Abstract:
A method of distributing fuel in a fuel nozzle comprises: providing at least two helical channels in the fuel nozzle, each having a channel exit port, providing a fuel inlet cavity in fluid communication with the helical channels, and flowing fuel in the fuel inlet cavity, the helical channels and the channel exit ports.

Description:
This application is a divisional of U.S. patent application Ser. No. 10/743,712, filed Dec. 24, 2003 U.S. Pat. No. 7,174,717 issued on Feb. 13, 2007. 

   FIELD OF THE INVENTION 
   The present invention relates to gas turbine engines, and more particularly to a fuel nozzle for such gas turbine engines. 
   BACKGROUND ART 
   Fuel nozzles of gas turbine engines usually comprise a fuel distributor for dividing the fuel in several equal streams in order to develop a uniform fuel film. The fuel distributor is often also responsible for swirling the fuel streams to obtain a good fuel spray distribution. 
   Fuel distributors usually comprise a sealed disk element having a plurality of circumferentially spaced apart small metering holes or slots. The disk is usually mounted on a cylindrical channel adapted to deliver the fuel. The small metering holes are drilled with an axial as well as a circumferential orientation in order to provide a swirl to the fuel passing therethrough. 
   This configuration poses several problems, one of which is the fact that drilling identical holes of such a small size can be very difficult. If sufficient similarity between metering hole sizes is not achieved, the fuel film is not uniform, causing a poor spray quality. In addition, holes of such a small size are very susceptible to contamination or plugging. 
   Another problem with the prior art is that the channels upstream of the metering holes are exposed to a high amount of heat input through adjacent walls due to external heat transfer from hot air to the cool walls. This can lead to coke formation and hole plugging. 
   Also, the resistance of the metering holes is often insufficient to reach the desired nozzle resistance value, and a tuning orifice is often required at the inlet of the nozzle to compensate. 
   Finally, the disk is usually sealed with braze to prevent unmetered fuel from escaping around the metering holes. This presents a risk in manufacturing since braze can run into the metering holes, blocking them after the braze sets. 
   Accordingly, there is a need for an improved fuel distributor that overcomes the above-mentioned problems of the prior art. 
   SUMMARY OF INVENTION 
   It is therefore an aim of the present invention to provide an improved fuel distributor. 
   Further in accordance with an aspect of the present invention, there is provided a method of fabricating a fuel distributor adapted to swirl fuel in a combustor assembly of a gas turbine engine, comprising: providing an elongated cylindrical member; forming at least two helical grooves along an axially extending outer surface of the elongated cylindrical member, each helical grooves defining at least one complete turn, forming one end of the elongated cylindrical member so as to produce a frustro-conical surface at the end, such that radially outwardly oriented channel exit ports are created where the helical grooves intersect the frustro-conical surface; and fitting the elongated cylindrical member into a tubular member such that the cooperation of an inner surface of the tubular member with the outer surface having helical grooves forms independent helical channels adapted to communicate fuel. Also in accordance with an aspect of the present invention, there is provided a method of distributing fuel in a fuel nozzle of a combustor assembly of a gas turbine engine, the method comprising: providing at least two helical channels in the fuel nozzle, each helical channels having a helix axis and a channel exit port axially aligned with the helix axis; and flowing fuel from a fuel inlet cavity, through the helical channels and the channel exit ports and into a surrounding flow of air. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will now be made to the accompanying drawings: 
       FIG. 1  is a side view of a gas turbine engine, in partial cross-section, exemplary of an embodiment of the present invention; 
       FIG. 2  is a simplified side view of a combustor of a gas turbine engine, in cross-section, exemplary of an embodiment of the present invention; 
       FIG. 3  is side view, in cross-section, of a fuel nozzle according to a preferred embodiment of the present invention; 
       FIG. 4  is a side view, in partial cross-section, of the fuel nozzle of  FIG. 3 ; and 
       FIG. 5  is a front view of a fuel distributor of the fuel nozzle of  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine  18  for extracting energy from the combustion gases. 
   Referring to  FIG. 2 , the combustor section  16  is shown. The combustor section  16  includes an annular casing  20  and an annular combustor tube  22  concentric with the turbine section  18  and defining a combustor chamber  23 . The turbine section  18  is shown with a typical rotor  24  having blades  26  and a stator vane  28  upstream from the blades  26 . 
   A fuel nozzle  30  is shown as being located at the end of the annular combustor tube  22  and directly axially thereof. The fuel nozzle  30  includes a fitting  32  to be connected to a typical fuel line. There may be several fuel nozzles  30  located on the wall of the combustion chamber, and they may be circumferentially spaced apart. For the purpose of the present description, only one fuel nozzle  30  will be described. 
   Referring to  FIGS. 3 and 4 , a fuel nozzle  30  according to a preferred embodiment of the invention is shown. The fuel nozzle  30  comprises an air swirler  34  and a fuel distributor  36 . The fuel nozzle also comprises a fuel filmer lip  37  having the function of generating a fuel film from the swirled fuel received from the fuel distributor  36 . 
   The air swirler  34  comprises a tubular body  38  including an inner surface  40  defining a central bore adapted to receive the fuel distributor  36 . The air swirler  34  also comprises outer air swirling means of a type similar to outer air swirling means of fuel injectors known in the art, such as is described in U.S. Pat. No. 6,082,113, issued Jul. 4, 2000 to the applicant, which is incorporated herein by reference. Preferably, the outer air swirling means include an air swirler frustro-conical ring  42  having a plurality of circumferentially spaced apart bores  44 . The axis of each bore  44  has an axial as well as a circumferential component so as to be able to swirl the air passing therethrough. 
