Abstract:
A U-tube diffusion flame burner assembly having improved flame stabilization with no undesirable acoustic effects. The burners are axial units comprised of flame holders and combustors. The flame holders includes secondary air tubes to support the flow of secondary air that have helical walls forming helical passageways along at least a portion of its inner diameter to impart a swirl to the secondary air. The helical passageways impart a swirl to the secondary air exiting the passageways while simultaneously acting as a heat exchanger to heat primary air. The flame holders includes fuel tubes having first ends connected to a fuel supply and second flame ends. A plurality of radially oriented apertures are located at the second flame ends to distribute fuel in a radial direction. Primary air tube surrounds at least the flame ends of the fuel tubes and extend axially from first air supply ends to second flame ends. Air is diverted from air supply ends of the flame burners into primary air tubes through radial apertures located at the first air supply ends of the primary air tubes. Secondary air tubes surround the primary air tubes extending from first air supply ends to second ends opposite the air supply, the secondary air tubes having inner diameters larger than the outer diameters of the primary air tubes. The secondary air tubes extend for a preselected distance beyond the second flame ends of the primary air tubes and are coupled to axially-oriented conical-shaped reducers. Flame at high velocity exits the restricted end of the reducers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This Application is a continuation-in-part of application Ser. No. 09/853,135, filed May 10, 2001 now abandoned. 

   FIELD OF THE INVENTION 
   The present invention is generally directed to a gas burners using a diffusion flame with a unique flame stabilization method and specifically to a burner system for a U-tube type burner using self-recuperative radiant tube burners. 
   BACKGROUND OF THE INVENTION 
   Diffusion flame (nozzle mixing) gas burners are used in various industrial kilns and furnaces. There are various types of units used in such furnaces, including but not limited to self-recuperative single-ended radiant tube burners. These burners have flame contained within the tube, which tube then radiates heat into the furnace. The units are designed so that no products of combustion enter the work processing chamber of the furnace. Substantially all of the products of combustion are exhausted rearward through the burner outside of the work processing chamber of the furnace and in a direction substantially opposite the flame direction. These exhaust gases preheat the incoming combustion air by extracting waste heat from the hot exhaust gases. 
   Diffusion flame burners such as self-recuperative single-ended radiant tube burners must have excellent flame stabilization. If the flame front is not anchored at a fixed start location, undesirable acoustics will develop in the tube. Self-recuperative burners additionally have an effective built-in heat exchanger to transfer waste heat from the exhaust gas to the incoming combustion air. These burners typically have an axial design comprised of a flame holder and an inner flame tube. The flame holder provides fuel through a central fuel tube. Primary air to support combustion of the fuel is provided at a low velocity along a first passageway coaxial with the central fuel tube. The primary air distributes fuel as the fuel exits the central fuel tube where it is spark-ignited. Secondary air is provided at greater volumes along a second passageway coaxially with the first passageway to mix with uncombusted fuel in a combustor to provide a flame. The secondary air is separated from the primary air by a tube whose tube wall forms a boundary between the primary air and the secondary air. The secondary air may be provided at higher pressure or at higher velocities or both. As the secondary air combines with the unmixed fuel, it is projected from a reducer at high velocities into the inner flame tube, which is located axially downstream from the reducer. The mixing of the primary air and the fuel is completed in the inner flame tube which heats the industrial furnace or kiln. An exemplary self-recuperative radiant tube burner is set forth in U.S. Pat. No. 4,705,022 to Collier. 
   Another type of burner assembly is a radiant tube burner with two legs, and a semi-toroidal linking section, commonly referred to as “U-tubes” because the tube is in the shape of an elongated “U.” The traditional method of U-tube design is to place the burner at one end of the tube and the recuperator at the other end of the tube. The usual result from this design is that the firing leg of the tube operates at a higher temperature than the exhaust leg of the tube as the energy of the flame is dissipated down the firing leg of the tube. 
   Various design changes have been introduced into burner designs to improve the combustion of the burners or to reduce NO x  emissions from the burners. One such design improvement is set forth in U.S. Pat. No. 5,700,143 (&#39;143 patent) to Irwin et al. and assigned to the assignee of the present invention. The complex tube design set forth in the &#39;143 patent swirls secondary air by introducing it into the secondary tube through spin vanes where a spin is imparted. The swirling air exits the secondary tube at the end of the primary air nozzle or slightly upstream of the primary air nozzle. The swirling secondary air assists in atomizing the fuel from the fuel/air mixture. 
   While the burner set forth in the &#39;143 patent is particularly effective in permitting a change from one fuel to another, it is complex and expensive, but can introduce undesirable acoustics. 
   A difficulty that can be encountered by burners that rely on swirling air to improve mixing is that the secondary air is introduced and swirled at a first end, but exits the secondary tube at a second end. Under steady state conditions, the swirl imparted to the air appears to be stable. The swirling air modifies the acoustics of the burner, producing undesirable acoustics that are very unpleasant, and potentially damaging noise, to anyone in the vicinity. Additionally, with changing conditions as the air flow is modified, the effects within the tube can change the nature of the swirl, causing the flame to be unstable. In certain extreme situations, this can impact the combustion process, such as by extinguishing the flame. 
   Another problem associated with burners is efficiency. The efficiency of the burners can be improved as the temperature of the secondary air is increased. In radiant burners such as the one described in the &#39;143 patent as well as other burners, the secondary air is frequently used to cool the metallic parts that comprise the burner, as the elevated temperatures of combustion can destroy these parts, if adequate cooling is not provided. Of course, the secondary air is also heated, but this effect is limited by the heat transfer characteristics of the assembly. 
   Another problem that impacts upon burners, as noted above, is flame stabilization. If the flow of secondary air is altered, either by increasing or decreasing air flow, it is possible to move the flame front and/or extinguish the flame that exists at the juncture of the primary air tube and fuel tube. Thus care must taken when adjusting air flow so as not to extinguish the flame. 
   What is needed is a diffusion flame burner that can provide a swirling component to secondary combustion air in a manner to stabilize the flame over a broad range transient conditions, and that does not produce undesirable acoustic effects. The burner should also allow changes in secondary air flow without destabilizing the flame. It should also heat the secondary combustion air to improve the efficiency of the combustion process. The burner should also be able to be used with retrofitted U-tube radiant burners or newly installed U-tube radiant burners. The diffusion flame burner of the present invention should be a simple design to construct, and should be resistant to damage resulting from long exposures to heat, high temperature and flame. 
   SUMMARY OF THE INVENTION 
   The present invention is a diffusion flame burner having improved flame stabilization with no undesirable acoustic effects. The burner is an axial unit comprised of a flame holder and combustor. In a single-ended, self-recuperative radiant tube burner, the flame holder and combustor are axially coupled to an inside flame tube. As defined herein, the term “self-recuperative” refers to burners that are entirely self-recuperative and at least partially self-recuperative. The flame tube is surrounded by the outer radiant tube. The flame holder includes a secondary air tube to support the flow of secondary air that has helical walls forming helical passageways along at least a portion of its inner diameter to impart a swirl to the secondary air. The helical passageways impart a swirl to the secondary air exiting the passageways while simultaneously acting as a heat exchanger to heat primary air. 
   The flame holder mechanism of the present invention comprises a fuel tube having a first end connected to a fuel supply and a second flame end. A plurality of radially oriented apertures are located at the second flame end to distribute fuel in a radial direction. A primary air tube surrounds at least the flame end of the fuel tube. The primary air tube extends axially from a first air supply end to a second flame end. Air is diverted from an air supply end of the flame burner into the primary air tube through a radial aperture located at the first air supply end of the primary air tube. 
   The secondary air tube surrounds the primary air tube extending from a first air supply end to a second end opposite the air supply, the secondary air tube having an inner diameter larger than the outer diameter of the primary air tube. The secondary air tube in turn is surrounded by an exhaust gas housing, which, in its simplest form is in the shape of a tube, the inner diameter of which is larger than an outer diameter of the secondary air tube. An annulus is formed between the inner diameter of the exhaust gas housing and the outer diameter of the secondary air tube, the gap size dictated by the differences in the two diameters. The secondary air tube extends for a preselected distance beyond the second flame end of the primary air tube and is coupled to an axially-oriented conical-shaped reducer, the conical-shaped reducer having a first opening at a first end where it is coupled to the secondary air tube and a second restricted opening at a second end opposite the first end of the reducer. The portion of the secondary air tube beyond the flame end of the primary air tube and the conical-shaped reducer constitute the combustor and the volume within this region supports combustion. Flame at high velocity exits the restricted end of the reducer. 
   When the diffusion flame burner is a single-ended self-recuperative radiant tube burner, flame exiting the restricted end of the conical-shape reducer then enters an inner flame tube to which it is coupled. The inner flame tube extends in an axial direction beyond the reducer, having a first end adjacent the reducer and a second end opposite the reducer. The inner flame tube is surrounded by an outer radiant tube, the outer radiant tube having an inner diameter larger than an outer diameter of the inner flame tube, with an annular space formed therebetween. 
   The helical walls located on the inside diameter of the secondary air tube act as an air swirl to deliver the secondary air to the combustor with a predetermined swirl, the swirl determined by the pitch of the helical walls. This air swirl forms a plurality of standing vortices in pockets of the helical passageways at the second end of the secondary air tube. This plurality of standing vortices impart stability to the flame. Even when one or more of these standing vortices are temporarily extinguished, the flame is perpetuated by remaining vortices, which reignite the extinguished vortices once they are re-established. The walls which project inward from the inside diameter of the secondary air tube additionally act as an efficient heat exchanger, the helical walls serving as fins to effect efficient heat transfer from hot exhaust gases flowing in the exhaust gas housing annulus, formed between the outer diameter of the secondary air tube and the inner diameter of the exhaust gas housing, and the secondary air tube. 
   Another application of the present invention is the use of the diffusion burner flame stabilization device in a burner with two legs. Typically, the two legs comprise each of the legs of a U-tube burner assembly. The burner of the present invention is able to function in a retrofitted U-tube burner assembly. Prior art U-tubes generally have a burner at one end of the U-tube and a recuperator at the other end of the tube, with fuel being burned in the burner and the hot gases of combustion being forced along the U-tube and exhausted from the recuperator. The old burner and recuperator can be removed from the U-tube and retrofitted with the burners of the present invention. Two burners of the present invention can be inserted into each end of the U-tube so that burners are firing at both ends of the tube. As the U-tube has three housing elements, two substantially parallel, linear tube elements joined by a semi-toroidal tube element, one burner can fit into each leg of the U-tube. 
   In some cases, the housing elements are a unitary housing element. In other cases, the U-tube housing elements are separate elements that are mechanically or metallurgically joined. As each burner fires, a portion of the hot gases of combustion are forced forward toward the other end of the U-tube housing, while a second portion of the hot gases of combustion are forced back toward the burner which produced the gases. In the U-tube burner assembly of the present invention, the pressure in the tip of the “U” portion of the tube, namely the semi toroidal element, is generally zero or negative at normal operating conditions, as the pressure from the hot gases of combustion are generally equivalent. 
   In the U-tube assembly, only a portion of the combustion gases generated by each burner are expelled through the recuperator of the respective burner. The other portion of the hot gases of combustion are processed through the recuperator of the burner at the other opposite leg of the U-tube. However, the entire U-tube assembly using the burners of the present invention is completely self-recuperative, as all of the hot gases of combustion are used to heat the incoming gas and air flows prior to combustion. In normal operating conditions, half of the hot gases of combustion are being generated by each burner. As the hot gases of combustion are somewhat equally distributed along the length of the U-tube, the U-tube assembly of the present invention has a much more consistent heat transfer along its length. In addition, the burner of the present invention can be used with newly manufactured U-tubes in the same manner as set forth above. 
   Certain U-tube assemblies have inner housing tube cross-sectional areas, or inner diameters, that are not amenable to the use of the burner and flame stabilization device of the present invention because the velocity of the hot gases of combustion created by the burner travel too slowly through the housing to adequately transfer heat from the hot gases of combustion to the air and gas flowing into and through the burner. At normal steady state operating conditions, if the hot gases of combustion are moving too slowly, the heat transfer between the hot gases of combustion and the air and fuel inflow is insufficient to properly preheat the air and fuel inflow prior to combustion. In these assemblies, sleeves are inserted into the housing to reduce the cross-sectional areas of the tubes thereby accelerating the flow of the hot gases. The sleeve may be a metal, a ceramic, or combination thereof. 
   The present invention is also a self-recuperative radiant tube burner assembly with improved flame stability comprising a U-tube housing having a first substantially linear tube housing element, a second substantially linear tube housing element and a toroidal tube housing element, where the first housing element has a first and a second end, where the second housing element has a first and a second end, where the third toroidal housing element has a first end and a second end, the second end of the first housing element being joined to the first end of the toroidal housing element, the second end of the second housing element being joined to the second end of the toroidal housing element, the U-tube housing having a central axis extending between the first end of the first housing element and the first end of the second housing element. The self-recuperative radiant tube burner assembly of the present invention also comprises two flame holders for a diffusion flame burner, each flame holder having a first end and a second end. The self-recuperative radiant tube burner assembly of the present invention also comprises a first flame holder positioned in the first end of the first housing element and a second flame holder positioned in the first end of the second housing element, where the first flame holder has a first inner flame tube axially spaced from the second end of the flame holder by a gap and is positioned at the second end of the first housing element. The second flame holder has a second inner flame tube axially spaced from the second end of the first flame holder by a gap and is positioned at the second end of the second housing element. 
   An advantage of the present invention is that it provides a more efficient combustion of the fuel as the secondary air is heated to provide a recuperative effect. Thus, the air from the secondary air tube is provided to the combustor at a higher temperature than if there were no recuperative effect. The walls of the secondary air tube act as a heat exchanger to more efficiently transfer heat to the secondary air and heat it to higher temperatures than heretofore possible. 
   Another advantage of the present invention is that it provides a more stable flame. The air is swirled in such a manner as to stabilize the flame within the recirculation zone, allowing combustion to continue even under transient conditions of secondary air flow, without concern about flame extinguishment as a result of the formation of a plurality of stationary vortices. The design of the unit does not produce undesirable acoustic effects normally associated with diffusion flame gas burners that incorporate swirled air. 
   Another advantage of the present invention is that it can perform at higher operating temperatures by delivering secondary air preheated to higher temperatures than similar self-recuperative burners. 
   Another advantage of the present invention is that it is resistant to thermal shock, so that it can undergo extreme temperature changes in short periods of time without suffering adverse effects. 
   A further advantage of the present invention is that it is able to be retrofitted into pre-existing U-tube burners after the burner and recuperator present in the U-tubes have been removed, which allows burners to fire at both ends of the U-tube. As both ends of the U-tube are burners, the heat transfer along the length of the U-tube within the furnace is more equally distributed than in standard prior art U-tube. 
   A further advantage of the present invention is that it is able to be used with newly manufactured U-tube housings. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of the radiant tube burner of the present invention extending through a furnace wall; 
       FIG. 2  is an enlarged cross-sectional view of the second end of the flame holder of the present invention; 
       FIG. 3  is a cross-section of the tube extending inside the radiant tube burner before the termination of its inside diameter, depicting the bounded channels; 
       FIG. 4  is a schematic of the tube extending inside the radiant tube burner after the termination of its inside diameter depicting the helical configuration and unbounded channels. 
