After burner

Abstract

An afterburner installation for a gas turbine engine including an expansion turbine from which, in operation, there is discharged a high-velocity high-temperature exhaust gas flow, which installation comprises an afterburner duct located downstream of such expansion turbine, through which duct the exhaust gas flow passes, a source of afterburner fuel, and at least one hollow structure arranged within the afterburner duct in order to be in a heat transfer relationship with the high-velocity high-temperature exhaust gas flow; such hollow structure, having its interior connected to the fuel source and comprising an entry portion with an intake orifice facing upstream relative to the direction of exhaust gas flow and through which a fraction from the exhaust gas flow is collected and penetrates into the interior of such hollow structure, the intake portion extending in the longitudinal direction of the afterburner duct, and a discharge portion comprising at least one exit orifice through which a high-temperature jet of a fuel/gas mixture escapes from the hollow structure and enters the afterburner duct where it spontaneously ignites and forms a stabilised flame front, such discharge portion extending transversely of the longitudinal direction of the duct and such exit orifice or orifices facing downstream relative to the direction of the exhaust gas flow in the afterburner duct.

Description

The present invention relates to an afterburner installation designed for fitting to a gas turbine engine comprising an expansion turbine from which, in operation, there is discharged a high-velocity high-temperature exhaust gas flow.

The invention relates more particularly to high-power installations for use in high-performance turbojet propulsion engines designed for the propulsion of aircraft at high speeds, in which the temperature of the exhaust gas flow can reach a very high level, for example in the order of 850.degree. to 950.degree.C, which is well above the level generally reached in conventional machines designed for the propulsion of aircraft at lower speeds.

The afterburner installation is of the kind comprising an after burner duct located downstream of the expansion turbine, through which duct the exhaust gas flow passes, a source of afterburner fuel, and at least one hollow structure arranged within the afterburner duct in order to be in a heat transfer relationship with the high-velocity high-temperature exhaust gas flow; hollow structure, the interior of which is connected to such fuel source, comprising an entry portion with an intake orifice facing upstream relative to the direction of exhaust gas flow and through which a fraction from the exhaust gas flow is collected and penetrates into the interior of the hollow structure, such intake portion extending in the longitudinal direction of the afterburner duct, and a discharge portion comprising at least one exit orifice through which a high-temperature jet of a mixture of gas and fuel escapes from the hollow structure and enters the afterburner duct where it spontaneously ignites and forms a stabilised flame front.

In conventional combustion installations, in order to stabilise the combustion of the fuel in a high-velocity gas flow, recourse is had to one or more obstacles referred to as "flame holders" immersed in the flow in order to locally slow it down and create, in their wake, one or more turbulent zones of low axial velocity, where combustion can establish. Thus, downstream of said obstacle or obstacles, a combustion zone is formed which is limited in the upstream direction by a flame front stabilised by the obstacles. An ignition device to furthermore generally provided in order to temporarily or permanently ensure the triggering of combustion.

However, it appears that the presence, within the body of such a gas flow, of stabiliser obstacles and of a special ignition device, is a source of serious drawbacks, as these elements give rise to substantial pressure losses. Furthermore, in particular in high performance installations of the kind referred to hereinbefore, they experience substantial overheating which can impair their operating life.

The object of the invention is to overcome the aforesaid drawbacks or difficulties by providing a device which makes it possible to trigger and maintain combustion of a fuel in a high-velocity high-temperature exhaust gas flow, without it being necessary to have recourse to stabilising obstacles and to a special ignition device.

To this end, in accordance with the invention, in an afterburner installation of the kind referred to hereinbefore, the discharge portion of the hollow structure extends transversely relative to the longitudinal direction of the afterburner duct, and the exit orifice or each of such orifices of the discharge portion faces downstream relative to the exhaust gas flow direction.

In accordance with one embodiment, the discharge portion of the hollow structure is disposed immediately in succession to the intake portion thereof.

