GB2125110A - Gas turbine augmentor fuel control system - Google Patents

Abstract

The system has multiple spray tubes or bars 20 each fed fuel from a common plenum 40 through a respective variable metering area 62 having a pressure regulator 100 associated therewith which maintains a substantially constant pressure drop across that metering area during operation. Each spray tube or bar 20 also has a respective shutoff valve 100. With this system the shutoff valves 100 to the individual spray devices 20 may be opened and closed without affecting the fuel flow rate through spray devices which are already operating. The fuel flow rate through each spray device is a direct function of its corresponding metering area 62. <IMAGE>

Description

SPECIFICATION Augmentor fuel control system Technical Field This invention relates to augmentors for gas turbine engines, and more particularly to fuel control systems for such augmentors.

Background Art Augmentors for gas turbine engines typically include a plurality of fuel spray tubes in the form of radially extending bars or concentric annular rings disposed in the exhaust duct a short distance upstream of one or more flameholders. The spray tubes, of whatever shape, each include a plurality of either fixed or variable area orifices through which fuel is sprayed into the gas stream. Variable area fuel outlet orifices in spray tubes are, in general, expensive to fabricate, and for that reason are undesirable.

In prior art augmentor systems individual spray tubes or groups of spray tubes have separate shutoff valves which may be opened in a particular sequence to facilitate lighting-off and other operational modes of the augmentor. A plurality of these spray tubes or groups of spray tubes are supplied fuel from a common manifold. A single main valve controls the total fuel flow rate into the manifold. The fuel flow rate through orifices in the individual spray tubes which are connected to the manifold is a function of the pressure drop across the orifices, which is a function of the back pressure of the system. During steady state operation the back pressure is substantially constant and the flow rates through the spray tubes is steady and predictable.During initiation of augmentation and during augmentor transients the shutoff valves are opened in sequence and the main fuel flow valve is opened to provide increased total fuel flow to the manifold according to an appropriate schedule. Since the fuel flow rate from each spray tube is sensitive to back pressure, each time another shutoff valve is opened there is a sudden change in back pressure which results in a sudden drop, increase, or fluctuation in the final flow rate through already flowing spray tubes. Thus, while total fuel flow is controlled through the main valve into the manifold, distribution of fuel flow amongst the spray tubes during transients is uncertain.

Disclosure of Invention One object of the present invention is an improved fuel control system for a gas turbine engine augmentor.

A further object of the present invention is to permit the turning on and off of individual augmentor spray devices without affecting the flow rate through other augmentor spray devices in the system.

Another object of the present invention is a fuel control system which has the capability of varying the fuel flow rate through individual spray tubes without the need for variable area fuel outlet orifices in the spray tubes.

Accordingly, an augmentor fuel control system includes a plurality of fuel spray devices upstream of a flameholder and which have fixed area fuel outlet orifices and are fed fuel from a common plenum through individual variable metering areas, one metering area corresponding to each spray device, wherein each metering area has its own pressure regulator which maintains a substantially constant pressure drop across that metering area, each spray device also including its own shutoff valve, said control system including means for opening and closing said shutoff valves.

In this control system, each augmentor spray device associated with the common fuel plenum sees a back pressure which is independent of the back pressures seen by other flowing spray devices. The shutoff valves to the individual spray devices may now be opened and closed as desired, such as by opening them in a predetermined sequence during light-up of the augmentor or during subsequent augmentor transients, without substantially affecting the fuel flow rate through any of the spray devices which may already be operating. The fuel flow rate through each spray device is a direct function of its corresponding metering area.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as shown in the accompanying drawing.

Brief Description of the Drawing The sole figure is a partly schematic, partly diagrammatic view of an augmentor fuel control system according to the teachings of the present invention.

Best Mode for Carrying Out the Invention In the drawing, a gas turbine engine casing 6 surrounds a turbine exhaust duct 8 and the engine tail cone 10. The duct 8 and tail cone 10 define an annular turbine exhaust gas flow path 12 about the engine centerline 14. An augmentor is generally represented by the numeral 1 6. The augmentor 1 6 comprises a flarneholder 18 and annular fuel spray tubes 20 (there are six shown in this embodiment) disposed upstream thereof within the gas flow path 12. Each ring 20 includes a plurality of circumferentially spaced apart fixed -area nozzle-like orifices or openings 22 through which fuel is sprayed during augmentation.

Although in the drawing the spray tubes are shown as annular rings, they could just as well be radially extending bars, or the like, or a combination of bars and rings.

The flameholder 1 8 may be constructed in accordance with designs and principles well known in the art. In the example shown in the drawing, the flameholder comprises three concentric annular V-shaped troughs 24 with a plurality of circumferentially spaced apart V-shaped troughs 26 extending radially inwardly and outwardly therefrom.

In accordance with the present invention, each of the spray rings 20 receives fuel through a separate fuel supply tube 38. The supply tubes 38 are fed fuel from a common plenum 40 of a fuel metering valve generally designated by the reference numeral 42. In this embodiment the fuel metering valve 42 is shown schematically as comprising an outer wall 44 having an inner cylindrical surface 46, and an inner hollow cylinder 48 having an outer cylindrical surface 50 concentric with said inner cylindrical surface 46 and in sliding engagement therewith. The cylinder 48 has closed ends 49, 51 and defines the hereinabove-referred to plenum 40. Fuel from a fuel supply 52 enters the plenum 40 through openings 54, 56 through the wall 44 and cylinder 48, respectively.Each fuel tube 38 is in fluid communication with the fuel plenum 40 through a corresponding pair of fixed area openings 58,60 in the outer wall 44, and cylinder 48, respectively, which openings can be brought into and out of register with each other by moving the cylinder 48 relative to the surface 46. Thus, variable metering areas 62 are defined at the interfaces between corresponding openings 58, 60.

