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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 formin

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.

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1. 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, said first housing element having a first an

1. 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, said first housing element having a first and a second end, said second housing element having a first and a second end, said third toroidal housing element having a first end and a second end, said second end of the first housing element being joined to the first end of the toroidal housing element, said 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; two flame holders for a diffusion flame burner, each flame holder having a first end and a second end, wherein the two flame holders are capable of holding a substantially equivalent flame of combustion simultaneously during normal burner operation; a first flame holder being positioned in the first end of the first housing element and a second flame holder being positioned in the first end of the second housing element; the first flame holder having a first inner flame tube axially spaced from the second end of the flame holder by a gap and positioned at the second end of the first housing element; and the second flame holder having a second inner flame tube axially spaced from the second end of the first flame holder by a gap and positioned at the second end of the second housing element. 2. 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, said first housing element having a first and a second end, said second housing element having a first and a second end, said third toroidal housing element having a first end and a second end, said second end of the first housing element being joined to the first end of the toroidal housing element, said 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; two flame holders for a diffusion flame burner, each flame holder having a first end and a second end; a first flame holder being positioned in the first end of the first housing element and a second flame holder being positioned in the first end of the second housing element; the first flame holder having a first inner flame tube axially spaced from the second end of the flame holder by a gap and positioned at the second end of the first housing element; and the second flame holder having a second inner flame tube axially spaced from the second end of the first flame holder by a gap and positioned at the second end of the second housing element, a secondary air tube axially extending from the first end of the flame holder toward the second end of the flame holder, the secondary air tube having an inside diameter and an outside diameter; a primary air tube extending from the first end of the flame holder toward the second end of the flame holder, the primary air tube being coaxial with the secondary air tube along at least a portion of the secondary air tube, the primary air tube having an outside diameter that is smaller than the inside diameter of the secondary air tube, the secondary air tube extending beyond a terminus of the primary air tube toward the second end of the flame holder; a fuel tube extending beyond the terminus of the primary air tube at the second end of the flame holder, the fuel tube having an outside diameter that is smaller than the inside diameter of the primary air tube and coaxial with the primary air tube along at least a portion of a central axis of the primary air tube at the second end of the flame holder, the fuel tube including a plurality of radially oriented apertures; an axially oriented reducer positioned at the second end of the flame holder coupled to the secondary air tube; a plurality of helical walls extending between the inside diameter of the secondary air tube and the outside diameter of the primary air tube, the plurality of helical walls, the inside diameter of the secondary air tube and the outside diameter of the primary air tube forming closed helical channels, and the inside diameter of the secondary air tube and the plurality of helical walls forming open helical channels for at least a portion of the secondary air tube axially extending beyond the primary air tube at the second end of the flame holder; and the helical channels imparting a swirl to secondary air from the secondary air tube mixing with fuel from the fuel tube to form stationary vortices of a fuel and air mixture to form flamelets that stabilize a flame upon an ignition sequence; and a combustion zone formed by the at least a portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder and the reducer coupled to the secondary air tube wherein combustion of a fuel and air mixture is propagated by the flamelets. 3. The U-tube burner assembly of claim 2, wherein a pitch of the helical channels is sufficient to swirl secondary air to cause formation of the stationary vortices.4. The U-tube burner assembly of claim 3 wherein the pitch of the helical walls is about one revolution of the inside diameter of the secondary tubes in 12 inches of an axial length of the secondary tubes to about one revolution of the inside diameters of the secondary tubes in 12 inches of the axial lengths of the secondary tubes.5. The U-tube burner assembly of claim 2 wherein at least the secondary air tubes are further comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.6. The U-tube burner assembly of claim 5 further including the primary air tubes and the reducers comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.7. The U-tube burner assembly of claim 5 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.8. The U-tube burner assembly of claim 5 wherein the material is coated with a protective coating.9. The U-tube burner assembly of claim 5 wherein the ceramic composite material is Si/SiC ceramic.10. The U-tube burner assembly of claim 6 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.11. The U-tube burner assembly of claim 2 wherein each primary air tube and secondary air tube are single units comprising a primary air tube joined to a secondary air tube.12. The U-tube burner assembly of claim 11 wherein each primary air tube and secondary air tube are joined by the respective plurality of helical walls extending from the inside diameter of the secondary air tube to the