, the respective backflow pressure margins may be balanced and thus the scheme optimised.

(33) FIG. 8 shows a fourth embodiment, which for the most part is substantially the same as or closely corresponds to that of FIG. 7. As in the embodiment of FIG. 7, the insert tube 485a is mounted in the forward cooling chamber 403F and supported therein by pin-fins 481 cast onto the internal walls of the NGV casting. It should be noted here that the supporting features 483 on either side of the leading edge forward sub-chambers 450a, 450b are not pin-fins, but continuous walls that seal the respective forward sub-chambers 450a, 450b, allowing them to be maintained at a higher pressure than the other mini-chambers or passages immediately adjacent the forward suction-side and pressure-side walls, as defined by the insert tube 485a. However the arrangement of FIG. 8 is slightly different from that of FIG. 7. Here, instead of the array of impingement holes in the leading edge of the insert tube 485a, in the embodiment of FIG. 8 the arrangement has a single row of large holes feeding the leading edge cavity. Generally, the plural impingement holes option of FIG. 7 may be favoured in practice when adequate feed pressure is available to provide jet cooling and adequate backflow pressure margin(s) to prevent hot gas ingestion into the cooling chamber 303F/403F.

(34) FIGS. 9(a) and 9(b) show a fifth embodiment, this being a “Wall Cooled” HPT NGV aerofoil cooling scheme. This cooling scheme again has a dual-feed forward cooling passage 503F supplying the leading edge showerhead holes 52Q and the pressure-side film cooling rows. The suction-side and trailing edge are separately fed—typically from an inboard source 536, which tends to be less contaminated—and the coolant air flows forward to the suction-side film cooling holes, and rearward to the trailing edge. The suction-side of the aerofoil is convectively cooled and the heat transfer rates are augmented using banks of pedestals 595 cast into the thin suction-side core wall 585, and film-cooled by spent coolant bled from the early suction-side films.

(35) The pressure-side cavity has a cast-in, preferably planar, baffle plate arrangement 570 to prevent blow-through from occurring which may reduce the local static pressure to dangerously low levels. In order to improve the “backflow pressure margin” a pair of partitioning elements 500a, 500b according to the invention are located at the respective entrances (outboard and inboard) to the forward cooling chamber 503F, adjacent the leading edge portion thereof. Each partitioning element 500a, 500b is similarly shaped to that in other embodiments described above, in particular the embodiment of FIG. 5(b), although other shapes are of course possible. In this embodiment however each partitioning element 500a, 500b is not sealed to the sidewall. The forward sub-chambers 550a, 550b supply coolant air to the showerhead cooling holes 220 at the extremities, i.e. close to the end-walls or entrances to the respective sub-chambers 550a, 550b. As in the other embodiments described above, the walls of the respective partitioning elements 500a, 500b are curved in order to reduce the pressure losses due to the sharp curvature at the entrances to the respective forward sub-chambers 550, 550b. In operation a proportion of coolant air passes into the forward sub-chambers 550a, 550b adjacent the leading edge and feed a proportion of the showerhead cooling holes 520 close to the entrances to the respective forward sub-chambers 550a, 550b, while the remainder of the cooling air flow feeds the larger rearward sub-chambers 560a, 560b. Again, by balancing the flow per unit area of each forward sub-chamber 550a, 550b with each other and that of the rear sub-chambers 560a, 560b, the respective backflow pressure margins may be balanced and thus the scheme optimised.

(36) As will already be apparent at least in part from the foregoing description, embodiments of the present invention may give rise to any one or more of several advantages over the prior art. For example: The use of the new partitioning element(s) may achieve reduced local coolant flow velocity at or close to the entrance to the forward cooling chamber by reducing the local “mass flow per unit area” values (W.sub.c/A.sub.flow).sub.local. The (W.sub.c/A.sub.flow).sub.local values may be reduced by introducing the new partitioning element(s) adjacent the leading edge portion of the component, especially the showerhead cooling holes (where employed). This (or each respective) newly created leading-edge (especially forward) sub-chamber feed passage within the existing forward cooling chamber only supplies approximately 10% of the total L/E showerhead flow; however the local flow area of the forward sub-chamber is greater than 20% of the total feed passage area of the forward cooling chamber. Therefore, the W.sub.c/A.sub.flow).sub.local value for the forward sub-chamber is kept at a lower level, ensuring that the local velocity is low and the static pressure remains at a high level close to the total feed pressure. The newly-created forward (L/E) sub-chamber(s) feed passages ideally may be incorporated into both inboard and outboard feed source locations which supply the forward cooling chamber, when a dual feed system is present in the cooling scheme design. By carefully balancing the quantity of coolant air entering each of the forward (L/E) sub-chambers with respect to its flow area, and/or balancing those flows with the quantity of coolant air entering the respective rearward sub-chambers (i.e. the main feed passage flow into the cooling chamber) with respect to its feed area, the minimum pressure margin can be arranged to exist in both forward (L/E) and rearward (main) sub-chambers. This combination may therefore give the minimum feed pressure required whilst ensuring that the minimum backflow pressure margin is achieved. The benefits of the pressure recovery system described may be utilised in one or more of the following ways: (a) For reducing the pressure drop across the combustor (P.sub.30-P.sub.40). (b) For reducing the local thickness of the aerofoil section. (c) Using the additional feed pressure to increase the local internal heat transfer at the aerofoil leading edge. This may be achieved by increasing the pressure ratio across the impingement cooling jets incorporated into the L/E feed cavity.

(37) Any one or more of the above improvements may ultimately improve the efficiency of the engine (e.g. gas turbine engine) and thus ultimately may improve the engine's SFC (specific fuel consumption).

(38) Moreover, the potential benefits of reducing the combustor pressure drop may be potentially very significant. A typical modern civil aircraft gas turbine engine combustor may have a pressure drop across it in the range of from about 2.3 to 2.7%, where:
Combustor Pressure Drop={(P.sub.30−P.sub.40)/P.sub.30}×100[%].

(39) Typical exchange rates suggest that a 1% change in percentage pressure drop is equivalent to 0.25 to 0.33% SFC.

(40) Whilst the described embodiments relate to the specific case of a nozzle guide vane, the skilled person will appreciate the inventive concept may also be applied to other components. For example (but without limitation) a turbine blade, a shroud or a hub which suffers the problems addressed by the invention.

(41) Whilst the specific embodiments relate to a vane having a forward and rearward channel, the invention is equally applicable to vanes having a single cooling chamber or a plurality of cooling chambers which is greater than 2.

(42) It is to be understood that the above description of embodiments and aspects of the invention has been by way of non-limiting examples only, and various modifications may be made from what has been specifically described and illustrated whilst remaining within the scope of the invention as defined in the appended claims.

(43) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and are not intended to (and do not) exclude other moieties, additives, components, integers or steps.

(44) Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(45) Furthermore, features, integers, components, elements, characteristics or properties described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith.