Abstract:
A dampening device for suppressing vibrations of a tube assembly in a catalytic combustor which includes, a plurality of closely oriented, parallel tubes with each tube having at least one expanded region and at least one narrow region. The expanded regions being structured to contact at least one adjacent tube, thus providing support and minimizing degradation of the joint connecting the tubes to the tube sheet, and degradation of the tubes themselves. Such degradation can result from vibration due to flow of cooling air inside of the tubes, flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and/or other system/engine vibrations.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a catalytic combustor for a combustion turbine and, more specifically, to a device for suppressing vibration in the plurality of cooling tubes which pass through the fuel/air mixture plenum within a catalytic combustor.  
           [0003]    2. Background Information  
           [0004]    Combustion turbines, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor compresses ambient air. The compressed air flows into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vanes and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine section forces the blades, and therefore the shaft, to rotate. The shaft may be connected to a generator.  
           [0005]    Typically, the combustor assembly creates a working gas at a temperature between 2,500 to 2,900 degrees Fahrenheit (1371 to 1593 degrees centigrade). At high temperatures, particularly above about 1,500 degrees centigrade, the oxygen and nitrogen within the working gas combine to form the pollutants NO and NO 2 , collectively known as NOx. The formation rate of NOx increases exponentially with flame temperature. Thus, for a given engine working gas temperature, the minimum NOx will be created by the combustor assembly when the flame is at a uniform temperature, that is, there are no hot spots in the combustor assembly. This is accomplished by premixing all of the fuel with all of the of air available for combustion (referred to as low NOx lean-premix combustion) so that the flame temperature within the combustor assembly is uniform and the NOx production is reduced.  
           [0006]    Lean pre-mixed flames are generally less stabile than non-well-mixed flames, as the high temperature/fuel rich regions of non-well-mixed flames add to a flame&#39;s stability. One method of stabilizing lean premixed flames is to react some of the fuel/air mixture in conjunction with a catalyst prior to the combustion zone. To utilize the catalyst, a fuel/air mixture is passed over a catalyst material, or catalyst bed, causing a pre-reaction of a portion of the mixture and creating radicals which aid in stabilizing combustion at a downstream location within the combustor assembly.  
           [0007]    Prior art catalytic combustors completely mix the fuel and the air prior to the catalyst. This provides a fuel lean mixture to the catalyst. However, with a fuel lean mixture, typical catalyst materials are not active at compressor discharge temperatures. As such, a preburner is required to heat the air prior to the catalyst adding cost and complexity to the design as well as generating NOx emissions, See e.g., U.S. Pat. No. 5,826,429. It is, therefore, desirable to have a combustor assembly that burns a fuel lean mixture, so that NOx is reduced, but passes a fuel rich mixture through the catalyst bed so that a preburner is not required. The preburner can be eliminated because the fuel rich mixture contains sufficient mixture strength, without being preheated, to activate the catalyst and create the necessary radicals to maintain a steady flame, when subjected to compressor discharge temperatures. As shown in U.S. patent application Ser. No. 09/670,035, which is incorporated by reference, this is accomplished by splitting the flow of compressed air through the combustor. One flow stream is mixed with fuel, as a fuel rich mixture, and passed over the catalyst bed. The other flow stream may be used to cool the catalyst bed.  
           [0008]    One disadvantage of using a catalyst is that the catalyst is subject to degradation when exposed to high temperatures. High temperatures may be created by the reaction between the catalyst and the fuel, pre-ignition within the catalyst bed, and/or flashback ignition from the downstream combustion zone extending into the catalyst bed. Prior art catalyst beds included tubes. These tubes were susceptible to vibration because they were cantilevered, being connected to a tube sheet at their upstream ends. The inner surface of the tubes were free of the catalyst material and allowed a portion of the compressed air to pass, unreacted, through the tubes. The fuel/air mixture passed over the tubes, and reacted with, the catalyst. Then, the compressed air and the fuel/air mixture were combined. The compressed air absorbed heat created by the reaction of the fuel with the catalyst and/or any ignition or flashback within the catalyst bed. See U.S. patent application Ser. No. 09/670,035.  
           [0009]    The disadvantage of such systems is susceptibility of the tubular configuration to vibration damage resulting from: (1) flow of cooling air inside of the tubes, (2) flow of the fuel/air mixture passing over the tubes transverse and longitudinal to the tube bundle, and (3) other system/engine vibrations. Such vibration has caused problems in the power generation field, including degradation of the joint (e.g. braze) connecting the tubes to the tubesheet and degradation of the tubes themselves, both resulting from tube to tube and/or tube to support structure impacting.  
