GB2252133A

GB2252133A - Gas turbines

Info

Publication number: GB2252133A
Application number: GB9101719A
Authority: GB
Inventors: Geoffrey Edward Bone; Ronald George Cook; Alan Earnest Massey; Nicholas Black
Original assignee: GEC AEROSPACE Ltd
Current assignee: GEC AEROSPACE Ltd
Priority date: 1991-01-25
Filing date: 1991-01-25
Publication date: 1992-07-29
Also published as: GB9101719D0; EP0496571A1

Description

2252133 GAS TURBINES This invention relates to gas turbines and is

especially applicable to gas turbines for use as an auxiliary power unit on aircraft.

The use of one or more gas turbines on an aircraft to drive a hydraulic pump or to supply on-demand or continuous power is well known. A typical use of such a gas turbine would be on a four-engined, long-haul jet aircraft in which one or more gas turbines would operate, for example, from engine bleed air. In such an application, in the past the efficiency of the gas turbine has not been particularly important because of the large amount of bleed air which is available. However, it is becoming common for long-haul aircraft to be provided with only two engines, instead of four, with less bleed air being available, and it is therefore important that the gas turbine should operate as efficiently as possible.

In prior art gas turbines it is known to provide so-called fixed-blade nozzles which drive the blades of the turbine wheel, where the fixedblade nozzles are designed to operate efficiently at, for example, the mid-point of the

P/8563/E34 operating range of the air supply. At any other point in the operating range the efficiency of the gas turbine decreases. An improvement in the fixed-blade nozzle system has been to provide variable angle nozzles which can improve the efficiency over a wider range, but not at the two extremes of the air supply range.

It is now becoming a requirement for the gas turbine to have high efficiency at the two extremes of the operating range, i.e. at low pressure (LP) and at high pressure (HP), the efficiency being less important at intermediate points in the range. This cannot easily be achieved using the variable angle nozzle technique. It would, of course, be possible to use two gas turbines, one designed to operate at low pressure and the other at high pressure, but this is obviously expensive to do especially when it is sometimes necessary to provide back-up gas turbines for use in the case of turbine break-down.

It is an object of the present invention to provide a single gas turbine which can be designed, to operate with high efficiency at both low and high pressures.

According to the present invention there is provided a gas turbine comprising a turbine wheel having a plurality of P/8563/E34 outwardly extending blades, a first set of nozzles for directing gas to at least some of said blades and a second set of nozzles for directing gas to at least some of said blades, a gas inlet, a first gas duct extending from said gas inlet for directing gas to said first set of nozzles and a second gas duct extending from said gas inlet for directing gas to said second set of nozzles, gas flow control means being provided for controlling the gas flows in said first and second gas ducts.

In a preferred arrangement for carrying out the invention it will be arranged that said first and second gas ducts are each of annular form, parts of each of which are disposed concentrically.

It may be arranged that said first set of nozzles are disposed in said first annular gas duct and said second set of nozzles are disposed in said second annular gas duct.

In an especially preferred arrangement it will be arranged that said turbine wheel comprises a first set of outwardly extending blades to which said first set of nozzles direct gas, and a second set of outwardly extending blades to which said second set of nozzles direct gas, said first set of blades being disposed radially outwards of said 1 P/8563/E34 - 4 second set of blades.

Advantageously, said first and second sets of blades are separated by a cylindrical separator.

It may be arranged that said gas flow control means is -disposed centrally within said first and second annular gas ducts, said first and second annular gas ducts having openings which are at least partially closeable by said gas flow control means for controlling the gas flows therein, the opening of said first duct preferably being disposed upstream of the opening of said second duct, said gas flow control -means being disposed downstream of the openings of said first and second ducts.

Preferably, said gas flow control means comprises a differential pressure piston actuator.

It may be arranged that the said first set of nozzles and said at least some of said blades on said turbine wheel are designed for low pressure gas supply operation and the said second set of nozzles and said at least some of said blades on said turbine wheel are designed for high pressure gas supply operation, whereby high efficiency operation at said low pressure and at said high pressure is achieved.

P/8563/E34 An exemplary embodiment of the invention will now be described reference being made to the accompanying drawings in which:

Fig. 1, is a cross-sectional side view of a gas turbine in accordance with the present invention; and Fig. 2, is a partially cut-away perspective view of the gas turbine of Fig. 1 with the turbine wheel removed.

The gas turbine shown in the drawing comprises a generally cylindrical turbine housing 1 having a reduced diameter gas/bleed air inlet 2, the gas or bleed air input to which is used to drive a turbine wheel 3. The turbine wheel 3 is mounted on a shaft 4 which may constitute part of a reduction gear-box (not shown) which itself may be used to drive a hydraulic pump (not shown) or other auxiliary equipment in well known manner.

