Patent Publication Number: US-7591648-B2

Title: Burner apparatus

Description:
BACKGROUND 
   The present disclosure relates to burner assemblies, and particularly to air-fuel burner assemblies. More particularly, the present disclosure relates to internally fired industrial gas burners. 
   SUMMARY 
   A burner assembly in accordance with the present disclosure includes a fuel nozzle and an air-fuel mixing cone coupled to the fuel nozzle. A mixing chamber provided in the air-fuel mixing cone is configured to receive and mix fuel discharged by the fuel nozzle with pressurized air extant in a nearby air plenum to generate a combustible air-fuel mixture. This mixture can be ignited to produce a flame. 
   The air-fuel mixing cone includes an inner end having an opening receiving the fuel nozzle, an outer end having a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends. The air-fuel mixing cone also includes an air-admission portal comprising various openings formed in the funnel-shaped size wall to conduct pressurized combustion air extant in the air plenum into the mixing chamber to mix with fuel discharged into the mixing chamber by the fuel nozzle. 
   In illustrative embodiments, the air-admission portal is formed in the funnel-shaped side wall and configured to decrease progressively in effective size (i.e., total open area) along a length of the funnel-shaped wall as the distance away from the fuel nozzle increases. This progressive decrease in the total open area of the openings formed in the funnel-shaped side wall to define the air-admission portal causes a greater volume of pressurized combustion air to pass from the air plenum through an “upstream” portion of the air-admission portal into a part of the mixing chamber located near to the fuel nozzle. This progressive decrease also causes a lesser volume of pressurized combustion air to pass from the air plenum through a “downstream” portion of the air-admission portal into other parts of the mixing chamber located farther away from the fuel nozzle. 
   In illustrative embodiments, the funnel-shaped side wall includes a perforated inlet section located near the fuel nozzle and formed to include the air-admission portal. A cold-temperature flame-quenching zone is formed in the perforated inlet section and this zone “contains” a first-stage air-and-fuel mixture characterized by a relatively low nitrogen oxide (NOx) content and a relatively high hydrocarbon (HC) content and a relatively high carbon monoxide (CO) content. 
   The funnel-shaped side wall also includes a “downstream” unperforated outlet section located between the perforated inlet section and the downstream combustion-discharge opening. A high-temperature emission-reduction burnout zone is formed in the unperforated outlet section to burn CO and HC included in the first-stage air-and-fuel mixture flowing from the cold-temperature flame-quenching zone of the perforated inlet section into the high-temperature emission-reduction burnout zone. In this emission-reduction burnout zone, CO and unburned HC are burned to produce a second-stage air-and-fuel mixture characterized by a low NOx content, a low CO content, and a low hydrocarbon (HC) content. No additional combustion air is added to the second-stage air-and-fuel mixture flowing through the high-temperature emission-reduction burnout zone formed in the unperforated outlet section of the funnel-shaped side wall. The absence of air at this stage raises the temperature and lowers CO and HC content of the air-and-fuel mixture flowing in the burnout zone to produce a second-stage air-and-fuel mixture in accordance with the present disclosure. 
   An igniter is used to ignite the combustible air-and-fuel mixture created in the mixing chamber to produce a flame. In illustrative embodiments, about 80 to 90 percent of the air needed for combustion is admitted into the mixing chamber through the air-admission portal that is configured to have a progressively smaller effective “open area” or size as the air-admission portal extends away from the fuel nozzle and along the length of the funnel-shaped side wall. In such embodiments, about 10 to 20 percent of the air needed for combustion is discharged into a downstream combustion zone provided in a burner housing configured to receive the second-stage air-and-fuel mixture exiting through the downstream combustion-discharge opening formed in the air-fuel mixing cone. 
