METHOD FOR INJECTING A HYDROGEN-AIR MIXTURE FOR A TURBINE ENGINE BURNER

Description

technical field

[0001] This disclosure relates to the process of supplying gas to injection devices for gas turbines such as aircraft turbomachinery powered by hydrogen and air. This includes, in particular, civil and military aeronautical applications: helicopters, VTOLs, drones, APUs, turbogenerators, fixed-wing aircraft for recreational, business, or commercial aviation, turbojets, and turboprops. Previous technique

[0002] The propulsion sector, and particularly the aeronautics sector, faces major environmental challenges. The appeal of using hydrogen combustion rather than kerosene is growing, as hydrogen combustion would avoid carbon-based pollutant emissions such as carbon dioxide, carbon monoxide, unburned hydrocarbons, and fine particles and smoke.

[0003] A principle for micro-mixing air-hydrogen burners is known. Burners based on this principle do not guarantee the absence of flashback in the hydrogen injection device and have a complex geometry. Such burners are expensive to manufacture, have a high pressure drop, and are specific to a given combustion chamber design.

[0004] At the level of injection and combustion, two main technological configurations exist for hydrogen-air injection systems applied to gas turbines, namely lean injection systems, and rich injection systems.

[0005] More generally, it is important to bear in mind that lean-burn fuel systems tend to generate significant thermoacoustic instabilities that can damage these systems, whereas stable combustion is necessary to avoid impairing engine performance. Rich-burn fuel systems, on the other hand, tend to emit more pollutants than lean-burn systems if they are not properly designed. In one particular technique, US patent 2010 / 330510 A1 proposes carrying out an exothermic catalytic reaction of a rich mixture in a channel before burning the resulting gas at the channel outlet with a supply of heated air. An injection method for an injection device in the combustion chamber of an aircraft turbomachine, according to the prior art, is known from US patent 8,197,249 B1. Technical problem

[0006] The use of hydrogen raises several issues that must be taken into consideration at the combustion chamber level:

[0007] Under equivalent thermodynamic conditions in pressure, temperature, and fuel-air ratio, the adiabatic temperature of the flame from a hydrogen-air combustion is higher than that of the flame from a kerosene-air combustion.

[0008] Similarly, the flame speeds from hydrogen-air combustion are higher than for kerosene-air flames. High flame speeds can lead to flashback problems in injection systems, particularly at the boundary layers, causing serious damage to these systems or creating safety hazards.

[0009] However, the flammability limits of hydrogen are more extensive than those of kerosene and allow a hydrogen-air mixture to be ignited at lower or higher air-fuel ratios than for kerosene, which can ultimately lead to lower flame temperatures than with the use of kerosene.

[0010] Finally, the combustion of hydrogen with air tends to emit much more noise than conventional kerosene combustion and can therefore generate significant noise pollution at airports. Description of the invention

[0011] This document proposes a premixed rich injection process dedicated to the combustion of dihydrogen and air, which addresses the technical problems presented previously.

[0012] More specifically, the present disclosure proposes an injection method according to claim 1.

[0013] The process in which injection is carried out continuously after ignition in order to operate the turbine makes it possible to reduce the temperature of the flame fronts which reduces the NOx content of the burnt gases and reduces wear on the injector.

[0014] The characteristics described in the following paragraphs correspond to embodiments that can be implemented independently of each other or in combination with each other as appropriate:

[0015] The dihydrogen-air mixture can have a hydrogen content greater than 2.

[0016] Advantageously, said dihydrogen-air mixture may have a hydrogen content greater than or equal to 4.

[0017] An air flow rate in the external annular channel can be chosen such that the overall richness at the outlet of the internal and external annular channel assembly is fixed between 0.15 and 0.5 depending on the operating points of the turbomachine.

[0018] The hydrogen content of the mixture can be chosen so that said rich combustion takes place with a flame front temperature below 1800 K, which preserves the combustion chamber.

[0019] The hydrogen content of the mixture can be chosen so that the first flame front is laminar and has a Lewis number greater than 1, limiting thermo-diffusive instabilities and thus avoiding flashback phenomena.

[0020] The mixture burned in the first flame front generates residual gases which are advantageously burned in the second flame front stabilized by the supply of air from the external annular channel.

