RU2468298C2 - Stage-by-stage fuel combustion in burner - Google Patents

Abstract

FIELD: power industry.SUBSTANCE: stage-by-stage fuel combustion method at start-up of burner for combustion chamber of gas turbine engine, which includes burner housing enveloping the burner that includes upstream and downstream end parts located in opposite axial direction and auxiliary combustion chamber located on upstream end of the above burner consists in the following: auxiliary combustion chamber is provided with fuel and air for fuel combustion to form the flow of unblocked concentration of radicals on unbalanced level and heat from auxiliary combustion zone, which is directed downstream along axial line of auxiliary combustion chamber through the throat at auxiliary combustion chamber outlet. In addition, burner is provided with the primary combustion space restricted with downstream refractory walls relative to outlet of auxiliary combustion chamber to keep the main flame and arrange the main recirculation zone for circulating unburnt radicals. The above method involves the following stages: fuel mixed with air is added to auxiliary combustion chamber. Mixture is ignited using an ignition device provided on upstream end of auxiliary combustion chamber for initiation of flame of poor mixture inside auxiliary combustion chamber and for supply of the above flow of radicals and heat to downstream combustion space. First vortex of air-and-fuel mixture is supplied on the outside at auxiliary combustion chamber outlet on upstream end of combustion space for creation and keeping of the main flame of poor mixture in the primary combustion space. Then, formation of the second vortex of air-and-fuel mixture is performed from outlet of the first channel for preliminary formation of downstream poor mixture relative to outlet of auxiliary combustion chamber and from outlet of the second channel for preliminary formation of downstream poor mixture relative to the above outlet of the first channel for preliminary formation of poor mixture. Step-by-step addition of the second vortex of air-and-fuel mixture is performed to provide the full-load stage at least to one of the above first and second channels for supply of the main air and fuel flow to the main flame of poor mixture.EFFECT: stabilisation of combustion in burner for burning in poor and rich zones of partially formed mixture with low contaminant matter, which is intended for combustion chamber of gas turbine in all engine operating modes.3 cl, 7 dwg

Description

FIELD OF THE INVENTION

The present invention relates to a burner, preferably intended for use in gas turbine engines, and, more specifically, to step-by-step (step) combustion of fuel in a burner adapted to stabilize the combustion process in the engine, and, moreover, to step-by-step (step) combustion of fuel in a burner that uses an auxiliary (pilot) combustion chamber to produce combustion products to stabilize the main process of burning pre-formed lean mixtures.

TECHNICAL BACKGROUND

Gas turbine engines are used in many different applications, including power generation, military and civil aviation, pipelines and shipping. In a gas turbine engine, which operates in the mode of burning pre-partially formed lean mixtures, fuel and air are supplied to the burner chamber, where they mix and ignite by means of a torch, as a result of which combustion is initiated. The main problems associated with the combustion process in gas turbine engines, in addition to thermal efficiency and proper mixing of fuel and air, are related to stabilization of the flame, elimination of pulsations and noise and the fight against polluting emissions, especially with nitrogen oxides (NO _x ), CO, unburned hydrocarbons, smoke and particulate emissions.

EP 1659339 A1 discloses a method for starting a burner, preferably for burning synthetic gas, which includes the operation of an auxiliary burner with a diffusion flame located in the center of the swirl nozzles for supplying a mixture of fuel and air. US Pat. No. 5,321,948 A1 discloses a combustion chamber with combustion zones of preformed mixtures and dry suppression of NO _x , comprising at least two concentric cylinders arranged with ledges between which a channel is formed for supplying a mixture of fuel and air to the combustion zone.

In industrial gas turbine engines, which operate in the mode of burning pre-partially formed lean mixtures, the flame temperature is reduced by adding more air than is required for the combustion process itself. Excessive air that does not react must be heated during combustion, and as a result, the flame temperature during combustion decreases (below the stoichiometric point) from approximately 2300 K to 1800 K and below. This reduction in flame temperature is required to significantly reduce nitrogen oxide emissions. The method that has been proven to be most successful in reducing NO _x emissions is to make the combustion process by burning a mixture so lean that the flame temperature drops to temperatures below the temperature at which the diatomic nitrogen and oxygen dissociate (N ₂ and O ₂ ) and recombining them to form NO and NO ₂ . Combustion flows during vortex flame stabilization are widely used in industrial gas turbine engines to stabilize combustion by creating, as indicated above, a return flow (a recirculation zone formed by a vortex) above the center line, as a result of which the return flow ensures the return of heat and free radicals back to the incoming unburned air-fuel mixture. Heat and free radicals from previously reacted fuels and air are necessary to initiate (to subject the fuel to pyrolysis and to initiate the branching process) and to maintain stable combustion / burning of a fresh unreacted air-fuel mixture. Stable combustion in gas turbine engines requires a cyclic combustion process that causes the formation of combustion products, which are transported back to the area located upstream to initiate the combustion process. The flame propagation front stabilizes in the boundary layer of the recirculation zone formed by the vortex. Within the boundary layer, the "Local velocity of the turbulent flame of the air-fuel mixture" must be higher than the "Local velocity of the air-fuel mixture", and as a result, the flame propagation front / combustion process can be stabilized.

