Abstract:
A method of assembling a fluid manifold includes: providing an array of spaced-apart manifold fittings, each manifold fitting having: a generally tubular neck; a pair of spaced-apart generally tubular arms extending away from a first end of the neck; and a coupling connected to a second end of the neck; and placing each manifold fitting in a predetermined angular orientation; and providing a plurality of curved tubes, and coupling one end of each tube to one arm of each of two adjacent ones of the manifold fittings, such that the assembled manifold has a predetermined first natural frequency.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to fluid handling, and more particularly to apparatus and methods for fluid manifolds in gas turbine engines. 
         [0002]    A gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. Depending on the engine&#39;s configuration, the core may be combined with a fan and low pressure turbine system to generate propulsive thrust, or with a work turbine to extract mechanical energy and turn a driveshaft or propeller. 
         [0003]    In conventional gas turbine engines, fuel is introduced to the combustor through an array of fuel nozzles which are coupled to an external manifold surrounding the combustor. In operation, pressurized fuel is fed to the manifold. The manifold then distributes the pressurized fuel to the individual fuel nozzles. Such manifolds are commonly manufactured from various tubes and fittings, and are secured to the combustor with brackets and other mounting hardware. Such manifolds experience significant vibration during engine operation. 
         [0004]    Thermal growth is a critical design criterion for these fuel manifolds. The cases that support the fuel nozzles grow as the engine warms, but the temperature of the fuel in the manifold stays relatively cool. This temperature difference, coupled with the different material growth rates of various components, creates a thermal loading on the manifold. To avoid fatigue failure, the manifold&#39;s properties such as stiffness, damping, etc. must be designed so as to avoid excitation of one or more of the manifold&#39;s natural frequencies within the engine operating range while providing proper flexibility for thermal growth. 
         [0005]    These manifolds are unique to each specific engine model. This requires a substantial design effort and testing iterations, leading to high engineering costs. Furthermore, the typical geometry and large part count leads to relatively high system weights. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    These and other shortcomings of the prior art are addressed by the present invention, which provides a method for assembling a frequency-tunable fluid manifold. 
         [0007]    According to one aspect of the invention, a method of assembling a fluid manifold includes: providing an array of spaced-apart manifold fittings, each manifold fitting comprising: a generally tubular neck; a pair of spaced-apart generally tubular arms extending away from a first end of the neck; and a coupling connected to a second end of the neck; and placing each manifold fitting in a predetermined angular orientation; and providing a plurality of curved tubes, and coupling one end of each tube to one arm of each of two adjacent ones of the manifold fittings, such that the assembled manifold has a predetermined first natural frequency. 
         [0008]    According to another aspect of the invention, a method is provided of configuring a fluid manifold including: (a) an annular array of manifold fittings, each manifold fitting comprising:(i) a generally tubular neck; (ii) a pair of spaced-apart generally tubular arms extending away from a first end of the neck; and (iii) a coupling connected to a second end of the neck ; and (b) a plurality of curved tubes. The method includes: determining a desired natural frequency for the manifold as a whole; and selecting a combination of: angular positions of the fittings; and corresponding lengths of the curved tubes permitting one end of each tube to be coupled to one arm of each of two adjacent manifold fittings, such that the combination will result in the desired natural frequency; and assembling the manifold using the selected combination of angular positions and curved tubes having the corresponding lengths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0010]      FIG. 1  is a schematic half-sectional view of a gas turbine engine incorporating a fluid manifold constructed in accordance with an aspect of the present invention; 
           [0011]      FIG. 2  is a partial perspective view of a combustor of the engine of  FIG. 1 , showing a fluid manifold mounted thereto; 
           [0012]      FIG. 3  is a plan view of the manifold shown in  FIG. 2 , with fluid fittings installed in a first position; 
           [0013]      FIG. 4  is a plan view of the manifold shown in  FIG. 2 , with fluid fittings installed in a second position; 
           [0014]      FIG. 5  is a cross-sectional view of one of the fluid fittings of the manifold; 
           [0015]      FIG. 6  is a plan view of the fitting of  FIG. 5 ; 
           [0016]      FIG. 7  is a side view of the fitting of  FIG. 5 ; 
           [0017]      FIG. 8  is a rear view of the fitting of  FIG. 5 ; 
           [0018]      FIG. 9  is a left side view of the fitting of  FIG. 5 ; and 
           [0019]      FIG. 10  is a right side view of the fitting of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts an exemplary gas turbine engine  10  having a fan  12 , a low pressure compressor or “booster”  14  and a low pressure turbine (“LPT”)  16  collectively referred to as a “low pressure system”, and a high pressure compressor (“HPC”)  18 , a combustor  20 , and a high pressure turbine (“HPT”)  22 , collectively referred to as a “gas generator” or “core”. Together, the high and low pressure systems are operable in a known manner to generate a primary or core flow as well as a fan flow or bypass flow. While the illustrated engine  10  is a high-bypass turbofan engine, the principles described herein are equally applicable to turboprop, turbojet, and turboshaft engines, as well as turbine engines used for other vehicles or in stationary applications. The principles of this invention are also equally applicable to other fields where a vibration-resistant fluid manifold is required. 