[0021] The richness of the second flame front is such that the second flame front can be maintained at a temperature below 1800K.

[0022] The air injected through the annular channel can be rotated by an annular screw so as to make the second flame front turbulent and so that this second flame front is not attached to the lip of the internal channel.

[0023] Advantageously, positioning the downstream end of the internal channel upstream of the downstream end of the external annular channel optimizes the mixing between the gases from the first combustion and the air injected through the external channel. Brief description of the drawings

[0024] Other features, details, and advantages of the invention will become apparent from the following detailed description of non-limiting embodiments and from the analysis of the accompanying drawings, in which: [ Fig. 1 ] shows a turbomachine comprising an injection device arranged in an annular bottom of an annular combustion chamber in three configurations; [ Fig. 2] shows a first schematic example in cross-sectional side view of an injection device to which the process of this disclosure applies; [ Fig. 3 ] shows a schematic view of the device of the figure 2 in a combustion situation; [ Fig. 4 ] shows a plurality of possible configurations (Figures A, B, C, D, E) of the internal channel of a device to which the method of this disclosure applies; [ Fig. 5 ] shows a plurality of examples of annular channel output configurations (Figures A, B, C) for a device to which the method of this disclosure applies. Description of the implementation methods

[0025] The drawings and description below contain elements that can not only help to better understand the invention, but also contribute to its definition, if necessary.

[0026] Reference is now being made to the figure 1which represents three examples of configurations for the installation of an injection device 2 on a turbomachine 1 depending on the orientation of the annular bottom of an annular combustion chamber 4, 4', 4" of the turbomachine: either the combustion chamber 4" is oriented substantially along a longitudinal axis X, or the combustion chamber 4 is oriented at an acute angle to this longitudinal axis, or the combustion chamber 4' is transverse to said longitudinal axis X. In all cases, the injection device 2 is installed between a compressor 101 and a high-pressure turbine 102, 103, 104, on an annular bottom of the annular combustion chamber 4, 4', 4" or on an external shell.

[0027] The injection device can be, as illustrated in figure 2, an injection device comprising an internal channel 6 and an external annular channel 8. The external channel 8 is centered on the internal channel 6, and in the case of tubular channels, the internal channel 6 and the external annular channel 8 are coaxial. These channels open into the combustion chamber 4, 4', 4" of the device figure 1 The internal and external channels have a circular cross-section. An ignition device (not shown) ignites the gases exiting the channels to initiate combustion.

[0028] This injection device 2 is used in the present disclosure in a configuration in which a rich dihydrogen-air mixture is injected into the internal channel 6 while air is injected into the external channel 8. As a result, the combustion includes a first combustion rich in dihydrogen at the outlet of the internal or central channel 6. and a second combustion called lean which is carried out around a flame created by the first combustion.

[0029] For injection into internal channel 6 and combustion at the outlet of this channel, injection and combustion are said to be rich when there is an excess of hydrogen compared to a stoichiometric combustion between hydrogen and atmospheric oxygen, and lean when there is an excess of oxygen compared to this stoichiometric combustion. Stoichiometric combustion is itself defined as combustion in which there are the correct number of hydrogen and oxygen atoms necessary to consume all the fuel, leaving only water and nitrogen in the combustion products.

[0030] According to the figure 3, the present invention thus provides an injection method which comprises an injection of a dihydrogen-air mixture 12a with a hydrogen content greater than the stoichiometric dosage into the internal channel 6 of the injection device and an injection of air 26a into the external annular channel 8 so as to produce, at the outlet of said internal channel 6, a first flame front 30 resulting from a rich combustion surrounded by a second flame front 31 resulting from a lean combustion.

[0031] The internal channel 6 then forms a hydrogen-air rich mixture injection tube 12a and the external annular channel 8 forms an air injection tube 26a.

[0032] The rich mixture 12a of air and dihydrogen is injected from an inlet 10 located at an upstream end of the internal channel 6.

[0033] The internal channel 6 has an internal diameter d. The choice of the internal diameter d of the channel depends on a desired thermal power.

[0034] Back to the figure 2 A downstream end 16 of the internal canal 6 is arranged upstream with respect to a downstream end 24 of the external annular canal 8. The downstream end 24 of the external annular canal 8 is arranged at a distance r from the downstream end 16 of the internal canal 6 downstream. This external annular canal 8 has an internal diameter D.