The burning of pre-formed lean mixtures is inherently less stable than diffusion flame burning, for the following reasons.

1. The amount of air required to reduce the flame temperature from 2300 K to 1700-1800 K is approximately twice the amount of air required for stoichiometric combustion. This makes the overall ratio of the components of the air-fuel mixture (Ф) very close (about or less than 0.5; Ф≥0.5) or similar to the ratio of the components of the air-fuel mixture, in which the flame of the lean preformed mixture is extinguished. Under these conditions, the flame may fade locally and reappear periodically.

2. Near the limit value corresponding to the extinction during the burning of a poor mixture, the flame propagation velocity of previously partially formed poor mixtures is very sensitive to deviations in the ratio of components. Fluctuations in the flame propagation velocity can lead to spatial deviations / movements of the flame propagation front (the recirculation zone formed by the vortex). A less stable, easily moving flame propagation front of the preformed mixture leads to periodic changes in the intensity of heat generation, which, in turn, leads to the movement of the flame, unstable hydroaerodynamic processes and the appearance of thermoacoustic instabilities.

3. Fluctuations in the ratio of the components are probably the most “common connecting mechanism,” linking unstable heat generation with unstable pressure fluctuations.

4. To make the burning of a lean combustion sufficient to allow a significant reduction of emissions of nitrogen oxides NO _x, almost all of the air used in the engine must pass through the nozzle and must be pre-mixed with the fuel. Therefore, the entire flow in the burners is potentially reactive and requires that the point at which combustion begins is fixed.

5. In the case where the heat required for the reaction to occur is a stability limiting factor, very insignificant temporary fluctuations in the ratios of the components of the air-fuel mixture (which can occur as a result of fluctuations in either the fuel flow or the air flow through the burner / nozzle) can cause partial extinguishing and re-occurrence of the flame.

6. An additional and very important reason for reducing the stability of the flame of the preformed mixture is that the sharp gradient of the mixing of fuel and air is eliminated from the combustion process. This creates the possibility of ignition of the pre-mixed stream in any place where there is sufficient temperature for the reaction. When a flame can occur more easily in many places, it becomes more unstable. The only way to stabilize the flame of the pre-formed mixture with its fixed position is based on the temperature gradient that occurs where the unburned, pre-mixed fuel and air mix with the hot combustion products (the flame cannot occur where the temperature is too low). This makes the temperature gradient, arising from the emission, radiation, heat dissipation and convective heat transfer, a means of stabilizing the flame of the preformed mixture. Radiative heating of the fluid does not create a sharp gradient; therefore, the source of stability should be the release, heat dissipation, and convective heat transfer to the pre-reacted zone. Dissipation causes a sharp gradient only in laminar and not turbulent flows, while only convective heat transfer and energy generation remain the means causing sharp gradients, which are desirable for stabilizing the flame, which in fact are the thermal gradient and the gradient of free radicals. Both heat and free radicals are formed, scattered and transferred due to convection with the provision of their gradient through the same mechanisms due to circulating combustion products within the recirculation zone formed by the vortex.

7. In pre-mixed flows, as well as in diffusion flows, rapid expansion causes separation and vortex circulating flows, both of which are widely used to form heat and free radical gradients and feed them into pre-reacted fuel and air.

WO 2005/040682 A2 describes a solution aimed at a burner for gas turbine engines that use an auxiliary flame to help maintain and stabilize the combustion process.

When the ignition device, as in the burners of the prior art, is located in the outer recirculation zone, the air-fuel mixture entering this zone often needs to be made rich in order to ensure a sufficiently high flame temperature to maintain stable combustion in this zone. In this case, the flame often cannot propagate into the main recirculation zone until the main stream of pre-mixed fuel and air becomes rich enough, hot and does not have enough free radicals, which occurs at higher fuel consumption. When a flame cannot propagate from the outer recirculation zone to the inner main recirculation zone shortly after ignition, it must propagate at a higher pressure after the engine speed begins to increase. This transfer of initiation of the main flame from the outer recirculation zone only after the pressure in the combustion chamber begins to increase leads to faster relaxation of free radicals to low equilibrium levels, which is an undesirable characteristic that is counterproductive for igniting the flame at the front braking point in the main zone recycling. Ignition in the main recirculation zone may not occur until the auxiliary combustion chamber provides a sufficient increase in the average volumetric temperature to a level at which the equilibrium levels of free radicals trapped in the main recirculation zone and the formation of additional free radicals in the preformed main fuel mixture and air will be sufficient to ignite the main recirculation zone. In the process of ensuring the spread of the flame from the outer to the main recirculation zone, significant amounts of fuel leave the engine without burning from the non-ignited pre-formed main air-fuel mixture. The problem arises if the flame propagates into the main recirculation zone in a certain burner before it spreads to the other burners in the same engine, since burners in which the flame is stabilized from the inside provide combustion at higher temperatures, since all fuel burns. This leads to temperature variations between the burners, which can cause damage to engine components.

