GB1605403A - Improvements in or relating to gas turbine engines - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS TURBINE ENGINES We, ROLLS-ROYCE LIMITED a British Company of Buckingham Gate, London SW1E 6AT, formerly ROLLS-ROYCE (1971) LIMITED, a British Company of Norfolk House, St. James's Square, London SWlY 4JR, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a seal clearance control mechanism for gas turbine engines and other fluid flow machines, and relates in particular to the improved control of sealing clearances between static and rotating parts within such engines.

One of the main problems encountered in such bladed fluid flow machines is that of leakage of compressed working fluid over the tips of the rotor blades. In the past it has not been possible to maintain an adequate, or the most efficient tip clearance under all engine conditions. With certain types of conventional tip seals for example, it has been necessary to provide an excessive blade tip clearance when the engine is not operating, or is running at slow speed, to avoid rubbing of the seals at other conditions.

This obviously is detrimental to engine performance.

It has been found that when an engine is quickly run up to operating speed the engine rotors undergo several stages of radial growth.

In the first instance the relatively thin rotor blades expand quickly in response to increases in temperature and centrifugal loading, and to this is added the radial growth of the rotor disc due to centrifugal loading. A further stage of thermal growth occurs when the relatively thick rotor disc heats up to operating temperature.

During all the above mentioned phases of expansion the casing surrounding the rotor grows at a steadily decreasing rate. Furthermore, there is the additional condition arising from a sudden shut-down, during which the blading and casing are cooled quickly but the relatively thick rotor wheel retains its heat. Thus the tip gap undergoes a sudden decrease with the danger of a blade tip rub unless an excessive gap has been provided. Therefore the tip clearance of the rotor blades must be calculated such as to tolerate all relative changes in growth and contraction of both the entire rotor and the casing.

The present invention concerns means whereby a sealing clearance in, for example, a rotor blade tip seal, may be controlled in order to accommodate a substantial portion of the thermal growth of the turbine blades and the rotor.

Accordingly the present invention provides in a fluid flow machine, a seal clearance control mechanism comprising a sealing ring spaced from rotating structure to form a sealing clearance therebetween, control means, portions of which are arcuate and are arranged circumferentially about the sealing ring, and restraining means adapted to restrain the ends of each of the arcuate portions of the control means from circumferential movement, parts of the sealing ring being secured to unrestrained parts of the arcuate portions of the control means, whereby upon expansion or contraction of said unrestrained parts they flex such that the sealing ring is displaced radially.

The control means may comprise a plurality of individual arcuate segments arranged around the sealing ring, or altematively may comprise a continuously extending member having arcuate portions. In the first case the restraining means may restrain each adjacent pair of ends of the arcuate segments against movement, and in the second case the restraining means may restrain the ends of equally spaced apart selected arcuate portions of the continuously extending member.

Preferably the control means are supplied with high pressure air from the compressor section of the engine when serves to control their rate of expansion or contraction, the high pressure air being controlled by valve means.

In the preferred embodiment of the invention the sealing ring comprises a plurality of segments each of which is secured at its respective ends to the mid-point of an adjacent arcuate segment of the control means.

Alternatively the sealing ring comprises a continuous ring of resilient material which is secured at different circumferential locations to adjacent arcuate segments of the control means.

An embodiment of the invention will now be more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a diagrammatic side elevation of a gas turbine engine having a broken away turbine casing portion disclosing the turbine which includes an embodiment of the present invention.

Figure 2 shows an enlarged cross-sectional view in greater detail of the embodiment shown diagrammatically at Figure 1.

Figure 3 shows a further cross-sectional view taken on line 3-3 at Figure 2.

Referring to Figure 1 of the drawings, a gas turbine engine comprises in flow series, a low pressure compressor 12, a high pressure compressor 13, combustion equipment 14, a high pressure turbine 15, a low pressure turbine 16, the engine terminating in an exhaust nozzle 17. Both the low pressure compressor 12 and low pressure turbine 16, and high pressure compressor 13 and high pressure turbine 15 are mounted in conventional manner on two co-axial shafts (not shown). The low pressure compressor 12 produces a by-pass stream which flows around the combustion equipment 14 and the turbines in known manner in a by-pass duct 8 (Fig. 2).