outside diameter of the primary air tube.13. The U-tube burner assembly of claim 2 wherein each primary air tube, secondary air tube and reducer are a single unit.14. The U-tube burner assembly of claim 2 wherein each primary air tube and reducer are a single unit.15. The U-tube burner assembly of claim 2 wherein the helical walls extend radially inward from at least a portion of each inside diameter of the secondary air tubes.16. The U-tube burner assembly of claim 2 wherein the helical walls extend radially inward from the inside diameters of the secondary air tubes along their complete axial length.17. The U-tube burner assembly of claim 2 wherein the plurality of helical walls includes at least 10 channels per set of helical walls.18. The U-tube burner assembly of claim 2 wherein the velocity of a combustion gas flowing through the housing is increased by modifying the cross-sectional area of the housing.19. The U-Tube burner assembly of claim 2 wherein sleeves are inserted into the housing to increase the velocity of a combustion gas flowing through the housing by modifying a cross-sectional area of the housing.20. The U-tube burner assembly of claim 19 wherein the sleeves comprise ceramic.21. The U-tube burner assembly of claim 19 wherein the sleeves comprise metal.22. The U-tube burner assembly of claim 19 wherein the sleeves are mechanically fastened to the U-tube housing.23. The U-tube burner assembly of claim 19 wherein the sleeves are mechanically fastened to the burners.24. A U-tube burner system having recuperative radiant tube burners 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, said first housing element having a first and a second end, said second housing element having a first and a second end, said third toroidal housing element having a first end and a second end, said second end of the first housing element being joined to the first end of the toroidal housing element, said 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; two flame holders for a diffusion flame burner, each flame holder having a first end and a second end; a first flame holder being positioned in the first end of the first housing element and a second flame holder being positioned in the first end of the second housing element; the first flame holder having a first inner flame tube axially spaced from the second end of the flame holder by a gap and positioned at the second end of the first housing element; and the second flame holder having a second inner flame tube axially spaced from the second end of the first flame holder by a gap and positioned at the second end of the second housing element; the first flame holder having a first secondary air tube extending from the first end of the first flame holder toward the second end of the first flame holder, a first fuel tube extending toward the second end of the first flame holder, and a first reducer nozzle positioned at the second end of the first flame holder, the first secondary air tube having an inside diameter surrounding the first fuel tube and an outside diameter, a first passageway defined within the inside diameter of the first secondary air tube for transport of primary air, the first passageway for the transport of primary air terminating at a terminus of the inside diameter of the first secondary air tube, the first reducer nozzle positioned to project a first flame from the first flame holder toward the first inside flame tube; the second flame holder having a second secondary air tube extending from the first end of the second flame holder toward the second end of the second flame holder, a second fuel tube extending toward the second end of the second flame holder, and a second reducer nozzle positioned at the second end of the second flame holder, the second secondary air tube having an inside diameter surrounding the second fuel tube and an outside diameter, a second passageway defined within the inside diameter of the second secondary air tube for transport of primary air, the second passageway for the transport of primary air terminating at a terminus of the inside diameter of the second secondary air tube, the second reducer nozzle positioned to project a second flame from the second flame holder toward the second inside flame tube; the first secondary air tube having a first plurality of closed helical channels located between its outside diameter and inside diameter and extending for a first preselected distance along at least a portion of the first secondary air tube terminating beyond the terminus of the inside diameter of the first secondary tube as a first plurality of open helical channels; the second secondary air tube having a second plurality of closed helical channels located between its outside diameter and inside diameter and extending for a second preselected distance along at least a portion of the second secondary air tube terminating beyond the terminus of the inside diameter of the second secondary tube as a second plurality of open helical channels; the first plurality of helical channels imparting a first swirl to the first secondary air to stabilize the first flame while a first portion of first exhaust gases is expelled toward the first end of the first housing element through a first gap formed between a first outer boundary comprised of the first flame holder and the first inner flame tube and a first inner boundary comprised of the first housing element and while a second portion of first exhaust gases is expelled toward the first end of the second housing element through the third housing element and through a second gap formed between a second outer boundary comprised of the second flame holder and the second inner flame tube and a second inner boundary comprised of the second housing element; and the second plurality of helical channels imparting a second swirl to the second secondary air to stabilize the second flame while a first portion of second exhaust gases is expelled toward the first end of the second housing element through the second gap and while a second portion of first exhaust gases is expelled toward the first end of the first housing element through the third housing element and through the first gap. 25. The U-tube burner assembly of claim 18 wherein the secondary tubes are comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.26. The U-tube burner assembly of claim 19 