The turbine housing 1 is provided with an outer (high pressure-HP) annular duct 5 disposed between the housing 1 and an intermediate shroud 6 and an inner (low pressure LP) annular duct 7 disposed between the intermediate shroud 6 and a generally cylindrical inner shroud 8. The HP and LP P/8563/E34 annular ducts 5 and 7 respectively have circumferential openings 9 and 10 respectively, disposed adjacent to one another in the inner shroud 8 and adjacent to the inlet 2, the opening 10 to the LP annular duct 7 being downstream of the HP annular duct 5. The intermediate shroud 6 is held in spaced relationship from the housing 1 by means of radially disposed spacer plates 11 and the inner shroud 8 is held in spaced relationship from the intermediate shroud 6 by means of radially disposed spacer plates 12.

Within the inner shroud 8 and downstream of the openings 9 and 10 therein is disposed a control valve 13 having a generally pointed front end 14, the control valve 13 being axially movable in the inner shroud 8 under the control of a differential pressure piston actuator 15 in order to control the flow of gas/bleed air from the inlet 2 to the openings 9 and 10 of the HP and LP annular ducts 5 and 7.

The control valve 13 is shown in the drawings in a position corresponding to a LP condition (Design Point 1) in which gas/bleed air from the inlet 2 is directed through the openings 9 and 10 of the HP and LP annular ducts 5 and 7. The control valve 13 is axially movable in the inner shroud 8 under the control of the differential piston actuator 15 P/8563/E34 - 7 towards the inlet 2 to a position 16 shown in dashed outline which corresponds to a HP condition (Design Point 2) in which the opening 10 of the LP annular duct 7 is virtually closed by the control valve 13, and gas/bleed air from the inlet 2 is directed mainly through the opening 9 of the HP annular duct 5. For operating conditions above the normal HP operating condition (Design Point 2) the control valve 13 is movable towards the inlet 2 to a position 17 shown in dashed outline in which the opening 9 of the HP annular duct 5 is virtually closed to regulate the flow of gas/bleed air to the HP annular duct 5.

In the ends of the HP and LP annular ducts 5 and 7 remote from the openings 9 and 10 respectively thereto is disposed respective sets of fixed nozzles 18 and 19, which direct gas/bleed air flowing in the respective annular duct 5 or 7 onto a respective set of HP and LP turbine blades 20 or 21 on the turbine wheel 3.

The turbine wheel 3 consists of a circular inner portion 22 from which the set of LP turbine blades 21 radially extend to an intermediate cylindrical separator 23, which aligns with the intermediate shroud 6 between the annular ducts 5 and 7, so that the LP turbine blades 21 align with the LP nozzles 19. The HP turbine blades 20 A P/8563/E34 extend from the separator 23 to the outer periphery of the turbine wheel 3 and align with the HP nozzles 18.

Exhaust gas/bleed air from the turbine blades 20 and 21 are directed to an exhaust duct 24 which is advantageously tailored to afford a jet pump effect from the gas/bleed air from the HP turbine blades 20.

It is required that the gas turbine which has been described has the capacity to drive a peak load at constant speed and to have a high efficiency when operating on (a) low pressure-LP (Design Point 1) and (b) high pressure (Design Point 2). Intermediate gas/bleed air supply conditions are, considered less important in terms of efficiency. This is achieved by tailoring the LP annular duct 7, the LP nozzles 19 and the LP turbine blades 21, such that when the control valve 13 is in the position shown in full lines in the drawings, i.e. with the openings 9 and 10 to the LP and HP annular ducts open, the required efficiency at low pressure - Design Point 1 is obtained, and by tailoring the HP annular duct 5, the HP nozzles 18 and the HP turbine blades 20, such that when the control valve 13 is in the position 16 shown in dashed lines in Fig. 1, i.e. with the opening 10 to the LP annular duct 7 closed, the required efficiency at high pressure-Design Point 2 is P/8563/E34 obtained.

Control of the speed of the turbine wheel 3 is effected by adjusting the axial movement of the control valve 13. This is achieved by means of the differential pressure piston actuator 15, one side of which is subjected to the pressure of the gas/bleed air supply and the other side of which is subjected to a modulated pressure signal which is derived from a pressure outlet 25 which is upstream of the control valve 13 and which is adjusted in accordance with the speed of the turbine wheel and is applied to a pressure inlet 26 which couples to the differential piston actuator 15.

At the low pressure Design Point 1, the control valve 13 is positioned such that the opening 10 to the LP annular duct 7 is almost fully open, permitting a minimum amount of throttling of the control valve 13 to maintaining constant turbine speed at peak turbine load. Under these conditions a small amount of flow takes place in the HP annular duct 5.

At the high pressure Design Point 2, the control valve 13 is positioned such that the opening 10 to the LP annular duct 7 is fully closed, although a small amount of controlled leakage to the LP nozzles 19 may be permitted to P/8563/E34 - 10 minimise losses, and the opening 9 to the HP annular duct 5 is almost fully open permitting a minimum amount of throttling of the control valve 13 to maintain constant turbine speed at peak turbine load.

