GB2065237A - Turbine blades - Google Patents

Abstract

A turbine rotor comprises blades 1 of refractory, e.g. ceramic, material and means to apply compressive stress to the blades. Such means can comprise either a tension ring 6 surrounding the blades or tension means extending through the blades. For the tension ring, cooling arrangements 7 and a heat insulating barrier 4 can be provided. Carbon fibre is a preferred tension-providing material. <IMAGE>

Description

SPECIFICATION Turbine Blades In gas or steam turbines, the higher the operating temperature, the higher the efficiency.

The practical upper limit to this temperature is set by the materials used in the turbine blades; at the moment blades are made of special ferrous alloys and blade temperatures may not exceed approximately 10000C.

There exist, however, several refractory ceramic materials capable of bearing compressive load even in the temperature range 1 5000C-20000C; their disadvantage is their low tensile strength and, above all, their brittleness which makes such tensile strength as they have undependable.

The present invention concerns the application of prestressing to refractory ceramics to enable them to support the tensile stresses set up in turbine blades. When this is done, the ceramic material being able to stand several hundred degrees centigrade higher than the metals currently employed, the efficiency of the turbine can be substantially increased.

The stresses in the turbine blade are of the following sorts: (1) Tensile, due to centrifugal force.

(2) Bending and shear, due to the incidence of gas on the blade.

(3) Torsional, due to the combination of shape of blade and distribution of gas pressure.

(4) Tensile, due to thermal shock caused by sudden changes of temperature, e.g. a sudden blast of hot air on starting up or cold air on shutting down.

Of these the tensile stresses due to centrifugal force are much the largest; they, in combination with those due to bending, can be wholly blanketed by a compressive prestress acting along the axis of the blade. On the other hand, the torsional stresses and the tensile stresses caused by thermal shock must be capable of being supported by the material itself.

The compressive force along the axis of the blade can be applied by either of two means: (1) a tensioned ring passing round the tips of the blades like a tyre around spokes.

(2) an individual wire lying inside the blade and anchored to its outer tip and in the interior of the rotor to which the blade is to be fixed.

Using the first method, however, the tensioned member, the enveloping ring, can be located well outside the hot zone and it is possible to interpose between the tips of the blades and the tensioned ring an insulating layer to reduce the flow of heat and, outside it again, a cooling zone in which a flow of cold air circulates and removes the heat.

By this combination of heat barrier and heat drain, the temperature of the tensioned ring can be maintained at any specified level.

By means of this tensioned ring so cooled, the tension caused by centrifugal force and that caused by gas bending can be blanketed by a compressive stress.

The tensile stresses caused by shear and torsion will be reduced by this axial compression but a tensile stress will remain. The sudden changes of temperature which occur at starting up and shutting off the engine (in the first case there is a sudden impact of hot air on a cold blade, and in the second of cold air on the hot blade) will also cause transient tensile stresses, whose value is related to the following characteristics of the material: modulus of elasticity (the bigger the higher the stresses) coefficient of thermal expansion (the bigger the higher the stresses) coefficient of thermal conductivity (the bigger the smaller the stresses) These three characteristics of a material combined with its ultimate tensile strength indicate its ability to support the tensile stresses caused by thermal shock, stresses which are largely unaffected by axial compression.

The most attractive material for this purpose at the moment is Silicon Nitride (Si3 N4).

For the tensioned ring there are two requirements -- high tensile strength and, since centrifugal force in the ring depends on its mass, low density; the specific strength, i.e. strength divided by density is thus the critical value.

The material presently available which combines in most eminent degree high tensile strength and low density is carbon fibre. It is also possible that compacts of "whiskers" of various materials might be suitable.

One preferred embodiment of the invention, forming an example of the first method of prestressing the blades, will not be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a view and end elevation of a portion of a turbine rotor, and Figure 2 is a section on the line A-A in Figure 1.

Referring to the drawing, ceramic rotor blades 1 are secured to a central rotor disc 2 surrounded by a composite ring 3. The latter consists of three portions, a radially inward insulating barrier 4, a cooling zone 5, and a tension ring 6. The cooling zone 5 contains a large number of openings extending parallel to the rotor axis between an annular fluid supply chamber on one side of the rotor and an annular fluid collecting chamber on the other. Cooling fluid supplied under pressure to one chamber flows through the holes 7 to the collecting chamber. As a result of the barrier and cooling zone, the tension ring is kept relatively cool. The tension ring is designed to apply compressive stress to the blades up to the maximum rotor speed.

The first method of applying compressive forces to the blades, the tension ring, is preferably carried out with carbon fibre as the effective stressing material in the ring. The method can, however, be used with tensile elements less resistant to heat because the ring is cooled. The second method of applying compressive forces to the blades, by wires inside the blades, has the disadvantage that the tension wire is close to the flow of hot gases: it is not practical to keep the wire at a temperature substantially lower than its surroundings, and it is necessary to use a material which retains its strength at the operating temperature of the turbine.However, it is possible to use carbon fibres since they can support very high temperatures -- they do not begin to lose strength until about 1 5000C and still have appreciable strength at 25000C.

In this arrangement, the carbon fibres lie in the blade itself extending along the blade into the root of the blade which would be anchored in the rotor in a manner similar to the classic "fir-tree" anchorage of metal blades. Other methods of anchorage are conceivable, such as an extension of the blade in the form of a rod passing through a hole in the rim of the rotor, tensioned (thus stressing the blade on to the rim) and then anchored well inside the rim.