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.27. A method for stabilizing flames in a U-tube burner assembly, comprising the steps of:providing two flame holders, each flame holder having a first end and a second end, the flame holders further comprising: a secondary air tube extending from the first end of the flame holder toward the second end of the flame holder, the secondary air tube having an inside diameter and an outside diameter, a primary air tube extending from the first end of the flame holder to the second end of the flame holder, the primary air tube being coaxial with the secondary air tube along at least a portion of the secondary air tube and having a terminus within the secondary air tube, the primary air tube having an outside diameter that is smaller than the inside diameter of the secondary air tube, a fuel tube extending beyond the primary air tube at the terminus of the primary air tube within secondary air tube toward the second end of the flame holder, the fuel tube having an outside diameter that is smaller than the inside diameter of the primary air tube and coaxial with the primary air tube along at least a portion of a central axis of the primary air tube at the second end of the flame holder, a portion of the fuel tube extending beyond the primary air tube including a plurality of radially oriented apertures, an axially-oriented reducer positioned at the second end of the flame holder coupled to the secondary air tube, and a plurality of helical walls extending between the inside diameter of the secondary air tube and the outside diameter of the primary air tube for at least a portion of the secondary air tube, the helical walls, the inner diameter of the secondary air tube and the outer diameter of the primary air tube forming closed helical channels, and the inner diameter of the secondary air tube and the helical walls forming open helical channels for at least a portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder; providing air to the primary air tube and the secondary air tube; providing fuel to the fuel tube; mixing the fuel and the air to form a fuel/air mixture as fuel exits the fuel tube apertures; igniting the fuel/air mixture to form a flame; imparting a swirl to secondary air from the secondary air tube mixed with fuel from the fuel tube to form stationary vortices of the fuel/air mixture to form flamelets that stabilize the flame, the flame being combusted in a combustion zone formed by the at least the portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder and the reducer wherein combustion of a fuel and air mixture is propagated by the flamelets.28. A flame holder for a diffusion flame burner, the flame holder having a first end and a second end, comprising:a secondary air tube axially extending from the first end of the flame holder toward the second end of the flame holder, the secondary air tube having an inside diameter and an outside diameter; a primary air tube extending from the first end of the flame holder toward the second end of the flame holder, the primary air tube being coaxial with the secondary air tube along at least a portion of the secondary air tube, the primary air tube having an outside diameter that is smaller than the inside diameter of the secondary air tube, the secondary air tube extending beyond a terminus of the primary air tube toward the second end of the flame holder; a fuel tube extending beyond the terminus of the primary air tube at the second end of the flame holder, the fuel tube having an outside diameter that is smaller than the inside diameter of the primary air tube and coaxial with the primary air tube along at least a portion of a central axis of the primary air tube at the second end of the flame holder, the fuel tube including a plurality of radially oriented apertures; an axially oriented reducer positioned at the second end of the flame holder coupled to the secondary air tube; a plurality of helical walls extending between the inside diameter of the secondary air tube and the outside diameter of the primary air tube, the plurality of helical walls, the inside diameter of the secondary air tube and the outside diameter of the primary air tube forming closed helical channels, and the inside diameter of the secondary air tube and the plurality of helical walls forming open helical channels for at least a portion of the secondary air tube axially extending beyond the primary air tube at the second end of the flame holder; the helical channels imparting a swirl to secondary air from the secondary air tube mixing with fuel from the fuel tube to form stationary vortices of a fuel and air mixture to form flamelets that stabilize a flame upon an ignition sequence; and a combustion zone formed by the at least a portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder and the reducer coupled to the secondary air tube wherein combustion of a fuel and air mixture is propagated by the flamelets. 29. The flame holder of claim 28 wherein a pitch of the helical channels is sufficient to swirl secondary air to cause formation of the stationary vortices.30. The flame holder of claim 29 wherein the pitch of the helical walls is about one revolution of the inside diameter of the secondary tube in 12 inches of an axial length of the secondary tube to about one revolution of the inside diameter of the secondary tube in 12 inches of the axial length of the secondary tube.31. The flame holder of claim 28 wherein at least the secondary air tube is further comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.32. The flame holder of claim 31 further including the primary air tube and the reducer comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.33. The flame holder of claim 31 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.34. The flame holder of claim 31 wherein the material is coated with a protective coating.35. The flame holder of claim 31 wherein the ceramic composite material is Si/SiC ceramic.36. The flame holder of claim 32 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.37. The flame holder of claim 28 wherein the primary air tube and the secondary air tube are a single unit comprising a primary air tube joined to the secondary air tube.38. The flame holder of claim 28 wherein the primary air tube and the secondary air tube are joined by the plurality of helical walls extending from the inside diameter