SUMMARY OF THE INVENTION

The aspects of the step-by-step fuel combustion in accordance with the present invention are described here as an example in connection with a burner for burning in the depleted and enriched zones of a partially preformed mixture with a low yield of harmful substances, intended for the combustion chamber of a gas turbine, which provides a stable ignition and combustion process all engine loading modes. This burner operates in accordance with the principle of “supplying” heat and free radicals in high concentration from the outlet of the auxiliary combustion chamber to the main flame burning in a vortex of a poorly pre-formed air-fuel mixture, as a result of which a fast and stable burning of the main flame of a poor pre-supported formed mixture. The auxiliary combustion chamber provides heat and the addition of free radicals in high concentration directly to the front braking point and the main recirculation zone formed by the vortex, where the main lean pre-mixed stream is mixed with hot gaseous combustion products provided by the auxiliary combustion chamber. This makes it possible to burn a poorer mixture and lower combustion temperatures of the vortex of the main preformed air-fuel mixture, which otherwise would not be self-sustaining in the vortex-stabilized circulating flows under operating conditions of the burner.

In accordance with a first aspect of the invention, there is provided a stepwise combustion method of fuel, characterized by the features of claim 1.

Additional aspects of the invention are presented in the dependent claims.

The burner uses:

an air / fuel vortex with a swirl coefficient (S _N ) exceeding 0.7 (i.e., exceeding the critical value S _N = 0.6) generated / supplied to the flow by means of a radial swirl;

active substances - nonequilibrium free radicals released near the front point of inhibition;

a special type of burner geometry with multicomponent refractory embrasure; and

internal step-by-step combustion of fuel and air in the burner at the stage to stabilize the combustion process under all operating modes of the gas turbine.

In short, an open burner provides a stable ignition and combustion process under all engine loading conditions. Some important features related to the burner of the invention are:

geometric arrangement of burner elements;

the amount of fuel and air divided into steps in the burner;

minimum amount of active substances - radicals formed and required under different operating conditions of the engine / burner / fuel profile;

mixing fuel and air under different engine operating conditions;

imparted degree of turbulence;

multicomponent design of refractory embrasure (consisting of at least two sections of refractory embrasure).

To achieve the lowest possible levels of harmful emissions, the problem solved by this design / invention is to ensure the presence of uniform mixing profiles at the outlet of the channels for the preliminary formation of poor mixtures. In the burner covered by this description, there are two separate combustion / combustion zones in which fuel is always burned simultaneously. Both combustion zones are stabilized by a vortex, and fuel and air are premixed before the combustion process. The main combustion process, during which more than 90% of the fuel is burned, is the process of burning a lean mixture. The supporting combustion process, which takes place in a small auxiliary combustion chamber, in which up to 1% of the total fuel flow is consumed, can be a lean fuel mixture burning process, a stoichiometric process and a lack of air combustion process (with a ratio of components in the mixture Ф = 1.4 and higher).

An important difference between the open burner and the burner presented in the prior art document is that a poorly streamlined body is not required in the auxiliary combustion chamber, since the present invention uses an unblocked radical stream directed upstream from the combustion zone of the auxiliary combustion chamber along the axial line of the auxiliary combustion chamber, while the specified flow of radicals is discharged through the entire cross-sectional area of the opening of the neck of the auxiliary combustion chamber anija output of the auxiliary combustion chamber.

The main reason why the supporting process of combustion in a small auxiliary combustion chamber can be a process of burning a lean fuel mixture, a stoichiometric process or a process of combustion with a lack of air and still provide a stable process of ignition and combustion under all engine loading conditions, is related to the efficiency of complete combustion. The combustion process that takes place in a small auxiliary combustion chamber has low efficiency due to the large surface area, which leads to the removal of flame heat into the wall of the auxiliary combustion chamber. An ineffective combustion process, regardless of whether it is a poor fuel mixture burning process, a stoichiometric process or a lack of air combustion process, can generate a large amount of active substances - radicals, which is necessary to increase the stability of the main flame of the lean mixture and is preferable for successful burner operation this design / invention. (Note: the flame occurring in the preformed lean air-fuel mixture is referred to herein as the lean mixture flame).

It is very difficult to maintain combustion (but not to initiate, since a small auxiliary combustion chamber can function as a flare ignition device) in the boundary layer of the main recirculation zone at parameters below the limit values at which the flame of the lean mixture breaks off at the main flame of the lean mixture (approximately T> 1350 K and Ф≥0.25). To ensure the operation of the engine at parameters below the limit values for the main flame of the lean mixture, at which the flame of the lean mixture occurs, this burner design uses / provides an additional "separation into steps" of the auxiliary combustion chamber. The air that is used to cool the inner walls of the small auxiliary combustion chamber (performed by a combination of shock and convective cooling) and which accounts for about 5-8% of the total air flow through the burner is pre-mixed with the fuel in front of the swirl. A relatively large amount of fuel can be added to the air to cool the small auxiliary combustion chamber, which corresponds to the component ratios (Φ> 3) characteristic of very rich mixtures. Swirling cooling air and fuel, and hot combustion products from a small auxiliary combustion chamber can very effectively support the combustion of the main flame of a lean mixture with parameters that are lower, at or above the limit values at which the flame of the lean mixture breaks off. The combustion process is very stable and efficient, since hot combustion products and very hot cooling air (with temperatures above 750 ° C), pre-mixed with fuel, provide heat and active substances (radicals) to the front braking point in the main flame recirculation zone. During this combustion process, a small auxiliary combustion chamber, combined with very hot cooling air (with temperatures above 750 ° C) pre-mixed with fuel, functions as a flameless burner in which reagents (oxygen and fuel) are pre-mixed with combustion products, and distributed flame is generated at the front braking point in the recirculation zone formed by the vortex.