           [0010]    There is, therefore, a need for a dampening device for a catalytic reactor assembly of a combustion turbine, which suppresses vibration of the plurality of closely oriented parallel tubes.  
           [0011]    There is further a need for a dampening device for a catalytic reactor assembly to effectively baffle and promote even distribution of the fuel/air mixture.  
           [0012]    There is further a need for a dampening device for a catalytic reactor assembly that provides a stronger, reinforced attachment of the tubes to the tubesheet.  
           [0013]    There is further a need for a dampening device for a catalytic reactor assembly that provides resistance to reverse flow of the fuel/air mixture caused by eddie currents, which in turn can lead to backflash (premature ignition of the fuel in the combustor).  
           [0014]    There is further a need for a dampening device for a catalytic reactor assembly that maintains appropriate pressure differential to promote uniform distribution of the fuel/air mixture and ensure adequate cooling is maintained.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention satisfies these needs, and others, by providing a dampening device with expanded regions on the tubes that maintain tube to tube contact and thus suppress vibration. The invention consists of at least one expanded region and at least one narrow region on each tube. The expanded region may be achieved by a localized increase in the nominal tube circumference, a sleeve or furrel placed over the tube and enlarging the circumference, or by machining or swaging the tube to create narrow regions. The localized expansions extend for a portion of the tube length, having a gradual transition between the nominal circumference and the center of expansion. If the tube is cut or swaged to create narrow regions in between the nominal tube circumference regions, the nominal tube circumference would serve as the expanded region. There may also be multiple expanded regions on a tube.  
           [0016]    The expanded regions may be symmetric along the tube length and/or around the tube circumference. Alternatively, the expansions could be non-symmetric, or even single-sided. Expansions located at the ends of the tubes are examples of single-sided expansions. Moreover, an expanded region on one tube may contact another expanded region on another tube, or alternatively, may be staggered so that an expanded region on one tube contacts the narrow region of an adjacent tube. The tubes and the expanded regions thereon could be a variety of shapes such as bulges, ridges, and/or helices, so long as the flow path around the tubes and desired pressure drop is maintained.  
           [0017]    By maintaining tube to tube contact, adjacent tubes support one another rather than impact one another during various modes of vibration. Moreover, expansion of the tubes to provide contact at a plane just downstream of the fuel/air inlet has been predicted analytically to effectively baffle and to promote even distribution of the fuel/air mixture.  
           [0018]    The upstream ends of the tubes may be bulged or expanded to provide additional support of the fragile joints (e.g. brazes) where the tubes attach to the tube sheet. Similarly, the tubes may be bulged at their downstream ends to provide resistance to reverse flow and therefore backflash, because eddie currents are eliminated by the gradual bulging profile. The expanded or flared inlet and outlet ends of the tubes also provide a substantial reduction (e.g. approximately 14 percent for a flared inlet, 22 percent for a flared outlet) in pressure differential between the air inside the tubes and the air/fuel mixture passing over them. Avoiding an excessive pressure differential allows more effective cooling. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:  
         [0020]    [0020]FIG. 1 is a cross sectional view of a combustion turbine.  
         [0021]    [0021]FIG. 2 is a partial cross sectional view of a combustor assembly shown on FIG. 1.  
         [0022]    [0022]FIG. 3 is an isometric view showing modular catalytic cores disposed about a central axis.  
         [0023]    FIGS.  4 A- 4 H are cross sectional, close-up views of the various embodiments of the invention. Each figure shows a different embodiment of two of the many cooling tubes within a catalytic combustor module. FIG. 4A is a side view of an embodiment in which symmetric localized expansions on one tube contact the expansions on an adjacent tube. FIG. 4B a side view of an embodiment with staggered localized expansions. FIG. 4C is a side view of tubes having furrels disposed symmetrically. FIG. 4D is a side view of tubes having furrels as staggered localized expansions. FIG. 4E is a side view a ridge embodiment in which the ridge is a helix. FIG. 4F is a side view of an embodiment with expanded regions of various widths, lengths and heights FIG. 4F′ is a cross-sectional view taken along line  4 F′- 4 F′ on FIG. 4F. FIG. 4G is an isometric view of a symmetric ridge expansion. FIG. 4G′ is a cross-sectional view taken along line  4 G′- 4 G′ on FIG. 4G. FIG. 4H is an isometric view of a non-symmetric ridge expansion. FIG. 4H′ is a cross-sectional view taken along line  4 H′- 4 H′ on FIG. 4H.  
