GB2327120A - Atomising nozzle for premix burners - Google Patents

Abstract

An atomizing nozzle (1) for atomizing fuel comprises a nozzle tube (3), the inlet area thereof being separated into two concentric annular channels (19, 20), the fuel provided by a supply device (8) being atomized and evaporated in the inner first annular channel (19) by means of untwisted fast-flowing air, and the incoming untwisted air being twisted by means of twisting elements (21). The fuel-air mixture (16) and the twisted air are mixed to a homogeneous mixture in the nozzle chamber (18) of the nozzle tube (3), which chamber is adjacent to the exit openings of the two annular channels (19, 20), the mixture then being led into a combustion chamber (5) via a chamfered premixer nozzle 3.

Description

2327120 Atomizing nozzle for atomizing fuel in burners The invention

relates to an atomizing nozzle for the lean pre-evaporated, premixed combustion of liquid fuels, especially in gas turbines.

The lean pre-evaporated, premixed combustion in gas turbines allows for a reduction of nitro gen oxide emissions to a large extent. In this combustion concept, the fuel is evaporated in the nozzle or a preevaporator stage and mixed with air before the mixture is burned in the combustion chamber. The expressions of "pre-evaporation" and "premixing" mean that evaporation and mixing take place prior to the combustion zone. In a complete evaporation and mixing process, the combustion occurs at lean mixture ratios, i. e., at an excess of air and low temperatures leading to low nitrogen oxide emissions. An incomplete combustion of the fuel leads to a combustion in an atomization flame around the drop of fuel. In this process, the combustion stabilizes in nearly stoichiometric areas, i. e., when the contents of oxygen and fuel are approximately equal. This results in a locally high combustion temperature with high nitrogen emissions.

Nozzles for the lean pre-evaporated and premixed combustion are known, for example, from DE 37 29 861. The liquid fuel is usually injected into the nozzle or atomized from a tear-off edge by air streams. In this primary atomization, a spray is formed in which the fuel is present in the air both in liquid form (drops) and in gaseous form. In the case of the known nozzles, the fuel is already atomized by twisted air. The air streams oppositely twisted in most cases are supposed to atomize the fuel and simultaneously homogenize it by means of high turbulences in the shearing layer of the two air streams.

A fluther such nozzle is known from EP 0 660 038, from which the preamble of claim 1 departs.

2 The invention is based on the objective to improve the lean preevaporated, premixed combustion in gas turbines such that nitrogen emissions are reduced.

According to the invention, this objective is achieved by the features of claim 1.

The atomizing nozzle according to the invention has a pressure chamber with air flowing therefrom. A first annular channel branches off from the pressure chamber, with fuel being introduced into the inlet thereof from a supply device, the fuel being atomized to a spray. The supply device can be arranged in or in front of the inlet of the first annular channel. The first annular channel is surrounded by a second annular channel into which air flows and which contains twisting elements for twisting the air. The outlets of the two annular channels open into a nozzle tube chamfering in the direction of flow. The fuel introduced is post-atomized in the first annular channel by a weakly turbulent untwisted atomizing air stream. In this secondary atomization process, almost all the fuel drops are atomized or evaporated, as a high relative speed between air and large drops can be maintained, due to the untwisted atomizing air stream, for a period long enough to atomize the large drops into small drops, which are then evaporated completely during their stay in the nozzle. The factors determining the degree of evaporation to be obtained are the distribution of drop size and the time of residence in the nozzle, which is limited by the time of spontaneous ignition of the fuel in the air at the given pressure and temperature conditions. In the case of small drops, a nearly complete evaporation takes place, which leads to a smaller nitrogen oxide emission in the subsequent combustion. At the outlets of the two annular channels, the untwisted fuel-air mixture from the first annular channel and the twisted mixed air stream from the second annular channel meet. Within the nozzle tube, the two air streams are mixed so that a homogeneous lean fuel-air mixture is formed. The mixture is homogenized in the nozzle by the turbulence in the shearing layer between untwisted atomizing air and twisted mixed air.

In the atomizing nozzle according to the invention, atomization and preevaporation, on the one hand, and premixing, on the other hand, occur in separate zones. Pre-evaporation occurs by means of an untwisted air stream of high speed within the first annular channel, while the subsequent premixing process occurs by means of a twisted air stream from the second annular channel in the nozzle tube. Thus, a homogeneous fuel- air mixture with completely evapo- 3 rated fuel is obtained and is combusted in a combustion chamber adjacent the atomizing nozzle with a low nitrogen oxide emission.

