Composite Elongate Member Having A Predetermined Effective Coefficient Of Linear Expansion

Abstract

A stator shroud ring assembly in which a turbine of a gas turbine engine is positioned with an annular gap of predetermined radial distance between the stator shroud ring and the periphery of the turbine rotor, the stator shroud ring assembly including a stator shroud ring constructed from at least one arcuate portion made of one material and at least one arcuate portion made of another material, the co-efficients of linear expansion of the materials employed and the total arcuate length employed of each material being such that the circumferential length of whole shroud ring will change with temperature by an amount corresponding to a predetermined effective co-efficient of linear expansion substantially equal to that of the rotor material.

Description

The invention relates to a composite elongate member having a predetermined effective coefficient of linear expansion and is particularly, but not exclusively, concerned with a stator shroud ring assembly to be arranged concentrically around a turbine rotor of a gas turbine engine.

A stator shroud ring is provided around the periphery of a turbine rotor to define the radially outer boundary of the working fluid passage through the turbine. The blades of the rotor may themselves be unshrouded or they may be provided with a shroud integral with the blades and concentric with the stator shroud ring. There must of course be running clearance provided by an annular gap of a predetermined radial thickness between the tips of the blades, where they are unshrouded, and the stator shroud ring or between the rotor shroud ring, where provided, and the stator shroud ring.

The turbine rotor, including the blades and the rotor shroud ring, where provided, is made of a material selected to withstand the necessary high working temperatures. Similarly, the stator shroud ring is subjected to the same working temperatures and is likewise made of a material capable of withstanding high temperature. Where the rotor and the stator shroud ring are made of the same material, it has been found that the stator shroud ring will expand more than the rotor and therefore the annular gap will increase in radial thickness as working temperature increases. This is because the temperature gradient radially of the rotor increases from the hub to the blade tips or the rotor shroud ring, where provided. The increase in the radial thickness of the gap results in loss of performance when the turbine has reached its operating temperature, because there must be a minimum clearance when the turbine is cold to prevent the rotor rubbing against the stator shroud ring. An object of the invention is to provide a stator shroud ring which will have substantially the same radial expansion characteristics as the rotor, thereby resulting in an annular gap between the rotor and the stator shroud ring of substantially uniform radial thickness throughout the range of operating temperature of the turbine.

According to the invention, a composite elongate member is constructed from at least one portion made of one material and at least one portion made of another material, the portions being assembled end-to-end and the co-efficients of linear expansion of the materials employed and the total length employed of each material being such that the whole member when subjected to a temperature change will change in length by an amount corresponding to a predetermined effective co-efficient of linear expansion.

The invention also provides a stator shroud ring assembly in which a turbine rotor of a gas turbine engine is to be positioned with an annular gap of predetermined radial distance between the stator shroud ring and the periphery of the turbine rotor, the stator shroud ring assembly including a stator shroud ring constructed from at least one arcuate portion made of one material and at least one arcuate portion made of another material, the arcuate portions being assembled end-to-end and the co-efficients of linear expansion of the materials employed and the total arcuate length employed of each material being such that the circumferential length of the whole shroud ring will change with temperature, within the range of working temperature for which the engine has been designed, by an amount corresponding to a predetermined effective co-efficient of linear expansion, whereby the total change of circumferential length of the stator shroud ring will substantially correspond to the alteration in diameter of the rotor, the radial dimension of the annular gap thereby being maintained at substantially the aforesaid predetermined value.

Conveniently the stator shroud ring is formed from arcuate portions of two different materials but arcuate portions made of more than two materials may be used if desired.

Conveniently, one of the materials from which the arcuate portions are made is the same as that from which the rotor is formed, the other material or materials having a lower co-efficient of expansion, whereby the effective expansion of the composite stator shroud ring will be substantially the same as that of the rotor.

The arcuate portions may have interengageable end faces.

The arcuate portions may be bounded by a concentric enveloping ring made of a material having a co-efficient of linear expansion substantially the same as the material from which the turbine rotor is formed. The material of the concentric enveloping ring may have inferior stress rupture properties compared with the material from which the rotor is made.

The invention also includes a gas turbine including a rotor and a stator shroud ring assembly in accordance with any one of the five immediately preceding paragraphs.

By way of example, a gas turbine having a composite stator shroud ring in accordance with the invention is now described with reference to the accompanying drawings, in which:

FIG. 1 is an axial section through a peripheral portion of the turbine;

FIG. 2 is an end view of the turbine shown in FIG. 1;

FIG. 3 shows a modified form of the stator shroud ring shown in FIG. 2, and

FIG. 4 is a view similar to FIG. 1 showing a further modification.