To ensure the proper functioning and stable operation of the burner disclosed in this application, it is required that the imparted degree of turbulence and the coefficient of turbulence be higher than critical (not lower than 0.6 and not higher than 0.8), in which a recirculation zone with vortex decay will form and will be stably within the multicomponent design of the refractory embrasure. The front braking point P shall be located within the refractory embrasure and at the exit of the auxiliary combustion chamber. The main reasons for the need to comply with this requirement are as follows:

If the imparted degree of swirl is low and the resulting swirl coefficient is less than 0.6, then with most burner geometries a weak recirculation zone will form and unstable combustion may occur.

A strong recirculation zone is necessary to ensure the possibility of transporting heat and free radicals from previously subjected to combustion of fuel and air back against the flow towards the flame propagation front. A zone of sufficiently stable and strong recirculation is required to create a boundary layer zone in which the velocity of the turbulent flame can be “matched” or can correspond to the local air-fuel mixture and a stable flame can be established. This flame propagation front formed in the boundary layer of the main recirculation zone must be stable, and no periodic movements or displacements of the flame propagation front should occur. The attached swirl coefficient can be of great importance, but should not exceed 0.8, since for a given value of the swirl coefficient and for values above it, more than 80% of the total volume of the flow will come back due to recirculation. A further increase in the swirl coefficient will not contribute to a larger increase in the amount of combustion products subjected to recirculation, and the flame in the boundary layer of the recirculation zone will undergo significant turbulence and stresses, which can lead to heat removal to the combustion chamber wall and partial extinction and re-occurrence of the flame. In the burner covered by this description, any type of vortex generator, radial, axial and axial radial, can be used. This description shows the radial configuration of the swirl.

The burner uses aerodynamic flame stabilization and limits the flame stabilization zone — the recirculation zone — in the multi-component design of the refractory embrasure. The multicomponent design of the refractory embrasure is an important feature of the design of the developed burner for the following reasons. Refractory embrasure (or also called diffuser):

- provides a flame propagation front (the main recirculation zone) with holding the flame in a certain place in space without the need to keep the flame on a solid surface / poorly streamlined body, and thus avoid a large heat load and problems associated with the mechanical integrity of the burner;

- geometric characteristics (half α angle and length L of the refractory embrasure) are important for controlling the size and shape of the recirculation zone together with the swirl coefficient. The length of the recirculation zone is approximately 2-2.5 times the length of the refractory embrasure;

- the optimal length L is of the order of L / D = 1 (D is the diameter of the neck of the refractory embrasure). The minimum length of the refractory embrasure should not be less than L / D = 0.5 and should not be more than L / D = 2;

- half α of the optimal angle of the refractory embrasure should be at least 20 and not more than 25 degrees;

- provides the possibility of a smaller vortex before there will be a decrease in stability, compared with a less limited front of propagation of the flame; and

- has the important task of regulating the size and shape of the recirculation zone, since the expansion of hot gases as a result of combustion provides a reduction in the transportation time of free radicals in the recirculation zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified sectional view schematically showing a burner in accordance with aspects of the invention enclosed in a housing without any details showing how the burner is configured inside said housing.

Figure 2 is a section of the burner, schematically showing a section above the axis of symmetry, while the rotation around the axis of symmetry provides the formation of a body of revolution that displays the layout of the burner.

Figure 3 shows a graph of the variation of the limits of stability of the flame depending on the coefficient of turbulence, the given degree of turbulence and the ratio of the components of the mixture.

Fig. 4a shows a diagram characterizing the aerodynamics in the near zone of the combustion chamber.

Fig.4b shows a diagram characterizing the aerodynamics in the near zone of the combustion chamber.

Figure 5 shows a graph of changes in the intensity of turbulence.

6 shows a graph of the relaxation time as a function of the combustion pressure.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a number of embodiments will be described in more detail with reference to the attached drawings.

Figure 1 shows a burner, while the burner 1 has a housing 2 surrounding the components of the burner.

Figure 2 shows for clarity the cross section of the burner above the axis of rotational symmetry. The main parts of the burner are a radial swirler 3, a multi-component refractory embrasure 4a, 4b, 4c and an auxiliary combustion chamber 5.

As stated, the burner 1 operates in accordance with the principle of “supplying” heat and free radicals in high concentration from the output part 6 of the auxiliary combustion chamber 5 to the main flame 7, burning in a vortex of a poorly pre-formed air-fuel mixture exiting from the first exit 8 of the first channel 10 for the preliminary formation of the lean mixture and from the second outlet 9 of the second channel 11 for the preliminary formation of the lean mixture, as a result of which the fast and stable burning of the main flame 7 of the lean redvaritelno formed mixture. The specified first channel 10 for the preliminary formation of a lean mixture is formed by the walls 4a and 4b and between the walls 4a and 4b of the multi-component refractory embrasure. The second channel 11 for the preliminary formation of the lean mixture is formed by the walls 4b and 4c and between the walls 4b and 4c of the multi-component refractory embrasure. The farthest from the center rotationally symmetric wall 4c of the multicomponent refractory embrasure is made with a protruding part 4c1 to ensure the optimal length of the multicomponent design of the refractory embrasure. The first 10 and second 11 channels for the preliminary formation of a lean mixture are equipped with swirl blades forming a swirl 3 to impart rotation of the air-fuel mixture passing through the channels.