In accordance with another embodiment, the hollow structure comprises four portions disposed in accordance with the sides of a quadrilateral and traversed in series by the mixture of gas and fuel, namely: a first portion constituted by the intake portion of the hollow structure, a second portion extending transversely of the longitudinal direction of the afterburner duct, a third portion extending in the longitudinal direction of the afterburner duct, and extending upstream of the second portion, and a fourth portion constituted by the discharge portion.

In accordance with one embodiment of the invention, the hollow structure comprises, over at least part of its length, a tubular structure. In accordance with an arrangement which is applicable in this particular case, the tubular structure comprises, over at least part of its length, at least two tubular portions supplied in parallel by the same intake orifice.

In accordance with one embodiment of the invention, the discharge portion of the hollow structure extends substantially radially relative to the axis of the afterburner duct. In accordance with an arrangement which is applicable in this case, the radial spacing between two successive exit orifices decreases progressively with increasing distance from the axis of the afterburner duct.

In accordance with another embodiment, the discharge portion of the hollow structure extends in the circumferential direction of the afterburner duct.

The gas turbine engine can be of the single-flow or multiflow type, for example of the dual-flow type, namely a primary high-temperature high-velocity gas flow escaping from the expansion turbine, and a secondary gas flow which is passed around the expansion turbine.

In accordance with an embodiment which is applicable to the particular case in which the primary and secondary flows mix in a junction zone located downstream of the expansion turbine, and in which the engine comprises, furthermore, a pilot combustion chamber arranged in the junction zone in order to trigger and stabilise combustion in the secondary gas flow, the hollow structure in accordance with the invention is designed to enable afterburning of fuel in the primary gas flow, downstream of the expansion turbine, and to this end is arranged in the junction zone in order to be in a heat transfer relationship on the one hand with the primary gas flow and on the other with the pilot combustion chamber. This arrangement makes it possible to exploit the presence of the pilot combustion chamber associated with the secondary flow, in order to reduce the time taken to achieve spontaneous ignition of the fuel/gas mixture in the primary gas flow.

The invention will now be further described, by way of non-restrictive example, with reference to the accompanying drawings, in which :-

FIGS. 1 and 2 are experimental graphs illustrating the conditions under which ignition of a mixture of fuel and comburrent agent injected into a high-velocity high-temperature flow of gaseous comburrent agent takes place.

FIG. 3 is a half-section in an axial plane, on the line III--III of FIG. 4, through an afterburner installation in accordance with a first embodiment of the invention.

FIG. 4 is a transverse section, on the line IV-IV, through the installation shown in FIG. 3.

FIG. 5 is a view on a larger scale and in section on the line V--V, illustrating a detail of the installation shown in FIG. 3.

FIG. 6 is a half-section in an axial plane, through an installation in accordance with a second embodiment of the invention.

FIG. 7 is a perspective view, partially cut-away, of an afterburner installation in accordance with a variant of the embodiment shown in FIG. 6.

FIG. 8 is a half-section in an axial plane, through an afterburner installation in accordance with a third embodiment of the invention.

FIG. 9 is a view in transverse section, of a variant of a detail of the invention.

The Applicants were confronted with the problem of achieving the triggering and maintenance of combustion of a fuel in a high-velocity, high-temperature flow of gaseous comburrent agent (the temperature being for example between 850.degree.C and 950.degree.C), without having to slow down the flow and without having to have recourse to a special ignition device.

To this end, the Applicants studied the conditions under which, in such a high-velocity, high-temperature flow, diffusion of a mixture of fuel and comburrent gaseous agent whose richness is in excess of the rich flame-out limit, takes place.

They considered, more particularly, the variation in the zone of diffusion, of parameters such as pressure, concentration of fuel and temperature, and observed (and this has been borne out by experience) that these parameters, at a certain distance from the location where the mixture of fuel and comburrent agent enters the high-temperature gas flow, acquire a value such that the conditions of spontaneous ignition of the fuel are locally satisfied.

