Patent Application
20060059914
The present invention relates to the general field of systems for injecting an air/fuel mixture into a turbomachine combustion chamber. More particularly, it relates to an injection system of the aerodynamic type provided with means for creating effervescence in the fuel prior to it being mixed with air.
The conventional process for designing and optimizing a turbomachine combustion chamber seeks mainly to reconcile implementing operational performance of the chamber (combustion efficiency, stability domain, ignition and re-ignition domain, lifetime of the combustion area, etc.) as a function of the intended mission for the airplane on which the turbomachine is mounted while minimizing emissions of pollution (nitrogen oxides, carbon monoxide, unburnt hydrocarbons, etc.). To do this, it is possible in particular to act on the nature and the performance of the injection system for injecting the air/fuel mixture into the combustion chamber, on the distribution of dilution air inside the combustion chamber, and on the dynamics of air/fuel mixing within the combustion chamber.
The combustion chamber of a turbomachine typically comprises an injection system for injecting an air/fuel mixture into a flame tube, a cooling system, and a dilution system. Combustion takes place mainly within a first portion of the flame tube (referred to as the “primary zone”) in which combustion is stabilized by means of air/fuel mixture recirculation zones induced by the flow of air coming from the injection system. In the second portion of the mixer tube (referred to as the “dilution zone”), the chemical activity that takes place is less intense and the flow is diluted by means of dilution holes.
In the primary zone of the flame tube, various physical phenomena are involved: injection and atomization into fine droplets of the fuel, evaporation of the droplets, mixing of the fuel vapor with air, and chemical reactions of the fuel being oxidized by means of the oxygen in the air.
These physical phenomena are governed by characteristic times. Atomization time thus represents the time needed by the air to disintegrate the sheet of fuel to form an air/fuel spray. It depends mainly on the performance and the technology of the injection system used and on the aerodynamics in the vicinity of the sheet of fuel. Evaporation time also depends on the injection system used. It is a function directly of the size of the droplets resulting from the disintegration of the sheet of fuel; the smaller the droplets, the shorter the evaporation time. Mixing time corresponds to the time needed for the fuel vapor coming from the evaporation of the droplets to mix with the air. It depends mainly on the level of turbulence inside the combustion area, and thus on the flow dynamics in the primary zone. Chemical time represents the time needed for the chemical reactions to develop. It depends on the pressures and temperatures at the inlet to the combustion area and on the nature of the fuel used.
The injection system used thus plays a fundamental role in the process of designing a combustion chamber, in particular when optimizing the times that are characteristic of fuel atomization and evaporation.
There exist two main families of injection systems: “aero-mechanical” systems in which the fuel is atomized as a result of a large pressure difference between the fuel and the air; and “aerodynamic” systems in which the fuel is atomized by being sheared between two sheets of air. The present invention relates more particularly to such aerodynamic systems.
Aerodynamic injection systems known in the prior art present numerous drawbacks. In particular, at low turbomachine speeds, fuel atomization becomes highly degraded, thereby decreasing the stability of combustion and running the risk of the combustion area going out while also increasing polluting emissions of the nitrogen oxide type.
A main aim of the present invention is thus to mitigate those drawbacks by proposing an aerodynamic injection system that enables the times characteristic of fuel atomization and evaporation to be shortened at all operating speeds of the turbomachine.
To this end, the invention provides an aerodynamic injection system for injecting an air/fuel mixture into a turbomachine combustion chamber, the system comprising: a tubular structure of axis XX′ that opens out at a downstream end for delivering the air/fuel mixture; at least one air feed channel that is connected to a compressor stage of the turbomachine and that opens out into the tubular structure in such a manner as to introduce air at a pressure PA into the tubular structure; and an annular fuel passage that is formed in the tubular structure around its axis XX′, that is connected to at least one fuel feed channel in which fuel flows at a pressure PC, and that opens out at a downstream end into the tubular structure, forming an enlargement therein; the system further comprising means for injecting gas into the at least one fuel feed channel, the gas being at a pressure PG that is greater than the pressure PA and greater than or equal to PC so as to create effervescence in the fuel on being introduced into the tubular structure.