Air 12 is supplied to the first 10 and second 11 channels at the inlet 13 of said first and second channels. In accordance with the shown embodiment, the swirl 3 is located next to the input 13 of the first and second channels. In addition, fuel 14 is introduced into the vortex of the air-fuel mixture through a pipe 15 made with small diffusion openings 15b, which is located at the air inlet 13 between the blades of the swirler 3, as a result of which the fuel is distributed in the form of an aerosol in the air stream through these holes and mixes well with airflow. Additional fuel may be added via a second pipe 16 extending into the first channel 10.

When the stream of the pre-formed lean air-fuel mixture is burned, the main flame 7 is formed. The flame 7 is formed in the form of a conical rotationally symmetrical boundary layer 18 around the main recirculation zone 20 (hereinafter the abbreviation RZ is sometimes used). The flame 7 is enclosed within the protruding portion 4c1 of the farthest section of the refractory embrasure from the center, in this example, the section 4c of the refractory embrasure.

The auxiliary combustion chamber 5 provides heat and the addition of free radicals in high concentration directly to the front braking point P and the boundary layer 18 of the main recirculation zone 20 formed by a vortex, where the main stream of lean pre-formed mixture is mixed with hot gaseous products of combustion formed during auxiliary combustion chamber 5.

The auxiliary combustion chamber 5 is made with walls 21 surrounding the combustion space for the auxiliary combustion zone 22. Air is supplied to the combustion space through the fuel channel 23 and the air channel 24. Around the walls 21 of the auxiliary combustion chamber 5 there is a distribution plate 25 made with holes on the surface of the plate. The specified distribution plate 25 is located at some distance from the specified walls 21, while forming a cooling spatial layer 25A. The cooling air 26 is drawn in through the cooling inlet 27 and collides with the outside of said distribution plate 25, after which the cooling air 26 is distributed along the walls 21 of the auxiliary combustion chamber to effectively cool said walls 21. The cooling air 26 is released through the second swirl 28 after said cooling located around the auxiliary refractory embrasure 29 of the auxiliary combustion chamber 5. Additional fuel can be added to the main flame 7 of the lean mixture during combustion by supplying fuel to a channel 30 located around and outside the cooling space layer 25a. The specified additional fuel is then released and passes into a second swirl 28, in which now the hot cooling air 26 and the fuel added through the channel 30 are effectively pre-mixed.

A relatively large amount of fuel can be added to the air, designed to cool a small auxiliary combustion chamber 5, which corresponds to the ratios of the components of very rich mixtures (Ф> 3). Swirling cooling air and fuel, and hot combustion products from a small auxiliary combustion chamber can very effectively support the combustion of the main flame 7 of the lean mixture with parameters that are lower, at or above the limit values at which the flame of the lean mixture breaks off. The combustion process is very stable and efficient, since hot combustion products and very hot cooling air (with temperatures above 750 ° C), pre-mixed with fuel, provide heat and active substances (radicals) to the front braking point P in the main recirculation zone 20 flame. During this combustion process, a small auxiliary combustion chamber 5, in combination with very hot cooling air (with temperatures above 750 ° C) pre-mixed with fuel, functions as a flameless burner in which reagents (oxygen and fuel) are pre-mixed with combustion products, and a distributed flame is generated at the front braking point P in the recirculation zone 20 formed by the vortex.

To ensure the proper functioning and stable operation of burner 1, disclosed in this application, it is required that the imparted degree of turbulence and the coefficient of turbulence be above critical (not less than 0.6 and not more than 0.8, see also figure 3), with of which a recirculation zone 20 with vortex decay will be formed and will stably stay within the multicomponent design of the refractory embrasure 4a, 4b, 4c. The front braking point P must be located within the refractory embrasure 4a, 4b, 4c and at the exit 6 of the auxiliary combustion chamber 5. Some of the main reasons for the need to comply with this requirement were mentioned above in the section "Summary of the invention." An additional reason is that:

If the swirl coefficient exceeds 0.8, the swirling flow will pass until it exits the combustion chamber, which may lead to overheating of the turbine guide vanes located further down.

Below are summarized requirements for the assigned degree of swirl and swirl coefficient. See also FIGS. 4a and 4b.

The attached degree of turbulence (the ratio between the tangential and axial moment) must be higher than critical (0.4-0.6) to ensure the formation of a stable central recirculation zone 20. The critical swirl coefficient S _N also depends on the geometry of the burner, which is the reason why it varies from 0.4 to 0.6. If the assigned swirl coefficient is ≤0.4 or is in the range of 0.4 to 0.6, the main recirculation zone 20 can be quenched at a low frequency (less than 150 Hz), and the resulting aerodynamics can be very unstable, which will lead to transient combustion.

