GB2251895A

GB2251895A - Gas turbine shroud ring

Info

Publication number: GB2251895A
Application number: GB8031989A
Authority: GB
Inventors: John Henry Roy Sadler
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-10-03
Filing date: 1980-10-03
Publication date: 1992-07-22
Anticipated expiration: 2000-10-03
Also published as: GB2251895B

Abstract

A shroud ring (28) suitable for the turbine of a gas turbine engine comprises an annular heat pipe (27) with radially inwardly directed flanges (28) which are bridged by a plurality of abutting tiles (30) so that together they define an annular chamber (33). The annular chamber (33) serves to provide a thermal barrier between the tiles (30) and a major portion of the heat pipe (27). The annular chamber (33) may contain air, either static or flowing, as an insulator. or may be filled with a solid insulating material. The operating temperature of the shroud ring (26) is thus reduced thereby ensuring reduced radial expansion. <IMAGE>

Description

GAS TURBINE ENGINE This invention relates to gas turbine engines and in particular to the turbines of such engines.

In the pursuit of better gas turbine engine specific fuel consumption, higher bypass ratios have been utilised which have in turn led to increased power from. the gas generator. oever, this increased gas generator power has resulted in higher turbine entry temperatures. Whilst such increased temperatures are desirable from the point of view of engine efficiency, it has proved to be increasingly difficult to provide turbine components which will withstand such temperatures over lorg periods of time without suffering some kind of structural failure. In particular, severe problems have been encountered with turbine components which are subjected in use to localised thermal gradients. penis results in thermal stress being involved in those components which can in turn cause heir distortion or cracking.

Thermal gradients are a particular problem in the case of turbine shroud rings. Shroud rings are commonly used in axial flow turbines to define a portion of the gas passage through the turbine and surround an annular array of rotary aerofoil blades. It is common for shroud rings to vary in temperature by as much as 300 C in an axial direction.

This often results in the distortion of the shroud ring, thereby causing sealing problems with the array of rotary aerofpil blades lith which it cooperates, It has been suggested to manufacture shroud rings in the form of heat pines 90 that in operation they are substantially thermal. This eliminates distortion due to thermal gradients but results in a high overall operating temperature for the shroud ring. Consequently during turbine operation cycles the shroud ring tends to radially expand and contract by amounts which have a detrimental effect on ,the clearances between the shroud ring and the tips of the rotary aerofoil blades.Thus whilst the clearances between the shroud ring and blade tipS should ideally be as small as possible, high shroud ring temperatures ensure that such small clearances cannot be maintained.

It is an object of the present invention to provide a shroud ring which includes a heat pipe and which is subject to reduced radial expansion.

According to the present invention, a shroud ring suitable for the turbine of a gas turbine engine comprises, in combination, means.adapted to define an annular, radially inwardly facin=-surPace, an annular heat pipe positioned radially outwardly of said surface defining means and spacing -eans adapted to radially space apart at leat a major portion of said heat pipe and said surface defining means in such a manner that an annular thermal barrier is provided by them.

Said thermal barrier is preferably in the form of an annular chamber which is defined by said heat pipe, said surface defining means and said spacing means and contains a thermal insulator.

said thermal insulator may be air.

Said annular chamber may be provided with an inlet and an outlet whereby an air flow may be maintained through said annular chamber.

Said spc1ng means preferably comprises radially - extending flanges which constitute portions of said heat pipe.

Said anr.ular heat pipe is preferably of generally U-shaped cross section to provide two radially extending flanges which constitute said spacing means, said surface defining means bridging said flanges to define said annular chamber.

Said surface defining means preferably comprises a plurality of abutting tiles which together define said annular radially inwardly facing surface.

Said tiles may be either metallic or ceramic.

The invention will now be cescribed by way of example with reference to the accompanying drawings in which: Figure 1 its a side view of a gas turbine engine incorporating a shroud ring in accorance with the present invention, Figure 2 is a sectioned side view of a portion of the turbine of the gas turbine engine' shown in Figure.1, Figure 3 is an enlarged view of a portion of the turbine shown in Figure 2, With reference to Figure 1, a gas turbine engine generally indicated at 1C consists of a compressor 11, combustion equipment 12 and a turbine 13. The gas turbine engine 10 operates in the conventional manner, that is, air compressed by the compressor 11 is mixed with fuel and combusted in the combustion equipment 12.The resultant hot gases expand through the turbine 13 to atmosphere, thereby driving the turbine 13 which in turn drives the compresSor 11.