   Additional features of the present disclosure will become apparent to those skilled in the art upon consideration of illustrative embodiments exemplifying the best mode of carrying out the disclosure as presently perceived. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The detailed description particularly refers to the accompanying figures in which: 
       FIG. 1  is a perspective view of an air-fuel burner, with portions broken away, showing a fuel nozzle including a cylindrical shell formed to include eight fuel-discharge ports and a fuel-transport passageway conducting fuel from a fuel supply to the fuel-discharge ports and an air-fuel mixing cone in accordance with the present disclosure mounted in a burner housing to mate with the fuel nozzle and configured to mix incoming fuel discharged by the fuel nozzle into a “mixing” chamber formed in the cone with “primary combustion” air discharged into the mixing chamber through various air-admission slots and ports formed in a perforated inlet section of the cone to produce a combustible air-fuel mixture in the mixing chamber of the air-fuel mixing cone; 
       FIG. 2  is a schematic diagram of the air-fuel mixing cone and fuel nozzle of  FIG. 1  located in an air plenum formed in the burner housing showing, in series, from left to right, formation of (1) an upstream cold-temperature flame-quenching zone arranged to extend from the fuel nozzle in a “downstream” direction, located in the perforated inlet section of the air-fuel mixing cone, and supplied with primary (combustion) air via an air-admission portal comprising air-admission ports and slots formed in the perforated inlet section, (2) a downstream high-temperature emission-reduction burnout zone located in an unperforated outlet section of the air-fuel mixing cone and not supplied with any combustion air, and (3) a downstream combustion zone arranged to lie outside the air-fuel mixing cone and communicate with an outer end of the air-fuel mixing cone, located in a cylindrical burner discharge sleeve included in the burner housing and supplied with secondary (combustion) air discharged through an annular space formed between a large-diameter outer rim defining the outer end of the air-fuel mixing cone and a surrounding portion of the cylindrical burner discharge sleeve; 
       FIG. 3  is an enlarged perspective view of an exterior surface of a funnel-shaped side wall included in the air-fuel mixing cone of  FIG. 1  showing formation, in the perforated inlet section of the cone, of an air-admission portal comprising eight spaced-apart air-admission slots (each air-admission slot being characterized by a relatively larger sized inner opening located near a circular upstream nozzle-receiver opening formed in the narrow-diameter inner end of the cone) and of eight spaced-apart sets of air-admission ports and showing that the air-admission ports are progressively reduced in size as they are located further away from the circular nozzle-receiver opening formed in the narrow-diameter inner end of the cone and that there are no air-admission slots or ports in the relatively wider unperforated outlet section of the cone; 
       FIG. 4  is an enlarged sectional view taken along line  4 - 4  of  FIG. 1  showing the air-fuel mixing cone mounted on a downstream end of the fuel nozzle and showing formation of the fuel nozzle to include a fuel-transport passageway leading to several fuel-discharge ports opening into the mixing chamber formed in the air-fuel mixing cone; 
       FIG. 5  is an elevation view taken generally along line  5 - 5  of  FIG. 4  showing eight circumferentially spaced-apart fuel-discharge ports formed in the fuel nozzle, eight “keyhole-shaped” air-admission slots formed in the perforated inlet section of the cone, and eight sets of air-admission ports also formed in the perforated inlet section of the cone and diagrammatically showing some air and gas flow into the mixing chamber formed in the cone during “low-fire” conditions; 
       FIG. 6  is an elevation view similar to  FIG. 5  diagrammatically showing relatively greater air and gas flow into the mixing chamber formed in the cone during “mid-fire” conditions; 
       FIG. 7  is an elevation view similar to  FIGS. 5 and 6  diagrammatically showing “crescent-shaped” flame attachment regions on the interior surface of the cone during “high-fire” conditions; 
       FIG. 8  is a graph showing that the effective size of the “openings” in the air-fuel mixing cone made in accordance with the present disclosure and defined by the air-admission slots and ports decreases as (1) the volume of the cone increases and (2) the distance from the fuel nozzle increases in marked contrast to an increasing effective size of openings provided in a “typical” air-fuel burner; 
       FIGS. 9 and 10  show an air-fuel mixing cone in accordance with a second embodiment of the present disclosure; 
       FIGS. 11 and 12  show an air-fuel mixing cone in accordance with a third embodiment of the present disclosure; 
       FIGS. 13 and 14  show an air-fuel mixing cone in accordance with a fourth embodiment of the present disclosure; and 
       FIGS. 15 and 16  show an air-fuel mixing cone in accordance with a fifth embodiment of the present disclosure. 