         [0021]    The combustor  20  includes a radial array of fuel nozzles  24  which are coupled to a manifold  26  surrounding the combustor  20 . In operation, pressurized fuel is fed to the manifold  26  by a fuel metering system such as a hydromechanical unit, FMU, PMU, or FADEC system of a known type (not shown). The fuel is then distributed by the manifold  26  to the individual fuel nozzles  24 . The illustrated example shows a single-stage manifold and fuel nozzles, but it will be understood that the principles of the present invention are applicable to multi-circuit systems as well. 
         [0022]    The manifold  26  is shown in more detail in  FIG. 2 . The casing  28  of the combustor  20  can be seen with the inlet stems  30  of the fuel nozzles  24  protruding therefrom. Each inlet stem  30  incorporates an inlet fitting  32  of a known type. In the illustrated example, the nozzle inlet stems  30  penetrate the case  28  in a generally radial direction, and the inlet fittings  32  extend in a generally axial direction. Coupled to each inlet fitting  32  is a manifold fitting  34 . The manifold fittings  34  are interconnected by tubes  36 . In the illustrated example each tube  36  is generally “C”-shaped when seen in plan view, and has a constant radius of curvature. One or more feed tubes  37  are coupled to the manifold  26  and serve to allow fuel flow into the manifold  26  from a fuel metering and control system of a conventional type (not shown). Most typically the manifold  26  and its constituent components would be made from a metallic alloy, such as an iron- or nickel-based alloy. 
         [0023]      FIGS. 5-10  illustrate one of the manifold fittings  34  in more detail. The manifold fitting  34  is generally “Y”-shaped with a tubular central neck  38  and two spaced-apart, generally parallel tubular arms  40  extending therefrom. As used herein, the term “tubular” denotes a member which has a wall that encloses a volume for fluid flow therethrough and does not necessarily imply a structure that has a purely circular cross-section or a constant wall thickness. The neck  38  is connected to a coupling  42 . 
         [0024]    The coupling  42  includes a tubular inner member  44  having a first end  46  connected to the neck  38 , and a second end which defines a seat  48 . When connected to the inlet stem  30 , the seat  48  receives a ball-nose  50  of the inlet fitting  32  which has a shape complementary to the seat  48 . A groove  53  is formed in the cylindrical surface of the inlet fitting  32  adjacent the ball-nose  50  and receives a resilient sealing element  55 , which seals against the inner member  44 . In the illustrated example the sealing element  55  is an O-ring. The outer surface of the inner member  44  includes an annular shoulder  51 . A collar  52  surrounds the inner member  44  and includes an annular, radially-inwardly-extending flange  54  that engages the shoulder  50 . The interior of the collar  52  includes threads  56  that engage mating threads  57  of the inlet fitting  32 . The exterior of the collar  52  is formed into polygonal flats or other suitable wrenching surfaces  60 . Other types of coupling configurations could be used to couple the manifold fitting  34  to the inlet fitting  32  so long as they provide a leak-free joint. 
