Patent Publication Number: US-2012023951-A1

Title: Fuel nozzle with air admission shroud

Description:
BACKGROUND OF THE INVENTION 
     Turbine engines used in the power generation industry typically utilize a plurality of combustors which are arranged in a concentric ring around the exterior of the compressor section of the turbine. Within each combustor, a plurality of fuel nozzles deliver fuel into a flow of compressed air. The air-fuel mixture is then ignited within the combustor, and the hot combustion gases are directed to the turbine section of the engine. 
     In many fuel nozzles, compressed air runs down the inside of the nozzle body, and fuel is added to the air while it is inside the nozzle. Some fuel nozzles also include swirler vanes which are arranged inside the nozzle body. The swirler vanes cause the air passing down the length of the interior of the fuel nozzle to swirl around the interior of the nozzle in a rotational fashion. This swirling movement helps to mix the fuel and the air, and this mixing or pre-mixing helps to prevent the generation of undesirable combustion byproducts such as NO x . 
     BRIEF DESCRIPTION OF THE INVENTION 
     In a first aspect, the invention may be embodied in a fuel nozzle for a combustor of a turbine engine that includes an outer housing, and an air admission shroud that is located at an intermediate point along a length of the outer housing. The air admission shroud includes a plurality of air admission apertures that allow air passing along an exterior of the outer housing to enter an interior of the outer housing. 
     In another aspect, the invention may be embodied in a fuel nozzle for a combustor of a turbine engine that includes an outer housing, an inner fuel passageway located at approximately the center of the outer housing, and a plurality of swirler vanes that are located in an annular space between an outer surface of the inner fuel passageway and an inner surface of the outer housing. The swirler vanes cause air passing down the annular space to swirl in a first rotational direction around the annular space. The fuel nozzle also includes an air admission shroud that is located at an intermediate point along a length of the outer housing, wherein the air admission shroud includes a plurality of air admission apertures that allow air passing along an exterior of the outer housing to enter the annular space at a location downstream of the swirler vanes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of a portion of a fuel nozzle; 
         FIG. 2  is a transverse cross-sectional view of the fuel nozzle illustrated in  FIG. 1 ; 
         FIG. 3  is a partial cross sectional view of a portion of the fuel nozzle illustrated in  FIG. 1 ; and 
         FIG. 4  is a cross sectional view illustrating an air admission shroud insert for a fuel nozzle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a downstream portion of a typical fuel injector which can be used in the combustor of a turbine engine. Such a fuel nozzle could include additional structures located upstream of the elements depicted in  FIG. 1 . 
     The fuel nozzle includes an outer housing  102  and an inner fuel passageway  104 . The fuel nozzle also includes a central fuel passageway  106  which passes down the center of the inner fuel passageway  104 . An annular space  113  is formed between the outer surface of the inner fuel passageway  104  and the inner surface of the outer housing  102 . Compressed air would flow down through this annular space  113  and mix with fuel before existing the nozzle. 
     A plurality of swirler vanes  110  extend radially from the outer surface of the inner fuel passage way to a location adjacent the inner surface of the outer housing  102  within the annular space  113 . The upstream ends of the swirler vanes extend parallel to the longitudinal axis of the fuel nozzle. However, the downstream ends of the swirler vanes curve to cause the air flowing down the annular space to swirl around the annular space  113  in a rotational fashion. 
     The swirler vanes  110  are also depicted in the transverse cross sectional view illustrated in  FIG. 2 .  FIG. 2  better illustrates how the downstream ends of the swirler vanes  110  are curved to induce a swirling motion in the air flowing down the length of the nozzle. 
     A plurality of fuel delivery apertures  112  may be formed in the swirler vanes  110 . Fuel would be emitted through the fuel delivery apertures  112  into the flow of air passing down the annular space  113  within the outer housing  102  of the fuel nozzle  100 . In addition, or alternatively, fuel could be delivered into the flow of air through different structures. The swirling motion induced by the curved ends of the swirler vanes  110  helps to mix the air and the fuel as it moves down the length of the fuel nozzle. 
     The fuel nozzle also includes an air admission shroud  120  which includes a plurality of air admission apertures  122  located on the upstream side of the air admission shroud  120 . Air passing down the exterior of the outer housing  102  will enter the air admission apertures  122 , and the air is then received in an annular passageway  124  within the air admission shroud  120 . The air will then be conducted through the annular passageway  124  into an annular space  130  located downstream of the swirler vanes  110 . 
     The air entering the annular space  130  inside the nozzle through the air admission apertures  122  and the annular passageway  124  will then mix with the fuel-air mixture swirling around the annular space  130  downstream of the swirler vanes  110 . The fuel-air mixture will then travel to the downstream end  125  of the fuel nozzle where it will exit the fuel nozzle. The fuel-air mixture exiting the fuel nozzle is then ignited within the combustor of the turbine engine. 
     An enlarged cross sectional view of a portion of the air admission shroud on the fuel nozzle is illustrated in  FIG. 3 . In some embodiments of the air admission shroud, the air admission apertures  122  extend at an angle with respect to a longitudinal axis of the fuel nozzle. As a result, the air passing through the air admission apertures  122  will enter the annular space  124  at an angle, which causes the air within the annular passageway  124  to swirl around the interior in a rotational fashion. This swirling airflow will then enter the annular space  130  downstream of the swirler vanes while it is still swirling in a rotational fashion. 
