Abstract:
A premixer for an industrial type gas turbine engine wherein the premixer includes a diffuser ring assembly made up of annular concentric rings and upstream of the diffuser ring assembly in the airflow path is a corresponding fuel manifold ring assembly, each ring in the manifold ring assembly corresponding to a passageway formed between the diffuser rings, and each manifold ring includes a downstream channel for feeding the fuel to the air as the air passes by the ring.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gas turbine engines, and more particularly, to an air/fuel mixer for a combustor. The type of gas turbine engine may be used in power plant applications. 
     2. Description of the Prior Art 
     Low NO x  emissions from a turbine engine, of below 10 volume parts per million (ppmv), are becoming important criteria in the selection of turbine engines for power plant applications. The current technology for achieving low NO x  emissions may involve a combination of a catalytic combustor with a fuel/air premixer. This technology is known as Dry-Low-Emissions (DLE) and offers a prospect for clean emissions combined with high engine efficiency. The technology relies on a higher air content in the fuel/air mixture. 
     However, flame stability decreases rapidly at these lean combustion conditions, and the combustor may be operating close to its blow-out limit. In addition, severe constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems, and richer than average pockets will lead to unacceptable high NO x  emissions. The emission of carbon monoxide as a tracer for combustion efficiency will increase at leaner mixtures for a given combustor due to the exponential decrease in chemical reaction kinetics. Engine reliability and durability are of major concern at lean combustion conditions due to high pressure fluctuations enforced by flame instabilities in the combustor. 
     In a DLE system, fuel and air are premixed prior to injection into the combustor, without diluant additions, aligned for significantly lower combustion temperatures, therefore minimizing the amount of nitrogen oxide formation. However, two problems have been observed. The first is the stability or engine operability which provides decreasing combustion efficiency and, therefore, high carbon monoxide emissions. The stability of the combustion process rapidly decreases at lean conditions because of the exponential temperature dependence of chemical reactions. This can lead to flame-out and local combustion instabilities which change the dynamic behavior of the combustion process, and endangers the mechanical integrity of the entire gas turbine engine. At the same time, a substantial increase in carbon monoxide and unburned hydrocarbon (UHC) emissions is observed, and a loss in engine efficiency can be found under these circumstances. 
     It has been found that a key requirement of a successful DLE catalytic combustion system is the reaction of a perfectly mixed gaseous fuel and air mixture that is less than 1% variation in mixture fraction. Constraints on the system include less than a 1% pressure drop across the mixer. It is also important to develop a flow which will not generate flash-back or auto-ignition of the combustible fuel/air mixture. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to provide a diffusion gas mixture which is capable of providing a fuel/air mixture with less than 1% variation. 
     It is a further aim of the present invention to provide a gas mixer that has a pressure drop of below 1% while reducing the risk of auto-ignition and flash-back. 
     A construction in accordance with the present invention comprises a fuel/air premixer for a gas turbine combustor, wherein the premixer comprises an annular diffuser assembly placed in the airflow path, upstream of an inlet to the combustor, the diffuser assembly having an upstream section and a downstream section relative to the airflow path and including a plurality of concentric rings wherein a diffuser passageway is formed between each adjacent ring in a pair of rings; the passageway so formed including a converging cross-sectional portion at the upstream section of the ring assembly and a diverging cross-sectional portion at the downstream section of the ring assembly, and a gap is defined at the narrowest part of the passageway formed by the adjacent rings; and an assembly of concentric fuel manifold rings is provided upstream of the diffuser assembly whereby each manifold ring is located in axial alignment with a corresponding diffuser passageway whereby the air flows around the manifold rings and through the diffuser passageways, and fuel is delivered from the manifold rings into the airflow. 
     In a more specific embodiment of the present invention, the gap defined between the diffuser rings is determined by the formula M=ACd{square root over (2+L ρΔP)}, where M is the mass flow, ACd is the effective flow area, ρ is density of the air, and ΔP is the pressure drop. 
     A feature resulting from the present invention is that the fuel is drawn into the airflow since the fuel is fed at very low pressures. Thus, the fuel is not being mixed into the airflow as in typical fuel nozzles where the fuel is fed under high pressure and relies on the fuel momentum for mixing, but instead it is the flow of the air around the manifold rings which draws the fuel and the air is mixed into the fuel. This method is very effective since more than 95% of the fluid is air and it is the air that is doing the work. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration, a preferred embodiment thereof, and in which: 
     FIG. 1 is a schematic axial cross-section of a combustor system in accordance with the present invention; 
     FIG. 2 is an enlarged fragmentary axial cross-section of a portion of the premixer portion; and 