         [0025]    As best seen in  FIG. 3 , the tubes  36  are continuously curved so as to form a “U” or “C” shape. In this example the tubes  36  have a constant radius of curvature, but this aspect may be varied as desired to suit a particular application. Each tube  36  has opposed ends  58  which are connected to the arms  40  of adjacent manifold fittings  34 . The tubes  36  may be connected to the manifold fittings  34  in any manner that provides a secure, leak-free joint, for example by the use of thermal or mechanical bonding, adhesives, or mechanical joints or fasteners. As illustrated, the tubes  36  form butt joints  62  (see  FIG. 5 ) with the manifold fittings  34  that are brazed or welded together in a known manner. 
         [0026]    The manifold configuration is “modular” in the sense that a single type of manifold fitting  34  may be coupled to the inlet stems  30  in a variety of different angular orientations and then interconnected with tubes  36  suitable for the selected orientation. By “twisting” the manifold fitting  34  clockwise or counter-clockwise from a nominal position, a designer may effectively increase or decrease the tubing length between neighboring fuel nozzles  24 , with the result of changing or “tuning” the manifold&#39;s natural frequency. Smaller engines generally have a higher frequency of operation, and generally experience less total thermal growth. Larger engines generally have a lower frequency of operation, and generally experience more total thermal growth. The ability to tune the manifold&#39;s natural frequency allows it to be designed to each engine&#39;s specific needs, without the typical systemic redesign seen in the prior art when comparing one engine model to another. 
         [0027]    For example,  FIG. 3  shows a portion of the manifold  26  with the manifold fittings  34  rotated or “clocked” to a first angular orientation. For the sake of illustration, an arrow depicts the plane in which the arms  40  lie. In this position, the lateral spacing between the connected arms  40  of two adjacent fittings  34 , denoted “S 1 ”, is relatively small and the radius of the tube  36  which interconnects the arms  40 , denoted “R 1 ”, is relatively small as well. As a result, the first natural frequency of the manifold  26  is relatively high. Because the tube  36  spans a relatively short point-to-point distance as compared to prior art designs, there is no need for a separate bracket to mount the tubes  36  to the casing  28 . In other words, the tubes  36  are physically supported only be their connection to the fittings  34 . 
         [0028]      FIG. 4  shows a portion of a manifold  26 ′ assembled using manifold fittings  34  of identical construction to those shown in  FIG. 3 . The manifold fittings  34  in  FIG. 4  are rotated or “clocked” to a second angular orientation. An arrow depicts the plane in which the arms  40  lie, which is about 60 degrees away from the position shown in  FIG. 3 . In this position, the lateral spacing between connected fitting arms  40 , denoted “S 2 ”, is larger than the spacing  51 , and the tube  36 ′ which interconnects the arms  40  is relatively larger than the tubes  36  as well. For example, the radius R 2  of the tube  36 ′ may be about 1.25 times the radius R 1  of the tube  36 . As a result, the first natural frequency of the manifold  26 ′ is computed to be about 25% lower than that of the manifold  26 . In this example, the stress induced by thermal loading on manifold  26 ′ with a tube radius of R 2  is computed to be about 15% lower in magnitude than the stress induced by thermal loading on manifold  26  with a tube radius of R 1 . The position of the manifold fittings  34  is infinitely variable as dictated by design requirements. 
         [0029]    As part of the design process, the manifold&#39;s vibration characteristics would be analyzed, for example using a tool such as finite element analysis software, and then a fitting orientation and tube radius would be selected based on the required natural frequency. The design process is vastly simplified compared to the prior art because the manifold fittings  34  are common to many different manifolds  26 . The tubes  36  may be produced in one or more “stock” lengths corresponding to several default orientations of the manifold fittings  34 . 
         [0030]    The fluid manifold  26  described herein has several advantages over a conventional design. Depending on the specific configuration, the manifold  26  may contain as few as one-tenth as many parts as prior art manifold system designs. It may weigh only about half as much as a prior art manifold system and has a reduced part envelope. Design cycle time will also be decreased because of the simplified nature of the design. 
         [0031]    The foregoing has described a frequency-tunable fluid manifold. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.