Cooling of a gas turbine engine housing

Abstract: A gas turbine engine has an assembly of circumferentially spaced rotor blades each having a radial tip. A plurality of seal segments mounted for radial movement circumscribe the rotor blades. Each segment is connected to the casing through a seal carrier provided with a flexible membrane which divides high pressure air from low pressure into two channels.

Description: This invention relates to cooling apparatus for use with a gas turbine engine. More particularly but not exclusively this invention relates to a clearance control apparatus for a gas turbine engine which controls the clearance between the casing of the engine and the tips of the turbine blades. It is important to ensure the clearance between the tips of each rotating turbine blade and the casing is kept to a minimum. The most efficient arrangement would have no clearance gap between the turbine blades and the casing or seal within the casing. In use, the turbine blades experience thermal expansion due to the working gases and also expand due to the rotational forces of the blades. The annular casing however is stationary and only expands due to being heated by the working gases. Thus it is essential that there is a clearance gap between the blade and the casing to accommodate such expansion. However this gap is necessarily controlled so as to provide a minimum clearance at normal cruise conditions, whilst also ensuring that the blade tips do not rub on the shroud during transient operating conditions. Conveniently, this may be achieved by the provision of cooling air flows to appropriate parts of the casing, shrouds or associated structure so that their temperatures are maintained at levels consistent with the maintenance of an appropriate clearance gap. In US-A-4 841 726 and US-A-3 742 705 there are described turbine seal segment support cooling systems in which cooling air is directed in series through two radially separated chambers positioned within the seal segment support structure. However, such arrangements only permit the use of a single supply of cooling air, which may not be appropriate in all circumstances. In gas turbine engines it is useful to utilise the high pressure air from the compressors to cool turbine discs and turbine blades. It is an important design consideration that the cooling air flow from the compressors is utilised to best effect. For example high pressure air from the high pressure compressor may be used to cool the combustion chamber and the high pressure turbine blade through internal cooling passages. Lower pressure air from the compressor region may be used to cool the turbine discs of the lower pressure turbine regions. It is an object of the present invention to provide an improved blade tip clearance control apparatus and/or to provide improvements which attempt to provide more effective use of cooling air in such apparatus. According to the present invention there is provided a gas turbine engine comprising an assembly of circumferentially spaced rotor blades each having a radial tip, a plurality of seal segments circumscribing said rotor blades, each segment mounted for radial movement and connected through attachment means to a casing wherein said attachment means comprises at least two cooling air chambers radially separated by a separation element, the separation element being arranged to render said cooling air chambers discrete, so preventing cross-contamination of cooling air within said chambers. Advantageously the provision of separate chambers ensures that, in use, two compressed air supplies do not become mixed and are able to perform their respective cooling functions more efficiently. Also radial movement of the seal and hence tip clearance control is provided efficiently. The present invention will now be described with reference to the accompanying drawings in which. Figure 1 is a schematic illustration of a gas turbine engine incorporating tip clearance control apparatus in accordance with the present invention. Figure 2 is a sectional view of the blade tip clearance control apparatus of the engine shown in figure 1. With reference to figure 1, a gas turbine engine 10 comprises a core engine contained within a casing 12. A fan 14 is driven by the core engine, the fan being contained within a fan casing 16 attached to the casing 12. The core engine comprises in flow series compressors 18 a combustor 20, high, intermediate and low pressure turbines 22, 24 and 26 respectively. The turbines are connected to the fan 14 and compressors 18 to provide drive. The engine 10 functions in the conventional manner with air being drawn in and compressed by the fan 14 and the compressors 18. The compressed air is mixed with fuel and air in the combustor. The resultant exhaust gases expand through the turbines 22, 24, and 26 and are exhausted through the exhaust nozzle 28. Propulsive thrust is provided by the exhaust flow through the nozzle 28 and through air from the fan 14 which bypasses the compressors 18. Referring now to figure 2 turbine blade 30 forms part of an annular array of rotary aerofoil blades. The turbine blade is hollow to permit the flow of cooling air from the compressor region 18 into the blade. A shroud 32 is mounted on the tip of each blade 30, although it is envisaged that the present invention is equally applicable to shrouded and unshrouded blades. A shroud liner 34 surrounds the blades and comprises a ring type structure. The radially inner surface 36 of the shroud liner segments is provided with a ceramic coating to reduce the cooling heat flux required to maintain the shroud liner segments 34 at a suitable temperature. A plurality of seal segments 38 are attached to each shroud liner 34. The seal segment comprises a number of channels 40. High pressure flow from the high pressure compressor 18 is directed through channels 40 to provide cooling to the seal segment 38. Each seal segment 38 is connected to a mounting element 42 through a U shaped slot 44. This mounting element is itself connected to the inner turbine casing 46. Seal segment 38 is also mounted on a seal carrier 48 which is itself mounted on the outer turbine casing 50. The outer turbine casing 50 is provided with an insulation coating 52 preferably comprising a ceramic coating, which helps to reduce heat input into the outer casing 50. The seal carrier 48 is located between the seal segment 38 and the outer casing 50 via castellated support legs 54. Advantageously these attachment features 54 provide a radial connection link without undesirable mechanical 'hoop' stiffness. The seal carrier 48 is provided with a flexible membrane 56, the main function of which is to divide the low pressure air from the low or intermediate pressure compressor into the channel 58, radially outward of the membrane 56, and high pressure air from the high pressure compressor which flows into the channel 60, radially adjacent the seal segment 38. 'Expensive' high pressure air from the high pressure compressor can therefore be usefully directed underneath membrane 56 and cool seal segment 38 through channels 40. Due to the provision of flexible membrane 56 it is not contaminated with lower pressure air flowing above the membrane to cool other parts of the engine structure. Also advantageously if a stream of hot combustion gases should escape from the combustor itself, this flame stream would be more likely to remain contained beneath the membrane 56 since this is at a lower pressure than that above the membrane 56. The outer turbine casing 50 also comprises a of pair of radially extending flanges 62 positioned centrally above the seal carrier 48. The radially extending flange of each pair abut each other so as to, in effect, form a ring structure 64 circumventing the rotor blades. This control ring 64 is positioned within a hollow U shaped manifold 66. This manifold is provided with a number of apertures 68 formed within the inner faces of the manifold. In use, cooling air ducted from the fan 14 is directed into the hollow manifold 66 and exits via apertures 68 so as to provide a cooling effect onto the control ring 64 and adjacent casing surfaces. The control of this cooling air is provided by an on/off valve operated via an electronic control system based on altitude and high pressure shaft speed. This ring then contracts which has the effect of moving the seal segment 34 closer to its associated blade tip via seal carrier 48. The coating 52 also helps to prevent heat input into the control ring 64 during cruise conditions which could counter the effect of cooling air flow from manifold 66. Movement of the seal segment 34 closer to the blade tip during certain engine conditions has the effect of minimising the tip clearance leakage through this gap, thus increasing efficiency.