Under conditions where supply pressure exceeds the HP Design Point 2, the control valve 13 causes the opening 9 to the HP annular duct 5 to be progressively closed, permitting additional throttling of the control valve 13 to maintain constant turbine speed at lower efficiency.

It will be appreciated that the gas turbine which has been described has been given by way of example only and may be modified to suit any particular application. For example, although two annular ducts 5, 7 with corresponding fixed nozzles 18, 19 and turbine blades 20, 21 have been described, it is envisaged that more than two annular ducts could be used. Also, although the turbine wheel 3 is provided with two sets of turbine blades 20, 21 it is envisaged that a single set of turbine blades could be provided corresponding to the fixed nozzles 18, 19. It is also possible where, for instance, a low reaction is employed, to eliminate the separator 23 of the turbine wheel 3.

P/8563/E34

Claims

1. A gas turbine comprising a turbine wheel having a plurality of outwardly extending blades, a first set of nozzles for directing gas to at least some of said blades and a second set of nozzles for directing gas to at least some of said blades, a gas inlet, a first gas duct extending from said gas inlet for directing gas to said first set of nozzles and a second gas duct extending from said gas inlet for directing gas to said second set of nozzles, gas flow control means being provided for controlling the gas flows in said first and second gas ducts.

2. A gas turbine as claimed in Claim 1, in which said first and second gas ducts are each of annular form, parts of each of which are disposed concentrically.

3. A gas turbine as claimed in claim 2, in which said first set of nozzles are disposed in said first annular gas duct and said second set of nozzles are disposed in said second annular gas duct.

4. A gas turbine as claimed in claim 2 or Claim 3, in which said turbine wheel comprises a first set of outwardly P/8563/E34 - 12 extending blades to which said first set of nozzles direct gas, and a second set of outwardly extending blades to which said second set of nozzles direct gas, said first set of blades being disposed radially outwards of said second set of blades.

5. A gas turbine as claimed in Claim 4, in which said first and second sets of blades are separated by a cylindrical separator.

6. A gas turbine as claimed in any of Claims 2 to 4, in which said gas flow control means is disposed centrally within said first and second annular gas ducts, said first and second annular gas ducts having openings which are at least partially closeable by said gas flow control means for controlling the gas flows therein.

7. A gas turbine as claimed in Claim 6, in which the opening of said first duct is disposed upstream of the opening of said second duct, said gas flow control means being disposed downstream of the openings of said first and second ducts.

8. A gas turbine substantially as hereinbefore described with reference to the accompanying drawings.

8. A gas turbine as claimed in any preceding claim, Or P/8563/E34 - 13 in which said gas flow control means differential pressure piston actuator.

comprises a

9. A gas turbine as claimed in any preceding claim, in which the said first set of nozzles and said at least some of said blades on said turbine wheel are designed for low pressure gas supply operation and the said second set of nozzles and said at least some of said blades on said turbine wheel are designed for high gas supply operation.

10. A gas turbine substantially as hereinbefore described with reference to the accompanying drawings.

P/8563/E34 iL - Amendments tO the claims have been filed as.!Dilows A gas turbine comprising a turbine wheel having a plurality of outwardly extending blades, a first set of nozzles for directing gas to at least some of said blades and a second set of nozzles for directing gas to at least some of said blades, a gas inlet, a first gas duct extending from said gas inlet for directing gas to said first set of nozzles and a second gas duct extending from said gas inlet for directing gas to said second set of nozzles, said first and second gas ducts each being of annular form, parts of each of which are disposed concentrically, and gas flow control means for controlling the gas flows in said first and second gas ducts, said gas flow control means being disposed centrally within said first and second annular gas ducts, said first and second annular gas ducts having openings which are at least partially closeable by said gas flow control means for controlling the gas flows therein.

2. A gas turbine as claimed in Claim 1, in which said first set of nozzles are disposed in said first annular gas duct and said second set of nozzles are disposed in said second annular gas duct.

P/8563/E34 1 - 3. A gas turbine as claimed in Claim 1 or claim 2, in which said turbine wheel comprises a first set of outwardly extending blades to which said first set of nozzles direct gas, and a second set of outwardly extending blades to which said second set of nozzles direct gas, said first set of blades being disposed radially outwards of said second set of blades.

4. A gas turbine as claimed in claim 3, in which said first and second sets of blades are separated by a cylindrical separator.

5. A gas turbine as claimed in any preceding claim, in which the opening of said first duct is disposed upstream of the opening of said second duct, said gas flow control means being disposed downstream of the openings of said first and second ducts, and being moveable to selectively close said openings.

6. A gas turbine as claimed in any preceding claim, in which said gas flow control means comprises a differential pressure piston actuator.

7. A gas turbine as claimed in claim 3, in which the said first set of nozzles and said first set of outwardly 1 P/8563/E34 - k 110 extending blades on said turbine wheel are designed for low pressure gas supply operation and the said second set of nozzles and said second set of outwardly extending blades on said turbine wheel are designed for high gas supply operation.