In the boundary layer 18 of a stable and stable recirculation zone 20 with a strong velocity gradient and turbulence levels, flame stabilization can occur if:

turbulent flame propagation velocity (ST)> local air-fuel mixture velocity (UF / A).

The circulating products, which are: a source of heat and active substances (symbolically indicated by arrows 1a and 1b), located within the recirculation zone 20, must be stationary in space and time downstream relative to the mixing section in the burner 1 in order to allow incoming pyrolysis mixtures of fuel and air. If a stable combustion process is not predominant, thermoacoustic instabilities will occur.

Vortex stabilized flames have a length that is at most five times less than the length of the jet flame, and has limit values at which the flame of the lean mixture breaks off, which are characteristic of significantly poorer mixtures than with the jet flame.

A vortex during the combustion of a preformed mixture or turbulent diffusion combustion provides an effective means of pre-mixing fuel and air.

The involvement of the air-fuel mixture in the boundary layer of the recirculation zone 20 corresponds to the resistance of the recirculation zone, the swirl coefficient and the characteristic speed URZ in the recirculation zone corresponds to the resistance of the recirculation zone, the swirl coefficient and the characteristic speed URZ in the recirculation zone.

The characteristic speed URZ in the recirculation zone can be expressed as follows:

URZ = UF / A f (MR, dF / A, cent / dF / A, S _N ),

Where:

MR = rcent (UF / A, cent) 2 / rF / A (UF / A) 2

Experiments (Driscoll 1990, Whitelaw 1991) showed that

recirculation zone resistance = (MR) exp -1/2 (dF / A / dF / A, cent) (URZ / UF / A) (b / dF / A),

and

MR must be <1.

The value (dF / A / dF / A, cent) is important only for a turbulent diffusion flame.

The ratio of the size / length of the recirculation zones is “fixed” and corresponds to 2-2.5 dF / A.

No more than about 80% of the mass circulates in the opposite direction with an S _N value in excess of 0.8, regardless of how much the S _N value increases in the future.

The addition of diverging walls of the refractory embrasure downstream of the burner embrasure enhances recirculation (Batchelor 67, Hallet 87, Lauckel 70, Whitelow 90), and Lauckel 70 found that the optimal geometric parameters were: α = 20-25 °, L / dF / A, min = 1 and higher.

This suggests that dquarl / dF / A = 2–3, but flame stability suggests that the limit values characterizing the flameout of a lean mixture and characteristic of poorer mixtures have been reached for values close to 2 (Whitelaw 90).

Based on experiments and practical experience, it is also believed that the UF / A ratio should be more than 30-50 m / s for the flame of the preformed mixture due to the risks of a flashback (Proctor 85).

If the back-cutting operation of the end face is performed at the exit of the refractory embrasure, an external recirculation zone will be formed. The LERZ length of the outer recirculation zone is usually 2/3 hERZ.

Active substances - radicals

In vortex-stabilized combustion, the process is initiated and stabilized by transporting heat and free radicals 31 from the previously burned fuel and air back in the opposite direction to the flow to the flame propagation front 7. If the combustion process is a process of burning a very lean mixture, as is the case with systems of burning poorly partially formed mixtures, and as a result, the combustion temperature is low, then the equilibrium levels of free radicals are also very low. In addition, at high pressures in the engine there is a rapid relaxation of free radicals formed due to the combustion process, see Fig.6, to an equilibrium level that corresponds to the temperature of the combustion products. This is due to the fact that the rate of this relaxation of free radicals to an equilibrium level increases exponentially with increasing pressure, while, on the other hand, the equilibrium level of free radicals decreases exponentially with decreasing temperature. The higher the level of free radicals available to initiate combustion, the greater the tendency towards a faster and more stable combustion process. At higher pressures at which burners in modern gas turbine engines burn in a lean, partially pre-formed mixture, the relaxation time of free radicals can be short compared to the “transportation” time required for convective transfer of free radicals (symbolically indicated by arrows 31) further along the flow, from the place where they were formed in the boundary layer 18 of the main recirculation zone 20, in the opposite direction against the flow, to the propagation front 7 the flame and the front braking point P in the main recirculation zone 20. As a result, by the time the radical stream 31 circulating in the reverse direction within the main recirculation zone 20 will ensure that free radicals 31 move back to the flame propagation front 7 and when they begin to mix with the incoming “fresh” pre-formed poor air-fuel mixture from the first 10 and the second 11 channels at the front braking point P for initiating / maintaining the combustion process, free radicals 31 may already reach low equilibrium levels.

