Devices For Prevaporising Liquid Fuel

Abstract

A device for prevaporising liquid fuel, designed for fitting to a combustion system comprising a combustion chamber, a source of liquid fuel and a source of gaseous comburrent agent, said device being of the type which comprises, projecting into the combustion chamber, a hollow structure incorporating a body and at least one arm attached to said body and having at least one portion extending transversely of the general direction of said body, the latter comprising an entry portion which communicates with said sources of liquid fuel and gaseous comburrent agent, and said arm having an exit portion which opens into the combustion chamber and through which, in operation, there is discharged a mixture of gaseous comburrent agent and at least partially vaporised fuel, wherein said hollow structure comprises, in a zone which incorporates the transverse portion of said arm, a double-walled system constituted by an internal wall and an external wall which together delimit two separate passages, namely a central passage communicating both with the source of gaseous comburrent agent and with the liquid fuel source, and a peripheral passage communicating solely with the source of gaseous agent.

Description

The present invention relates to a device for prevaporising liquid fuel, designed for fitting to a combustion system comprising a combustion chamber, a liquid fuel source and a source of gaseous comburrent agent, said device being of the kind comprising, projecting into said combustion chamber, a hollow structure incorporating a body and at least one arm fixed to said body and having at least one portion extending transversely of the general direction of said body, the latter comprising an input portion communicating with said sources of liquid fuel and gaseous comburrent agent, and said arm having an exit portion which opens into the combustion chamber and through which, in operation, there is discharged a mixture of gaseous comburrent agent and at least partially vaporised fuel.

The combustion system in question may, in particular, form part of a gas turbine engine such as a turbojet engine.

One of the problems posed by the application of a hollow structure of the aforedescribed kind, is that of protecting it against local overheating, in particular of the zone of such arm or arms. This structure is subjected to the extremely high temperatures which prevail in the upstream part of the combustion chamber, and it can also happen that inside said structure premature combustion of the fuel flowing through it takes place.

It should be borne in mind in this context that a particularly effective coolant for the hollow structure is constituted by fuel in the course of prevaporisation, which is present inside said structure.

However, it may happen that for one reason or another, the flow of liquid fuel injected into the hollow structure, may be weaker than intended. In particular, it may happen that the fuel injector becomes partially or wholly obstructed. The direct consequence of this situation is the absence, inside said structure, of an adequate quantity of fuel in the process of vaporisation, and therefore a lack of cooling of the structure.

As far as the risk of premature combustion of the fuel inside the hollow structure is concerned, this happens when the richness of the mixture of fuel and comburrent agent flowing through said structure drops below a certain limit, i.e., in particular also in the situation of partial obstruction of the injector.

The object of the present invention, in a general way, is to diminish the risk of overheating of the hollow structure due to one or the other of the two aforesaid causes.

To this end, in a prevaporisation device in accordance with the invention, the hollow structure (which is of the kind defined hereinbefore) comprises, in a zone which includes the transverse portion of the arm, a double-walled system constituted by an internal wall and an external wall together delimiting two separate passages, namely a central passage communicating both with the source of gaseous comburrent agent and with the liquid fuel source, and a peripheral passage communicating solely with the source of gaseous comburrent agent.

In accordance with a preferred embodiment, the prevaporisation device of the invention comprises, arranged inside said peripheral passage, at least within a zone of said passage which is located along the transverse portion of said arm, at least one vane or blade extending in a direction which has a component parallel to the general direction of said peripheral passage.

In accordance with another embodiment of the invention, the external wall which delimits the peripheral channel aforementioned, is pierced, at least in the zone of said channel located along said arm, by a plurality of sweat-pores through which a fraction of the flow of gaseous comburrent agent passing through said peripheral passage, can escape into the combustion chamber.

The arrangements hereinbefore described, make it possible to ensure cooling of the hollow structure even in the event of partial or possibly complete blockage of the fuel injector. In other words, the peripheral passage, in all situations, passes a cooling gas flow which makes it possible to dissipate towards the combustion chamber the heat absorbed by the hollow structure. As will be appreciated, the function of the blades or vanes is to promote the transfer of said heat to the peripheral gas flow.