Claims: A gas turbine engine comprising an assembly of circumferentially spaced rotor blades (30) each having a radial tip, a plurality of seal segments (38) circumscribing said rotor blades (30), each segment (38) mounted for radial movement and connected through attachment means (48) to a casing (50), said attachment means (48) comprising at least two cooling air chambers (58,60) radially separated by a separation element (56), characterised in that said separation element (56) is arranged to render said cooling air chambers (58,60) discrete, so preventing cross-contamination of cooling air within said chambers (58,60). A gas turbine engine according to claim 1 wherein said said discrete chambers (58,60) are separated by a diaphragm (56). A gas turbine engine according to claim 2 wherein said diaphragm (56) comprises a flexible material. A gas turbine engine according to claim 1 wherein a rail assembly (64) is provided on the casing (50). A gas turbine engine according to claim 4 wherein said rail assembly (64) comprises a ring structure circumscribing the casing (50). A gas turbine engine according to claim 4 or claim 5 wherein at least one manifold (66) is positioned around said rail assembly (64). A gas turbine engine according to claim 6 wherein cooling air is directed to said manifold (66) and said manifold (66) is provided apertures (68) for directing the cooling air onto said rail assembly (64) to provide impingement cooling of the rail assembly. A gas turbine engine according to claims 4 to 7 wherein said rail assembly (64) is provided with insulating material (52) on its radially inner surface. A gas turbine engine according to claim 1 wherein said attachment means (48) comprises castellated leg portions (54).

Inventors: MAGUIRE ALAN ROBERT, RICHARDS MARTYN, MAGUIRE, ALAN ROBERT, RICHARDS, MARTYN
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1998
Main classification: F01D11
IPC codes: F01D11, F01D25