FR2694799A1 - Conventional annular combustion chamber with several injectors - includes base mounted circumferentially alternately for pilot and full gas providing idling or take off power with air blown to fuel cone - Google Patents

Abstract

The combustion chamber has axial walls (1,2) joined at a base (3), with an open downstream end (16) for gas output. The base has numerous orifices mounting fuel injectors (5) circumferentially in the same plane. Fuel is provided by a first "pilot" injector group to an idling head (6) and a second "full gas" group to a take off head (7). These injectors are mounted alternately. Each head has a tubular burner (11) surrounding the corresponding "full gas" injector, extending into the combustion chamber. Entries in the tubular burner blow air into the fuel cone (10). USE/ADVANTAGE - Turbojet aircraft engines, for Rich Quick Lean operation. Reduces nitrogen oxide pollutants, in reasonable size.

Description

ANNULAR COMBUSTION CHAMBER
CONVENTIONAL WITH SEVERAL INJECTORS
The present invention relates to the field of turbomachinery and in particular aviation turbojets.

 Aviation turbojets can be subjected to a variety of regimes, including idling, take-off and cruising. On the other hand, these turbojet engines are asked to emit few polluting gases and in particular few nitrogen oxides, whatever their regime, while maintaining flame stability in order to avoid extinction of the combustion chamber in critical flight phases, in particular when descending during the landing phase.

 It is known that the production of nitrogen oxides is notably high, at high temperature, when the air forms a stoichiometric mixture with the fuel and that this production of nitrogen oxides is all the more important as the stoichiometric mixture long resides in the combustion chamber.

These conditions are met at full speed in traditional turbojet engines because the combustion chamber has a notably high volume in order to guarantee combustion stability performance of relighting at low speed.

 It has already been proposed to equip the combustion chamber with a turbojet engine with idling heads and takeoff heads offset axially and radially and opening into a common area. If this type of chamber is satisfactory from the point of view of the reduction of pollution, it nevertheless has some drawbacks, such as for example its radial dimensioning linked to the superimposition of the idle heads and the take-off heads, and the large number of injectors, which increases the cost and mass.

Another drawback appears in the temperature map when leaving the room when only the idle head is operating. Indeed, the high pressure turbine can then be subjected to temperatures approaching 18000K at the blade heads while the feet are only around 900K. This causes efficiency losses to the turbine.

 To illustrate the technique of combustion chambers producing little nitrogen oxides, mention will be made of document US 4,898,001 in which an internal cone 13 is provided at the level of the injectors so as to reduce the combustion volume, document EP 0 441 542 in which the combustion volume is reduced by an internal cone and an air intake zone is provided at the end of the cone for the rapid passage of a rich mixture into a lean mixture, as well as the documents US 4,944,149 and FR 2,446,443 which describe tubular chambers with a zone of high dilution for a reduction in pollution.

In addition, among the concepts of combustion stoves potentially leading to a significant reduction in nitrogen oxide emissions, the rich combustion and rapid dilution hearth called RB-QQ (Rich Burn-Quick Quench) or
RQL (Rich Quick Lean) is known in the field of combustion.

 The object of the present invention is to provide an annular combustion chamber of the conventional type, that is to say in which the fuel injectors are left circumferentially at equal distance from the axis of the turbomachine, which makes it possible to reduce the production of nitrogen oxides, to maintain a homogeneous temperature of the gases at the inlet of the turbines, whatever the speed and while maintaining a reasonable radial and axial dimensioning in said chamber.

 The invention therefore relates to an annular combustion chamber for a turbomachine, in particular for an aviation turbojet, comprising axial walls joined by a chamber bottom and an open downstream end for the flow of gases, said chamber bottom having a plurality of holes for mounting the fuel injectors arranged circumferentially in the same plane.

 According to the invention, the combustion chamber is characterized in that said fuel injectors are divided into a first group of so-called "pilot" injectors each supplying fuel to an idling head, and into a second group of so-called injectors "full gas" each supplying fuel to a take-off head, said "full gas" injectors alternating with said "pilot" injectors, each take-off head comprising a tubular hearth extending from said chamber bottom into the volume of said chamber and surrounding the corresponding "full gas" injector, said tubular hearth being equipped at its downstream end with means for blowing air into the fuel cone coming from the corresponding full gas injector.

 Thanks to this structure, the fuel injectors are all in the same plane and arranged circumferentially. This allows a classic configuration to be preserved at the bottom of the room. The presence of the tubular hearths in the internal volume of the chamber makes it possible to reduce the volume of the "full gas" modules and thereby the production of nitrogen oxides due to the limitation of the residence time of the gases in the "full module" gas".