The combustion equipment 12 comprises an annular combustion chamber 1X, a portion of the downstream end of which can be seen in Figure 2. Hot gases from the combustion chamber 14 are directed into the high pressure section 15 of the turbine 13 by an annular array of stationary nozzle guide vanes, one of which can be seen at 16. In this particular case, the high pressure turbine 15 consists of a single stage.

of rotatably mounted turbine blades one of which can be seen at 17. The high pressure turbine 15 is drivingly connected by a suitable shaft (not shown). to the high pressure portion of the compressor 11. - ~- The hot gases issued from the high pressure turbine 15 are then directed into the low pressure turbine 18 by a second annular array of stationary nozzle guide vanes, one of which can be seen at 19. As in the case of the high pressure turbine 15, the low pressure turbine 18 consists of a single stage of rotatably mounted turbine' blades, one of which can be seen at 20. The low pressure turbine is drivinly connected by a further suitable shaft (not shown) to the low pressure portion of the compressor 11.The hot gases issued from the low pressure turbine 18 then pass through an annular array of outlet guide vanes (not shown) before being exhausted to atospere.

In order to ensure that as much as possible of the hot gases directed by the high pressure nozzle guide vanes 16 pass over the aernfoil portions of the high pressure turbine blades 17, the tips 25 of the turbine blades 17 are arranged to pass as closely as possible to an annular shroud ring 26. @ The shroud ring 26 can be seen more clearly in Figure 3.

It comprises an annular heat pipe 27 which is of generally U-shaped cross-section so as to provide two radially inwardly extending flanges 28.

Throughout this specification the term "heat pipe" is to be understood as meaning a heat transfer device comprising a sealed container which encloses both a condensable vapour and capillary means capable of causing the transport of the condensed vapour from a cooler area of the container to a hotter area, the conciensable vapour being trar.sporìed from the hotter area to the cooler area by the vapour pressure gradient between the two areas, the vapour being condensed in the cooler area.

The variation of vapour pressure with temperature oft such substances as water. ammonia, mercury, caesium, potassium, sodium, lithum and lead is such that a change in temperature of only 1 or 2 C gives a very large change in their vapour pressure. Consequently the temperature differences occurring over the length of a heat pipe containing one of these substances as the condensable vapour are so small as to render the heat pipe substantially isothermal. In practice, the effective thermal conductivity of a heat pipe can be as much as' 500 times greater than that of a solid copper rod having the same mass.The principles behind heat pipes are more thoroughly set out in "Struc-tures of Very High Thermal Conductance", Graver, Cotter and Erickson, Journal of plied Physics Vol 35, 1990 (June 1961).

The internal wall of the heat pipe 27 is covered with a stainless steel mesh 29 which functions on the heat pipe @@@ capillary. It will be appreciated, however, that other alternative capillary materials such as porous glass, metal or ceramic could be utilised.

In order to enable the heat pipe 27 to function, it is evacuated and contains a small amount of sodium as the condensable vapour. Other materials could, however, be utilised as The condensable vapour as will be apparent from the above description@@@ The radially inwardly extending -flanges 28 of the heat pipe 27 are bridged by a series of similar abutting tiles 30.

Location ribs 31 on the tiles 30 ensure their correct positioning with respect to the flanges 28. The tiles 32 extend around the whole of the radially inner eripher of the heat tipe 27 to define an annular radially inwardly facing surface 32 which is adjacent the rotary blade tips 25. The tiles 30 are metallic and boning to the heat pipe flanges 28. The tiles could, however, be formed from a suitable ceramic material, It would of course, be necessary to mount ceramic tiles in such a manner thlt differences in the ratos of thermal expansion of the heat pipe 27 and tiles 30 do not result in undesirably high strcss loads being imposed upon the tiles 30.

The heat pipe 28 and tiles 30 together define an annular chamber 7,3 which serves to space apart a major portion of the heat pipe 27 away from the tiles 30.

The annular chamber 33 @erves to provide a thermal barrier between a major portion of the heat pipe 27 and the tiles .30.

Thus although the tiles 30 will heat up during turbine; operation, the temperature of the heat pipe 27 will be generally lower than would be the case if more directly exposed to thee hot gas stream passing through the turbine 13. It follows therefore thst the heat pipe 27 and thus the shroud ring 26 will expand radially by a reduced amount, thereby maintaining the clearances between the tips 25 of the turbine blade 17 and the. inner shroud surface 32 at comparatively lo levels.Moreover since the majority of the shroud rir5 26 is constituted by the heat pipe 27, it will tend to be isothermal during turbine operation, thereby avoiding undes@rable distortion inducing thermal gradients.