In addition, the risk of premature combustion in the hollow structure is very much diminished by the fact that the richness of the mixture of fuel and comburrent agent, in the central passage, is radically increased as a consequence of the splitting of the comburrent agent flow into two sub-flows only one of which actually participates in combustion.

The description which now follows, given in relation to the attached drawing, provided purely by way of non-limitative example, will indicate how the invention may be put into effect.

FIG. 1 is an axial half-section through a combustion system equipped with fuel prevaporisation devices;

FIG. 2 is a view in partial transverse section, on the line II--II of FIG. 1, of said system;

FIG. 3 is a view on a larger scale, in section on the line III--III of FIG. 2, of a fuel prevaporisation device of known kind;

FIG. 4 is a view similar to that of FIG. 3, showing an improved prevaporisation device in accordance with the invention; and

FIG. 5 is a sectional view, taken on the line V--V, of part of the prevaporisation device shown in FIG. 4.

In FIGS. 1 and 2, the general reference 7 has been utilised to designate liquid fuel prevaporisation devices fitted to a combustion system forming part, for example, of a gas turbine engine such as turbojet engine.

This installation which is of the kind known per se, comprises for example, a combustion chamber of axial X'-X, delimited by an external casing 1a and an internal casing 1b which are substantially coaxial with one another. These two casings together define an annular space within which two walls 2a-2b, disposed substantially coaxially vis-a-vis the two casings, delimit an annular flame tube constituting the combustion space proper. This latter is closed off at its upstream end, by an annular wall 3 or dome, inside which there is disposed an annular supporting structure 4. The dome 3 and the annular structure 4 are pierced by holes 5-6 distributed uniformly about the axis X'-X of the chamber, each hole 6 being arranged in extension of a hole 5. A prevaporisation device 7 is assembled with a certain clearance, on the axis of each of the holes 6. The combustion chamber is connected at its upstream end to a source of comburrent agent such as compressed air, schematically represented by a line 8. This air circulates through the annular spaces defined respectively between the casing 1a and the wall 2a, and the casing 1b and the wall 2b, and enters the combustion space in the form of primary air, through the holes 5, and in the form of secondary air for cooling and dilution purposes, through holes 9a-9b, 10a-10b and 11a-11b. The reference 20 signifies a source of liquid fuel.

Each prevaporisation device 7 comprises (see also FIG. 3) a hollow structure projecting into the combustion chamber from a hole 6. This hollow structure comprises a tubular body 12 coaxial with the hole 6, and two arms 13, 14 fixed to said body, the assembly of the body and the two arms presenting the appearance of a generally T-shaped cane or the like.

The body 12 has an entry portion 12x communicating with the source 8 of gaseous comburrent agent, into which there also opens an injector 17 supplied with liquid fuel from the source 20.

Each of the arms 13, 14 has a portion 13x, 14x standing transversely to the general direction of the body 12, and an exit portion 13y, 14y which opens into the combustion chamber and through which, in operation, there is discharged a mixture of gaseous comburrent agent and at least partially vaporised fuel. It will be observed that each of the arms 13, 14 is bent so that its exit portion 13y, 14y extends in a direction having a component parallel to the general direction of the body 12.

The hole 6 has a larger cross-section than that of the body 12 in order to enable the assembly, around said body, of a sleeve 18 which may possibly be integral with said body. Said sleeve, in cooperation with the body 12, delimits an annular passage 19 through which direct admission into the combustion space, of a certain fraction of the primary airflow, takes place.

In operation (see FIG. 3) the major fraction (marked by the reference A) of the primary air flow, enters, at the same time as the liquid fuel (marked by the reference C), the hollow structure or cane 7, the walls of which, at their internal surfaces, are subjected to the action of the flame so that the fuel vaporises. The mixture of primary air and vaporised fuel (marked AC) if discharged substantially axially through the exit portions 13y, 14y of the arms 13, 14, in the reverse direction to the general direction of the flow of combustion gases, which general direction has been indicated in FIG. 1 by the arrow G. The smaller fraction (marked by the reference "a") of the air flow entering at 19 around the body 12, serves primarily to ensure a certain degree of thermal insulation of the upstream portion of the body 12 of the hollow structure 7.