       FIG. 5  is a cross-sectional view of two radiant tube burners of the present invention extending through a furnace wall into a U-tube burner. 
       FIG. 6  is a partial section view of two radiant tube burners of the present invention extending into a furnace. 
       FIG. 7  is a partial section view of an alternate embodiment of two radiant tube burners of the present invention extending into a furnace. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts the present invention included as a flame holder and combustor in a self-recuperative single-ended radiant tube burner. Self-recuperative burners require an effective built-in heat exchange to transfer heat which is otherwise lost as waste form the exhaust gas to the incoming combustion air. Ideally, the flame retaining mechanism and the heat exchanger should be as simple in construction as possible while also being resistant to heat and flames. Referring now to  FIG. 1 , the self-recuperative, single-ended radiant tube burner  100  of the present invention includes a central axis  102  and is comprised of a flame holder  200  and an inner flame tube  300 . The self-recuperative, single ended radiant tube burner  100  has a first end  110 , a second end  120  and an exhaust housing  130  that extends between the first end  110  and the second end  120 . Housing  130  extends through an aperture  104  in a furnace wall  106 . The portion of housing  130  extending through furnace wall  106  is referred to as outer radiant tube  400  and may be constructed integral with exhaust housing  130  or as a separate piece assembled to housing  130 . A support structure  108  extends between the first end  110  of radial tube burner  100  and furnace wall  106  firmly attaches and stabilizes burner  100  to furnace wall  106 . A flame (not shown in  FIG. 1 ) is generated within self-recuperative single-ended tube burner  100  and heat is radiated from radiant tube  400  positioned within the furnace or kiln. 
   Flame holder  200  has a first end  202  that extend within exhaust gas housing  130  from the first end  110  of burner  100  and terminates at a second end  204  within exhaust gas housing  130 . Flame holder  200  comprises a fuel tube  206  that extends from the first end  110  of tube burner  100  and, as shown in  FIG. 1 , is substantially coaxial with burner  100 . A primary air tube  210  surrounds fuel tube  206 . The inside diameter  212  of primary air tube  210  is larger than the outside diameter of fuel tube  206 , so that a natural passageway  214  is formed between fuel tube  206  and inside diameter  212 . Fuel is provided to fuel tube  206  by means of a connection to a fuel supply (not shown) at first end  110  of burner  100 . Passageway  214 , also referred to as the primary air passageway, supplies air to mix with fuel from fuel tube  206 , which is supplied from first end  110  of burner  100 . Primary air tube  210  terminates within flame holder  200 . A secondary air tube  222  has an outside diameter  216  surrounding tube  210  and extends beyond tube  210 . Fuel tube  206  terminates within flame holder  200  just beyond primary air tube  210 . Fuel tube  206  includes a plurality of apertures  220  having at least a partial radial orientation to form a nozzle that distribute fuel inside tube  210 , best illustrated in  FIG. 2 , an enlarged section of the second end  204  of flame holder  200 . 
   Referring now to  FIG. 2 , a spark initiator  224  in the form of a spark plug is positioned proximate to fuel tube  206 . The spark initiator  224  is connected to a power source to provide the requisite voltage by means of wiring (not shown) carried inside first protective tube  226  that extends toward first end  110 . A flame sensor  230  extends beyond the termination of tube  210 . Flame sensor  230  is connected to an indicator, preferably on a control panel (not shown) by means of wiring (not shown) carried inside second protective tube  232  that extends toward first end  110 . Conveniently, a single protective tube may be used to carry the wiring if desired. Primary cooling air transported along passageway  214  cools the electrical wiring in the protective tubes. 
   As depicted in the preferred embodiment of  FIG. 1 , the central axis of fuel tube  206  is coincident with burner central axis  102 , but this geometric configuration is not required. The only requirement is that the fuel tube must deliver fuel proximate to primary air and spark initiator. 
   Combustion is initially started by injecting a fuel from the plurality of apertures  220  forming a nozzle while simultaneously supplying air along primary air tube  214 . Ignition is initiated by supplying an ignition voltage at spark initiator  224 , a spark plug. An electric spark from the spark plug having its sparking electrode extending into the fuel stream causes an ignition of the fuel-air mixture. After ignition, combustion normally is a self-sustaining process, as long as the fuel and air feed the combustion flame. Flame sensor  230  detects the presence of a flame and, via appropriate indication, when the flame has been extinguished. 
   Referring again to  FIG. 1 , secondary air tube  222  includes a plurality of channels  240  that extend along its length. These channels are formed by helical walls  242  that extend inward from tube  222 . As best seen in  FIG. 2 , beyond the termination of primary air tube  210 , these walls extend away from secondary air tube  222 , projecting inwardly toward the central axis  102  of the burner. These channels form the secondary air passageways  240 . The radial inward boundary of these passageways  240  is the outer diameter of primary air tube  210 . Air is introduced into the secondary air tube at the first end  110  of radiant burner tube  100 . The air is supplied under pressure or it may be injected under pressure, typically from a combustion air blower. 