 According to a first variant, the hearth is conical in shape and has an inner wall and an outer wall joined at their downstream ends and delimiting between them an annular air passage.

The means for blowing air into the fuel cone having calibrated orifices provided in the interior wall in the vicinity of its downstream end. A spin can be arranged in said air passage.

 According to a second variant, the hearth is cylindrical in shape and comprises air intake means at its upstream end, as well as helical channels extending from upstream to downstream and intended to direct the air from the intake means of air to the means for blowing air into the fuel cone.

 The combustion chamber advantageously has an annular neck in the vicinity of the downstream end of the tubular hearths, which divides the internal volume of the combustion chamber into a rich combustion zone and a rapid dilution zone.

 The combustion chamber then consists of a group of tubular hearths with rich combustion and rapid dilution distributed circumferentially in said chamber and a group of pilot injection systems alternating with said tubular hearths.

Other characteristics and advantages of the present invention will appear better on reading the following description given by way of examples and with reference to the appended drawings in which:
- Figure 1 is a front view of the combustion chamber according to the invention;
- Figure 2 is an axial section along the line II-II of Figure 1 of an idle head;
- Figure 3 is an axial section along line III-III of Figure1 of a takeoff head according to a first embodiment
- Figure 4 is an axial section along line III-ffi of the figurel of a take-off head according to a second embodiment;
- Figure 5 is a perspective view of the tubular hearth of Figure 4;
- Figure 6 shows in detail the downstream end of a sheet used for the manufacture of the tubular hearth of Figure 5 ;;
- Figure 7 schematically shows an axial section of a tubular hearth of the RQL type; and
- Figure 8 shows an axial section of a pilot system in an RQL type home.

 The gas turbine combustion chamber shown in the drawings is of the annular type, delimited by an external annular wall 1, an internal annular wall 2, a transverse chamber bottom 3 joining said walls 1 and 2. The bottom is pierced with several orifices 4 inside each of which is placed a fuel injector 5. The fuel injectors 5 are substantially all equidistant from the axis of the combustion chamber and they are regularly spaced on the bottom of the chamber 3. L the downstream end 16 of the combustion chamber is open to allow the flow of gases.

 The injectors 5 are divided into two groups. The injectors 5 of the first group, called pilot injectors, supply fuel to the idle heads 6 adapted to operation at idle speed. The injectors 5 of the second group, called full-gas or take-off injectors, supply fuel to take-off heads 7 adapted to operating at full load, corresponding for example to the take-off of an aircraft or to its displacement while cruising.

 Each orifice 4 is associated with an air intake device in the combustion chamber 8 which notably comprises intake tendrils 9 having, in known manner, the property of entraining the air admitted around the fuel injectors 4 in the form of vortices inside the combustion chamber 8, in order to favor the vaporization of the fuel emitted, in the combustion chamber 8, by the fuel injector 5, in the form of a substantially conical jet of fuel 10 .

 Idle heads 6 alternate with takeoff heads 7.

Each take-off head 7 comprises a tubular hearth 11 which surrounds the corresponding full gas injector 5 and which extends into the internal volume of the combustion chamber 8. This tubular hearth 11 has at its downstream end 12 means for blowing the air in the fuel cone 10.

 According to a first embodiment of the tubular hearth 11, the latter comprises an outer wall 13 and an inner wall 14, joined at their downstream end 12 and forming a conical assembly open at its downstream end 12 and whose cross section decreases in the direction 15 of the gas flow. The upstream edges 18 and 19 of the walls 13 and 14 are linked to the bottom of the chamber 3 by brazing. The walls 13 and 14 define between them an annular air passage 20 around the fuel injector 5. This air is used, on the one hand, for the internal and external cooling of the walls 13 and 14 and, on the other hand, to the primary air supply to the take-off head 7 at the downstream end 12. For this purpose, the interior wall 14 comprises, in the vicinity of the downstream end 12, calibrated orifices 21. A spin 22 is advantageously placed in the annular passage 20. Perforations 23 are provided in the walls 13 and 14 for the circulation of the cooling air.

 It should be noted that the calibrated orifices 21 breathe primary air, practically perpendicularly, into the fuel cone 10 coming from the corresponding injector 5. They thus provide a function of controlling the opening of the ply of fuel.

 The air flow which escapes from the perforations of the external wall 13 ensures, in addition to its cooling function, the intercommunication of the flame between the injectors 5 of the idle heads 6.