The casing of the engine is shown broken away adjacent the turbines indicating the position of the improved seal of the present invention which is shown in greater detail in Figure 2.

Referring to Figures 2 and 3 there is shown a broken away portion of one high pressure turbine blade 18, radially outwardly of which is provided a shroud ring 19, a sealing clearance 20 being defined between the blade and the ring 19. The shroud ring 19, as illustrated, takes the form of a plurality of segments each of which has its respective ends slidably secured to the ends of adjacent segments by means of common intermediate pieces 21 which are shown in Figure 3. The intermediate pieces project into slots provided within the respective ends of the segments constituting the shroud ring 19.

Although the shroud ring illustrated takes the form of a segmented ring it is envisaged that it could equally well take the form of a continuous ring manufactured from a resilient material such as that sold under the Registered Trade Mark "Felt Metal".

The control means are arranged radially outwardly of the shroud ring 19, the whole mechanism being contained within the turbine casing 24. In the embodiment of the invention illustrated in Figure 3, the control means take the form of a plurality of arched segments 23, the respective ends of which are secured together by means of a plurality of radially extending bolts one of which is shown at 22. The shroud ring segments are connected to the arched control segments 23 by tee-shaped hooks 40 on the ends of radial flanges 41 depending from the central portion of each arched segment. The hooks engage in slots 42 in the ends of the shroud ring segments. Each of the respective segments 23 which together constitute the control means, is provided with a slot 25 in each of its ends.The slots 25 are provided such that the segments may be located and restrained from movement by tee-shaped portions 26 formed upon the radially innermost ends of the bolts 22. The bolts 22 are themselves restrained by being secured to an outer support structure 27.

As will be seen from Figure 3 of the drawings, the support structure 27 is in the form of a polygonal structure with the bolts 22 at its apices.

The effect of this is that any uneven load, or expansion or contraction, occurring either in the bolts 22 or in different portions of the structure 27 will be absorbed without significant deformation of the structure.

In order to minimise the expansions and contractions of the polygonal structure 27 it may be made of a material which is different to that of the turbine casing 24, having a lower coefficient of thermal expansion. In addition the structure 27 is disposed radially outwardly of the turbine casing 24 so that the relatively cool air flow in the by-pass passage flows over it to keep it relatively cool. By this means the structure remains relatively rigid and unchanged during the different operational modes of the engine, and the bolts 22 can be used to support the segments 23 and to restrain the ends thereof to prevent circumferential movement thereof during changes in engine conditions. The structure 27 is itself bolted to the turbine casing 24 by bolts 28.

In order to control the rate of expansion of the segments 23 an air supply is provided to a chamber 32 surrounding the control means, the air being high pressure air which is taken from the high pressure section of the engine. The air is metered prior to reaching the control means by a thermally responsive valve which is shown generally at 29 in Figure 2. The valve comprises a ring 30 which has a relatively small cross-section for rapid thermal expansion and contraction, (hereinafter referred to as the fast ring) and which is provided with a circumferentially extending array of holes, one of which is shown at 31, for the passage therethrough of air. The ring 30 is supported between an annular member 33 and a second ring 34. The member 33 is attached to a part of the turbine casing 24, and the ring 34 which has a relatively large cross-sectional area, for relatively slow thermal expansion and contraction, is supported within the turbine casing 24. The ring 34 is hereinafter referred to as the slow ring.

The ring 34 has defined therein an orifice 35 through which the compressor air can pass into further apertures 36 in the turbine casing 24 to get to the chamber 32. The ring 30 is positioned so as to be capable of varying the area of the orifice 35 under various conditions of the engine, but under steady state operation of the engine the orifice will be partly opened to allow a bleed of compressor air into the chamber 32 to maintain a steady temperature of the segments 23.

During an acceleration of the engine, however, the high pressure air supplied to the valve will be hotter and because of its relatively thin cross-section, the fast ring 30 will tend to heat up and expand more quickly than the slow ring 34 which has a relatively large cross-section. This will therefore uncover more of the orifice 35 in the ring 34 and allow an increased flow of air to pass therethrough into the chamber 32.