Combustion is triggered, therefore, without the need for a special ignition device and is maintained in the gas flow without it being necessary to have recourse to stabilizing obstacles in order to slow down the flow.

The Applicants were subsequently confronted with the problem of increasing the combustion efficiency of an installation operating in accordance with this principle. They observed that, for a given length or geometry on the part of the combustion space, it was relevant to reduce the distance separating the flame front (i.e., the upstream limit of the combustion zone) from the location where the mixture of fuel and comburrent agent enters the high-velocity, high-temperature gas flow. However, this distance can be considered as the sum of the distance required for diffusion of the mixture of fuel and comburrent agent through the gas flow, and a distance corresponding to the time taken to achieve spontaneous ignition of the fuel in the aforesaid diffusion zone. It is therefore advantageous to reduce as far as possible the time taken to achieve spontaneous ignition.

The Applicants noticed tha the chief factor in this context is the temperature T.sub.1 of the mixture of fuel and conburrent agent, at the instant at which it enters the high-velocity, high-temperature gas flow. In the following, the reference T will signify the temperature of the gas flow after injection of the fuel.

The graphs of FIGS. 1 and 2, illustrate the influence of this temperature T.sub.1, on the one hand upon the time t (in seconds) required to achieve spontaneous ignition, and on the other hand upon the rate of the spontaneous ignition, for the same temperature (850.degree.C) of the high-velocity high-temperature flow of gaseous comburrent agent. These graphs demonstrate very well that it is essential, in order for the spontaneous ignition time to be as short as possible, that the temperature T.sub.1 of the mixture of fuel and comburrent agent, should be as high as possible.

The afterburner installation illustrated in FIGS. 3 and 4 makes it possible to satisfy the aforesaid conditions.

In these Figures, the general reference 1 has been used to designate an afterburner installation designed to operate in a gas turbine engine such as a turbojet propulsion engine, comprising an expansion turbine 2 from which, in operation, there is discharged a high-velocity, high-temperature exhaust gas flow F containing a certain proportion of comburrent agent oxygen F. This installation comprises, downstream of the turbine 2, an afterburner duct 3 having an axis X'-X, externally delimited by a wall 4. At its upstream portion, the afterburner duct 3 has a divergent configuration due to the presence of a fixed turbine cone 5 supported by the wall 4 through the medium of profiled struts 6.

The afterburner installation likewise comprises a source 7 supplying afterburner fuel, connected to an annular collector or manifold 8 which externally girdles the afterburner duct 3. This fuel may be liquid or gaseous.

A hollow structure 9 is arranged within the afterburner duct 3 in order to be in a heat transfer relationship with the high-temperature, high-velocity exhaust gas flow F escaping from the turbine 2. This hollow structure is represented (see FIG. 3) in the form of an L-shaped tubular structure held in position, in relation to the wall 4, by means of suspension struts (not shown).

The hollow structure 9 comprises a first tubular portion 9a extending in the longitudinal direction of the afterburner duct (for example parallel to the axis X'-X) and a second tubular portion 9b extending transversely relative to the longitudinal direction (for example substantially radial).

At its upstream portion, the tubular section 9a is equipped with an intake orifice 9aa facing upstream relative to the direction of the gas flow F, through which a fraction f of the flow is collected and enters, under its own dynamic pressure, the interior of the hollow structure 9.

The tubular portion 9b advantageously has a streamlined contour (see FIG. 5) in order that the gas flow F is disturbed as little as possible. It is equipped with one or more exit orifices 9ba each of which faces downstream relative to the direction of the gas flow F, through which jets of a mixture of gas and fuel can escape from the hollow structure 9 to enter the afterburner duct 3. As FIGS. 3 and 4 show, the exit orifices 9ba are radially staggered so that the distance separating two successive orifices decreases progressively with increasing distance from the axis X'--X of the duct.