By injecting gas into the fuel duct at a pressure that is greater than or equal to the pressure of the fuel, liquid/gas mixing is caused to take place at the pressure PC prior to the fuel being introduced into the main structure in which it is dispersed. During the expansion of this mixture from the pressure PC to the internal pressure in the main structure, the sudden expansion of the gaseous phase causes the sheet of fuel to disintegrate: this is effervescence. As a result, the times characteristic of the fuel atomization and evaporation at the outlet from the injection system can be considerably reduced.
These shortenings of time thus make it possible at slow operating speeds of the turbomachine to increase combustion efficiency and to increase the ability of the combustion area to avoid going out, while at full-throttle speed of turbomachine operation, they enable the formation of polluting emissions of the nitrogen oxide and soot types to be limited.
More particularly, the injection system includes at least one gas injection channel that opens out into the fuel feed channel(s) and that is connected to a gas feed duct.
Advantageously, the gas injection channel opens out substantially perpendicularly into the fuel feed channel(s).
The injection system may comprise an annular gas distribution cavity that is formed in the tubular structure around the fuel passage, that is connected to the gas feed duct, and that opens out into the gas injection channel.
The injection system may also include an annular fuel distribution cavity that is formed in the tubular structure, that is connected to a fuel feed duct, and that opens out into the fuel feed channel.
In an embodiment of the invention, the air feed channel opens out into the tubular structure at an upstream end thereof. The injection system may include an outer air swirler that is disposed around the tubular structure, that is offset radially relative to the fuel passage, and that serves to inject air at the outlet from the tubular structure along a direction that is substantially axial. The outer air swirler may be connected to a compressor stage of the turbomachine, and a bowl that forms a divergent portion may be mounted downstream from the tubular structure.
In another embodiment of the invention, the air feed channel is disposed around the tubular structure and opens out axially into the fuel passage at an upstream end thereof. The annular fuel passage may present a narrowing of section in the fuel flow direction in order to accelerate the flow of fuel through the tubular structure.
According to an advantageous characteristic of the invention, the gas used is air which is preferably taken from a compressor stage of the turbomachine prior to being compressed.
According to another advantageous characteristic of the invention, a device is provided for controlling the flow rate of the gas injected into the fuel feed channel.
Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings which show an embodiment that has no limiting character. In the figures:
With reference to
The injection system 2, 2′ includes at least one air/feed channel 6, 6′ that is connected to a compressor stage (not shown) of the turbomachine and that opens out into the tubular structure 4. Air is thus introduced into the tubular structure 4 via said channel(s) 6, 6′ at a pressure PA, e.g. of the order of 0.5 to 50 bar.
The injection system 2, 2′ also includes an annular fuel passage 8 that is formed in the tubular structure about its axis XX′. The downstream end 8b of the fuel passage 8 opens out into the tubular structure 4 and forms a sudden enlargement therein.
The fuel passage 8, which is centered on the axis XX′ of the tubular structure 4, is connected to at least one fuel feed channel 10 having fuel flowing therein at a pressure PC. The passage 8 enables fuel to be introduced into the tubular structure 4 along the axial direction XX′. By way of example, the pressure PC of the fuel flowing in the fuel feed channel 10 is about 4 bar to 80 bar.
As shown in
The fuel feed channels 10 are preferably inclined tangentially relative to the annular fuel passage 8, e.g. an angle of abut 45° (
According to the invention, the injection system 2, 2′ further comprises at least one gas injection channel 12 that opens out into the fuel feed channels 10 and that is connected to a gas feed duct 14.
As shown in
Still according to the invention, the gas is introduced into the fuel feed channel(s) at a pressure PG that is greater than the pressure PA of the air introduced into the tubular structure 4 via the air feed channel(s) 6, 6′, and that is greater than or approximately equal to the pressure PC of the fuel flowing in the fuel feed channel(s) 10.
Introducing gas into the fuel feed channel(s) 10 at a pressure PG greater than the pressure PA and greater than or equal to the pressure PC serves to create a liquid/gas mixture at the pressure PC prior to the mixture being introduced into the tubular structure 4. Effervescence in the fuel is characterized by the fuel being atomized due to the gas expanding suddenly on being introduced into the tubular structure 4.