The present invention uses high nonequilibrium levels of free radicals 32 to stabilize the main combustion 7 of the lean mixture. In the present invention, the size of the small auxiliary combustion chamber 5 is kept small, and most of the fuel combustion occurs in the main combustion chamber of the lean preformed mixture (at the locations indicated 7 and 18), and not in the small auxiliary combustion chamber 5. The small combustion chamber 5 can be kept small, since free radicals 32 are released near the front braking point P in the main recirculation zone 20. This usually represents the most effective place for supplying additional heat and free radicals to the combustion process (7) stabilized by vortex. Since the exit 6 of the small auxiliary combustion chamber 5 is located at the front braking point P of the main lean recycle stream 20, the time interval between blocking and using free radicals 32 is very short and does not allow the free radicals 32 to relax to low equilibrium levels. The front braking point P in the main lean mixture recirculation zone 20 is maintained and aerodynamically stabilized in the refractory embrasure section (4a), at the exit 6 of the small auxiliary combustion chamber 5. To ensure that the distance and time from combustion of the poor, stoichiometric or rich mixture (zone 22) within the small auxiliary combustion chamber 5 is as short and straight as possible, the output of the small auxiliary combustion chamber 5 is located on the center line and at the neck 33 of the small auxiliary combustion chamber 5. On the center line at the throat 33 of the minor auxiliary combustion chamber 5 and within the refractory embrasure section 4a, free radicals 32 are mixed with the poor mixture combustion products 31 that are strongly preheated with a mixture of fuel and air from channel 30 and space 25a and subsequently with pre-mixed fuel 14 and air 12 in the boundary layer 18 of the main zone 20 of the recirculation of the lean mixture. This is very preferable for gas turbine engines with high pressure gas turbines, which by their nature are characterized by the most serious thermoacoustic instabilities. In addition, since the free radicals and heat generated by the small auxiliary combustion chamber 5 are used efficiently, its size may be small and a blocking process is not required. The ability to maintain the small size of the auxiliary combustion chamber 5 also has a positive effect on emissions of harmful substances.

BURNER GEOMETRY WITH MULTICOMPONENT DESIGN OF FIRE-RESISTANT AMBRAZURE

The burner uses aerodynamic flame stabilization and the flame stabilization zone is limited - the recirculation zone (5) in the multicomponent design (4a, 4b and 4c) of the refractory embrasure. The multicomponent design of the refractory embrasure is an important feature of the design of an open burner for the reasons listed below. Refractory embrasure (or sometimes called a diffuser):

provides a flame propagation front 7 (the main recirculation zone 20 is held without having to hold the flame on a solid surface / poorly streamlined body, and thus, a large heat load and problems associated with the mechanical integrity of the burner are avoided;

geometric characteristics (half α angle and length L of the refractory embrasure) are important for controlling the size and shape of the recirculation zone 20 together with the swirl coefficient. The length of the recirculation zone 20 is approximately 2-2.5 times the length L of the refractory embrasure;

the optimal length is about L / D = 1 (D is the diameter of the neck of the refractory embrasure). The minimum length of the refractory embrasure should not be less than 0.5 and not more than 2 (Ref. 1: The influence of Burner Geometry and Flow Rates on the Stability and Symmetry of Swirl-Stabilized Nonpremixed Flames; V. Milosavljevic et al .; Combustion and Flame 80, pages 196-208, 1990);

half α of the optimum angle (Ref. 1) of the refractory embrasure should be at least 20 and not more than 25 degrees;

provides the possibility of a lower turbulence coefficient before a decrease in stability occurs, compared with a less limited flame propagation front;

has the important task of regulating the size and shape of the recirculation zone, since expansion as a result of combustion reduces the transportation time of free radicals in the recirculation zone.

CHANGE BURNER SIZES

The refractory embrasure (or diffuser) and the attached swirl provide the ability to easily change the geometric characteristics of the open burner for different burner powers.

To reduce burner size (example):

channel 11 must be removed, and the casing forming section 4c of the refractory embrasure should thus replace the previously forming casing section 4b of the refractory embrasure that has been removed; the geometric characteristics of the refractory embrasure section 4c should be the same as the geometric characteristics of the pre-existing refractory embrasure section 4b;

the swirl coefficient in channel 10 should remain the same;

all other parts of the burner should be the same;

the separation of the fuel inside the burner should remain the same or similar.

To increase the size of the burner:

channels 10 and 11 should remain as they are;

refractory embrasure section 4c should be designed in the same way as refractory embrasure section 4b (formed as a thin dividing plate);

a new third channel (in this case fictitiously named 11b and not opened) should be located outside and around the second channel 11, and a new section 4d of the refractory embrasure (not shown in the drawings) should be located outside and around the second channel 11, as a result of which the outer wall of the third channel; the shape of the new refractory embrasure section 4d should be similar to the shape previously farthest from the center of the refractory embrasure section 4c;

the turbulence coefficients in the channels should be as follows: S _N , 10> S _N , 11> S _N , 11b, but all of them should exceed S _N = 0.6 and be no more than 0.8;

stay the same or similar.

STEADY BURNING OF FUEL AND BURNER OPERATION

The present invention also makes it possible to ignite the main combustion zone 7 at the front braking point P in the main recirculation zone 20. In most gas turbine engines, the outer recirculation zone, see fig. 4b, should be used as the place where the spark or flare ignition device ignites in the engine. Ignition can occur only if stable burning can also take place; otherwise, the flame will blow off immediately after ignition. The inner or main recirculation zone 22, as in the present invention, generally provides more successful flame stabilization, since the recirculated gas 31 moves backward and the heat from the combustion products in the recirculated gas 31 is concentrated in a small area at the front braking point P in main recycling zone 20. The combustion zone — the flame propagation front 7 also extends outward with a conical shape from a given braking front point P, as illustrated in FIG. This conical expansion upstream allows the heat and free radicals 32 formed upstream to maintain combustion upstream, which allows the flame propagation front 7 to expand as it moves further downstream. The refractory embrasure (4a, 4b, 4c) illustrated in FIG. 2, when compared with vortex-stabilized combustion without a refractory embrasure, shows how the refractory embrasure gives the flame a more conical shape and a shape less resembling a hemisphere in nature. The flame propagation front, which is more conical in shape, allows a point heat source to effectively initiate the combustion of the entire flow field.