In operation, the gas flow F escaping from the expansion turbine 2 is at a high temperature, ranging for example between 850.degree. and 950.degree.C. The fraction f of this flow which enters the hollow structure 9 mixes with the fuel flow injected into the structure. Its chief role is to act as a carrier for the fuel and prevent the latter from coking. To avoid any risk of premature ignition of the fuel inside the hollow structure, the flow rate of the collected fraction f of the gas flow is arranged in such a fashion that the richness of the fuel/gas mixture flowing through the structure is greater than the rich flame-out limit.

The gas which enters the interior of the hollow structure 9 commences by cooling in contact with the fuel (this is especially so if the latter is injected in the liquid state and vaporises inside the hollow structure). Being in a heat transfer relationship with the high temperature gas flow F, the fuel/gas mixture flowing through the hollow structure 9 subsequently becomes preheated to a temperature close to the temperature of self-ignition of the fuel.

The thus preheated mixture escapes through the exit orifices 9ba in the form of high-temperature jets which enter the afterburner duct 3 where they spontaneously ignite. Thus, in the duct, a combustion zone is developed which is delimited upstream by a stabilized flame front S located downstream of the exit orifices 9ba. The radial stagger, shown in FIGS. 3 and 4, of the orifices 9ba, as might be expected, favours the formation of a combustion zone which is uniform in transverse section.

As we have seen earlier, the distance d separating the flame front S from the exit orifices 9ba is a function of the time taken to achieve spontaneous ignition of the fuel/gas mixture entering the gas flow F, this time itself being a function of the temperature T.sub.1 of this mixture.

Other things being equal, the temperature T.sub.1 referred to hereinbefore depends upon the heat transfer efficiency of the hollow structure 9 which will therefore be shaped and dimensioned accordingly. Thus, one or more (see FIG. 4) hollow structures 9 could be used whose length and diameter are calculated to take account of the temperature T.sub.1 which is to be achieved.

The arrangement shown in FIG. 3, in accordance with which the fuel/gas mixture is preheated by heat exchange with the exhaust gas flow F before the temperature of the flow has been increased as a consequence of afterburning in the duct 3, is particularly suitable for situations where the temperature of the exhaust flow from the expansion turbine 2 is quite high, higher than 900.degree.C for example.

If this temperature is lower, the temperature T.sub.1 of the fuel/gas mixture entering the afterburner duct 3, will diminish correspondingly so that the distance d (and, consequently, the length of the afterburner duct) will increase.

If it is required to reduce this distance d, it is possible to utilise part of the heat liberated by the afterburning process to preheat the fuel/gas mixture flowing through the hollow structure 9.

FIG. 6 illustrates an arrangement which makes it possible to preheat the fuel/gas mixture by heat exchange with the high-velocity high-temperature exhaust gas flow F, after the temperature of the gas flow has been increased as a consequence of after-burning of fuel in the flow.

To this end, the hollow structure 9 comprises four tubular portions 9a, 9b, 9c, 9d, arranged in series with one another. The portions 9a and 9c each extend in the longitudinal direction of the afterburner duct 3, whilst the portions 9b and 9d both extent transversely relative to the longitudinal direction.

The hollow structure 9 is equipped as before with an intake orifice 9aa. The exit orifices 9da are arranged in the portion 9d, that is to say upstream of the rest of the hollow structure 9. Like the orifices 9ba (FIG. 3) they face downstream relative to the direction of flow of the gas flow F.

In operation, the jets of the fuel/gas mixture escaping from the exit orifices 9da, ignite spontaneously in the gas flow F giving rise to a flame front S' located at a distance d' from the exit orifices 9da. Under the effect of the heat liberated by the combustion of the fuel in the afterburner duct 3, the fuel/gas mixture flowing through the hollow structure 9 experiences a substantially greater temperature rise than is the case with the embodiment illustrated in relation to FIG. 3, so that the ignition time and the ignition d', are considerably reduced.

