Patent Application
20130283810
The invention relates generally to combustors, and more specifically, to a combustion nozzle for injecting an air-fuel mixture into a combustion zone within the combustor.
A combustor is a component or area of an engine where combustion takes place. In a gas turbine engine, for example, a compressor feeds high pressure air to the combustor or combustion chamber. The combustor then heats the air along with a fuel at a constant pressure. After combustion, the generated exhaust gases are fed from the combustor to the turbine via the nozzle guide vanes. Such an engine employed in a gas turbine plant or a combined cycle plant, for example, is operated to achieve higher operational efficiency under higher temperature and higher pressure conditions, and tends to increase emissions (for example, NOx) in an exhaust gas stream. Although various factors for generation of NOx are known, the dominant one is flame temperature in a combustor. NOx emissions are proportional to the flame temperature in a combustor.
Increasing turbulence intensities within the combustion zone reduces NOx emissions. NOx is reduced at higher turbulence levels due to the enhanced mixing of products in the flame zone, effectively lowering the local flame temperature in the combustion zone. One way of increasing turbulence is to increase the velocity of the jet. In such a scenario, the levels of turbulence are directly correlated to the jet velocity.
In accordance with one embodiment of the present invention, an exemplary combustion nozzle is disclosed. The combustion nozzle includes at least one passage having a mixing section and an exit section. The mixing section includes an air inlet, and a fuel inlet. The mixing section has a first length and a first diameter. The exit section has a second length different from the first length, and a second diameter different from the first diameter.
In accordance with another exemplary embodiment of the present invention, a gas turbine is disclosed. The gas turbine includes a compressor, an combustor having an exemplary combustion nozzle and a turbine.
In accordance with another exemplary embodiment of the present invention, a method includes mixing air and a fuel within a mixing section of a combustion nozzle to generate an air-fuel mixture. The mixing section has a first length and a first diameter. The method further includes injecting the air-fuel mixture via an exit section of the combustion nozzle to a combustion zone disposed downstream of the combustion nozzle. The exit section has a second length different from the first length, and a second diameter different from the first diameter. The method also includes generating a turbulent flow of the air-fuel mixture in the combustion zone.
In accordance with another exemplary embodiment of the present invention, a method includes forming at least one passage in a combustion nozzle. one passage having a mixing section and an exit section. The mixing section includes an air inlet, and a fuel inlet. The mixing section has a first length and a first diameter. The exit section has a second length different from the first length, and a second diameter different from the first diameter.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
In accordance with the embodiments of the present invention, a combustion nozzle is disclosed. The exemplary combustion nozzle includes at least one passage including a mixing section and an exit section. The mixing section has a first length and a first diameter and the exit section has a second length different from the first length, and a second diameter different from the first diameter. Air and fuel are mixed in the mixing section and the exit section generates turbulent flow of the air-fuel mixture in a combustion zone of a combustor. In other words, the degree of fuel/air mixedness in the mixing section is decoupled from the turbulence intensity levels entering the combustion zone of the combustor. Such generation of turbulence level is useful for reducing NOx emissions, especially at higher flame temperatures in the combustor. Turbulence level for reducing NOx, can be controlled based on length to diameter ratio of the nozzle exit section. Additionally, diameter and length of the mixing section may be independently designed for optimal mixing of air/fuel for further NOx reduction compared to a scenario where air and fuel are unmixed.
Turning now to drawings and referring first to
The combustor 12 includes a combustion nozzle for receiving compressed air 11 and the fuel stream, mix the air 11 and the fuel stream to generate an air-fuel mixture, and inject the air-fuel mixture to a combustion zone. The combustion nozzle is explained in greater detail with reference to subsequent figures.
Referring to
A combustion nozzle 28 is disposed within the combustion chamber 24. The exemplary combustion nozzle 28 includes at least one passage (tube) 30 having a mixing section 32 and an exit section 34. In the illustrated embodiment, four passages 30 are shown. In other embodiments, the number of passages 30 varies depending on the application. The mixing section 32 has a first length and a first diameter, and the exit section 34 has a second length different from the first length, and a second diameter different from the first diameter. The combustor 12 further includes a fuel plenum 36 for supplying a fuel to the combustion chamber 24. The fuel enters the combustion nozzle 28 via a plurality of fuel inlet holes 38 provided in the mixing section 32 of each passage 30. The passage 30 also has an air inlet 40 for receiving the air stream 11 from the compressor.
In certain embodiments, the fuel may include hydrocarbons, natural gas, or high hydrogen gas, or hydrogen, or biogas, or carbon monoxide, or syngas, or inert gas, or water vapor, or oxidizers along with predetermined amount of diluents. Diluents may include nitrogen, carbon dioxide, water, steam, or the like.
The mixing section 32 of the combustion nozzle 28 is configured to mix air 11 and the fuel and generate an air-fuel mixture. The exit section 34 is configured to receive the air-fuel mixture and inject the air-fuel mixture to a combustion zone 42 of the combustion chamber 24. The exit section 34 is configured to generate turbulent flow of the air-fuel mixture in the combustion zone 42 of the combustor 12. The dimensions of the exit section 34 is different from that of the mixing section 32 such that fuel/air mixing in the mixing section 32 is decoupled from the turbulence generated in the combustion zone 42 of the combustor 12. Such generation of turbulence level is useful for reducing NOx emissions, especially at higher flame temperatures in the combustor 12.
As discussed previously, increasing turbulence intensities within the combustion zone reduces NOx emissions. NOx is reduced at higher turbulence levels due to the enhanced mixing of combustion products and fresh reactants in the flame zone, effectively lowering the local flame temperature in the combustion zone. Such a phenomenon is effectively a combustion exhaust gas recirculation (EGR) at micro (very small) scales. One way of increasing turbulence is to increase the velocity of the jet. In such a scenario, the levels of turbulence are directly correlated to the jet velocity.
In accordance with the embodiments of the present invention, the generation of the turbulence via the exit section 34 allows turbulence intensification and mixing enhancements to be carried out in parallel. Specifically, a modified length to diameter ratio of the exit section allows the shear rates of the air-fuel mixture jet entering the combustion zone 42 to be controlled. For example, lower length to diameter ratio are experimentally observed to yield lower NOx emissions through enhanced shearing introduced into the combustion zone 42. By decoupling turbulence from reactant mixedness levels, NOx emissions reduction can be obtained.
Referring to
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where μ is viscosity of flow, u is the fluid velocity, du/dy is representative of a velocity gradient. Curve 44 shows du/dy is larger at a wall surface of the exit section 34 and is representative of a higher shear rate. The flame in the combustion zone stabilizes in the shear layer as the jets expand into the combustor. Increased shear rates from higher turbulence amplifies the mixing of exhaust gases in the flame zone contributing to NOx reductions.
A velocity distribution of the air-fuel mixture at a downstream location of the exit section 34 is represented by a curve 46. Curve 46 shows du/dy is relatively lower at a wall surface of the exit section 34 and is representative of a relatively lower shear rate. As noted above, a modified length to diameter ratio of the exit section 34 allows the shear rates of the air-fuel mixture jet entering the combustion zone to be controlled. Lower length to diameter ratio yield lower NOx emissions through enhanced shearing introduced into the combustion zone 42.
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In certain embodiments, all permutations and combinations discussed of features discussed with reference to
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