Any suitable means may be used to control the relative position of the openings 58, 60, and thus the size of the metering areas 62. In the embodiment shown in the drawing, the cylinder 48 is moved axially left and right with respect to the cylindrical surface 46 by a control valve 64 which receives commands from a computer which is not shown. A valve position sensor 65 provides a position feedback signal to the computer. During operation a chamber 66 adjacent the right-hand end of the cylinder 48 is filled with servo fluid which is maintained at a constant servo supply pressure represented by the box 67. This maintains a constant pressure on the right end 49 of the cylinder 48. To the left of the cylinder 48 is a piston 68 connected directly to the left end 51 of the cylinder 48 by means of a shaft 70.A chamber 72 is defined between the piston 68 and the cylinder 48. The chamber is filled with servo fluid maintained at a constant servo drain pressure depicted by the box 74, which is considerably lower than the servo supplying pressure 67. A chamber 76 is defined on the left side of the piston 68 and is filled with servo fluid at a pressure which may be varied between the supply pressure 67 and the drain pressure 74 by means of the control valve 64. Thus, by controlling the pressure of the fluid in the chamber 76, the cylinder 48 may be moved left or right to vary the metering areas 62 as desired.

It should also be apparent that the metering areas 62 may also be varied by rotating the inner cylinder relative to the outer wall. Additionally, other types of metering valves having a plurality of variable metering areas opening into a common plenum may also be used in the control system of the present invention.

Further in accordance with the present invention, associated with each fuel supply tube 38 is a separate shutoff valve 1 00. To initiate augmentation these valves are opened in a predetermined sequence by a control 102, the construction of which can take any well known form (electrical, hydromechanical, etc.). The control 102 is also used to open and close the valves 100 to achieve other modes of augmentor operation.

Associated with each supply tube 38 is a pressure regulating valve 104 located between the tube's metering area 62 and corresponding shutoff valve 1 00. Each valve 104 maintains a nearly constant pressure drop across the metering area 62 with which it is associated. Pressure regulating valves of this type are well known in the art. Each valve 104 is provided with a working fluid which is at the same pressure as the fuel in the plenum 40. In the control system shown in the drawing this working fluid is fuel supplied from the plenum 40 to the pressure regulating valves 104 via a system of conduits 106. The present invention does not require the pressure drop across each metering area 62 be the the same.

Thus, in the augmentor fuel control system of the present invention, the fuel pressure drop across each metering area 62 is controlled independently from the pressure drop across other metering areas 62. Turning on or off any spray ring 20 by means of its shutoff valve 100, such as during sequencing of the spray rings 20, thereby has no effect on the fuel flow rate to other rings 20. The rate of fuel flow to each spray ring 20 is dependent solely upon the size of the metering area 62 associated with that spray ring and the value of the pressure drop maintained across the metering area 62. The rate of fuel flow into the plenum 40 is the sum of the individual fuel flows maintained across the individual metering areas 62.As each additional spray ring is opened during sequencing, additional fuel is supplied from the fuel supply 52 to the plenum 40 to assure a constant flow through any already open valves 100 as well as through the valve 100 which is newly opened.

Although in the drawing each fuel supply tube 38 provides fuel to an individual spray ring 20, each supply tube 38 could equally as well provide fuel to a group of spray rings or spray bars or to a combination of rings and bars deemed appropriate by the particular configuration of the augmentor to which the control system of the present invention is applied. The augmentor fuel control system of the present invention is equally well suited to such an arrangement.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that other various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Claims (3)

1. An augmentor fuel control system for a gas turbine engine having an exhaust duct with a flameholder disposed therein and comprising: a plurality of spray tube means disposed within said exhaust duct upstream of said flameholder, each of said tube means having a plurality of orifices through which fuel is sprayed into said duct; fuel metering valve means including means defining a fuel plenum, said valve means including a plurality of separate, variable, fuel metering areas, each of said metering areas corresponding to one of said spray tube means and providing fluid communication between the said plenum and said spray tube means; fuel supply means in a fluid communication with said fuel plenum; pressure regulator means associated with each of said spray tube means and said valve means for maintaining a substantially constant pressure drop across each of said metering areas;; a plurality of shutoff valves, one valve corresponding to each one of said spray tube means for interrupting or allowing fuel flow from said metering valve means to the spray tube means corresponding to said shutoff valve; and control means associated with said plurality of shutoff valves for opening and closing said shutoff valves.

2. The control system according to claim 1 wherein said tube means orifices are fixed area orifices.

3. The control system according to claim 1 or 2 wherein said fuel metering valve means comprises a first wall defining a fuel plenum and having an outer cylinder surface, and a second wall having an inner cylindrical surface, said first wall being surrounded by said second wall, said inner and outer cylindrical wall surfaces being concentric and in sliding engagement, said second wall having a first plurality of openings therethrough and said first wall having a second plurality of openings therethrough, said walls being movable with respect to each other to enable bringing said first and second openings into register with each other to define said metering areas; and wherein said control means associated with said metering valve means includes means for controlling the relative movement of said first and second walls to vary the area of said metering areas.