   

   DETAILED DESCRIPTION 
   An illustrative burner assembly  10  for combining air from an air supply  12  and fuel from a fuel supply  14  to produce a flame (not shown) in a flame chamber  16  in a burner housing  18  is shown in  FIG. 1 . An air-fuel mixing cone  20  in accordance with the represent disclosure is shown illustratively in FIGS.  1  and  3 - 7  and diagrammatically in  FIG. 2 . A second illustrative air-fuel mixing cone  220  is shown in  FIGS. 9-10 . A third illustrative air-fuel mixing cone  320  is shown in  FIGS. 11-12 . A fourth illustrative air-fuel mixing cone  420  is shown in  FIGS. 13-14 . A fifth illustrative air-fuel mixing cone is shown in  FIGS. 15-16 . 
   Each of air-fuel mixing cones  20 ,  220 ,  320 ,  420 , and  520  is configured in accordance with the present disclosure to regulate flow of combustion air from air supply  12  into a mixing chamber containing fuel from fuel supply  14 . Each cone is formed to add a lot of combustion air into an upstream region of the mixing chamber near the fuel nozzle, then progressively decrease the amount of combustion air added into the mixing chamber as distance from the fuel nozzle increases, and finally block admission of any combustion air into a downstream region of the mixing chamber. By managing admission of combustion air in accordance with the present disclosure, it is possible to discharge from the mixing chambers provided in air-fuel mixing cones  20 ,  220 ,  320 ,  420 , and  520  an air-fuel mixture  102  characterized by a low nitrogen oxide (NOx) content, a low carbon monoxide (CO) content, and a low hydrocarbon (HC) content as suggested in  FIG. 2 . 
   As shown in  FIG. 1 , burner assembly  10  includes an air inlet duct  22  formed to include an air intake opening  24 , an air plenum  26  formed to include an air plenum chamber  28  arranged to receive combustion air  30  discharged through an air exhaust opening  34  formed in air inlet duct  22 , and a fuel nozzle  36  coupled to fuel supply  14  via a conduit  38  and arranged to extend into air plenum chamber  28  of air plenum  26  to mate with air-fuel mixing cone  20 . Air inlet duct  22  includes an air-conducting passageway  25  extending from air intake opening  24  to air exhaust opening  34  as suggested in  FIG. 1 . An air flow regulator  40  comprising an air intake valve  41 , an air intake valve controller  42 , and a valve-mover linkage  43  interconnecting air intake valve  41  and air intake valve controller  42  is coupled to burner housing  18  to regulate the flow of combustion air  30  discharged into air plenum chamber  28 . Valve-mover linkage  43  is also coupled to a fuel intake valve  31  (not shown) associated with conduit  38  and a fuel linkage  33  as suggested in  FIG. 1 . Air intake valve  41  and fuel intake valve  31  are linked via valve-mover linkage  43  and cooperate to regulate flow of combustion air  30  discharged into air plenum chamber  28  and the flow of fuel into fuel nozzle  36 . An impeller  44  turned by a motor  45  and located in an airflow conduit  46  interconnecting air supply  12  and air intake opening  24  of air inlet duct  22  is used to discharge combustion air  30  into air plenum chamber  28  via air inlet duct  22 . 
   Burner housing  18  also includes a burner discharge sleeve  50  formed to include an interior region  51  and coupled to air plenum  26  as shown, for example, in  FIGS. 1 ,  9 ,  11 ,  13 , and  15 . A cone support mount  52  is included in burner housing  18  and used to support air-fuel mixing cone  20  partly in air plenum chamber  28  and partly in interior region  51  of burner discharge sleeve  50 . It is within the scope of this disclosure to adjust the position of air-fuel mixing cone  20  in directions  53  or  54  and relative to air plenum  26  and burner discharge sleeve  50  as needed. In an illustrative embodiment, cone support mount  52  is formed to include air-flow passageways  54  interconnecting air plenum chamber  28  and interior region  51  in fluid communication. 