         [0024]    [0024]FIG. 5A shows an isometric view of a furrel that may be used as an expanded region of the tube.  
         [0025]    [0025]FIG. 5B shows an isometric view of furrels disposed on the tubes.  
         [0026]    [0026]FIG. 5C shows an isometric view of an alternate furrel.  
         [0027]    [0027]FIG. 6 is an end view of the invention looking along the longitudinal axis of one of the combustor tube modules. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    As is well known in the art and shown in FIG. 1, a combustion turbine  1  includes a compressor assembly  2 , a catalytic combustor assembly  3 , a transition section  4 , and a turbine assembly  5 . A flow path  10  exists through the compressor  2 , catalytic combustor assembly  3 , transition section  4 , and turbine assembly  5 . The turbine assembly  5  may be mechanically coupled to the compressor assembly  2  by a central shaft  6 . Typically, an outer casing  7  encloses a plurality of catalytic combustor assemblies  3  and transition sections  4 . Outer casing  7  creates a compressed air plenum  8 . The catalytic combustor assemblies  3  and transition sections  4  are disposed within the compressed air plenum  8 . The catalytic combustor assemblies  3  are, preferably, disposed circumferentiality about the central shaft  6 .  
         [0029]    In operation, the compressor assembly  2  inducts ambient air and compresses it. The compressed air travels through the flow path  10  to the compressed air plenum  8  defined by casing  7 . Compressed air within the compressed air plenum  8  enters a catalytic combustor assembly  3  where, as will be detailed below, the compressed air is mixed with a fuel and ignited to create a working gas. The working gas passes from the catalytic combustor assembly  3  through transition section  4  and into the turbine assembly  5 . In the turbine assembly  5  the working gas is expanded through a series of rotatable blades  9  which are attached to shaft  6  and the stationary vanes  11 . As the working gas passes through the turbine assembly  5 , the blades  9  and shaft  6  rotate creating a mechanical force. The turbine assembly  5  can be coupled to a generator to produce electricity.  
         [0030]    As shown in FIG. 2, the catalytic combustor assembly  3  includes a fuel source  12 , a support frame  14 , an igniter assembly  16 , fuel tubes  18 , and a catalytic reactor assembly  20 . The catalytic reactor assembly  20  includes a catalytic core  21 , an inlet nozzle  22 , and an outer shell  24 . The catalytic core  21  includes an inner shell  26 , a tube sheet  28 , a plurality of elongated tubes  30 , and an inner wall  32 . The catalytic core  21  is an elongated toroid which is disposed axially about the igniter assembly  16 . Inner wall  32  is disposed adjacent to igniter assembly  16 . Both the inner shell  26  and the inner wall  32  have interior surfaces  27 ,  33  respectively, located within the fuel/air plenum  38  (described below).  
         [0031]    Outer shell  24  is in a spaced relation to inner shell  26  thereby creating a first plenum  34 . The first plenum  34  has a compressed air inlet  36 . The compressed air inlet  36  is in fluid communication with an air source, preferably the compressed air plenum  8 . A fuel inlet  37  penetrates outer shell  24 . Fuel inlet  37  is located downstream of air inlet  36 . The fuel inlet  37  is in fluid communication with a fuel tube  18 . The fuel tube  18  is in fluid communication with the fuel source  12 .  
         [0032]    A fuel/air plenum  38  is defined by tube sheet  28 , inner shell  26 , and inner wall  32 . There is at least one fuel/air mixture inlet  40  on inner shell  26 , which allows fluid communication between first plenum  34  and fuel/air plenum  38 . The fuel/air plenum  38  has a downstream end  42 , which is in fluid communication with a mixing chamber  44 .  