The orifices 9a-9b, make it possible to produce two groups of secondary air jets Fa-Fb which are substantially radially and mutually oppositely directed. These jets meet one another in the neighbourhood of the exit portions 13y, 14y of the arms 13, 14. Part of their flow then recirculates towards the upstream part of the chamber in order to produce, in the region adjacent the dome 3, a turbulent zone which promotes ignition and maintenance of combustion, whilst the other part of the flow passes directly downstream in the direction of the arrow G, for example towards an expansion turbine which has not been shown.

As mentioned hereinbefore, partial or total blockage of the fuel injector 17 is dangerous for two reasons. On the one hand, in the absence inside the hollow structure 7 of a sufficient quantity of fuel in the course of vaporisation, this structure may be insufficiently cooled. On the other hand, the richness of the air-fuel mixture flowing through the hollow structure 7 may then have diminished in such a proportion that premature ignition of the fuel becomes possible, whilst the fuel is still inside the said structure. In either case, the consequence is a risk of an unacceptable degree of overheating of the hollow structure 7.

FIGS. 4 and 5 illustrate an example of a modified prevaporisation device in accordance with the invention, the modification being designed to reduce the risk of overheating referred to just now.

This device, designated by the general reference 107, comprises a hollow, bent, T-shaped structure, designed to project into the combustion chamber. In the same way as the hollow structure 7 hereinbefore described, the hollow structure 107 comprises a body 112 and one or more arms 113, 114. The body 112 has an entry portion 112x communicating with the source of gaseous comburrent agent, and into which there opens a liquid fuel injector 117. Each of the arms 113, 114 has a portion 113x, 114x, extending transversely of the general direction of the body 112, and an exit portion 113y, 114y which opens into the combustion chamber and through which, in operation, there is discharged a mixture AC of gaseous comburrent agent and at least partially vaporised fuel. The exit portion 113y, 114y is preferably orientated in a direction having a component parallel to the general direction of the body 112.

In accordance with the invention, the hollow structure 107 comprises at least in a zone which incorporates the transverse part 113x, 114x of the arm 113, 114 as well as a portion of the body 112, located near said transverse part, a double-walled system constituted by an internal wall S and an external wall T, together delimiting two separate passages, namely a central passage K.sub.1 and a peripheral passage K.sub.2. The two passages K.sub.1 and K.sub.2 communicate with the source of gaseous comburrent agent, but only the central passage K.sub.1 is supplied with liquid fuel through the injector 117.

One or more blades or vanes 125 are arranged inside the peripheral passage K.sub.2, at least in a zone thereof situated along the transverse portion 113x, 114x of the arm 113, 114. As FIGS. 4 and 5 show, each of the blades or vanes 125 is in contact with the internal S and external T walls which delimit the peripheral passage K.sub.2 and extends in a direction having a component parallel to the general direction of said peripheral passage. This latter is thus sub-divided into at least two elementary passages K.sub.21, K.sub.22 ... through which there flow in parallel respective elementary flows of the gaseous comburrent agent.

The external wall T which delimits the peripheral passage K.sub.2 is pierced, at least in a zone of said passage disposed along the arm 113, 114 by a plurality of sweat-pores 126 through which a fraction of the gaseous comburrent agent flow passing through said peripheral passage, can escape into the combustion chamber.

A sleeve 118, similar to that 18 hereinbefore described in relation to FIG. 3, can furthermore be arranged around the external wall T of the body 112 of the hollow structure, in a zone which comprises at least the entry portion 112x of said body. Said sleeve 118 and the external wall T thus together delimit a supplementary passage 119 having an entry end 119x communicating with the source of gaseous comburrent agent, and an exit end 119y opening into the combustion chamber.