   As suggested in  FIGS. 1 and 4 , fuel nozzle  36  includes a shell  56  having an outer end  58  formed to include several circumferentially spaced-apart fuel-discharge ports  60 . Shell  56  also is formed to include a fuel-transport passageway  62  arranged to communicate fuel from fuel supply conduit  38  to fuel-discharge ports  60  to cause a stream  61  of fuel (see FIGS.  2  and  5 - 7 ) to be discharged from fuel-transport passageway  62  through each of fuel-discharge ports  60  into a mixing chamber  66  formed in air-fuel mixing cone  20 . In the illustrated embodiment, a base  57  of shell  56  is coupled to burner housing  18  and most of fuel nozzle  36  is arranged to lie in air plenum chamber  28  as suggested in  FIG. 1 . 
   Mixing means  21  is provided for mixing the streams  61  of fuel discharged through fuel-discharge ports  60  formed in fuel nozzle  36  with primary (combustion) air  31  taken from combustion air  30  extant in air plenum  26  associated with fuel nozzle  36  to produce an air-and-fuel mixture  100  that can be ignited in mixing chamber  66  to produce a flame (not shown) as suggested in  FIG. 1 . Mixing means  21  comprises air-fuel mixing cone  20  and cone support mount  52 . As suggested in  FIGS. 2 and 3 , air-fuel mixing cone  20  is formed to include an inner end  70  defining an upstream nozzle-receiver opening  71 , an outer end  74  defining a downstream combustion-discharge opening  75 , and a funnel-shaped side wall  72  extending between inner and outer ends  70 ,  74  to define mixing chamber  66  therebetween. Fuel nozzle  36  is arranged to communicate with mixing chamber  66  via upstream nozzle-receiver opening  71  to discharge streams  61  of fuel into mixing chamber  66 . 
   As suggested in  FIGS. 2 and 4 , funnel-shaped side wall  72  of air-fuel mixing cone  20  includes a perforated inlet section  73  and an unperforated outlet section  76 . Perforated inlet section  73  extends from upstream nozzle-receiver opening  71  to unperforated outlet section  76 . Unperforated outlet section  76  terminates at downstream combustion-discharge opening  75  and defines an outer region  80  of mixing chamber  66 . Perforated inlet section  76  is formed to include an upstream territory  77  located adjacent to fuel nozzle  36  and a downstream territory  78  interposed between upstream territory  77  and unperforated outlet section  76 . Downstream territory  78  is arranged to cooperate with upstream territory  77  to define an inner region  79  of mixing chamber  66  as suggested diagrammatically in  FIG. 2  and illustratively in  FIG. 4 . 
   As suggested in  FIGS. 1-4 , perforated inlet section  73  of funnel-shaped side wall  72  is formed to include air-admission port means for defining an air-admission portal  82  exposed to pressurized air  30  extant in air plenum chamber  28  of air plenum  26 . Air-admission portal  82  is configured to extend away from upstream nozzle-receiver opening  71 . In illustrative embodiments, air-admission portal  82  comprises slots, apertures, or both formed in funnel-shaped side wall  72  of air-fuel mixing cone  20 . 
   Air-admission portal  82  (i.e., total open area of all of the slots and/or apertures cooperating to define air-admission portal  82 ) is configured to decrease in effective size along a length of funnel-shaped side wall  66  as distance from upstream nozzle-receiver opening  71  increases in direction  81  as suggested, for example, in  FIGS. 1-4 . This progressively smaller effective size causes a greater volume of pressurized air  31  to pass through an upstream portion of air-admission portal  82  into upstream territory  77  of inner region  79  of mixing chamber  66  in close proximity to fuel nozzle  36  to mix with the streams  61  of fuel discharged by fuel nozzle  36  to produce a combustible fuel-rich air-and-fuel mixture in upstream territory  77 . This progressively smaller effective size of air-admission portal  82  also causes a relatively smaller lesser volume of pressurized air  31  to pass through a downstream portion of air-admission portal  82  into downstream territory  78  of inner region  79  of mixing chamber  66  to generate a first-stage air-and-fuel mixture  101  in downstream territory  78 . First-stage air-and-fuel mixture  101  is characterized by a low nitrogen oxide (NOx) content, a high hydrocarbon (HC) content, and a high carbon monoxide (CO) content so that a cold-temperature flame-quenching zone  83  is established in inner region  79  of mixing chamber  66  and carbon monoxide and unburned hydrocarbon included in first-stage air-and-fuel mixture  101  flow from inner region  79  of mixing chamber  66  into outer region  80  of mixing chamber  66  formed in unperforated outlet section  76 . 