         [0033]    The plurality of tubes  30  each have a first end  46 , a medial portion  47  and a second end  48 . Each tube first end  46  extends through tube sheet  28  and is in fluid communication with inlet nozzle  22 . The tube first ends  46 , which are the upstream ends, are isolated from the fuel inlet  37 . Thus, fuel cannot enter the first end  46  of the tubes  30 . Each tube second end  48  is in fluid communication with mixing chamber  44 . The tubes  30  have an interior surface  29  and an exterior surface  31 . Each tube  30  has at least one expanded region  140 , at least one narrow region  160  and at least one transition region  135 . The narrow region  160  is typically the tube nominal diameter, however, as set forth below, the nominal tube diameter can be the expanded region  140  when the tube  30  is swaged to reduce the diameter in the narrow region  160 . A catalytic material  30   a  may be bonded to the tube outer surface  31 . Possible catalytic materials  30   a  include, but are not limited to, platinum, palladium, rhodium, iridium, osmium, ruthenium or other precious metal based combinations of elements with for example, and not limited to, cobalt, nickel or iron. Additionally, the catalytic material  30   a  may be bonded to the interior surface  27  of inner shell  26  and the interior surface  33  of inner wall  32 . Thus, the surfaces within the fuel/air plenum  38  are, generally, coated with a catalytic material. In the preferred embodiment, the tubes  30  are tubular members. The tubes  30  may, however, be of any shape and may be constructed of members such as plates. The mixing chamber  44  has a downstream end  49 , which is in fluid communication with a flame zone  60 . Flame zone  60  is also in fluid communication with igniter assembly  16 .  
         [0034]    The igniter assembly  16  includes an outer wall  17 , which defines an annular passage  15 . The annular passage  15  is in fluid communication with compressed air plenum  8 . The igniter assembly  16  is in further communication with a fuel tube  18 . The igniter assembly  16  mixes compressed air from annular passage  15  and fuel from tube  18  and ignites the mixture initially with either a spark igniter or a igniter flame (not shown). The compressed air in annular passage  15  is swirled by vanes in annular passage  15 . The angular momentum of the swirl causes a vortex flow with a low-pressure region along the centerline of the igniter assembly  16 . Hot combustion products from flame zone  60  are re-circulated upstream along the low-pressure region and continuously ignite the incoming fuel air mixture to create a stabile pilot flame. Alternately, a spark igniter could be used instead of the pilot flame.  
         [0035]    In operation, air from an air source, which is fed to the combustor, such as the compressed air plenum  8 , is divided into at least two portions; a first portion, which is about 10 to 20 percent of the compressed air in the flow path  10 , flows through air inlet  36  into the first plenum  34 . A second portion of air, which is about 75 to 85 percent of the compressed air within the flow path  10 , flows through inlet  22  into tubes  30 . A third portion of air, which is about 5 percent of the compressed air in the flow path  10 , may flow through the igniter assembly  16 .  
         [0036]    The first portion of air enters the first plenum  34 . Within first plenum  34  the compressed air is mixed with a fuel that enters first plenum  34  through fuel inlet  37  thereby creating a fuel/air mixture. The fuel/air mixture is, preferably, fuel rich. The fuel rich fuel/air mixture passes through fuel/air inlet  40  into the fuel/air plenum  38 . As the fuel rich fuel/air mixture, which is created in first plenum  34 , enters the fuel/air plenum  38 , the fuel/air mixture reacts with the catalytic material disposed on the tube outer surfaces  31 , inner shell interior surface  27 , and inner wall interior surface  33 . The reacted fuel/air mixture exits the fuel/air plenum  38  into mixing chamber  44 .  
         [0037]    The second portion of air travels through inlet  22  and enters the tube first ends  46 , traveling through tubes  30  to the tube second end  48 . Air which has traveled through tubes  30  also enters mixing chamber  44 . As the air travels through tubes  30 , it absorbs heat created by the reaction of the fuel/air mixture with the catalytic material. Within mixing chamber  44 , the reacted fuel/air mixture and compressed air is further mixed to create a fuel lean pre-ignition gas. The fuel lean pre-ignition gas exits the downstream end of the mixing chamber  49  and enters the flame zone  60 . Within flame zone  60  the fuel lean pre-ignition gas is ignited by ignition assembly  16  thereby creating a working gas.  
         [0038]    As shown in FIG. 3, for ease of construction the catalytic reactor assembly may be separated into modules  50  that are disposed about a central axis  100 . Each module  50  includes inner shell  26   a,  an inner wall  32   a  and sidewalls  52 ,  54 . A plurality of tubes  30  are enclosed by inner shell  26   a,  inner wall  32   a  and sidewalls  52 ,  54 . Each module  50  also has a tube sheet  28   a,  an outer shell  24   a  and a fuel inlet  37   a.  The rhomboid tube sheet  28   a  is coupled to the inner shell  26   a,  inner wall  32   a  and sidewalls  52 ,  54  of the upstream end of the module  50  by a fastening process (e.g. brazing). The tube sheet  28  is segmented, supporting a plurality of tubes  30  passing therethrough at the tubes  30  upstream ends  46 . As shown, six modules  50  form a generally hexagonal shape about the central axis  100 . Of course, any number of modules  50  of various shapes could be used.  