The amount of air supply provided to the chamber 32 will obviously depend upon the thermally responsive valve 29 which is adapted to meter the air in accordance with the rate of change of its temperature variation. As the ends of each respective segment are restrained by the bolts 22 the expansion will displace the unrestrained portions of the segments which will grow radially outwards.

Because the bolts 22 restrain the ends of the segments 23, this results in a greater radial growth of the unrestrained portions than would normally be the case, and this magnified radial movement of the unrestrained portions is transmitted to the shroud ring segments via the connecting hooks 40.

Since the temperature of the high pressure air is proportional to the temperature of the working fluid in the turbine, the movement of the shroud segments radially outwardly can be arranged to be sufficiently rapid that the radially outward growth of the turbine blades under the increased temperature of the working fluid passing therethrough during the acceleration period never catches up with the shroud movement and thus the clearance 20 is never reduced to zero.

Once the temperature of the compressor air has stopped increasing at the end of the acceleration, the slow ring 34 continues to expand to a new steady state condition in which it catches up with the expansion of the fast ring 30 thus covering more of the orifice 35 and reducing the flow into the chamber 32, which reduces the rate of expansion of the control segments 23, until steady state conditions are attained.

During deceleration of the engine the compressor air reaching the valve is cooler and the fast ring 30 cools quickly covering the orifice 35 and cutting down or even stopping the air supply to the chamber 32. The air in the chamber 32 thus tends to stagnate preventing too rapid cooling and contraction of the control segments.

This prevents a too rapid radially inward movement of the shroud ring 19 during the relatively longer period which it takes for the turbine rotor disc (not shown) which carries the blades 18 to cool down, thus maintaining the clearance 20 during the deceleration.

The rate at which the shroud ring moves can be varied experimentally by varying the mass flow of the compressor air bled to the valve, by varying the thermal response of the fast ring 30, and by varying the magnification factor of the movement of the unrestrained portion of the arched segments by varying the lengths and curvature of the arches.

Thus it can be seen that by adopting the above-described method of controlling the shroud movement during accelerations of the engine the possibility of a rubbing condition occurring between the blade tips and the shroud can be minimised, or even eliminated, and by choosing an appropriate value for the rate at which the shroud ring moves, the tip clearance 20 can be controlled at a smaller value, thus improving the engine performance.

What we claim is: 1. Seal clearance control mechanism for a fluid flow machine comprising a sealing ring spaced from rotating structure to form a sealing clearance therebetween, control means, portions of which are arcuate and are arranged circumferentially about the sealing ring, and restraining means adapted to restrain the ends of each of the arcuate portions of the control means from circumferential movement, parts of the sealing ring being secured to unrestrained parts of the arcuate portions of the control means, whereby upon expansion or contraction of said unrestrained parts they flex such that the sealing ring is displaced radially.

2. Seal clearance control mechanism as claimed in claim 1 and in which the control means comprises a plurality of individual arcuate segments surrounding the sealing ring.

3. Seal clearance control mechanism as claimed in claim 2 and in which the sealing ring is formed in segments, and the ends of the segments of the sealing ring are secured to the mid-points of the adjacent arched segments of the control means.

4. Seal clearance control mechanism as claimed in any preceding claim and in which working fluid of the fluid flow machine is supplied to the region of the machine in which the control means is located, said working fluid being of variable temperature during operation of the machine to increase the rate at which unrestrained parts of the control means expand or contract.

5. Seal clearance control mechanism as claimed in claim 4 and in which the working fluid is supplied through a metering valve which is responsive to the temperature of the fluid passing therethrough to increase or decrease the rate of fluid supply to said region.