   Unperforated outlet section  76  of funnel-shaped side wall  72  is separated from air plenum  26  to block admission of pressurized air  30  from air plenum  26  into outer region  80  of mixing chamber  66  to establish a high-temperature emission-reduction burnout zone  84  in outer region  80  of mixing chamber  66  causing carbon monoxide and hydrocarbon admitted into outer region  80  to be burned therein to generate in outer region  80  of mixing chamber  66  a second-stage air-and-fuel mixture  102  as suggested in  FIG. 2 . Second-stage air-and-fuel mixture  102  is characterized by a relatively low nitrogen oxide content, a relatively low hydrocarbon content, and a relatively low carbon monoxide content and is discharged from outer region  80  of mixing chamber  66  through combustion-discharge opening  75  formed in outer end  74  of air-fuel mixing cone  20 . 
   Air-admission portal  82  comprises a series of air-admission slots  90  formed in perforated inlet section  73  of funnel-shaped side wall  72  of air-fuel mixing cone  20 . Each of the air-admission slots  90  is arranged to extend in a downstream direction  81  along a portion of the length of funnel-shaped side wall  72 . Each of air-admission slots  90  is characterized by a lateral width that varies along a length of the slot and widens in places closer to inner end  71  of air-fuel mixing cone  20 . 
   Each air-admission slot  90  is defined by first and second flame-anchor edges  91 ,  92  and a concave curved edge  93  having a first end coupled to first flame-anchor edge  91  and a second end coupled to second flame-anchor edge  92  as suggested in  FIGS. 2 and 4 . First and second flame-anchor edges  91 ,  92  are arranged to lie in spaced-apart relation to one another to define a downstream air-transferring channel  94  therebetween. Concave curved edge  93  is located in a space  95  provided between the first and second flame-anchor edges  91 ,  92  and upstream nozzle-receiving opening  71  of inner end  70  of air-fuel mixing cone  20  to define an upstream air-transferring aperture  96  communicating with downstream air-transferring channel  94 . 
   First and second flame-anchor edges  91 ,  92  are separated by a uniform width dimension and concave curved edge  93  is defined by an arcuate section of a circle having a diameter that is greater than the uniform width dimension provided between first and second flame-anchor edges  91 ,  92  as suggested in  FIGS. 2-4 . Each of first and second flame-anchor edges  91 ,  92  has a length that is about 3.5 times the diameter of the circle described above. Concave curved edge  93  is arranged to intersect in two places (A and B) a first reference line  131  coincident with first flame-anchor edge  91  and to intersect in two places (C and D) a second reference line  132  coincident with second flame-anchor edge  92  as suggested in  FIG. 3 . Concave curved edge  93  circumscribes an arc of about 300 degrees and in illustrative embodiments, an arc within a range of about 250-320 degrees 
   As suggested in  FIG. 1 , burner housing  18  includes an interior region comprising at least air-conducting passageway  25  in air duct  22 , air plenum chamber  28  in air plenum  26 , and the interior region provided in burner discharge sleeve  50 . Air-fuel mixing cone  20  is located in the interior region of burner housing  18  to expose air-admission portal  82  to primary (combustion) air  31  derived from combustion air  30  extant in air plenum chamber  28  of air plenum  26 . As suggested in  FIGS. 1 and 2 , funnel-shaped side wall  72  of air-fuel mixing cone  20  includes an exterior surface  97  that terminates at a large-diameter outer rim  98  and cooperates with a surrounding wall included, for example, in burner discharge sleeve  50  included in burner housing  18  to define means for diverting pressurized combustion air  30  from air plenum  26  to generate a stream of secondary (combustion) air  32  flowing past unperforated outlet section  76  of funnel-shaped side wall  72  to cool funnel-shaped side wall  72  of air-fuel mixing cone  20  and flowing through a secondary air channel  99  defined between large-diameter outer rim  98  and surrounding wall  50  into a combustion zone  103 . Combustion zone  103  is provided in burner housing  18  and arranged also to receive second-stage air-and-fuel mixture  102  discharged from outer region  80  of mixing chamber  66  through combustion-discharge opening  75  formed in outer end  74  of air-fuel mixing cone  20 . 