         [0039]    The use of the catalytic material  30   a  allows a controlled reaction of the rich fuel/air mixture at a relatively low temperature such that almost no NOx is created in fuel/air plenum  38 . The reaction of a portion of the fuel and air preheats the fuel/air mixture which aids in stabilizing the downstream flame in flame zone  60 . When the fuel rich mixture is combined with the air, from the second portion of compressed air, a fuel lean pre-ignition gas is created. Because the pre-ignition gas is fuel-lean, the amount of NOx created by the combustor assembly  3  is reduced. Because compressed air only travels through the tubes  30 , there is no chance that a fuel air mixture will ignite within the tubes  30 . Thus, the tubes  30  will always be effective to remove heat from the fuel/air plenum  38  thereby extending the working life of the catalytic material  30   a.    
         [0040]    A vibration dampening device  120 , shown in FIGS.  4 A- 4 G, consists of at least one expanded region  140  and at least one narrow region  160  on one or more of the tubes  30 . The narrow region  160 , in most of the embodiments, is simply the unexpanded part of the tube or the nominal tube circumference. The expanded region  140  permits the plurality of closely oriented and parallel tubes  30  to remain in contact with one another, thus suppressing vibration. At least one expanded region  140  on each tube  30  is located on the tube medial portion  47 .  
         [0041]    The expanded regions  140  may be formed numerous ways, including but not limited to, a localized expansion  130  of the nominal tube circumference with a gradual transition region  135  between the nominal tube circumference and the center of expansion, as shown in FIG. 4A; a sleeve or furrel  130   a  placed over the tube  30 , thus enlarging the circumference as shown in FIG. 4C; or by using the nominal circumference as the expanded region  140  after machining or swaging the tube  30  to remove tube material and create narrow regions  160 . The expanded region  140  does not extend the entire length of the tube  30  but there may be more than one expanded region  140  on each tube  30 . As discussed in more detail below, the expanded region  140  may be symmetric  230  (FIG. 4G) along the tube length and/or around the tube circumference. Alternatively, the expansions could be non-symmetric  330 , single-sided expansions  430  (FIG. 4H), or any combination thereof. The catalyst material  30   a  may cover the entire tube  30  or only the narrow regions  160 , in which case the contacting expanded regions  140  are not coated.  
         [0042]    As shown in FIG. 4A, in one embodiment, each tube  30  has an expanded region  140  at its first end  46 , which is the upstream end of the tube  30 , at least one expanded region  140  at the tube medial portion  47  and an expanded region  140  at it&#39;s second end  48 , which is the downstream end of the tube  30 . The upstream end  46  expanded region  140  help provide additional strength and support at the vibration susceptible tube sheet  28  junctions between the tubes  30  and the inner shell  26 , inner wall  32 , and side walls  52 ,  54 . At the point where the tubes  30  pass through the tube sheet  28 , the expanded regions  140  do not contact each other. That is, to allow the tube sheet  28  to be contiguous, the expanded regions  140  are spaced from each other at the tube sheet  28 . Both expanded region  140  located at the first end and the second end  46 ,  48  also help to generate the desired flow path around the tubes  30  and the desired minimal pressure drop within the module  50 .  