In operation, the prevaporisation device 107 passes three coaxial flows of gaseous comburrent agent, namely a central flow A.sub.1 passing through the central passage K.sub.1, a peripheral flow A.sub.2 passing through the peripheral passage K.sub.2 and a supplementary flow "a" passing through the supplementary passage 119. Only the central flow A.sub.1 contains fuel.

In the event that the injector 117 becomes blocked, the three passages K.sub.1, K.sub.2, 119, continue to pass the gaseous comburrent agent flows. The peripheral flow A.sub.2, a fraction s' of which escapes through the sweat pores 126, thus makes it possible to dissipate to the combustion chamber the heat absorbed by the hollow structure, the transfer of this heat to the said flow being accelerated by the presence of the blades or vanes 125.

The vent holes 126 furthermore ensure the development, in the neighbourhood of the external wall of the hollow structure, of a protective film of gaseous comburrent agent. Another protective film is produced by the supplementary flow "a" of gaseous comburrent agent, escaping from the supplementary passage 119.

As far as the risk of premature combustion of the fuel inside the hollow structure is concerned, it will be seen that it is now very much reduced because of the fact that the richness of the mixture of fuel and comburrent agent is increased as a consequence of the sub-division of the comburrent agent flow into two sub-flows only one (the central flow A.sub.1) of which contains any fuel.

Claims

We claim:

1. A device for prevaporising liquid fuel, designed for fitting to a combustion system comprising a combustion chamber, a source of liquid fuel and a source of gaseous comburrent agent, said device being of the kind which comprises, projecting into the combustion chamber, a hollow structure incorporating a body and at least one arm attached to said body and having at least one portion extending transversely of the general direction of said body, the latter comprising an entry portion which communicates with said sources of liquid fuel and gaseous comburrent agent, and said arm having an exit portion which opens into the combustion chamber and through which, in operation, there is discharged a mixture of gaseous comburrent agent and at least partially vaporised fuel, in which said hollow structure comprises, in a zone which incorporates the transverse portion of said arm, a double-walled system comprising an internal wall and an external wall which together delimit two separate passages, namely a central passage communicating both with the source of gaseous comburrent agent and with the liquid fuel source, and a peripheral passage communicating solely with the source of gaseous comburrent agent.

2. A prevaporisation device as claimed in claim 1, and further comprising, arranged inside said peripheral passage at least in a zone thereof situated along the transverse portion of said arm, at least one blade or vane extending in a direction having a component parallel to the general direction of flow in said peripheral passage.

3. A prevaporisation device as claimed in claim 2, in which said blade or vane is in contact with the external wall which delimits said peripheral passage.

4. A prevaporisation device as claimed in claim 3, in which said blade or vane is likewise in contact with the internal wall which delimits said peripheral passage.

5. A prevaporisation device as claimed in claim 1, in which the external wall which delimits said peripheral passage is pierced, at least in a zone thereof situated along said arm, by a plurality of sweat-pores through which a fraction of the gaseous comburrent agent flow passing through said peripheral passage, can escape into the combustion chamber.

6. A device for prevaporizing liquid fueld adapted to be fitted to a combustion system comprising a combustion chamber, a source of liquid fueld and a source of gaseous comburrent agent, said device comprising, projecting into the combustion chamber, a hollow structure incorporating a body and at least one arm attached to said body and having at least one portion extending transversely of the general direction of said body, said body comprising an entry portion which communicates with said sources of liquid fuel and gaseous comburrent agent, and said arm having an exit portion which opens into the combustion chamber and through which, in operation, there is discharged a mixture of gaseous comburrent agent and at least partially vaporized fuel, in which said hollow structure comprises, in a zone which incorporates the transverse portion of said arm, a double-walled system comprising an internal wall and an external wall which together delimit two separate passages, namely a central passage communicating both with the source of gaseous comburrent agent and with the liquid fueld source, and a peripheral passage communicating solely with the source of gaseous comburrent agent, said external wall being pierced by a plurality of sweat-pores through which a fraction of the gaseous comburrent agent flow passing through said peripheral passage can escape into the combustion chamber, and at least one blade or vane arranged inside said peripheral passage and extending in a direction having a component parallel to the general direction of flow in said peripheral passage.