   The fuel filmer lip  37  is located at the junction of the inner surface  40  and frustro-conical ring  42  of the air swirler. 
   The fuel distributor  36  comprises a tubular body  46  having a frustro-conical end  48 . The tubular body  46  includes an inner surface  50  defining a cylindrical core air passage  52 . The tubular body  46  also includes an outer surface  54  having a plurality of helical grooves  56 . In a preferred embodiment, three helical grooves  56  are defined in the outer surface  54  and are helically parallel to one another, i.e. the grooves are interlaced so that three successive grooves along an axial line will belong respectively to the first, second and third helical groove. Once the fuel distributor  36  is fitted into the air swirler  34 , the inner surface  40  of the air swirler  34  cooperates with the outer surface  54  of the fuel distributor  36  so that each helical groove  56  defines a closed helical channel. Each helical channel is in fluid communication with an inlet fuel cavity  60  receiving fuel from a fuel inlet  62 . The intersection of a surface of the frustro-conical end  48  with an end of each helical groove  56  creates channel exit ports  58 , as can best be seen in  FIG. 5 . The shape of the channel exit ports  58  contributes to the swirl of the fuel in a fuel swirling chamber  59  defined between the frustro-conical end  48  of the fuel distributor  36  and the fuel filmer lip  37 . 
   The helical grooves  56  and frustro-conical end  48  are preferably formed by standard turning operations. The fuel distributor  36  is preferably shrink-fit into the air swirler  34 . The shrink-fit allows the inner surface  40  of the air swirler  34  and the outer surface  54  of the fuel distributor  36  to cooperate so that the helical grooves  56  can define sealed fuel channels without the need for braze. 
   It is considered to provide helical grooves  56  with a depth progressively shallower toward the frustro-conical end  48  in order to decrease the pressure drop in the beginning of each channel (i.e. near the fuel inlet  60 ) and increase it toward the end thereof (i.e. near the frustro-conical end  48 ). The channel exit ports  58  can be designed so as to have an exit flow area similar to that provided by the metering holes of the prior art in order to obtain similar filming of fuel. 
   It is also contemplated to define the helical grooves into the inner surface  40  of the air swirler  34  to obtain the closed helical channels in cooperation with the outer surface  54  of the fuel distributor  36 , the outer surface  54  being continuous. Alternatively, both the air swirler inner surface  40  and fuel distributor outer surface  54  can have helical grooves defined therein to form the helical channels. 
   During operation, the pressurized fuel enters the fuel inlet  60  and fills the fuel inlet cavity  62 . The fuel pressure than forces the fuel in the helical channels defined by the helical grooves  56 . The fuel in each helical channel exits through the corresponding channel exit port  58 . The helical motion of the fuel through the helical channels and the shape of the channel exit ports  58  both contribute to producing a swirl in the fuel exiting the fuel distributor  36  and entering the fuel swirling chamber  59 . The swirling fuel is then transformed into a fuel film in a manner similar to standard fuel nozzles, by the interaction of the fuel swirling out of the swirling chamber  59  through an opening defined by the fuel filmer lip  37  with air exiting the core air passage  52 . The fuel film is then atomized by contact with swirling air coming from the bores  44  of the frustro conical ring  42  of the air swirler  34 . It is also possible to omit the fuel filmer lip  37  so that the fuel exiting from the exit ports  58  is directly atomized by the swirling air without being transformed into a fuel film. 
   The present invention presents several improvements over the prior art. Since the flow resistance of the nozzle is distributed over the length of the channels rather than across metering holes, a better uniformity of resistance can be achieved which results in a more accurate fuel division. Also, since the helical grooves  56  are formed by standard turning operations, the dimensions of the helical channels can be highly accurate and the operation is less expensive than drilling small metering holes. Forming the channels through standard turning operations allows for easy selection of the length of the channels, which is a function of the pitch of the helical grooves, and of the depth of the channels, whether constant or variable along the channel length. The depth and length of the channels can therefore be chosen so as to tune the pressure drop of the fuel flowing therethrough, and this pressure drop distribution will have several effects on the fuel flow. Tuning the overall pressure drop of a nozzle provides tuning of its resistance with respect to the other nozzles of the combustor. This allows for balancing the flow among various nozzles without the need for a traditional tuning orifice, which reduces fabrication costs. The pressure drop of an individual channel can also be set so as to balance the resistance, thus the fuel flow, among the channels of a same nozzle. The channel length also as a great influence on the rate of heat transfer of the fuel flowing therethrough. Helical channels have the advantage of being much longer than straight channels, which provides for greater heat transfer along the channel. This contributes to reducing fabrication costs since heat transfer in the nozzle tip is reduced, eliminating requirement for additional heat shields. Finally, the depth of each channel can be selected in order to obtain a desired fuel velocity. Since smaller channels will induce a higher fuel velocity, the helical fuel channels, which are smaller then conventional channels, will provide a higher fuel velocity, thus less coke deposition on the channel walls. 
   The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. For example, any desired depth profile and groove cross-section may be used, and not all grooves need to be the same. Any number of grooves may be provided, and they may be provided by any suitable manufacturing method. Other apparatus may be provided having the described groovelike effect. The present distributor may be used alone, or in conjunction with prior art or other distribution and/or swirler apparatus. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.