of the secondary air tube to the outside diameter of the primary air tube.39. The flame holder of claim 28 wherein the primary air tube, the secondary air tube and the reducer are a single unit.40. The flame holder of claim 28 wherein the primary air tube and the reducer are a single unit.41. The flame holder of claim 28 wherein the helical walls extend radially inward from at least a portion of the inside diameter of the secondary air tube.42. The flame holder of claim 28 wherein the helical walls extend radially inward from the inside diameter of the secondary air tube along its complete axial length.43. The flame holder of claim 28 wherein the plurality of helical walls includes at least 10 channels.44. The flame holder of claim 43 wherein the plurality of helical walls includes at least 16 channels.45. The flame holder of claim 28 wherein a radial thickness of the secondary air tube is between about ⅛″-½″.46. The flame holder of claim 45 wherein the radial thickness of the secondary tube is between about ⅛″-¼″.47. The flame holder of claim 28 wherein a radial dimension of the plurality of helical walls is between about ⅛″-¼″.48. The flame holder of claim 28 wherein the helical channels have a circumferential dimension that is about two times a radial dimension of the helical walls.49. The flame holder of claim 28 further including means for igniting a fuel and air mixture.50. The flame holder of claim 49 wherein the means for igniting is a sparking device.51. A recuperative radiant tube burner having improved flame stability, comprising:a first end, a second end, and a housing having a central axis extending between the first end and the second end; a flame holder, having a first end and a second end, positioned within the first end of the burner and extending axially within the housing toward the second end of the burner; an inner flame tube axially spaced from the second end of the flame holder by a gap and positioned at the second end of the housing; the flame holder having a secondary air tube extending from the first end of the flame holder to the second end of the flame holder, a fuel tube extending toward the second end of the flame holder, and a reducer nozzle positioned at the second end of the flame holder, the secondary air tube having an inside diameter surrounding the fuel tube and an outside diameter, a passageway defined within the inside diameter of the secondary air tube for transport of primary air, the passageway for the transport of primary air terminating at a terminus of the inside diameter of the secondary air tube, the reducer nozzle positioned to project a flame from the flame holder toward the inside flame tube; the secondary air tube having a plurality of closed helical channels located between its outside diameter and inside diameter and extending for a preselected distance along at least a portion of the secondary air tube terminating beyond the terminus of the inside diameter of the secondary tube as a plurality of open helical channels; the plurality of helical channels imparting a swirl to the secondary air to stabilize the flame while exhaust gases are expelled toward the first end of the housing through a gap formed between an outer boundary comprised of the flame holder and the inner flame tube and an inner boundary comprised of the housing. 52. The burner of claim 51 wherein the secondary tube is comprised of a material that is thermally shock resistant and suitable for use in an oxidative environment.53. The burner of claim 52 wherein the material is selected from the group consisting of carbon-carbon materials, cordierite, ceramic materials, ceramic composite materials, and ceramic matrix composite materials.54. The burner of claim 51 further including an ignition device.55. A method for stabilizing a flame in a diffusion flame burner, comprising the steps of:providing a flame holder having a first end and a second end, the flame holder further comprising: a secondary air tube extending from the first end of the flame holder toward the second end of the flame holder, the secondary air tube having an inside diameter and an outside diameter; a primary air tube extending from the first end of the flame holder to the second end of the flame holder, the primary air tube being coaxial with the secondary air tube along at least a portion of the secondary air tube and having a terminus within the secondary air tube, the primary air tube having an outside diameter that is smaller than the inside diameter of the secondary air tube; a fuel tube extending beyond the primary air tube at the terminus of the primary air tube within secondary air tube toward the second end of the flame holder, the fuel tube having an outside diameter that is smaller than the inside diameter of the primary air tube and coaxial with the primary air tube along at least a portion of a central axis of the primary air tube at the second end of the flame holder, a portion of the fuel tube extending beyond the primary air tube including a plurality of radially oriented apertures; an axially-oriented reducer positioned at the second end of the flame holder coupled to the secondary air tube; and a plurality of helical walls extending between the inside diameter of the secondary air tube and the outside diameter of the primary air tube for at least a portion of the secondary air tube, the helical walls, the inner diameter of the secondary air tube and the outer diameter of the primary air tube forming closed helical channels, and the inner diameter of the secondary air tube and the helical walls forming open helical channels for at least a portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder; providing air to the primary air tube and the secondary air tube; providing fuel to the fuel tube; mixing the fuel and the air to form a fuel/air mixture as fuel exits the fuel tube apertures; igniting the fuel/air mixture to form a flame; and imparting a swirl to secondary air from the secondary air tube mixed with fuel from the fuel tube to form stationary vortices of the fuel/air mixture to form flamelets that stabilize the flame, the flame being combusted in a combustion zone formed by the at least the portion of the secondary air tube that axially extends beyond the primary air tube at the second end of the flame holder and the reducer wherein combustion of a fuel and air mixture is propagated by the flamelets.