TURBINE CASING

Abstract: A turbine casing (20) is surrounded by a cowling (29) so that a gap (31) is defined between them for the flow of cooling air. The gap (31) between the cowling (29) and the casing (20) is locally reduced adjacent thickened casing portions (27) in order to provide increased velocity cooling air flow over those casing portions (27).

Description: TURBINE CASING This invention relates to a turbine casing and is particularly concerned with the cooling of such a casing. The turbine of a gas turbine engine typically comprises a circular cross-section casing which encloses axially alternate annular arrays of aerofoil blades and vanes. During the operation of the engine, hot gases exhausted from the engine combustion equipment are passed through the turbine in order to provide rotation of the annular arrays of turbine blades. Since the gases are very hot, they naturally provide some degree of heating of the turbine casing. In order to permit the casing to withstand this heating, it is usual to manufacture the casing from a high temperature resistant alloy. However, notwithstanding this, the casing can reach undesirably high temperatures, thereby making it necessary to provide cooling. One way of achieving such cooling is by the provision of cooling air manifolds around the exterior surface of the casing. Apertures in the. manifolds direct a flow of cooling air on to the casing surface. While such cooling air manifolds can be effective in providing casing cooling, they tend to be complicated and costly to produce. Moreover, their positioning adjacent the casing has to be accurate to ensure that the desired degree of cooling is achieved. It is an object of the present invention to provide a turbine casing cooling system which is simple. According to the present invention, a turbine casing is at least partially enclosed by a cowling so that a gap is defined between them for the flow of a cooling air, the magnitude of said gap varying in proportion to the local cooling requirements of said turbine casing so that appropriate local velocity variations in each flow of cooling air is facilitated. The present invention will now be described, by way of example, with reference to the accompanying drawings in which: . Figure 1 is a sectioned side view of the upper half of a ducted fan gas turbine engine have a turbine casing in accordance with the present invention; Figure 2 is a sectioned side view, on an enlarged scale, - of a portion of the- turbine casing of the ducted fan gas turbine engine shown in figure 1. With reference to figure 1, a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 11, a propulsive fan 12, an 10 intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust nozzle 19. The gas turbine engine 10 works in the conventional 5 manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second flow which provides propulsive thrust. The intermediate pressure compressor 13 compresses the air flow directed into it before 0 delivering that air to the high pressure compressor 14 where further compression takes place. The 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 5 resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16,17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16,17 and 18 0 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts. A portion of the casing 20 of the low pressure turbine 18 can be seen in greater detail if reference is now made to 5 figure 2. The casing 20 is of generally frustoconical configuration and is provided with an annular flange 21 at its upstream end for attachment to a corresponding flange 22 provided on the downstream end of the casing of the intermediate pressure turbine 17. A further flange (not shown) is provided on the downstream end of the casing 20 to provide' support for the nozzle 19. The casing 20 contains axially alternate annular arrays stator aerofoil vanes 23 and rotor aerofoil blades 24. The rotor aerofoil blades are mounted in the conventional manner on the peripheries of discs contained within the casing 20. ■ j Annular shrouds 25 are mounted on the internal surface of the casing 20 to cooperate with the radially outer tips 26 of the rotcr aerofoil blades 24 so that a gas seal is defined between them. The edges of the annular shrouds 25 are located in slots 15 provided in thickened support regions 27 which are formed integrally with the casing 20. The thickened support regions 27 additionally provide support for the radially outer extents of the stator vanes 23. The turbine casing 20 inevitably gets hot during normal 0 engine operation and requires a certain degree of cooling in order to ensure that its temperature remains within acceptable limits. That cooling is provided by a flow of cooling air over the exterior surface of the casing 20 as indicated by the arrows 28. The air is derived from the low 5 pressure compressor 12 and is constrained to flow in a generally axial direction by an annular cowling 29 which surrounds the casing 20. The cowling 29 is attached to the casing 20 by a series of bolt and bracket assemblies 30. It generally follows the 0 configuration of the casing 20 so that a radial gap 31 of generally constant magnitude is defined between cowling 29 and the casing 20 for the cooling air flow 28. However, those regions of the cowling 29 which surround the thickened casing portion 27 are deformed so that they define 5 circumferentially extending channels 32. The channels 32 serve to provide local reductions in the magnitude of the radial gap 31 adjacent the thickened casing portions 27. This ensures that as the cooling air flow 28 passes through the gap 31 its velocity locally increases through the narrow portions of the gap 31 to provide enhanced cooling of the thickened casing portions 27. Consequently the cooling air flow 28 is able to provide variable cooling of the turbine casing 20: those thickened casing portions 27 which require a greater degree of cooling being provided with a higher velocity cooling air flow than the remainder. The turbine casing 20 is therefore cooled in a uniform manner and this helps to ensure that it maintains its configuration during engine operation. This in turn means that the radial clearances between the tips 26 of the rotor aerofoil blades 24 and the annular shroud 25 can be maintained at smaller values than would be the case if the casing 20 did not maintain its configuration. Such reduced clearances ensure greater overall turbine efficiency. A further benefit from the provision of the cowling channels 32 is that they enhance the stiffness of the cowling 29. The cowling 29 can be therefore formed from thinner, and therefore lighter, material than would otherwise be the case. Although the present invention has been described with reference to a turbine casing 20 provided with a cowling 29 which is configured so as to ensure a cooling air flow velocity increase in the regions of the thickened casing portions 27, it will be appreciated that other configurations could be used if so desired. Such alternative configurations would of course be determined by the cooling requirements of the casing.

Claims: Claims 1. A turbine casing at least partially enclosed by a cowling so that a gap is defined between them for the flow of cooling air characterised in that, the magnitude of said gap (31) varies in proportion to the local cooling requirements of said casing (20) so that appropriate local velocity variation in said flow of cooling air is facilitated. 2. A turbine casing as claimed in claim 1 characterised in that said casing (20) is provided with regions (27) which are of greater thickness than the remainder thereof, the gap (31) between said cowling (29) and said regions of greater thickness (27) being of lesser magnitude than that between said cowling (29) and the remainder of said casing (20) so as to provide a local increase in the velocity of said cooling air flow adjacent said regions (27) of increased casing thickness, said gap (31) between said cowling (29) and said remainder of said casing (20) being of substantially constant magnitude. 3. A turbine casing as claimed in claim 2 characterised in that said casing regions (27) of increased thickness provide support for shroud members (25) and stator vanes (23) located within said turbine casing (20). 4. A turbine casing as claimed in any one preceding claim characterised in that said cowling (29) is provided with channel-shaped portions (32) in order to define said variations in said gap between said cowling (29) and said turbine casing (20). 5. A turbine casing as claimed in claim 4 characterised in that said channel-shaped portions (32) are additionally so configured as to provide enhanced cowling (29) stiffness. 6. A turbine casing as claimed in any one preceding claim characterised in that said casing (20) is that of the low pressure turbine (18) of a ducted fan gas turbine engine (10).

Inventors: HUTCHINSON DAVID, HUTCHINSON, DAVID
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1992
Main classification: F01D11
IPC codes: F01D11, F01D25, F02C7