   At the second end  204  of flame holder  200  is a conically shaped reducer  250  that acts as a nozzle to discharge the flame. Reducer  250  has a first end  252  that is received within tube  222 . A second end  254  of discharge nozzle  250  has an aperture  256  to direct the flame toward the inner flame tube  300 . Discharge nozzle  250  can be any desired shape, but in a preferred embodiment shown in  FIGS. 1 and 2 , is conically shaped to concentrate the flame, increasing the flame velocity as it directs the flame toward inner flame tube  300 . 
   Inner flame tube  300  is located within outer radiant tube  400  receiving flame from reducer  250 . Combustion is completed in inner flame tube  300 . The hot products of combustion are turned 180° at second end  120  of inner flame tube  300  and flow in a reverse direction toward first end  110  of burner through the annular space or gap  260  between inner flame tube  300  and outer radiant tube  400 . A second annular space  270  is present between the flame holder  200  and outer radiant tube  400 . The hot products of combustion are turned 180° because the end of outer radiant tube  400  is closed. 
   The configuration of inner flame tube  300  as depicted in  FIG. 1  is conventional. It is located at end  120  of self-recuperative single-ended radiant tube burner  100 , receiving the flame from reducer  250  where combustion is completed. Heat is radiated form outer radiant tube  400  into the vessel that is being heated, for example a kiln or industrial furnace. The flame may expand as it is projected from discharge nozzle into inner flame tube  300  and any additional combustion may be completed as any uncombusted fuel and air combine in this portion of the burner prior to discharge into the vessel. 
   As shown in  FIG. 1 , secondary air tube  222  is formed with a plurality of helical channels  240  formed by helical walls  242  that serve as a conduit for secondary air. These helical walls  242  form closed channels  240  that are bounded passageways when a primary air tube  210  is inserted within secondary air tube  222 . The helical walls extend beyond the termination of primary air tube  210 . The helical walls  242  essentially act as heat exchanger fins along their entire length. Below the termination of primary air tube  210 , the helical channels are no longer completely bounded passageways. It will be understood by those skilled in the art that although the combustor is shown comprising three elements, a primary air tube  210 , a secondary air tube  222  having helical walls, and a reducer, these elements may be combined to form a single piece or as a single tube having the requisite characteristics coupled to a reducer. 
   Air is introduced into the channels at preselected location at the first end  110  of self-recuperative single-ended tube burner  100 . The air may be introduced axially or radially. The method of introduction of air is not a part of this invention. However, as exhaust gases exit the burner at first end  110 , air may be introduced in any satisfactory manner that does not interfere with the discharge of exhaust gases. The air enters helical channels  240  which run the length of tube  222  in the preferred embodiment. It will be understood that the helical channels are not required to extend the entire length of tube  222  to be effective, nor are the walls  242  required to extend the entire length of tube  222 . However, as the walls  242  act as fins, a maximum of efficiency is obtained when they extend the entire length of tube  222 . Other configurations of the walls  242  and channels  240  are within the scope of the invention. 
     FIG. 3  represents a cross-section of primary air tube  210  and secondary air tube  222  to better illustrate passageways  240 . This cross-section is located above the combustor portion of the flame holder, that is, between the end of inside diameter  212  and first end  110  (not shown in FIG.  3 ). As can best be seen from  FIG. 3 , tube can be made as a monolithic material, with walls  242  formed integrally into tube.  FIG. 3  illustrates clearly the bounded nature of channels  240  in this portion of tube. As a cross-section,  FIG. 3  does not illustrate the helical aspects of channels  240  and walls  242 . 
     FIG. 4  is a schematic view of secondary air tube  222  in the combustor portion of flame holder immediately preceding reducer  250 . After air from first end  110  (not shown in  FIG. 4 ) is introduced into helical channels, it is transmitted along the length of tube  222 . The air is imparted with a swirl, that is, in addition to a velocity in the axial direction of the tube, the air also has a velocity in the circumferential direction, or with a tangential component. In a preferred embodiment, the pitch on the helical walls is between about one turn in 12″ and one turn in 20′. As the air exits the passageways beyond the termination of primary air tube  210  (not shown in FIG.  4 ), where fuel is injected into the air flow established by helical walls  242 , a flame front is established stabilized by the swirl of air by helical channels  240  and the bluff bodies made by the walls of the fins. Airflow over the walls  242  plus the swirl imparted to the air by the helical channels cause a plurality of flame vortices to form within the valleys between the walls  242 . These flame vortices or flamelets are substantially stationary. These flamelets ignite the balance of the faster moving air-gas mixture thereby providing stabilization to the flame as it accelerates through the orifice or aperture  256  of reducer  250 . The flamelets provide stabilization to the flame front since even if one or more are temporarily extinguished by transient conditions, others remain to perpetuate the main flame until steady state conditions are re-established and the extinguished flame vortices are reformed. 
