The present invention relates generally to gas turbine combustors and more specifically to a fuel and air mixing device in a gas turbine combustor.
In a gas turbine engine, the combustion section contains a reaction that occurs when fuel and compressed air are mixed together and react after being ignited by an ignition source. Compressed air is directed to one or more combustion chambers from the engine compressor. Fuel injection devices inject a fuel, either liquid or gas, into the compressed air stream and the mixture undergoes a chemical reaction once being exposed to a heat source, such as an igniter.
Some examples of prior art mixer devices are shown in
In order to control emissions levels of oxides of nitrogen (NOx) and carbon monoxide (CO), it is critical that the fuel molecules burn as completely as possible such as to not leave any unburned hydrocarbons to pass into the atmosphere. In order for the fuel to completely burn a number of issues must be addressed, one of which is fuel and air mixedness prior to ignition. Fuel and air mixedness is controlled by factors such as swirl, fuel injection location, and mixing time prior to ignition. Therefore, for the lowest possible emissions, it is most desirable to provide a mixer for a gas turbine combustor that optimizes swirl, fuel injection location, and mixing time such that the combustion process will be as complete as possible.
The present invention provides a mixer for a gas turbine combustor wherein the mixer comprises a plurality of annular walls containing at least a plurality of first vanes oriented at a first angle in between said annular walls, thereby creating a shear layer. A fuel injector is positioned adjacent the vanes to inject a fuel such that the fuel jet penetrates the shear layer for optimum mixing. Furthermore, the annular walls of the mixer are configured such that sufficient time and distance is provided in order to obtain optimum mixing prior to ignition of the fuel/air mixture.
The preferred embodiment of the present invention will now be described in detail with particular reference made to
Mixer 75, which serves to provide a region for thorough fuel and air mixing prior to ignition, comprises multiple components depending on the desired level of fuel and air mixedness. For a complete understanding of the invention, all components of mixer 75 are shown in
An additional feature of mixer 75 is fuel injector 85, which is located adjacent second generally annular wall 77 for injecting a fuel into the shear layer formed adjacent first generally annular wall 76. In the preferred embodiment of the present invention, fuel injector 85 comprises an annular manifold 86 having a plurality of injection locations 87 around annular manifold 86. Furthermore, injection locations 87 are oriented generally perpendicular to center axis A-A.
As a result of the radial and axial positions of the generally annular walls 76, 77, and 80 as well as position of combustion liner 73, a mixing passage 88 is created. Mixing passage 88 is formed between second generally annular wall 77 and combustion liner 73 and serves as a region of extended length for mixing fuel and air, due to bend 78 in second portion 77B of second generally annular wall 77.
An additional feature of mixer 75 is its ability to compensate for thermal expansion of combustion liner 73. Combustion liner 73 contains a spring seal 89 that is fixed to the outer surface of combustion liner 73 at a first seal end and is free at a second seal end. The third generally annular wall 80 of mixer 75 engages spring seal 89 proximate its second seal end to provide a means for maintaining the dimensions of mixing passage 88 that is compliant to various thermal changes between combustion liner 73 and mixer 75.
In operation, having provided the aforementioned combustor and mixer geometry, a flow of air is provided to mixer 75. The airflow is then split with a first portion of air being directed through first vanes 81 and a second portion being directed through second vanes 82. The airflow portions are swirlered at their respective angles by their respective vanes and form a shear layer, or more specifically, a layer of air in between two rotating flows of different degrees. This shear layer has a thickness, which is attributed to the thickness of first generally annular wall 76 directly upstream of the shear layer. Fuel is then injected into the shear layer to form a premixture in mixing passage 88. The premixture is directed through bend 78 and into the combustor for ignition.
As a result of the swirl vane configuration and orientation, the fuel injection from a manifold configuration into the shear layer, coupled with the mixing passage distance and time, computational analysis has predicted an extremely high rate of mixedness prior to ignition. A plot of this analysis can be seen in
In a first alternate embodiment of the present invention, mixer 85 contains only plurality of first vanes 81 between first generally annular wall 76 and second generally annular wall 77. In this embodiment, the shear layer is formed between the angle of first vanes 81 and the flow passing through a passageway formed by first generally annular wall 76 and combustion liner 73. While this configuration is simpler to manufacturer and can be manufactured at a lower cost due to the simplified geometry, the mixing benefits associated with the shear layer are not as great given the limited shear generated by the interaction from a single set of vanes and an axial flow. This first alternate embodiment is advantageous if radial space for mixing is limited or sufficient mixing can be achieved with a single set of vanes.
A second alternate embodiment maintains the benefits of the preferred embodiment with respect to the shear generated by opposing flow angles from the plurality of first and second vanes, but eliminates seal 89. Removing seal 89 from the mixer geometry simplifies the manufacturing and reduces the associated cost by eliminating a component that is manufactured from a higher cost alloy having spring capability. However, while removing seal 89 simplifies the manufacturing process and can reduce cost, it does allow for additional thermal movement between combustion liner 73 and mixer 75 than if seal 89 were present, thereby affecting dimensions of mixing passage 88. Depending on the operating conditions and temperatures of combustor 70, eliminating seal 89 may not have adverse affects on fuel and air mixing and combustor performance.
While the invention has been described in what is known as presently the preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, is intended to cover various modifications and equivalent arrangements within the scope of the following claims.