If necessary, the operating temperature of the shroud rin-. 26 may be further reduced by providing an air flow through the annular chamber 33. This would of course neces@itate the provision of a suitable inlet and outlet to the annular chamber 33.

Although tne resent invention has been described with reference to a shroud ring 26 comprising 2 heat pipe 27 with flanges 28, it may be appreciated that other configurations are possible without departing from the general corcept of the present invention. Thus for instance, the flanges 28 on the heat pipe 27 could be omitted and the tiles 50 prided with substitute flanges which are not in the form of heat ripen. The annular chamber 33 would be retained, thereby providing the necessary spacing between the major portion of the heat pipe 27 and the surface defining portions of the tiles 30.

The present invention has been described with reference to an annular chamber 33 which contains air, either static or flowing, as on insulator. However, it may be convenient in certain circumstances to fill the chamber with a suitable solid insulating material.

Claims

What we claim is:

1. h shroud ring suitable for the turbine of a gas turbine engine comprising, in combination, means adapted to define an annular radially inwardly facing surface, an annular heat pipe positioned radially outwardly of said surface defining means and spacing means adapted to radially space apart at least a major porticn of said heat pipe and said surface defining means in such a manner that an annular thermal barrier is provided between them.

2. A shroud ring as claimed in claim 1 wherein said thermal barrier is in the for:n o an annular chamber which is defined by said heat pipe, said surface defining means and said spacing means, and contains a thermal insulator.

3. A shroud ring as claimed in claim. 2 wherein said thermal insulator 's air.

4. A shroud ring a claimed in clair 3 wherein said annular chamber is provi@ed with an inlet and an outlet whereby an air flow may be maintained through said annular chamber.

5. @ shroud ring as claimed in any one preceding c'air'-.

wherein $said spacing means comprises radially extending flanges which constitute portions of said heat pipe.

6. A shroud ring as claimed in claim 5 wherein said annular heat nine is of generally U-shape crosr;-section to provide two radially extending flanges which constitute said spacing means, said sirface defining means bridging said flanges to define said annular chamber

7. A shroud ring as claimed in any rne preceding clain wherein said surface defining means comprises a plurality of abutting tiles which together define said annular radially Inwardly facing surface.

8. A shroud ring as claimed ir claim 7 'Therein said tiles are either metallic or ceramic.

9. A shroud ring substantially as hereinbefore described wlth reference to and as shown in Figures and 3 of the accompanying drawings.

10. A gas turbine engine provided with a shroud ring as claimed in any one preceding claim.

10. A gas turbine engine provided with a shroud ring as claimed in any one preceding claim. @ amendments to the claims have been filed as follows

1. A shroud ring suitable for the turbine of a gas turbine engine comprising, in combination, means adapted to define an an annular radially inwardly facing surface, an annular heat pipe positioned radially outwardly of said surface defining means and spacing means adapted to radially space apart at least a major portion of said heat pipe and said surface defining means in such a manner that an annular thermal barrier is provided between said at least a major portion of said heat pipe and said surface defining means.

2. A shroud ring as claimed in claim 1 wherein said thermal barrier is in the form of an annular chamber which is defined by said heat pipe, said surface defining means and said spacing means, and contains a thermal insulator..

3. A shroud ring as claimed in claim 2 wherein said thermal insulator is air.

4. A shroud ring as claimed in claim 3 wherein said annular chamber is provided with an inlet and an outlet whereby an air flow may be maintained through said annular chamber.

5. A shroud ring as claimed in any one preceding claim wherein said spacing means comprises radially extending flanges which constitute portions of said heat pipe.

6. A shroud ring as claimed in claim 5 wherein said annular heat pipe is of generally U-shape cross-section to provide two radially extending flanges which constitute said spacing means, said surface defining means 'bridging said flanges to define said annular chamber.

7. A shroud ring as claimed in any rne preceding claim wherein said surface defining means comprises a plurality of abutting tiles which together define said annular radially inwardly facing surface.

8. A shroud ring as claimed ir claim 7 wherein said tiles are either metallic or ceramic.

9, A shroud ring substantially as hereinbefore described with reference to and as shown in Figures @ and 3 of the accompanying drawings.