In the present invention, the combustion process in the burner 1 is divided into stages (stages). In the first stage, that is, the ignition stage, the lean mixture flame 35 is initiated in the small auxiliary combustion chamber 5 by adding fuel 23 mixed with air 24 and igniting the mixture by using an ignition device 34. After ignition, the ratio of the components of the flame 35 in the small auxiliary chamber 5 combustion is regulated for combustion conditions or lean mixture (with a ratio of components of less than 1, and with a ratio of components of approximately 0.8), or rich mixture (with a ratio components in excess of 1, and with a component ratio of approximately 1.4 to 1.6). The reason that the ratio of the components in the small auxiliary combustion chamber 5 during the rich mixture combustion mode is in the range from 1.4 to 1.6 is the emission levels of harmful substances. There is the possibility of operating and maintaining the flame 35 in a small auxiliary combustion chamber 5 under stoichiometric conditions (with a ratio of components equal to 1), but this option is not recommended, since it can lead to high levels of harmful substances and a greater thermal load on the walls 21. Advantage work and is recommended, since it can lead to high levels of harmful substances and a greater thermal load on the walls 21. The advantage of working and maintaining the flame 35 in a small auxiliary Leray combustion when combustion mode or the lean or rich mixture is generated that pollutant emissions are minor and the thermal load on the wall 21 is low.

In the next stage, the second stage with a low load, the fuel is added through the channel 30 to the cooling air 27, and it is informed of the swirling motion in the swirl 28. In this way, the main flame 7 of the lean mixture is very efficiently burned at parameters lower than and above the limit values at which the flame breaks out of the lean mixture. The amount of fuel that can be added to the hot cooling air (preheated to temperatures well above 750 ° C) may correspond to component ratios greater than 3.

In the next stage of the burner operation, the third fuel part 15a, corresponding to the full-load stage, is gradually added to the air 12, which is the main air stream entering the main flame 7.

Claims (3)

1. The method of step-by-step operation when starting the burner for the combustion chamber of a gas turbine engine, comprising a burner housing (2) surrounding the burner (1) having axially opposite, upstream and downstream end parts and an auxiliary combustion chamber (5), located on the upstream end of said burner (1);
wherein said auxiliary combustion chamber (5) is supplied with fuel and air for burning said fuel to form a stream of unblocked concentration of radicals (32) at an nonequilibrium level and heat from the auxiliary combustion zone (22) directed downstream along the axial line of the auxiliary chamber (5) ) combustion through the neck at the outlet (6) of the auxiliary combustion chamber (5); wherein said burner (1) is additionally provided with a main combustion space bounded by the walls of the refractory embrasure (4a, 4b, 4c) downstream of the outlet (6) of said auxiliary combustion chamber (5) to hold the main flame (7) and the location of the main zone (20) of recirculation for circulating unburned radicals (31); wherein the method includes the following steps:
- adding fuel (23) mixed with air (24) to the auxiliary combustion chamber (5);
- igniting the mixture by using an ignition device (34) provided at the upstream end of the auxiliary combustion chamber (5) to initiate a flame (35) of the lean mixture inside the auxiliary combustion chamber (5) and to supply said stream of said radicals (32) and heat to said downstream combustion space;
- supplying the first vortex of mixed fuel and air (27) from the outside at the outlet (6) of the auxiliary combustion chamber (5) at the upstream end of the specified combustion space to create and maintain the main flame (7) of the lean mixture in the specified main space for combustion;
- the formation of a second vortex of mixed air (12) and fuel (14) from the outlet (8) of the first channel (10) for the preliminary formation of a lean mixture downstream relative to the specified exit (6) of the auxiliary combustion chamber, and from the exit (9) of the second channel (11) for pre-forming the lean mixture downstream of the specified output (8) of the first channel for pre-forming the lean mixture,
- the gradual addition of the specified second vortex of mixed air (12) and fuel (14) to provide a stage with a full load in at least one of the first (10) and second (11) channels for supplying the main flow of air (12) and fuel ( 14) into the indicated main flame (7) of the lean mixture. 2. The method according to claim 1, further comprising the following steps:
- regulation - after ignition - the ratio of the components of the flame (35) in the auxiliary combustion chamber so that it was:
a) or a lean mixture flame with a component ratio of less than 1, and preferably with a component ratio of approximately 0.8;
b) or a flame of a rich mixture with a ratio of components greater than 1, and preferably with a ratio of components of 1.4 to 1.6. 3. The method according to claim 2, further comprising the following steps:
- supply of cooling air (26) for cooling the walls (21) of the auxiliary combustion chamber (5);
- preheating said cooling air (26) to a temperature above 750 ° C by said cooling of said walls (21);
adding air to said cooling air to an amount that corresponds to component ratios greater than 3.

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