 According to a second embodiment, the tubular hearth 11 is of cylindrical shape. It can be produced by two sheets 30 and 31 linked together by brazing. Before the connection by brazing, grooves were etched on the contacting faces of the two sheets 30 and 31 so as to form helical channels 32. These helical channels 32 are supplied with air by air intakes 33 provided upstream from the combustion chamber at the bottom of the chamber 3. They open in the vicinity of the downstream end 12 in an annular cavity 34 also etched between the two sheets 31 and 32. The air circulating in the channels 32 makes it possible to cool the tubular hearth 11 then, after having circulated in the cavity 34, it is directed by a plurality of calibrated orifices 21 perpendicularly to the fuel cone 12 coming from the injector 5. The calibrated orifices 21 also play the piloting function here opening of the fuel table. At the upstream periphery of the cylindrical hearth 11, a plurality of orifices 35 are arranged convergently to the sheet metal 31. The air entering through the orifices 35 cools the sheet metal 31 by film.

 According to an advantageous arrangement of the invention, the combustion chamber 8 has an annular neck 40 immediately downstream of the tubular hearths 11. The annular neck 40 shares the internal volume of the combustion chamber 8 in a rich combustion zone 41 located in the side of the injectors 5 and a lean combustion zone 42 situated on the side of the downstream end 16, said zones 41 and 42 being separated by a rapid dilution zone 43 at the level of the annular neck 40. This concept of rich combustion hearth and rapid dilution known as RB-QQ or RQL is known in the field of combustion, and potentially leads to a significant reduction in nitrogen oxide emissions. The combustion chamber 8 of the present invention thus comprises nested RQL 44 type tubular hearths in a conventional annular chamber, that is to say not radially stepped, and alternating with pilot injection systems 45.

 The neck 40 makes it possible to reduce the volume of the rapid combustion mixture at the outlet of the tubular hearths of type RQL 44 and to limit the sudden bursting at the outlet of these hearths, while ensuring conditions of rapid dilution. Ports 46 for dilution air inlet are provided in the walls 1 and 2 at the neck 40.

 Between each RQL 44 tubular hearth, the pilot injection systems 45 make it possible to stagger the fuel flow between the idling and full gas conditions in order to control the pollution and the temperature profile throughout the operating range of the turbojet, as well as the re-ignition ceiling and flame stability.

Claims (7)

 1. Annular combustion chamber comprising axial walls (1, 2) joined by a chamber bottom (3) and an open downstream end (16) for the flow of gases, said chamber bottom (3) having a plurality of orifices (4) for mounting fuel injectors (5) arranged circumferentially in the same plane, characterized in that said fuel injectors (5) are distributed in a first group of so-called "pilot" injectors each supplying fuel at an idle head (6), and in a second group of so-called “full throttle” injectors each supplying fuel to a takeoff head (7), said “full throttle” injectors alternating with said “pilot” injectors, each take-off head (7) comprising a tubular hearth (11) surrounding the corresponding "full gas" injector (5) and extending in the internal volume (8) of said combustion chamber, said tubular hearth (11) being equipped at its downstream end (12) means (21, 22 ) to inject air into the fuel cone (10) from the corresponding "full gas" injector (5).  2. Combustion chamber according to claim 1, characterized in that said tubular hearth (11) is conical in shape and has an inner wall (14) and an outer wall (13) joined at their downstream end (12) and delimiting between they an annular air passage (20), and in that the means for blowing air into the fuel cone (10) having calibrated orifices (21) formed in said inner wall (14) in the vicinity of its downstream end (12).  3. Combustion chamber according to claim 2, characterized in that the tubular hearth (11) further comprises a twist (22) in said air passage.  4. Combustion chamber according to claim 2, characterized in that said inner wall (14) and said outer wall t13) further comprise perforations (23) allowing the circulation of cooling air.  5. Combustion chamber according to claim l, characterized in that the tubular hearth (11) is of cylindrical shape and comprises air intake means (33) at the upstream end, as well as helical channels (32) s extending from upstream to downstream and intended to direct the air from said air intake means (33) to the means (21) for blowing air into the fuel cone (10).  6. Combustion chamber according to any one of claims 1 to 5, characterized in that said combustion chamber (8) has an annular neck (40) in the vicinity of the downstream end (12) of the tubular hearths (11) .  7. Combustion chamber according to claim 6, characterized in that there are provided orifices (46) for air intake in the neck (40).