More particularly, effervescence takes place in the fuel when the following conditions are satisfied: the gas is at a pressure PG that is substantially equal to the pressure PC of the fuel (or at a pressure that is slightly greater), and the mixing of the gas with the fuel takes place in a space that is substantially confined (specifically mixing takes place in the zone of confluence between the gas injection channels 12 and the fuel feed channels 10).
Effervescence in the fuel is characterized by the presence of bubbles of gas in the sheet of fuel that flows in the fuel passage 8. The expansion of the gas bubbles during introduction of the mixture into the tubular structure 4 thus facilitates subsequent atomization thereof. The times characteristic of fuel atomization and evaporation are thus shortened.
The gas is preferably an inert gas that has no direct influence on the combustion of the air/fuel mixture. For example, the gas is air that is taken from a compressor stage of the turbomachine and that is further compressed in order to reach a pressure PG greater than the pressure PA of the air feeding the air feed channel(s) 6, 6′.
According to an advantageous characteristic of the invention, the gas injection channel(s) 12 opens out substantially perpendicularly into the fuel feed channel(s) 10. This particular arrangement serves to encourage the appearance of effervescence in the fuel.
An annular gas cavity 16 may be formed in the tubular structure 4 around the fuel passage 8. Such a gas cavity 16 is centered on the axis XX′ of the tubular structure 4 so as to be coaxial with the fuel passage 8. It is connected to the gas feed duct 14 and opens out into the gas injection channel(s) 12. This gas cavity 16 thus acts as a gas distribution cavity.
Similarly, an annular fuel cavity 18 may be formed in the tubular structure 4. As shown in the figures, this fuel cavity 18 is also centered on the axis XX′ of the tubular structure 4 so as to be coaxial with the fuel passage 8 and the gas cavity 16. It is connected to a fuel feed duct 20 and opens out into the fuel duct channel(s) 10. This fuel cavity 18 thus acts as a fuel distribution cavity.
According to another advantageous characteristic of the invention, the injection system 2, 2′ further comprises a device 22 for controlling the flow rate of the gas injected into the fuel feed channel 10. Such a device 22 thus serves to control the flow rate of the gas needed for injection in order to achieve effervescence in the fuel. For example, the gas flow rate may be controlled as a function of the flow rate and the pressure PC of the fuel.
Particular features of the embodiment of the injection system 2 of the invention as shown in
In this embodiment, the injection system 2 may have two rows of air feed channels 6 that are axially spaced apart from each other and that are regularly distributed around the entire circumference of the tubular structure 4. These channels 6 may open out into the upstream end 4a of the tubular structure 4.
The air introduced via the channel(s) 6 at a pressure PA thus flows in the tubular structure 4 in the axial direction XX′ to the downstream end 4b of the structure accompanied by a rotational effect inside the tubular structure 4.
Furthermore, the injection system 2 preferably includes an outer air swirler 24 that is disposed around the tubular structure 4 and that is radially offset relative to the fuel passage 8. This outer air swirler 24 serves to inject air at the outlet of the tubular structure 4 in a direction that is substantially axial and likewise accompanied by a rotary effect. Thus, the effervescent fuel that is introduced into the tubular structure 4 via the fuel passage 8 is atomized by the effect of the shear between the air coming from the air speed channel 6 and from the outer air swirler 24.
The air feeding the outer air swirler 24 is preferably taken from a compressor stage of the turbomachine, e.g. from the same stage as the air that is introduced into the tubular structure 4 via the air feed channel(s) 6. In addition, still in this embodiment of the invention, a bowl 26 forming a diverging portion can be mounted downstream from the tubular structure 4.
The particular features of the embodiment of the injection system 2′ shown in
In this embodiment, the injection system 2′ has a single air feed channel 6′. This channel is annular; it is placed around the tubular structure 4 and opens out axially into the fuel passage 8 at an upstream end 8a thereof. The air introduced via the channel 6′ at a pressure PA thus flows in the fuel passage 8 prior to being introduced into the tubular structure 4 via an enlargement thereof.
Furthermore, the fuel passage 8 preferably presents a narrowing of section 8c in the fuel flow direction in order to accelerate the flow of fuel in the tubular structure 4.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0410052 | Sep 2004 | FR | national |