6. A gas turbine engine including seal clearance control mechanism as claimed in any preceding claim and disposed around the tips of rotatable blading thereof to control the flow of working fluid over the tips of the blading.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. orifice will be partly opened to allow a bleed of compressor air into the chamber 32 to maintain a steady temperature of the segments 23. During an acceleration of the engine, however, the high pressure air supplied to the valve will be hotter and because of its relatively thin cross-section, the fast ring 30 will tend to heat up and expand more quickly than the slow ring 34 which has a relatively large cross-section. This will therefore uncover more of the orifice 35 in the ring 34 and allow an increased flow of air to pass therethrough into the chamber 32. The amount of air supply provided to the chamber 32 will obviously depend upon the thermally responsive valve 29 which is adapted to meter the air in accordance with the rate of change of its temperature variation. As the ends of each respective segment are restrained by the bolts 22 the expansion will displace the unrestrained portions of the segments which will grow radially outwards. Because the bolts 22 restrain the ends of the segments 23, this results in a greater radial growth of the unrestrained portions than would normally be the case, and this magnified radial movement of the unrestrained portions is transmitted to the shroud ring segments via the connecting hooks 40. Since the temperature of the high pressure air is proportional to the temperature of the working fluid in the turbine, the movement of the shroud segments radially outwardly can be arranged to be sufficiently rapid that the radially outward growth of the turbine blades under the increased temperature of the working fluid passing therethrough during the acceleration period never catches up with the shroud movement and thus the clearance 20 is never reduced to zero. Once the temperature of the compressor air has stopped increasing at the end of the acceleration, the slow ring 34 continues to expand to a new steady state condition in which it catches up with the expansion of the fast ring 30 thus covering more of the orifice 35 and reducing the flow into the chamber 32, which reduces the rate of expansion of the control segments 23, until steady state conditions are attained. During deceleration of the engine the compressor air reaching the valve is cooler and the fast ring 30 cools quickly covering the orifice 35 and cutting down or even stopping the air supply to the chamber 32. The air in the chamber 32 thus tends to stagnate preventing too rapid cooling and contraction of the control segments. This prevents a too rapid radially inward movement of the shroud ring 19 during the relatively longer period which it takes for the turbine rotor disc (not shown) which carries the blades 18 to cool down, thus maintaining the clearance 20 during the deceleration. The rate at which the shroud ring moves can be varied experimentally by varying the mass flow of the compressor air bled to the valve, by varying the thermal response of the fast ring 30, and by varying the magnification factor of the movement of the unrestrained portion of the arched segments by varying the lengths and curvature of the arches. Thus it can be seen that by adopting the above-described method of controlling the shroud movement during accelerations of the engine the possibility of a rubbing condition occurring between the blade tips and the shroud can be minimised, or even eliminated, and by choosing an appropriate value for the rate at which the shroud ring moves, the tip clearance 20 can be controlled at a smaller value, thus improving the engine performance. What we claim is:

1. Seal clearance control mechanism for a fluid flow machine comprising a sealing ring spaced from rotating structure to form a sealing clearance therebetween, control means, portions of which are arcuate and are arranged circumferentially about the sealing ring, and restraining means adapted to restrain the ends of each of the arcuate portions of the control means from circumferential movement, parts of the sealing ring being secured to unrestrained parts of the arcuate portions of the control means, whereby upon expansion or contraction of said unrestrained parts they flex such that the sealing ring is displaced radially.

2. Seal clearance control mechanism as claimed in claim 1 and in which the control means comprises a plurality of individual arcuate segments surrounding the sealing ring.

3. Seal clearance control mechanism as claimed in claim 2 and in which the sealing ring is formed in segments, and the ends of the segments of the sealing ring are secured to the mid-points of the adjacent arched segments of the control means.

4. Seal clearance control mechanism as claimed in any preceding claim and in which working fluid of the fluid flow machine is supplied to the region of the machine in which the control means is located, said working fluid being of variable temperature during operation of the machine to increase the rate at which unrestrained parts of the control means expand or contract.

5. Seal clearance control mechanism as claimed in claim 4 and in which the working fluid is supplied through a metering valve which is responsive to the temperature of the fluid passing therethrough to increase or decrease the rate of fluid supply to said region.

6. A gas turbine engine including seal clearance control mechanism as claimed in any preceding claim and disposed around the tips of rotatable blading thereof to control the flow of working fluid over the tips of the blading.

7. Seal clearance control mechanism

substantially as hereinbefore described with reference to the accompanying drawings.