     In  FIG. 3 , a longitudinal axis of one of the air admission apertures  122  is identified with reference numeral  130 . A line parallel to the central longitudinal axis of the fuel nozzle is identified with reference numeral  132 . The longitudinal axis line  130  and the line  132  parallel to the longitudinal axis of the fuel nozzle are both located in a plane that is parallel to a plane which is tangent to the outer cylindrical surface of the air admission shroud  120  at a location just above the air admission aperture  122 . As illustrated in  FIG. 3 , an angle θ 2  is formed between the longitudinal axis  130  of the air admission aperture  122  and the line  132  parallel to the longitudinal axis of the fuel nozzle. 
     Then the angle θ 2  is relatively small, the air entering the annular passageway  124  will only swirl a small amount. As the angle θ 2  becomes greater, the air entering the annular passageway  124  will be induced to swirl at a greater rotational velocity around the annular passageway  124 . 
       FIG. 3  also illustrates that the walls of the annular passageway  124  are angled inward with respect to a longitudinal axis of the fuel nozzle. As shown in  FIG. 3 , the inner surface of the outer wall  127  of the annular passageway  124  forms an angle θ 1  with respect to a line  135  which is parallel to a central longitudinal axis of the fuel nozzle. As a result, the air passing through the annular passageway  124  is directed down into the annular space  130  located downstream of the swirler vanes  110 . The slight convergence provided by the angle θ 1  increases the axial of the fuel-air mixture, which helps to avoid problems with flame holding just downstream of the swirler vanes  110 . 
     It is desirable for the air entering the fuel nozzle through the air admission shroud to swirl around the interior of the fuel nozzle in a rotational direction which is opposite to the swirling direction of the air which has passed over the swirler vanes  110 . Causing the airflow entering the fuel nozzle through the air admission shroud to swirl in a rotational direction which is opposite to the air-fuel mixture which is already swirling around the interior of the fuel nozzle helps to induce better mixing of the air and the fuel within the nozzle. And the better mixing of the air and fuel leads to a reduction in undesirable combustion byproducts such as NO x . 
     As noted above,  FIG. 2  depicts a transverse cross sectional view of the fuel nozzle as seen from an upstream end of the fuel nozzle. Accordingly, air passing down the length of the fuel nozzle will be passing into the plane of the page illustrated in  FIG. 2 . Because of the way the swirler vanes  110  are curved, air passing across the swirler vanes  110  will swirl in a counterclockwise direction, as viewed from the upstream end of the fuel nozzle. 
     Accordingly, it is desirable for the air admission apertures  122  of the air admission shroud  120  to induce the air entering through the air admission shroud  120  to swirl in a rotational direction which is clockwise, as seen from the upstream end of the fuel nozzle. Causing the air entering the fuel nozzle through the air admission shroud to swirl in a clockwise direction, which is opposite to the swirl direction induced by the swirl vanes  110 , helps to better mix the fuel and air within the fuel nozzle. Also, differences in the longitudinal velocities between the two airstreams creates a shear layer between the two airstreams which also enhances mixing of the air and fuel. 
     In some embodiments, the air admission shroud can be configured as an insert which is inserted into the length of a fuel nozzle.  FIG. 4  illustrates such an embodiment. As shown in  FIG. 4 , the air admission shroud  120  is actually an insert which is inserted between an upstream end  102   a  of the fuel nozzle and a downstream end  102   b  of the fuel nozzle. 
     As shown in  FIG. 4 , a plurality of air admission apertures  122  admit air which is passing down the exterior of the upstream end  102   a  of the fuel nozzle into an annular passageway  124 . The air admission holes  122  are angled with respect to a longitudinal axis of the fuel nozzle. As a result, the air entering the annular passageway  124  tends to swirl around the interior of the air admission shroud in a rotational fashion. 
     In some embodiments, a plurality of turbulence inducing projections  126  may also be located on surfaces of the annular passageway  124 . Some turbulence inducing projections  126  can be located on the surface of the inner side  121  of the annular passageway  124 . Turbulence inducing projections  129  could also be located on the surface of the exterior wall  127  of the annular passageway  124 . The turbulence induced by the turbulence inducting projections would further help to mix the air and the fuel within the nozzle. 
     In some embodiments, the turbulence inducing projections would be arranged in a concentric ring around one or both of the walls of the annular passageway  124 . In other embodiments, the turbulence inducing projections could be located in other types of patterns on the walls of the annular passageway. The turbulence inducing projections may also be located in a pattern that helps to preserve the swirling motion of the air passing through the annular passageway  124 . Also, the turbulence inducing projections may also have a shape that helps to preserve the swirling motion of the air passing through the annular passageway  124 . 
     The provision of the air admission apertures  122  can also have a beneficial effect on combustor dynamics. The space within head end of the combustor can act as an absorption volume. By selectively varying the number, position and aperture size of the air admission apertures  122 , one can cause selected undesirable vibration frequencies to be absorbed. Varying the number, position and aperture size of the air admission apertures  122 , allows one to target certain specific frequencies for absorption. 
     While the invention has been described in connection with what is presently considered to be the most practical and 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 included within the spirit and scope of the appended claims.