   Air-admission portal  82  is sized to provide primary air means for admitting from air plenum chamber  28  of air plenum  26  about 80 to 90 percent of combustion air needed for combustion into mixing chamber  66  in illustrative embodiments of the present disclosure. Secondary air channel  99  defined between large-diameter outer rim  98  and surrounding wall  50  is sized to provide secondary air means for admitting from air plenum chamber  28  of air plenum  26  about 10 to 20 percent of combustion air needed for combustion in combustion zone  103  also in illustrative embodiments of the present disclosure. 
   As suggested diagrammatically in  FIG. 2 , air-admission portal  82  comprises first and second air-admission slots  111 ,  112  formed in perforated inlet section  73  of funnel-shaped side wall  72  and arranged to lie in spaced-apart relation to one another to define a field  113  located therebetween. A first small-size air-admission port  114  is formed in field  113  in perforated inlet section  73  of funnel-shaped side wall  72  and located in spaced-apart relation to upstream nozzle-receiving opening  71  and characterized by a first open-area size. A large-size air admission port  116  is formed in field  113  to lie between upstream nozzle-receiving opening  71  and first small-size air-admission port  114  and characterized by a second open-area size that is greater than the first open-area size. Air-admission portal  82  further comprises a second small-size air-admission port  115  formed in field  113  and located between first small-size air-admission port  114  and first air-admission slot  111 . Second small-size air-admission port  115  is characterized by the first open-area size. It is within the scope of this disclosure to provide air-admission ports in varying numbers, shapes, patterns, and locations in field  113 . 
   As suggested in  FIG. 2 , each of the air-admission slots  111 ,  112  is arranged to extend in a downstream direction along a portion of the length of funnel-shaped side wall  72 . Each of air-admission slots  111 ,  112  is characterized by a lateral width that varies along a length of the slot and widens in places closer to inner end  71  of air-fuel mixing cone  20 . Air-admission ports  116 ,  115 ,  114  are progressively reduced in size as distance away from upstream nozzle-receiving opening  71  increases in direction  81  as suggested in  FIG. 2 . 
   As suggested in  FIG. 2 , an upstream air-admission port  116  is formed in field  113  along a bifurcation reference line  117  that is arranged to bifurcate field  113  to define a first field section  118  between first air-admission slot  111  and bifurcation reference line  117  and a second field section  119  between second air-admission slot  112  and bifurcation reference line  117 . First downstream air-admission port  114  is formed in first field section  118  to locate upstream air-admission port  116  between first downstream air-admission port  114  and upstream nozzle-receiving opening  71 . Second downstream air-admission port  115  is formed in second field section  119  to locate upstream air-admission port  116  between second downstream air-admission port  115  and upstream nozzle-receiving opening  71 . One of the fuel-discharge ports  60  is oriented to discharge a stream  61  of fuel into upstream territory  77  of mixing chamber  66  along bifurcation reference line  117  as suggested in  FIG. 2 . Upstream air-admission port  116  provides an opening of a first size and each of the first and second downstream air-admission ports  114 ,  115  provides an opening of a relatively smaller second size as suggested in  FIG. 2 . 
   An air-mixing cone  220  in accordance with a second embodiment of the present disclosure is shown, for example, in  FIGS. 9 and 10 . Air-fuel mixing cone  220  is formed to include an inner end  270  defining an upstream nozzle-receiver opening  271 , an outer end  274  defining a downstream combustion-discharge opening  275 , and a funnel-shaped side wall  272  extending between inner and outer ends  270 ,  274  to define mixing chamber  266  therebetween. Fuel nozzle  36  is arranged to communicate with mixing chamber  266  via upstream nozzle-receiver opening  271  to discharge streams of fuel into mixing chamber  266 . 
   Air-mixing cone  220  is formed to include an air-admission portal  282  comprising only a series of spaced-apart air-admission slots  290  as shown, for example, in  FIGS. 9 and 10 . It is, however, within the scope of the present disclosure to form air-mixing cone  220  to include air-admission ports or other openings in the fields  213  between adjacent air-admission slots  290 . 