         [0043]    In this embodiment, the expanded regions  140  are localized expansions  130  of the nominal outside tube circumference. The localized expansions  130  have at least one transition region  135 , forming a gradual angle between the nominal outside tube circumference and the center of the expanded region  140 . The gradual transition  135  and subtle expansion profile  130  are necessary to promote even flow through the module  50  and prevent an excessive pressure drop. An abrupt transition  135  and/or expansion  140  would likely create eddie currents which have damaging consequences such as back flash. The tubes  30  upstream ends  46  and downstream ends  48  are both expanded and each of the expanded regions  140  of one tube  30  contact the expanded regions  140  of the adjacent tubes  30 . The catalyst  30   a  is only covering the unexpanded or narrow regions  160  of the tube  30 . A flow path  138 , corresponding to the fuel/air plenum  38 , exists between the adjacent tubes  30 . The flow path  138  is structured to avoid excessive pressure drop within, and promote uniform flow through, the module  50   
         [0044]    In another embodiment, shown in FIG. 4B, the localized expansions  130  of one tube  30  are staggered with respect to the localized expansions  130  of at least one other, adjacent tube  30 , so that the narrow region  160  of one tube contacts the localized expansion  130  of the adjacent tube  30 . In this embodiment a different flow path  138  is created. As shown in FIG. 4B, the flow path  138  gaps are smaller but more numerous. However, the same beneficial uniform flow and minimal pressure drop can be achieved. Additionally, all of the tubes  30  do not have the same expansion pattern. As seen in FIG. 4B, every other tube does not have expansions  140  at the upstream  46  and downstream  48  ends. The end expansion  140  on one tube  30  supports the nominal tube circumference or narrow region  160 , of the adjacent tube  30   
         [0045]    In another embodiment, shown in FIGS. 4C, 4D,  5 A,  5 B, and  5 C, a furrel  130   a  is disposed over the tube  30 , thus creating an expanded region  240 . A furrel  130   a  is a separate sleeve or piece of material having a greater outside diameter than the nominal diameter of the tube  30 . As shown in FIG. 5A, the furrels  130   a  may be various lengths and shapes as long as a flow path  138  is formed between the expanded regions  240 . The furrels  130   a  may be held in place on the tube  30  by any commonly used fastening means such as brazing, or a setscrew  131  (FIG. 5C). The preferred furrel  130   a  shape, shown in FIG. 5A, is a sleeve tapered on both sides to form a gradual transition region  135  between the tube nominal circumference and the region with the greatest diameter on the furrel  130   a.  As shown in FIG. 5C, the furrel  130   a  may be formed without a transition. As before, the catalyst material  30   a  may cover the entire tube  30  or only the narrow regions  160 , and the furrels  130   a  of one tube  30  may contact the furrels  130   a  of the adjacent tubes  30  as shown in FIG. 4C or they may be staggered as shown in FIG. 4D.  
         [0046]    FIGS.  4 E- 4 G show another embodiment in which the expanded regions  140  comprise a narrow ridge  340  expansion, extending longitudinally along the tube  30  and extending radially beyond the nominal diameter of the tube  30 . As shown in FIG. 4E, the ridge  340  may form a helix  330 A as it wraps around the tube  30 . The helix  330 A would touch the helix  330 A of the adjacent tubes  30 , thus providing support. Moreover, the helix shape  330 A may enhance the flow path  138  around the tubes  30  and through the module  50  to improve catalytic reaction and achieve the best balance of fuel/air mixture combining with the cooling air exiting the tubes  30  at the downstream ends  48 . Alternatively, as shown in FIGS.  4 F,  4 F′,  4 G, and  4 H the ridge  330 B may be generally straight, that is, extending in a direction parallel to, but spaced from, the tube axis. The ridges  330 B may have various lengths, widths and heights. Additionally, the ridges  330 B may be disposed at various locations around the circumference of the tubes  30 . FIGS.  4 G and  4 G′ illustrates symmetric ridges  330 B, with the ridges  330 B spaced generally 90 degrees apart around the circumference of the tube  30 . FIGS.  4 H and  4 H′ show non-symmetric ridges  330 C wherein the ridge  330 C is located on one side of the tube  30 . FIG. 4H also shows varying the pattern of the expanded region  340  depending on the tube  30  location within the module  50 . That is, ridge  330 D is configured for a tube  30  located in a corner of a module  50 , where for example the inner shell  26  and one of the side walls  52  connect. Various tube  30  size, shape, location and symmetry combinations could be utilized to benefit from the best amalgamation of tube  30  support, module  50  flow rate, and pressure drop within the module  50 .  
         [0047]    As FIG. 6 shows the tubes  30  in a module  50 . The expanded regions  140  contact each other where the tubes  30  are adjacent to other tubes  30 , or contact the interior shell surface  27  or inner wall surface  33  where the tubes  30  are located adjacent to either the interior shell  26  or inner wall  32 . The tubes  30  support each other and therefore reduce vibration. The fuel/air mixture flows past the expanded regions  140  through the plenum gaps constituting the flow path  138  and then combines with the cooling air exiting the tubes  30  at the tube downstream ends  48 . FIG. 5 shows the medial portion of the module  50 , looking down the longitudinal tube axis, of the embodiment in which the expansions  140  are localized tube expansions  130  of the nominal tube circumference.  
         [0048]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, although the tubes  30  have been shown to be circular, various shapes could be used. For example the tubes could be oval or any other shape so long as the contacting surfaces preserve a flow path  138  for the fuel rich mixture to traverse and the benefit of minimal pressure drop is sustained. Accordingly, the particular arrangements disclosed, are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.