It will be observed, too, that by virtue of the fact that the afterburner duct 3, in its upstream section, has a divergent form, the static pressure prevailing in the duct at the entry part of the divergent section, adjacent the exit orifices 9da (see FIG. 6), is lower than that at the exit part of the divergent section, adjacent the exit orifices 9ba (see FIG. 3). Thus, between the intake orifice 9aa and the exit orifices 9da (see FIG. 6), there is a greater pressure difference than that existing between the intake orifice 9aa and the exit orifices 9ba (see FIG. 3). This greater pressure difference thus makes it possible to produces circulation of the fuel/gas mixture through a hollow structure 9 having a higher internal aerodynamic resistance in the case of FIG. 6, than is so in the case of FIG. 3. Thus, in the case shown in FIG. 6, it is possible to use a longer tube than in the case shown in FIG. 3, for the hollow structure 9. For the same reason, (and taking into account, furthermore, the fact that the main gas flow F flowing around the hollow structure 9 is at a higher temperature in the case of FIG. 6, than it is in the case of FIG. 3) it is possible to reduce the diameter of the tube constituting the hollow structure shown in FIG. 6.

This circumstance constitutes an advantage because of the fact that the smaller the aforesaid diameter, the smaller the pressure loss produced in the gas flow F by the hollow structure 9. To further reduce this pressure loss, it is also possible to use streamlined tubular sections 9b and 9d, as shown in FIG. 5.

FIG. 7 shows a variant embodiment of the invention, in accordance with which the hollow structure 9 comprises, over at least part of its length, at least two tubular portions 9 c.sub.1 -9d.sub.1 and 9c.sub.2 -9d.sub.2 supplied in parallel by the same intake orifices 9aa and each equipped with exit orifices 9da.sub.1 and 9da.sub.2. This arrangement has the advantage of making it possible to reduce the diameter of the radial tubular portions 9d.sub.1 and 9d.sub.2, so that there is less disturbance to the high-velocity high-temperature gas flow F.

FIG. 8 shows a further variant embodiment of the invention in accordance with which the fuel/gas mixture is preheated, in part at least, by heat exchange with an auxiliary heat source.

In this FIG., there is shown a portion of a by-pass or dual-flow gas turbine engine. A primary high-velocity high-temperature gas flow F escapes from an expansion turbine 22, and a secondary gas flow or cold flow Z, passes around the expansion turbine. The two flows F and Z mix in a junction zone located downstream of the turbine 22.

An auxiliary combustion chamber 23 delimited by walls 20 and 21 and extended by V-shaped flame-holders 28, is arranged in the aforesaid junction zone. It is supplied on the one hand with fuel by means of injectors 24, connected to a fuel manifold 25, and on the other hand with high-temperature comburrent gas through an annular orifice 26 which picks up a fraction of the primary gas flow F.

In operation, the auxiliary combustion chamber 23 directs into the secondary gas flow Z, through the flame-holders 28, gas jets in the course of combustion, and thus acts as a pilot combustion chamber making it possible to trigger and stabilize combustion in the secondary flow. This chamber serves, at the same time, as an auxiliary heat source, as will be explained hereinafter.

In order to permit afterburning of fuel in the high-velocity high-temperature primary gas flow F, the invention, as in the preceding cases, provides a hollow structure 29 which makes it possible to very substantially preheat the fuel/gas mixture.

This hollow structure comprises a portion 29a of generally annular shape, delimited, on the one hand, by the wall 20 of the pilot combustion chamber 23, and, on the other hand, by a wall 40. The portion 29a contains an intake orifice 29aa likewise of annular form, through which a fraction f of the primary gas flow F can enter the interior of the hollow structure 29. Into the interior of the same portion 29a there open fuel injectors 30 connected by channels 31 to a fuel manifold 32 itself connected to a fuel source (not shown). The lines 31 pass through the pilot combustion chamber 23 inside streamlined struts 33.

The protion 29a of the hollow structure 29 is closed at its downstream end but extends in the form of a certain number of tubes 29b which project transversely into the primary gas flow F and each of which is equipped with one or more exit orifices 29ba.