   Secondary air tube  222  as well as other burner parts exposed to flame or high temperature exhaust gas such as reducer  250 , other radiant tube  400 , primary air tube  210 , exhaust gas housing and inner flame tube  300  are ideally manufactured of a material that has good heat transfer properties and that is thermally shock resistant, but which is suitable for continued use in an oxidative environment at sustained temperatures as high as about 2400° F. While oxidation-resistant high temperature metallic alloys may be used, such as high temperature stainless steels or high temperature superalloys, particularly alloys that have been coated with protective coatings for additional oxidation, corrosion and/or thermal protection, ceramic materials that are capable of being manufactured as tubes of the above configuration are preferred. Some of these ceramic materials may be ceramic composites that may be coated to improve their thermal properties or their oxidation resistance. A carbon-carbon material that has been coated with a protective coating is an example. Alumina may also be used, however, it does not have optimal thermal and conductive properties, and its use is limited to conditions in which the burner does not require thermal shock resistance. However, the preferred material is siliconized silicon carbide, also referred to as Si/SiC ceramic. A protective coating may be added to this tube, but it is not necessary. Another acceptable material is cordierite. 
   Tube  222  may be of any desired length, its length being dictated by the operating requirements for the radiant tube burner into which it is to be fitted. In a preferred embodiment, the wall thickness of the tube may vary from less than about ⅛″ to about ½″, measured away from walls  242 . More preferably, the wall thickness is in the range of ⅛″ to about ¼″. This radial thickness of the walls  242  in a preferred embodiment is about ⅛″, but may be increased or decreased based on design considerations dictated by factors such as finite element analysis or manufacturing techniques. This radial thickness also establishes one of the radial channel dimension, so that the tube thickness from the outside diameter to the inside diameter is increased by the radial thickness of the walls when measured at walls  242 . The width of the channel, that is, its circumferential dimension between adjacent walls  242 , may vary or may remain constant along the axial length of tube  222 . Clearly, if the channel size is increased, the flow of air will decrease and the velocity of the flame will be reduced. Channel size can be controlled by controlling the number of walls  242  or the circumferential thickness of the walls. This dimension also preferably is ⅛″. Alternatively, as the channel size is decreased, the opposite effect will occur. In a preferred embodiment, the circumferential dimension of channel  240  is two times the wall thickness, or about ¼″ when the wall thickness is about ⅛″. 
   The novel aspects of the flame holder mechanism are achieved as the walls  242  on secondary air tube act as an air swirl and as bluff body flame front stabilizers. Fuel is injected radially from the fuel tube  206  into channels  240 . Flame is spark ignited using the primary air flowing through the primary air tube. This flame propagates to the fuel and air being mixed in the region of the channels below primary tube  210 . The airflow causes standing vortices of fuel/air mixture to form in the channels between the helical fins that, when ignited, form flamelets. These flamelets ignite the balance of the mixing fuel and air and the stable flame exits at high velocity through the restricted nozzle. Instead of the combustion process being dependent on one pilot flame to ignite the fuel, after the initial “lighting” of the flame, the flame can be sustained by one or more of the flamelets, making the flame much more stable. 
   While the preferred embodiment is described in conjunction as a self-recuperative, single-ended radiant tube burner, the novel aspects of the present invention, particularly the flame stabilization aspects, are not restricted to use with a-self-recuperative, single-ended radiant tube burner and may be used in other types of burners, for example, with a U-tube burner assembly. 
     FIG. 5  depicts a cross-section of the U-tube self-recuperative burner assembly  500  of the present invention. The two self-recuperative, radiant tube burner assemblies  600  are shown fitted into a U-Tube housing, which serves as both the exhaust housing  510 ,  520  the burner assemblies  600  and as the outer radiant tube of the U-tube burner assembly  500 . The housing of the U-tube burner assembly  500  itself comprises two substantially parallel, linear U-tube elements  520 , each element  520  having a first end  522  and a second end  524  connected at one end by a semi-toroidal tube element  510 , said element having two ends,  512  and  514 . These elements are joined together so that the entire U-tube housing forms a hollow elongated “U.” These three elements  510  and  520  may be of a unitary construction or the elements  510  and  520  may be separate tubes that are mechanically or metallurgically joined together. The semi-toroidal tube element  510  is physically supported by a brace or bracket  550  in the furnace. Each self-recuperative, radiant tube burner  600  has a first end  610  and a second end  620 . In the case of the U-tube assembly, since there are two burner assemblies  600 , there are two first ends  610  and two-second ends  620 . 
   The radiant U-tube assembly  500  extends through two apertures  504  in a furnace wall  506 . Support structures  508  extend between the first ends  610  of radial tube burners  600  and the first ends of the linear U-Tube elements  522  and firmly attaches and stabilizes burners  600  to the first ends of linear U-tube elements  522 . A flame (not shown) is generated within self-recuperative single-ended tube burners  600  and heat is radiated from radiant U-tube elements  510  and  520  positioned within the furnace or kiln. The fuel enters the burners  500  through the fuel inlet lines (not shown) and the air enters the burners through the air inlet lines  530 . The exhaust leaves the burners through the exhaust lines  540 . 
   The two elements of the radiant burner assemblies  500  are the flame holders  200  and the inner flame tubes  310 . Each burner assembly has a first end  610  and a second end  620 . Each inner flame tube has a first open end  315  and a second open end  320 . Inner flame tubes  310 , are located within U-tube housing elements  520  receiving flame from reducers  250  at the first ends of the inner flame tubes  315 . A portion of the hot products of combustion continue out the open end  320  of the inner flame tube  310  (which are also the second ends of linear U-tube elements  524  and the second end of the burner assemblies  620 ) and into the semi-toroidal tube element  510 . A portion of the combustion gases flow in reverse direction toward the first end of the burners through the space created between the inner flame tube  310  and the U-tube element  520 . 