Carbon fibres must, however, be protected from oxygen at temperatures higher than about 3000C-4000C. This can be achieved if the ceramic body of the blade is free of porosity, requiring the sort of complete compaction which is obtained by vitrification. When this can be obtained, the blade would be manufactured by tensioning the fibres, compacting the ceramic around it and then firing the whole.

Where such compaction is difficult to obtain, the carbon fibres may be made up into thin rods formed of a fully vitrified ceramic compacted around the fibres; these rods are tensioned and the ceramic body of the main blade is compacted around them and fired. Ideally, the vitrified ceramic around the carbon fibres would soften at a temperature slightly lower than that of the firing temperature of the blade ceramic, whereby it would release during firing the restraint on the tensile strain of the carbon fibres, ensure that they are fully protected and, during the process of cooling, grip the fibres once more and, on release of the tension in the fibres, transmit the tensile force in them to the blade ceramic.

Since in this prestressed union between fibre and ceramic the one is acting against the other, rather than each endeavouring to act with the other, differences between coefficients of thermal expansion are translated into variations of compressive stress in the ceramic rather than in tensile stresses with the accompanying risk of cracking. The combination is made more favourable in that carbon fibres have a negative coefficient of expansion and thus exert more prestress at higher temperatures than at low.

The penetration of hot oxidising gas through the ceramic on to the carbon fibres is a function of the thickness of the ceramic and of its porosity, which may be related so as to prevent the flow of gas on to and around the fibres, as distinct from static penetration. When this is possible, a metallic or other coating on the fibres will be adequate to protect against oxidation.

If it is possible to cool a tensioned ring to 3000C000C, a temperature which gives a good differential with respect to ambient and permits of efficient cooling, it is possible to protect the fibres with thermosetting resins which can survive in this temperature.

Cooling to this temperature may be possible in stationary engines, but in aeroengines the temperature of air flowing around the turbine may be of the same order of magnitude and positive cooling means will be needed whose consumption of energy may offset so much of the gain in efficiency as to be unattractive. A dense ceramic ring is then necessary; it is possible to supplement the protection of the dense ceramic by a protective coating on the fibres.

If a ceramic body of this sort, capable of fully protecting the fibres, is formed, then the ceramic/fibre ring will need no cooling and may bear directly upon the ends of the turbine blades.

The expenditure of energy on cooling and its structural complications are thus avoided.

One of the secondary advantages of prestressing the turbine blades by a tensioned ring is that the stresses in the blades are supported at their tips not at their roots. The complications of "fir-tree" roots are thus replaced by a simple butt joint and the problems of the rotor disc are transformed -- it is now sufficient to design it to support a moment and a shear applied at the blade roots under maximum service conditions and a compression when at rest; a wire wheel combined with a ceramic ring are an attractive expedient.

The butt joints between blade and ring and blade and rotor disc present a problem; it is not feasible to obtain a perfect joint and any jointing material must resist stress, high temperature and scour. An attractive materials for making the joints is high-purity high aluminous cement made by fusing lime and alumina. This material sets hydraulically at normal temperatures; as the temperature rises, the hydraulic crystalline bond is destroyed and progressively replaced by a ceramic bond which does not collapse until temperatures of about 1 8000C are reached.

Claims (14)

1. A turbine rotor comprising a hub and a series of blades, the blades being of a refractory material weak in tension, and means strong in tension which is located to exert radial force on the blade.

2. A turbine rotor comprising a hub and a series of blades, the blades being of refractory material, and a tension ring surrounding the blades and exerting a radial compressive force on the blades at all operating speeds.

3. A turbine rotor as claimed in Claim 2, wherein the tension ring is separated radially from the blades by annular means enabling cooling whereby in operation the tension ring is at a lower temperature than the blades.

4. A turbine rotor as claimed in Claim 3, wherein the annular cooling means includes a heat barrier surrounding the blades and a cooling ring between the barrier and the tension ring to be cooled by fluid passing through it.

5. A turbine rotor as claimed in either of Claims 3 or 4, wherein the tension ring comprises carbon fibre as the tension providing material.

6. A turbine rotor as claimed in Claim 2, wherein the tension ring comprises carbon fibres on the tension providing material, the fibres being enclosed by vitrified ceramic to prevent access of oxygen.

7. A turbine rotor as claimed in Claim 6, the tension ring being butt-jointed to the blades with the aid of high aluminous cement.

8. A turbine rotor comprising a hub and a series of blades, the blades being of rafractory material and having within them tension means anchored to the hub and exerting a radial compressive force on the blades at all operating speeds.

9. A turbine rotor as claimed in Claim 6, wherein the tension means comprises carbon fibres protected from access by oxygen.

10. A turbine rotor as claimed in Claim 9, wherein the carbon fibre is enclosed by fully vitrified ceramic.

1 A turbine rotor as claimed in Claim 8, wherein the blade material is ceramic, the fibre protecting ceramic softening at a temperature slightly below the softening temperature of the blade-enclosing ceramic, the whole forming an integral unit.

12. A turbine rotor as claimed in any of Claims 8 to 11, wherein each blade is mounted to the hub by a "fir tree" connection.

1 3. A turbine rotor as claimed in any of Claims 8 to 11, wherein each blade is mounted to the hub by an extension passing through the hub and anchored within it.

14. The turbine rotor herein described with reference to the accompanying drawing.