In operation, the hollow structure 29 passes a fuel/gas mixture which is heated up by heat exchange, on the one hand with the primary gas flow F, and on the other hand with the auxiliary heat source constituted by the pilot combustion chamber 23. The high intensity of combustion in the pilot combustion chamber contributes to a substantial increase in the temperature of the jets of fuel/gas mixture escaping, through the exit orifices 29ba, into the primary gas flow F. In this fashion, a very short ignition time for the afterburning of the fuel in the primary gas flow, is achieved. It will be observed in this context that apart from its function as an auxiliary heat source, the pilot combustion chamber 23 plays no other part whatsoever in the aforesaid afterburning process. It will be noted, by contrast, that the hollow structure 29 through which the fuel/gas mixture in the course of preheating flows, acts as a heat shield for the wall 20 of the pilot combustion chamber 23.

It has been assumed, thus far, that the exit orifices 9ba (or 9da) for the preheated jets of fuel/gas mixture, are radially staggered. However, this is not a mandatory arrangement. Thus, in FIG. 9, a variant detail is shown in which the portion 9b (or 9d) of the hollow structure 9 equipped with the exit orifices extends in the circumferential direction of the afterburner duct.

Claims

What is claimed is:

1. In and for a gas turbine engine comprising an expansion turbine discharging a high-velocity, high-temperature exhaust gas flow, an afterburner installation comprising means bounding an afterburner duct located downstream of said expansion turbine and through which duct said exhaust gas flow passes, a source of afterburner fuel, and a system for supplying the afterburner duct with afterburner fuel from said fuel source to bring about combustion of said fuel in said afterburner duct and for stabilizing said combustion, said supplying and stabilizing system consisting of a plurality of hollow tubular structures each distinct from said means bounding the afterburner duct and arranged totally within said afterburner duct, so as to be immersed in said high-velocity, high-temperature exhaust gas flow, each said tubular structure comprising four portions arranged in series relationship including:

a first entry portion extending peripherally in the longitudinal direction of the afterburner duct and having one intake orifice which faces upstream relative to the direction of said exhaust gas flow, and through which a fraction from said exhaust gas flow is collected and penetrates into the interior of said tubular structure; the interior of said entry portion being further connected, in the vicinity of said intake orifice, with said source of afterburner fuel;

a second portion extending generally transversely of the longitudinal direction of the afterburner duct;

a third portion extending generally in the longitudinal direction of the afterburner duct, upstream of said second portion relative to said exhaust gas flow direction; and

a fourth discharge portion extending generally transversely of the longitudinal direction of said duct, and having a plurality of exit orifices each of which faces downstream relative to said exhaust gas flow direction; whereby, in operation, each tubular structure is traversed, along substantially its whole length extending from its first portion to its fourth portion, by a mixture of gas and fuel which is preheated by heat-transfer from the high-velocity, high-temperature exhaust gas flow in which said structure is immersed, which preheated mixture escapes in the form of a plurality of preheated jets which spontaneously ignite after entering the afterburner duct and form a stabilized flame front, without any necessity for recourse to any other film stabilizing means.

2. An installation according to claim 1 wherein said afterburner duct comprises a section having a divergent configuration relative to said exhaust gas flow direction, and said fourth portion of the tubular structure is located in an upstream region of said divergent section.

3. An installation according to claim 1 wherein said tubular structure comprises at least two tubular portions supplied in parallel by the same intake orifice.

4. An installation according to claim 1 wherein said fourth portion extends substantially radially relative to the axis of the afterburner duct.

5. An installation according to claim 4 wherein the spacing, in the radial direction, between two successive exit orifices diminishes progressively with increasing distance away from the axis of the afterburner duct.

6. An installation according to claim 1 wherein said fourth portion extends in the circumferential direction of said afterburner duct.

7. An installation according to claim 1 wherein said four portions of said tubular structure are arranged in the form of a quadrilateral.