   An air-mixing cone  320  in accordance with a third embodiment of the present disclosure is shown, for example, in  FIGS. 11 and 12 . Air-fuel mixing cone  320  is formed to include an inner end  370  defining an upstream nozzle-receiver opening  371 , an outer end  374  defining a downstream combustion-discharge opening  375 , and a funnel-shaped side wall  372  extending between inner and outer ends  370 ,  374  to define mixing chamber  366  therebetween. Fuel nozzle  36  is arranged to communicate with mixing chamber  366  via upstream nozzle-receiver opening  371  to discharge streams of fuel into mixing chamber  366 . 
   Air-mixing cone  320  is formed to include an air-admission portal  382  comprising only a series of spaced-apart air-admission slots  390  as shown, for example, in  FIGS. 11 and 12 . It is, however, within the scope of the present disclosure to form air-mixing cone  320  to include air-admission ports or other openings in the fields  313  between adjacent air-admission slots  390 . 
   As suggested in the embodiment of  FIGS. 11 and 12 , first and second flame-anchor edges  391 ,  391  are arranged to diverge in an upstream direction toward a concave curved edge  313 . This arrangement causes the air-admission slot  390  bounded by the first and second flame-anchor edges  391 ,  392  to have a lateral width that narrows as distance away from concave curved edge  393  increases. Each air-admission slot  390  is also bounded by a concave curved edge  393  located between the upstream nozzle-receiving opening  371  and the first and second flame-anchor edges  391 ,  392  and arranged to interconnect upstream ends of first and second flame-anchor edges  391 ,  392 . Concave curved edge  393  is arranged to lie wholly in a space provided between a first reference line coincident with first flame-anchor edge  391  and a second reference line coincident with second flame-anchor edge  392 . 
   An air-mixing cone  420  in accordance with a fourth embodiment of the present disclosure is shown, for example, in  FIGS. 13 and 14 . Air-fuel mixing cone  420  is formed to include an inner end  470  defining an upstream nozzle-receiver opening  471 , an outer end  474  defining a downstream combustion-discharge opening  475 , and a funnel-shaped side wall  472  extending between inner and outer ends  470 ,  474  to define mixing chamber  466  therebetween. Fuel nozzle  36  is arranged to communicate with mixing chamber  466  via upstream nozzle-receiver opening  471  to discharge streams of fuel into mixing chamber  466 . 
   Air-mixing cone  420  is formed to include an air-admission portal  482  comprising only a series of spaced-apart air-admission slots  490  as shown, for example, in  FIGS. 13 and 14 . It is, however, within the scope of the present disclosure to form air-mixing cone  420  to include air-admission ports or other openings in the fields  413  between adjacent air-admission slots  490 . 
   As suggested in the embodiment of  FIGS. 13 and 14 , each first and second flame-anchor edge  491 ,  492  includes an upstream end located in close proximity to the upstream nozzle-receiving opening  471  and an opposite downstream end located between a companion upstream end and downstream combustion-discharge opening  475  formed in outer end  474  of air-fuel mixing cone  420 . First and second flame-anchor edges  491 ,  492  intersect at the downstream ends thereof at point  495 . Each air-admission slot  490  is also bounded by an interior edge  493  formed in funnel-shaped side wall  420  and arranged to interconnect the upstream ends of first and second flame-anchor edges  491 ,  492 . In the illustrated embodiment, each of edges  491 ,  492 ,  493  are straight and edges  491 ,  492 ,  493  cooperate to form an Isosceles triangle. 
   An air-mixing cone  520  in accordance with a fifth embodiment of the present disclosure is shown, for example, in  FIGS. 15 and 16 . Air-fuel mixing cone  520  is formed to include an inner end  570  defining an upstream nozzle-receiver opening  571 , an outer end  574  defining a downstream combustion-discharge opening  575 , and a funnel-shaped side wall  572  extending between inner and outer ends  570 ,  574  to define mixing chamber  566  therebetween. Fuel nozzle  36  is arranged to communicate with mixing chamber  566  via upstream nozzle-receiver opening  571  to discharge streams of fuel into mixing chamber  566 . 
   Air-mixing cone  520  is formed to include an air-admission portal  582  comprising only a series of spaced-apart air-admission slots  590  as shown, for example, in  FIGS. 15 and 16 . It is, however, within the scope of the present disclosure to form air-mixing cone  520  to include air-admission ports or other openings in the fields  513  between adjacent air-admission slots  590 . As suggested in the embodiment of  FIGS. 15 and 16 , each of first and second flame anchor edges  591 ,  592  intersects a narrow-diameter inner rim  570  defining upstream nozzle-receiving opening  571 . 