Referring to FIGS. 1 and 2 the turbine has a stator shroud ring formed from a plurality of arcuate portions 1, 2 two or more abutted together circumferentially to form a complete annulus. The segmental portions 1, 2 are made of two different materials but where there are more than two arcuate portions more than two materials may be used. One or more of the arcuate portions 1 are made of the same material as the rotor 12, e.g., a nickel-based alloy, and the other arcuate portion or portions 2 are made of a ceramic or ceramic-like material, such as silicon nitride. The arcuate portions are held within an enveloping concentric outer ring 3 which may be made of a material having the same co-efficient of linear expansion as the nickel-based alloy; but which may be made of a cheaper material having an inferior stress rupture property, which itself would not be capable of withstanding the stresses to which the stator shroud ring will be subjected during operation of the turbine. The ratio of the total circumferential length of the arcuate portions 2 to the total circumferential length of the alloy portions 1 is such that the effective co-efficient of linear expansion of the composite stator shroud ring is substantially equal to the co-efficient of linear expansion of the material of the rotor. For example, where the relative co-efficient of linear expansion of the alloy is 18 and the relative co-efficient of linear expansion of the ceramic material is 3 and the thermal gradient radially of the turbine rotor results in the effective relative co-efficient of linear expansion of the rotor being 12, the effective relative co-efficient of linear expansion of the composite stator ring could also be made to be 12 by making the ratio of the circumferential lengths of the alloy to ceramic segmental portions in the ratio of 3:2.

By making the effective linear co-efficient of expansion of the composite stator shroud ring substantially equal to the co-efficient of linear expansion of the material from which the rotor is made, the radial dimension of the gap X between the stator shroud ring and the tips of the rotor blades, or the rotor blade shroud ring, where provided, is maintained substantially constant over the range of operating temperatures for which the turbine has been designed.

The external concentric enveloping ring 3 may be located from movement in the axial direction by clamping it between two housing parts 4 and 5, such as the nozzle guide vane supporting ring and another annular shroud positioned between the turbine rotor and an adjacent rotor. The ring 3 may make a shrink fit in one or both of the housing parts.

The composite shroud ring 1, 2 may extend over a ring of nozzle guide vanes as well as the rotor blades.

As shown in FIG. 2, the arcuate portions 1, 2 may have interengaging ends, such as ribs and notches 6 to maintain correct relative location of the portions. Alternatively the opposite circumferential ends of each of the portions may be made concave and convex respectively as shown at 7 in FIG. 3. This construction would also maintain adjacent ends of the portions at the correct radial locations.

FIG. 4 shows a modification of the construction shown in FIG. 2 in which the concentric enveloping ring 8 is tapered in the axial direction and is engaged with a frusto-conical face of a housing member 9. A spring 10 acts betwen the ring 8 and a second housing member 11 and thereby urges the ring 8 by wedging action radially inwardly against the composite ring 1, 2. Hence the arcuate portions 1, 2 will be maintained in end-to-end abutment.

Claims

What I claim as my invention and desire to secure by Letters Patent of the United States is:

1. A stator shroud ring assembly in which a turbine of a gas turbine engine is to be positioned with an annular gap of predetermined radial distance between the stator shroud ring and the periphery of the turbine rotor, the stator shroud ring assembly including a stator shroud ring constructed from at least one arcuate portion made of one material and at least one arcuate portion made of another material, the arcuate portions being assembled end-to-end and the co-efficients of linear expansion of the material employed and the total arcuate length employed of each material being such that the circumferential length of the whole shroud ring will change with temperature, within the range of working temperature for which the engine has been designed, by an amount corresponding to a predetermined effective co-efficient of linear expansion, whereby the total change in circumferential length of the stator shroud ring will substantially correspond to the alteration in diameter of the rotor, the radial dimension of said annular gap thereby being maintained at substantially the aforesaid predetermined value.

2. A stator shroud ring assembly as claimed in claim 1 in which the stator shroud ring is formed from arcuate portions of two different materials.

3. A stator shroud ring assembly as claimed in claim 1 in which one of the materials from which the arcuate portions are made is the same as that from which the rotor is formed, the material of the remaining arcuate portions having a lower co-efficient of expansion, whereby the effective expansion of the composite stator shroud ring is substantially the same as that of the rotor.

4. A stator shroud ring assembly as claimed in claim 1 in which the arcuate portions have interengageable end faces.

5. A stator shroud ring assembly as claimed in claim 1 in which a concentric enveloping ring made of a material having a co-efficient of linear expansion substantially the same as the material from which the turbine rotor is made bounds the arcuate portions of the stator shroud ring.

6. A gas turbine engine including a bladed turbine rotor and a stator shroud ring assembly, the latter defining a stator shroud ring surrounding the rotor with an annular gap of predetermined radial distance between the stator shroud ring and the periphery of the turbine rotor, the stator shroud ring being constructed from at least one arcuate portion made of one material and at least one arcuate portion made of another material, the arcuate portions being assembled end-to-end and the co-efficients of linear expansion of the materials employed and the total arcuate length employed of each material being such that the circumferential length of the whole shroud ring will change with temperature, within the range of working temperature for which the engine has been designed, by an amount corresponding to a predetermined effective co-efficient of linear expansion, whereby the total change in circumferential length of the stator shroud ring will substantially correspond to the alteration in diameter of the rotor, the radial dimension of said annular gap thereby being maintained at substantially the aforesaid predetermined value.