     FIG. 3 is an end elevation taken upstream of another embodiment of the premixer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and particularly to FIG. 1, the combustor system  10  is shown with a combustion chamber  12  within an engine casing  14 . The compressed air flow, in the present embodiment, moves from right to left in FIG. 1 in the direction of the inlet  13  of the combustion chamber  12 . A fuel/air premixer  16  is provided within the housing  18 , defining the passageway of the airflow. A plurality of premixers may be provided for a single combustion chamber  12  with a premixer corresponding to each inlet  13  of the combustion chamber  12 . FIGS. 1 and 2 show in detail the structure of the premixer  16 . 
     The premixer  16  includes a diffuser ring assembly  20  made up of concentric diffuser rings which are identical in cross-section. In the present case, there are four diffuser rings  22   a  to  22   d . On the inner and outer walls of the housing  18 , half diffuser rings  22   e  and  22   f  are provided. Each diffuser ring defines, with an adjacent concentric diffuser ring  22 , a diffusing passageway made up of converging surfaces  24  and  25 , in the upstream portion of the diffuser ring assembly  20 , and diverging diffuser ring surfaces  26  and  27 , in the downstream portion. Thus, a cross-section of the diffuser ring  22  is somewhat of an elongated quadri-lateral in the form of two isosceles triangles with a common base at the widest portion of the ring. The widest portion of each diffuser ring  22  defines a gap  28  with an adjacent annular ring. There is no limit to the number of diffuser rings  22  which might be used as a ring assembly. 
     The degree of homogeneous mixing of the fuel/air mixture, as will be described, is dependent on the length of the downstream passageway mixing area  30 . Since this area is limited, the angle and length of the divergent surfaces  26  and  27  can be adjusted. 
     As can be seen in FIGS. 2 and 3, there is a manifold assembly  32  upstream of the diffuser assembly  16 . Each of these annular manifold rings  34   a  to  34   e  is provided with individual fuel supply pipes  36   a  to  36   e . In FIG. 3, only three rings, namely, rings  34   a ,  34   b , and  34   c , are shown, but these are representative of the five rings  34   a  to  34   e  which can be provided in the apparatus. 
     As shown in FIG. 2, each of the manifold rings  34   a  to  34   e  includes a fuel chamber  38  which extends throughout the manifold. Each ring  34  defines a channel  40  in a downstream portion thereof. Tangential openings  42  extend between the chamber  38  and the channel  40  to permit the fuel to flow through from the chamber  38  into the channel  40 . The fuel is fed in gaseous form through the pipes  36   a  to  36   e  into the fuel chamber  38  of each ring  34   a  to  34   e , and the fuel is then distributed into the channel  40  of each ring tangentially, such that there is a circular component to the flow of the gaseous fuel in the channel  40 . The fuel advances along the walls of the channel  40  to be sheared at the edges of the manifold ring  34  at  41  where the fuel is mixed by the air passing around the manifold rings  34  in the passageway and towards the area formed by converging surfaces  24  and  25  of the diffuser rings  22 . 
     A similar construction could be used for liquid fuel, but the air would then be under a higher pressure drop. 
     The fuel/air mixture passes through the constrained gaps  28  and then is diffused as the diverging surfaces  26  and  27  of the diffuser rings  22  spread out, causing the homogeneous mix of the fuel and the air. As the mixture advances through the diffusion area  30 , downstream of the diffuser assembly, the mixing of the fuel and air is completed prior to passing through the inlet  13  into the combustion chamber  12 . 
     The shape and location of the diffuser rings  22  cause the fuel and air mixture to accelerate through the converging portion in the upstream portion of the diffuser assembly  20 , minimizing the risk of flash-back and auto-ignition. The aerodynamic diffusion accelerates the natural chemical diffusion of the mixture. The mixture was analytically demonstrated to have a mix with a variation of less than 1% throughout the area downstream of the diffuser assembly area  30  downstream of the diffuser assembly. The fuel is fed at low pressure. A pressure drop of below 1% was realized on the airflow across the inlet. 
     Depending on the size of the engine to which the premixer is to be adapted, the dimensions of the plates and particularly the gap size  28  might vary. To determine the gap area, the following formula should be followed: 
     
       
         
           M=ACd{square root over (2 ρΔP +L )} 
         
       
     
     wherein M=mass flow 
     ACd=effective flow area 
     ρ=density of the air 
     ΔP=pressure drop 
     As previously mentioned, the diffusion of the mixing gases can be adjusted by varying the angles of the converging and diffusing surfaces  24 ,  25 ,  26 , and  27 . 
     The manifold assembly  32  made up by the manifold rings  34   a  to  34   e  is mounted within the housing, and the concentric rings  34   a  to  34   e  are mounted together by means of fins  44  which are staggered at 90° in order to cause the least amount of drag on the air flow. These fins can be seen in FIG.  3 . 
     The diffuser assembly  20  is placed downstream of the manifold assembly  32 . Each diffuser ring  22   a  to  22   d  is individually mounted to the manifold assembly by means of elongated bolts  46  and brackets  48  as seen in FIG.  2 . Each bolt  46  has a bolt head  47 . The bracket  48  includes further bolts  50  which can be screwed onto the manifold rings. 
     A catalyst (not shown) may be provided in the area  30  downstream of the diffuser ring assembly.