[0035] The external annular channel 8 is configured to receive a second gas, which is air 26a. This gas enters the external annular channel through an inlet 26a arranged at the upstream end of said external annular channel.

[0036] An annular spiral 28 is housed at the upstream end of the external annular channel 8. This spiral can be radial or axial. This annular spiral 28 is positioned at a distance L from the downstream end 36 of the external annular channel 8. The air 26a passing through the external annular channel is set into rotation by the external spiral 28. This generates a vortex assembly that will help to detach the second flame front from the outlet of the central channel.

[0037] The hydrogen-air premix 12a is injected into the internal channel 6, formed by a tube creating a central injection conduit. The premix has a richness greater than two, i.e., greater than 2 masses of hydrogen to 1 mass of air, and can even have a richness greater than four in certain operating configurations.

[0038] The pure air 26a injected into the annular channel 8 around the internal channel 6 is injected in a calculated quantity to target an overall injection richness between 0.15 and 0.5, which corresponds to an overall lean combustion. The pure air 26a is set in rotation within the annular channel 8 by the external axial or radial auger 28 located upstream of the outlet plane 16a of the downstream end 16 of the central injection duct for the rich dihydrogen-air mixture.

[0039] The lip 16 of the internal canal 6 is here in retreat relative to the exit plane 24a of the annular canal 8.

[0040] The operation of the injection device is described below, based on the Figure 3 :

[0041] The injection of the rich dihydrogen-air premix 12a into the internal channel 6 allows, after ignition, the creation of a first flame front 30 at the outlet of the internal channel 6, resulting from the rich combustion of said mixture. This flame front adheres to the lip 16 of the internal channel 6. This rich combustion, for example with an air-fuel ratio greater than 2, occurs with a flame front temperature below 1800 K in order to avoid generating nitrogen oxides. This flame front 30 is laminar and is not subject to thermal diffusion instabilities due to a Lewis number greater than 1.

[0042] The injection of air 26a at the external annular channel 8 allows for the rapid dilution and confinement of the exhaust gases from the combustion of the rich premix. The presence of a strong turbulent shear layer reduces the local richness. This mixture is then burned, generating a second flame front 31 of lean combustion. This flame front remains stabilized thanks to the vortex quench caused by the air supply and the high reactivity of dihydrogen, despite the significant stretching imposed on the flame. This second flame front, resulting from lean combustion, is also at a temperature below 1800 K, limiting the formation of nitrogen oxides. The flame front 31 is turbulent and is not attached to the lip 16 of the internal channel 6.The length of the flame will depend on the conditions of entry of the fuels and oxidants and in particular on the ratio of the quantities of momentum, the indentation of the internal channel relative to the external annular channel, the presence of vortices in the flame.

[0043] The creation of these two flame fronts, a rich flame 30 and a lean flame 31, allows the thermo-acoustic load from combustion to be distributed over a larger surface area, thus reducing combustion noise. Similarly, stabilizing the two flame fronts at the burner lips divides the thermo-acoustic loads associated with combustion and reduces the noise generated.

[0044] The combustion process of this document thus achieves a staged combustion of hydrogen in order to bypass the nitrogen oxide formation zone by means of the combustion of the dihydrogen-rich air premix in a first zone, the internal flame 30, and the combustion of the residual gases in a second zone, the flame 31 around the flame 30.

[0045] The risk of flashback is limited with rich combustion because the initial flame front does not exhibit thermal diffusion instabilities. Therefore, the speed of the initial flame is not accelerated by these instabilities.

[0046] The integrity of the combustion chamber is also ensured because, by performing combustion at high and low air-fuel ratios, the flame temperatures are lower than under stoichiometric conditions. Potential flame fronts originating from stoichiometric zones that might be present are not attached to the injector lips, thus limiting injector damage.

[0047] An example of an embodiment provides, for operation under typical conditions of a gas turbine of a turboprop, a rich zone richness of the order of 4 for the dihydrogen-air mixture injected by the internal channel 6, i.e. a richness well above the stoichiometric dosage of richness 1, and an air supply by means of the annular channel 8 in such quantity that the overall richness is fixed between 0.15 and 0.50 depending on the operating points of the turboprop.