   Some U-tube assemblies have housings that have inner cross-sectional areas that are too large for the proper use of the self-recuperative burners of the present invention. In these burners, because the cross-sectional is too large, the velocity of the combustion gases is slower than is required for proper heat transfer between the hot gases of combustion and the incoming air and fuel. For these U-tube assemblies, it is necessary to fit a metal or ceramic sleeve  272  into the U-tube housings so as to reduce the cross-sectional area of the housing as far as the flame holders extend into the housings so that the space created between flame tube  310  and the U-tube element  520  is also reduced. By reducing the cross-sectional area of the housing, the velocity of the hot gases of combustion is increased to a velocity that enables adequate heat transfer between the hot gases of combustion and the incoming air and fuel. When a sleeve is used to decrease the cross-sectional area of the U-tube element, the combustion gases flow through the space created by the inner flame tube  310  and the sleeve. The sleeve can be used in any design to tailor the size of the space through which combustion gases flow. Because the combustion gases from both burners assemblies  600  are flowing into the semi-toroidal tube element  510 , the pressure may be zero or negative in sections of the semi-toroidal tube element  510 . Under normal operating conditions the amount of fuel and air that is supplied to the burners  600  will be about equal. Therefore, both burners will be producing about the same amount of combustion gases. Therefore, the volume of combustion gases that is flowing through one end of the toroidal element  512  toward the other end  514  is substantially the same amount of combustion gases that are flowing toward the other way, from end  514  toward end  512 . Thus, under normal operating conditions, the volume of combustion gases flowing out each of the exhaust lines  540  is substantially equivalent. 
   Since the heat transfer of the combustion gases to the furnace are equally generated by the two burners  600 , the combustion gases generally distribute more uniformly along the length of the three U-tube elements  510  and  520  than in prior art U-tubes having one burner and one recuperator. 
     FIG. 6  depicts the present invention included as a flame holder and combustor  600  in each end a self-recuperative radiant U-tube burner. Each self-recuperative radiant tube burner assembly  600  of the present invention includes a central axis  602  and is comprised of a flame holder  200  and an inner flame tube  310 . Each self-recuperative radiant tube burner assembly  600  has a first end  610 , a second end  620  and a substantially linear U-tube housing element  520  into which the burner assembly  600  is fitted. In retrofitting, the existing U-tube housing element  520  is already installed, as discussed above. The entire U-tube housing comprises three separate elements, a first substantially linear tube housing element  520 , a second substantially linear tube housing element  520  and a third toroidal housing element  510 , which are shown  FIG. 5   
   As both U-tube housing elements  520  have substantially identical self-recuperative tube burners assemblies  600 ,  FIG. 5  illustrates the burner assemblies  600  installed in both U-tube housing elements  520 . U-tube housing element  520  extends through an aperture  504  in a furnace wall  506 . A support structure  503  extends between the first end  610  of radial tube burner  600  and furnace wall  506  firmly attaches and stabilizes burner  600  to furnace wall  506 . In an alternative embodiment, support structure  508  extends between the first end  610  of radial tube burner assembly  600  and the end of U-tube housing element  620 , which firmly attaches and stabilizes burner assembly  600  to U-tube housing element  520 . In another alternative embodiment as shown in  FIG. 7 , support structure  508  extends between the first end  610  of radial tube burner assembly  600  and the sleeve  272 . In addition, U-tube housing elements  520  may have different physical structure, requiring support structures of differing geometries so that any convenient geometry may be used. A flame (not shown in  FIG. 5  is generated within self-recuperative tube burner assembly  600  and heat is radiated from the three U-tube housing elements  510  and  520  (as shown in  FIG. 5  into the furnace. 
   Flame holder  200  has a first end  202  that extends within exhaust U-tube housing element  520  from the first end  610  of burner assembly  600  and terminates at a second end  204 , typically positioned inside the furnace wall and within U-tube housing element  520 . Flame holder  200  comprises a fuel tube  206  that extends from the first end  610  of tube burner assembly  600  and, as shown in  FIG. 6 , is substantially coaxial with burner assembly  600 . A primary air tube  210  surrounds fuel tube  206 . The inside diameter  212  of primary air tube  210  is larger than the outside diameter of fuel tube  206 ,  50  that a natural passageway  214  is formed between fuel tube  206  and inside diameter  212 . Fuel is provided to fuel tube  206  by means of a connection to a fuel supply (not shown in  FIG. 5 ) at first end  610  of burner assembly  600 . Passageway  214 , also referred to as the primary air passageway, supplies air to mix with fuel from fuel tube  206 , which is supplied from first end  610  of burner assembly  600 . Primary air tube  210  terminates within flame holder  200 . A secondary air tube  222  has an outside diameter  216  surrounding tube  210  and extends beyond tube  210 . Fuel tube  206  terminates within flame holder  200  just beyond primary air tube  210 . Fuel tube  206  includes a plurality of apertures  220  having at least a partial radial orientation to form a nozzle that distribute fuel inside tube  210 , best illustrated in  FIG. 2 , an enlarged section of the second end  204  of flame holder  200 . 