Further advantageous embodiments of the invention result from the subclaims and the drawing.

There now follows a more detailed description of an exemplary embodiment of the invention with regard to the drawings.

Fig. 1 shows the atomizing nozzle in a longitudinal section, and Fig. 2 shows a separation body with twisting elements.

The atomizing nozzle 1 shown in Fig. 1 is connected by means of an air inlet 2 to a pressure source, not represented, such as a compressor, which provides heated air at a temperature of about 350 to 700 'C at a speed of 80 to 130 nVsec. The air streams into the atomizing nozzle at a pressure of 3 to over 50 bars. The air inlet 2 is formed annularly, with the cross section chamfering downstream, i. e. away from the pressure source, to speed up the air further. Connected to the air inlet 2 is a nozzle tube 3 the diameter of which continuously chamfers in the course of the stream. A diffusor 4 is connected to the downstream end of the nozzle tube 3, the cross section of the diffusor enlarging in the direction of the stream to slow down the air stream and to produce swirls for the combustion chamber 5 adjacent to the diffusor 4. The transitions between the air inlet 2, the nozzle tube 3 and the diffusor 4 are continuous, i. e., no edges obstructing the flow are present.

An axially arranged interior body 6 guiding the air stream in the interior area of the air inlet 2 and the nozzle tube 3 extends in the air inlet 2 and the nozzle tube 3. A supply device 8 for the liquid fuel is provided in the annular space 7 between the interior wall of the air inlet 2 and the interior body 6. The supply device 8 consists of an annular body 9 surrounding the interior body 6 and being fixed on the air inlet 2 by means of supporting stays 10. The supporting stays 10 also support the interior body 6.

In at least one of the supporting stays 10, a fuel conduit 11 extends as far as into the annular body 9. There an annular channel 12 circumferential within the annular body 9 is connected 4 thereto receiving the fuel. Downstream, the annular body 9 terminates in a tongue 13 ending in a tear-off edge 14. The tear-off edge 14 is located in the transitional area between the air inlet 2 and the nozzle tube 3. The annular fuel channel 12 continues downstream within the annular body 9 and comprises an annular opening 15 at the beginning of the tongue 13, the fuel exiting from the opening and covering the tongue with a film. The annular opening 15 is located on the outside of the annular body 9. The interior space 7 of the air inlet 2 has its smallest cross section in the area of the tongue 13 covered by the fuel film. In this area, the interior body 6 has its largest diameter, the width of the passage area for the air being approximately halved in comparison to the inlet area of the air inlet 2. The incoming air thus has an increased speed in this place. The incoming untwisted air stream flows around the annular body 9 and the tongue 13 on the inside and the outside and drives the fuel on the outside of the tongue 13 towards the tear-off edge 14. The air stream releases the fuel on the tear-off edge 14, producing a spray 16 of air and fuel drops.

In the inlet area of the nozzle tube 3 an annular separation body 17 is located separating the nozzle space 18 between the interior body 6 and the nozzle tube 3 into an interior first annular channel 19 and an exterior second annular channel 20. The first annular channel 19 is limited by the interior body 6 and the separation body 17. The second annular channel 20 arranged concentrically to the first annular channel 19 is limited by the separation body 17 and the nozzle tube 3. At their upstream ends, the two annular channels 19, 20 are in contact with the air inlet 2. At their downstream ends, the two annular channels 19, 20 are in contact with the nozzle chamber 18.

The tear-off edge 14 is arranged in the upstream entrance area of the first annular channel 19. The spray 16 forming behind the tear-off edge 14 is moved through the first annular channel 19 by means of the air stream. In the course of the fust annular channel 19, the spray mixture 16 spreads out towards the walls. The height of the first annular channel 19 is designed such that no fuel drops cover the walls. Covering interior parts of the atomizing nozzle 1 with fuel would lead to the fuel remaining in the atomizing nozzle 1 for too long a time and igniting while still within the atomizing nozzle 1 due to the high temperature and pressure of the entering air, and not igniting only in the combustion chamber 5.