[0048] There figure 4shows various possible embodiments of the premix injection outlet of the internal channel 6. The shape and thickness of the outlet 16 of the internal channel, 16a, 16b, 16c, can be adjusted relative to the basic shape 16 of the internal channel shown in figure 4(A) In figure 4(B) , the end 16a of the internal canal is formed in a re-entrant bevel, in figure 4(C) The 16b end always flares out at a bevel. figure 4(D) The end 16c of the internal channel flares out but has a terminal face perpendicular to the longitudinal axis of the channel. These different configurations allow the attachment of the first flame front 30 to the lip 16 to be managed according to the configurations of the injection system.

[0049] In figure 4(E) a screw 17 is added in the internal channel 6 in order to homogenize the dihydrogen-air premix.

[0050] As for the external channel 8, it can open from a wall 240 as shown in figure 5 and have different channel outlet lip configurations:

[0051] Right exit 24 in figure 5(A) , angled flared conical outlet 24a in figure 5(B) or conical outlet closing 24b as in figure 5(C) These different configurations allow for adjustment of the exit speed of the air surrounding the rich flame exiting the internal channel 6.

[0052] This disclosure relates to a process for injecting pre-mixed dihydrogen with air into an aeronautical gas turbine based on staged combustion, in which: a. A high-rich hydrogen-air premix combustion takes place in a first region and generates a first flame front attached to the injector lips; b. A rapid mixing of the combustion products via air injection to be burned in a second region generating a second stable flame front.

[0053] This process allows, in particular: a. To obtain aerodynamically stabilized flames over a wide operating range, b. To achieve combustion with very low nitrogen oxide emissions, c. To avoid the risk of flashback of the second flame front, d. To reduce noise pollution related to hydrogen combustion, e. To guarantee the integrity and lifespan of the injector.

[0054] The method as defined in the claims is not limited to the above description and can in particular be applied to injection systems disposed in rear walls of combustion chambers or protruding from such walls.

Claims

1. Injection method, for an injection device in a combustion chamber (4, 4') of an aircraft turbine engine (1), said injection device comprising an internal channel (6) surrounded by an external annular channel (8), said channels leading into said combustion chamber (4, 4') of said gas turbine, characterized in that it comprises an injection of a gaseous hydrogen / air mixture (12a) having a greater hydrogen richness level than the stoichiometric amount into said internal channel (6) and an injection of air into said external annular channel (8), producing at the outlet of said internal channel (6), after ignition of said mixture at the outlet of the internal channel, a first flame front (30) resulting from rich combustion, external to said internal channel and attached to a lip at the outlet of the internal channel, said first flame front being surrounded by a second flame front (31) resulting from lean combustion with the air exiting said external channel.

2. Injection method according to claim 1, wherein said gaseous hydrogen / air mixture (12a) has a hydrogen richness level that is greater than 2.

3. Injection method according to claim 1, wherein said gaseous hydrogen / air mixture (12a) has a hydrogen richness level that is greater than or equal to 4.

4. Injection method according to claim 1, 2 or 3, wherein an air flow rate in the external annular channel is chosen such that the overall richness level at the outlet of the internal channel (6) / external annular channel (8) assembly is set between 0.15 and 0.5 depending on the operating points of the turbine engine.

5. Injection method according to any one of the preceding claims, wherein the injection of the gaseous hydrogen / air mixture (12a) and the device are configured to create, at the outlet of the internal channel (6), said first flame front (30) resulting from a rich combustion of said mixture, and to attach it to a lip (16) of the internal channel (6) after ignition of said mixture.

6. Injection method according to claim 5, wherein the gaseous-hydrogen richness of the mixture is chosen so that said rich combustion is carried out with a flame front temperature of less than 1800 K.

7. Injection method according to claim 5 or 6, wherein the gaseous-hydrogen richness of the mixture is chosen so that the first flame front (30) is laminar and has a Lewis number greater than 1, limiting diffusive-thermal instabilities.

8. Injection method according to claim 5, 6 or 7, wherein the mixture burned in the first flame front generates residual gases burned in the second flame front (31) which is stabilized by the supply of air from the external annular channel.

9. Injection method according to claim 8, wherein the second flame front is maintained at a temperature below 1800K.

10. Injection method according to claim 8 or 9, wherein the air injected by the annular channel is rotated by an annular swirler (28).