GB2099516A

GB2099516A - Idle braking in marine gas turbine engines

Info

Publication number: GB2099516A
Application number: GB8116605A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1981-05-30
Filing date: 1981-05-30
Publication date: 1982-12-08

Abstract

A marine gas turbine engine (10) has a bleed duct (42), a nozzle box (38) and nozzles (40) to allow excess air produced by a compressor (12) at engine idle, to apply a retarding force to a power turbine (18). This arrangement allows the gas turbine to idle but the power turbine has an insignificant output torque. With unshrouded blades the excess air nozzles may be in the plane of the downstream rotor (Fig. 3). <IMAGE>

Description

SPECIFICATION Marine gas turbine engine This invention relates to gas turbine engines of the type derived from gas turbines used for aircraft propulsion. Such marine engines comprise a gas generator supplying power via a hot gas duct to a power turbine. The drive to the propeller or propellers, is then through a suitable gearbox.

All the gas from the gas generator passes to the power turbine and so, even when the gas generator is adjusted to idle, there is still a significant flow of hot gas to the power turbine.

The power turbine thus provides a residual torque causing the gearbox and propeller assembly to continue rotating. Unless the propeller is of the variable pitch type enabling the blades to be set to zero pitch, it is likely that the residual propeller rotation will cause thrust, leading to unwanted movement of the ship. In some cases any propeller movement is undesirable particularly if the propeller becomes entangled or there is a manoverboard.

The present invention seeks to provide a marine gas turbine in which the residual torque developed by the power turbine is significantly reduced. This reduction can be such as to reduce the propeller speed to insignificance, or since there is always a considerable friction in stern shaft assemblies, the residual torque may not be sufficient to cause shaft rotation.

Methods are available for reducing propeller speeds to insignificance or zero, such as providing a neutral in the gearbox or a gearbox disc brake. A neutral does not entirely solve the problem because of the large drag in the clutch and a brake has to dissipate large amounts of heat, requires a high pressure fluid supply system and a considerable space because of the large discs required.

The present invention proposes the use of aerodynamic energy and has no rubbing components to wear with time. Many modern gas turbines incorporate compressors having a narrow operating range, which means that large volumes of compressed air have to be discharged to atmosphere in various engine handling conditions, particularly idling.

Accordingly the present inventions makes use of this compressed air at idle and provides a marine gas turbine engine having a gear generator which includes a compressor, a power turbine mounted in a housing and hot gas ducting to conduct the exhaust gases from the gas generator to the power turbine, the power turbine housing having a nozzle box arranged to receive excess air from the compressor through a bleed duct and a valve, excess air also being capable of being discharged to atmosphere via the bleed duct and a compressor bleed valve, the nozzle box having a number of nozzles arranged to receive the excess compressed air and to discharge the air onto at least some of the rotating blades of the power turbine to retard the rotation of the power turbine caused by the gas flow through the power turbine.

The nozzles may direct the excess air onto one or all of the rotating blade stages of the power turbine, and either an arc of nozzles or a complete circumferential row may be provided.

In the case of unshrouded power turbine blades, the nozzles can be in the plane of the rotating stage or stages and inclined so that the jets of excess air impinge on the backs or convex surfaces of the rotating blades. In the case of shrounded blades, the nozzles will have to be located downstream of the last rotating blade stage and set at angle so that the excess air can be effective under the blade shrouds. The valve is arranged so that under normal engine operating conditions, the valve is closed to prevent any bleed, whilst the valve can be opened either to discharge excess air to atmosphere or to the nozzle box when required.

The present invention will now be more particularly described with reference to the accompanying drawings in which: Figure 1 is a schematic view of one form of marine gas turbine engine according to the present invention, Figure 2 is a side elevation showing part of the engine in Figure 1, and Figure 3 shows a modified form of the engine shown in Figure 1.

Referring to Figures 1 and 2, a marine gas turbine engine 10 has a gas generator which includes a compressor 12, a combustion system 14, which receives compressed air from the compressor 12 and a supply of fuel, a turbine 16 which receives the combustion gases from the combustion system and a power turbine 1 8. A hot gas duct 20 conducts the gases from the turbine 1 6 to the power turbine 18 and after passing through the power turbine, the gases exhaust to atmosphere through a duct 22.

The power turbine 18 which in this case has two rotating shrouded blade stages 24, 26 and two stator blade stages 28, 30 is mounted on a shaft 32 in bearings 34 and the power is transmitted to the propellers of the vessel through a gear box, all of which are not shown.

The power turbine 1 8 is mounted in a casing 36 which has a nozzle box 38 attached to it, the nozzle box includes a number of nozzles 40 extending around at least a part of the circumference of the housing and inclined towards the last stage of the power turbine rotating stages.

A bleed duct 42 connects a bleed valve 44 on the compressor 12 to the nozzle box 38 via a twoway valve 46, and the bleed duct also has a duct 48 to atmosphere via the valve 46. During normal engine operation, the valves 44 and 46 are closed and there is no bleed either to atmosphere or the nozzle box. When excess air has to be vented the compressor bleed valve 44 is opened and the valve 46 is opened either to allow the excess air to vent to atmosphere or if it is required to brake the power turbine at engine idle the excess air is ducted to the nozzle box 38.

In the latter case, the excess air is directed by the nozzles 40 onto the backs of the blades in the rotating blade stage 26, inwardly of the blade shrouds 27 (Figure 2) so applying a retarding torque to the power turbine. The action of the jets is one of simple momentum transfer, as the back blade shape is not conducive to precision gas guidance.

Referring to Figure 2, the rotor blades of the power turbine are unshrouded and the nozzles 40 are in the plane of the last rotor stage 26.

Although the nozzles 40 have been shown as only extending over a part of the circumference of the last rotor blade stage, they can extend over the whole circumference, and the nozzles can be applied any one, or all of the rotor blade stages of the power turbine. The nozzles 40 are arranged so that they do not intrude into the normal gas path to avoid interference and performance loss in normal power turbine operation.

Claims

1. A marine gas turbine engine having a gas generator which includes a compressor, a power turbine mounted in a housing and hot gas ducting to conduct the exhaust gases from the gas generator to the power turbine, the power turbine housing having a nozzle box arranged to receive excess airfrom the compressor through a bleed duct and a valve the excess air also being capable of being discharged to atmosphere via the bleed duct and a compressor bleed valve, the nozzle box having a number of nozzles arranged to receive the excess compressed air and to discharge the air onto at least some of the rotating blades of the power turbine to retard the rotation of the power turbine caused by the gas flow through the power turbine.

2. An engine as claimed in claim 1 in which the rotor blades of the power turbine are unshrouded and the nozzles are arranged in the plane of at least one of the rotor stages of the power turbine and inclined at an angle to direct the air onto the convex surfaces of the blades on the or each rotor stage.

3. An engine as claimed in claim 1 in which the rotor blades of the power turbine are shrouded and the nozzles are located downstream of the last rotor stage, the nozzles being directed in an upstream direction to direct the air onto the convex surfaces of the blades of the last rotor stage radially inward of the blade shrouds.

4. An engine as claimed in any one of the preceding claims in which the nozzles are provided around a part of the circumference of the power turbine housing.

5. An engine as claimed in any one of the preceding claims 1 to 3 in which the nozzles extend around the whole of the circumference of the power turbine housing.

6. A marine gas turbine engine constructed and arranged for use and operation substantially as herein described, and with reference to Figure 1, and Figures 2 and 3 of the accompanying drawings.