Releasing the fuel film on the tear-off edge 14 is called primary atomization. This leads to more or less large drops of fuel. In the first annular channel 19, the so-called secondary at- omization of the fuel takes place. Due to the high relative speed between entering untwisted air and the drops of fuel, the drops of fuel are atomized in the first annular channel 19 to increasingly smaller drops of fuel. The air flowing past the two sides of the tear-off edge 14 encloses the fuel-air mixture 16 forming behind the tear-off edge 14. This reduces the danger of fuel precipitation on the walls of the first annular channel 19. The smaller drops of fuel evaporate due to the air temperature, which is why the first annular channel 19 is also called pre- evaporation zone W. The length of the first annular channel 19 is dimensioned such that as many drops of fuel evaporate as possible, which can be obtained by means of a long time of residence of the fuel, the time of residence, however, not being as long as to cause a spontaneous combustion of the fuel within the atomizing nozzle 1 - In the second annular channel 20, there are twisting elements 21 setting the air flowing axially into the second annular channel 20 into rotation, i. e., giving a circumferential component to the air flow. Fig. 2 shows a top plan view of a detail of the separation body 17. The twisting elements 21 are formed as bent guiding plates, the degree of redirection determining the amount of twisting of the air. The twisted air enters the nozzle chamber 18 at the downstream end of the second annular channel 20, where it meets the untwisted flowing fuel-air mixture 16. In the nozzle chamber 18 forming the premixing zone MZ, the fuel-air mixture 16 and the twisted air mix to form a homogeneous mixture, with an air ratio of approximately 2 desired for the lean combustion being obtained according to the design of the gas turbine. An "air ratio of T' means that twice the amount of air is present as in the case of a stoichiometric combustion. By the continuous reduction of the cross-sectional area of the nozzle chamber 18, the fuel-air mixture is continuously accelerated so that no burble and no backflows into the nozzle chamber 18 are possible.

At the downstream end of the nozzle tube 3, the interior body 6 terminates in a tip. This tip comprises an air outlet 22 in connection with an inlet channel 23 and an interior hollow chamber 24 of the interior body 6. Air enters into the inlet channel 23 in the area of the supply device 8 and is guided into the hollow chamber 24. There the air is accelerated due to the reduction of the cross section of the hollow chamber 24 so that the speed thereof is adapted to the air flowing around the interior body 6. At the end of the hollow space 24, the air exits out of the air outlet 22 and prevents a burble and a swirl of the flow behind the tip of the interior body 6. The interior body 6, the interior profile of the nozzle tube 3 and the twisting elements 6 21 are designed such that the fuel-air mixture 16 does neither get onto the interior body 16 nor onto the nozzle tube 3 when the air stream is appropriate.

In the diffusor 4, the flowing fuel-air mixture is expanded and swirled into the combustion chamber 5, where it is ignited.

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Claims (1)

1. Claims

   1. Atomizing nozzle (1) for atomizing fuel in burners, especially for gas turbines, having a pressure chamber, a first annular channel (19) branching off from the pressure chamber and comprising an inlet on one end and an outlet on the other end, a supply device (8) introducing fuel into the first annular channel (19), a second annular channel (20) surrounding the first annular channel (19) and comprising a pressurized air inlet and twisting elements (2 1), the outlet thereof surrounding the one of the first annular channel (19), and a nozzle tube (3) adjacent to the outlet of the second annular channel (20) and chamfering in the direction of flow and forming a premixer (18), characterized in that the supply device (8) is arranged in the area of the inlet of the first annular channel, the first annular channel (19) forming a pre-evaporator in which essentially all the fuel supplied is evaporated.

   2. Atomizing nozzle according to claim 1, characterized in that the first annular channel (19) surrounds an axial interior body (6) extending as far as into the nozzle tube (3).

   Atomizing nozzle according to claim 2, characterized in that the interior body (6) comprises, at the downstream end thereof, at least one air opening (22) preventing the fuel-air mixture from tearing off.

   4. Atomizing nozzle according to any one of claims 1-3, characterized in that the supply device (8) comprises an annular body (9) around which untwisted air flows and which merges at the downstream end into an annular tongue (13) with a tear-off edge (14), at least one exit opening (15) for the fuel being provided in a circumferentially distributed manner on the annular body (9) so that the air spreads the fuel on the tongue (13) and releases it on the tear-off edge (14).

   5. Atomizing nozzle according to any one of claims 1-4, characterized in that the incoming air is preheated to a temperature of 350 - 700 T.

   8 6. Atomizing nozzle according to any one of claims 1-5, characterized in that the speed of the incoming air amounts to 80 - 130 m/sec.

   7. Atomizing nozzle according to any one of claims 1-6, characterized in that the air flows in at a pressure from 3 to over 50 bars.

   8. Atomizing nozzle according to any one of claims 1-7, characterized in that the air inlets and the supply device (8) are formed such that the fuel-air mixture has an air ratio of about 2.