   As depicted in the preferred embodiment of  FIG. 6 , the central axis of fuel tube  206  is coincident with burner central axis  602 , but this geometric configuration is not required. The only requirement is that the fuel tube must deliver fuel proximate to primary air and spark initiator. 
   As shown in  FIG. 2 , combustion is initially started by injecting a fuel from the plurality of apertures  220  forming a nozzle while simultaneously supplying air along primary air tube  214 . Ignition is initiated by supplying an ignition voltage at spark initiator  224 , a spark plug. An electric spark from the spark plug having its sparking electrode extending into the fuel stream causes an ignition of the fuel-air mixture. After ignition, combustion normally is a self-sustaining process, as long as the fuel and air feed the combustion flame. Flame sensor  230  detects the presence of a flame and, via appropriate indication, when the flame has been extinguished. 
   Referring back to  FIG. 6 , secondary air tube  222  includes a plurality of channels  240  that extend along its length. These channels are formed by helical walls  242  that extend inward from tube  222 . As best seen in  FIG. 2 , beyond the termination of primary air tube  210 , these walls extend away from secondary air tube  222 , projecting inwardly toward the central axis  602  of the burner. These channels form the secondary air passageways  240 . The radial inward boundary of these passageways  240  is the outer diameter of primary air tube  210 . Air is introduced into the secondary air tube at the first end  610  of radiant burner tube  600 . The air is supplied under pressure or it may be injected under pressure, typically from a combustion air blower. 
   At the second end  204  of flame holder  200  is a conically shaped reducer  250  that acts as a nozzle to discharge the flame. Reducer  250  has a first end  252  that is received within tube  222 . A second end  254  of discharge nozzle  250  has an aperture  256  to direct the flame toward the inner flame tube  310 . Discharge nozzle  250  can be any desired shape, but in a preferred embodiment shown in  FIGS. 6 and 2 , is conically shaped to concentrate the flame, increasing the flame velocity as it directs the flame toward inner flame tube  310 . 
   Inner flame tube  310  is located within U-tube housing element  520  receiving flame from reducer  250 . Combustion is completed in inner flame tube  310 . Unlike the embodiment set forth in  FIG. 1 , in which the end of outer radiant tube  400  is closed, not all of the hot products of combustion are turned 180° at second end  620  of inner flame tube  310 . Rather, only a portion of the hot products of combustion are turned 180° and flow in a reverse direction toward first end  610  of burner through the annular space or gap  260  between inner flame tube  310  and outer radiant tube (and U-tube housing element)  520 . A second annular space  270  is present between the flame holder  200  and U-tube housing element  520 . The remaining elements of combustion continue out the open end of inner flame tube  315  and into the third toroidal U-tube element  510  (shown in FIG.  5 ). 
   The configuration of inner flame tube  310  as depicted in  FIG. 5  is conventional. It is located at end  620  of self-recuperative radiant tube burner assembly  600 , receiving the flame from reducer  250  where combustion is completed. Heat is radiated from the three U-tube housing elements  510  and  520  into the vessel that is being heated, for example a kiln or industrial furnace. The flame may expand as it is projected from discharge nozzle into inner flame tube  310  and any additional combustion may be completed as any uncombusted fuel and air combine in this portion of the burner. 
   As shown in  FIG. 6 , secondary air tube  222  is formed with a plurality of helical channels  240  formed by helical walls  242  that serve as a conduit for secondary air. These helical walls  242  form closed channels  240  that are bounded passageways when a primary air tube  210  is inserted within secondary air tube  222 . The helical walls extend beyond the termination of primary air tube  210 . The helical walls  242  essentially act as heat exchanger fins along their entire length. Below the termination of primary air tube  210 , the helical channels are no longer completely bounded passageways. It will be understood by those skilled in the art that although the combustor is shown comprising three elements, a primary air tube  210 , a secondary air tube  222  having helical walls, and a reducer, these elements may be combined to form a single piece or as a single tube having the requisite characteristics coupled to a reducer. 
   Air is introduced into the channels at a preselected location at the first end  610  of self-recuperative single-ended tube burner  600 . The air may be introduced axially or radially or both. The method of introduction of air is not a part of this invention. However, as exhaust gases exit the burner at first end  610 , air should be introduced in any satisfactory manner that does not interfere with the discharge of exhaust gases. The air enters helical channels  240  which run the length of tube  222  in the preferred embodiment. It will be understood that the helical channels are not required to extend the entire length of tube  222  to be effective, nor are the walls  242  required to extend the entire length of tube  222 . However, as the walls  242  act as fins, a maximum of efficiency is obtained when they extend the entire length of tube  222 . One alternative embodiment includes walls  242  that are not helical at first end  610  of burner assembly  600 , but which become helical at or near the termination of tube  210 . Another embodiment includes no walls  242  along tube  222  toward first end  510 , but helical walls  242  are included between primary air tube  210  and secondary air tube  222  at an intermediate location along the length of tube  210  away from first end  610  of burner assembly  600 . Other configurations of the walls  242  and channels  240  similarly are within the scope of the invention. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.