   The design of mixing cones  20 ,  220 ,  320 ,  420 , and  520  in accordance with the present disclosure allows for mid to low emission performance without sacrificing burner turndown. The burner emissions can be controlled and regulated easily by simply increasing or decreasing excess air. Air-fuel mixing cones  20 ,  220 ,  320 ,  420 , and  520  can be scaled easily to a larger or smaller burner while maintaining same flame characteristics and emission performance. Each air-fuel mixing cone is made out of stainless steel material and provided with holes or slots. The slots are sized for an optimal open area through which air passes and enters the cone. The cone is located inside of a burner discharge sleeve  50  and is mounted on a fuel nozzle  36 . 
   The fuel nozzle  36  delivers fuel into the air-fuel mixing cone and injects fuel  61  between the air-opening slots  90 ,  290 ,  390 ,  490 , or  590 . The slots are sized and shaped to allow for the largest volume of air to enter the cone next to fuel nozzle  36  at the throat of the cone and are smaller as the cone opens. The cone openings extend to only half of the cone length. The remaining portion of the cone without openings serves as a protective zone. 
   The reason for the opening size and shape is to provide flame with a cold-temperature flame-quenching zone  83  where the flame temperature is minimized, thus reducing the emission of thermal NOx. The latter part of the cone without the openings exists to burn out the CO created by the quenched flame in the first zone of the cone. 
   The shape and size of the openings are defined to allow for maximum volume of air near fuel nozzle  36  without sacrificing flame stability. The fuel  61  is injected between the cone openings at the same or slightly larger angle as the cone, allowing the fuel jet to flow parallel to the cone area between the openings and to progressively mix with air. This enhances the fuel-air mixing, as well as provides an anchor for the flame at low-fire conditions. 
   The area in fields  113 ,  213 ,  313 ,  413 , and  513  between the slots provides a retention zone where the flame can stabilize near the fuel nozzle and is not directly in the air stream. At mid-to-high fire conditions, the area between the slots offers a medium for gas to progressively mix with air and to penetrate deeper into the cone. The negative pressure around the edge of the slots, produced by the air stream entering the cone, creates an eddy effect which enhances the mixing of fuel  61  and air  31 . The eddy effect not only helps in mixing of fuel and air, but also creates an effective anchor where flame can establish. Depending on the intensity of the air stream, the flame anchor can either encompass the entire circumference of the slot opening or can shift and move to the end of the slot opening. 
   At high-fire conditions the intensity of air stream moves the flame to the end of the slots and anchors the flame in the base of the cone protective zone  84  defined by unperforated outlet section  76 . In the protective zone  84 , the velocity of the air stream greatly decelerates, allowing the flame to establish and to float with minimum flame retention. The flame is still anchored to the slot openings. However, a majority of the flame is lifted and burns almost as a premixed flame. The anchored flame serves as a supply of ignition for the main flame. As the base of the flame shifts and moves away from the gas nozzle, the fuel and air are partly mixed before burning. The openings (e.g., air-admission ports  114 ,  115 ,  116 ) between the slots provide additional means to quench the flame by injecting air into the base of the flame and also a way to split the fuel and force it to mix with the air flowing form the slots. 
   Nearly all of the combustion air (80 to 90 percent) enters the air-fuel mixing cone throughout the slots and holes at the base of the cone. The rest of the air is directed around the cone and enters combustion zone  103  outside of the cone as secondary air  32 . The secondary air  32  around the cone is used to cool the cone and to provide additional and final flame quenching. The amount of secondary air  32  is controlled by the gap  99  provided between the cone and a discharge sleeve in which the cone is located. 
   The slots/openings are sized and shaped to allow the largest volume of air to enter the cone adjacent to the nozzle at the base of the cone and are smaller as the cone opens. The cone opening lengths are sized to extend half of the cone length. The remaining portion of the cone without openings serves as a protective burnout zone  84 . One reason for the opening size and shape is to provide flame with a cold temperature flame-quenching zone  83  where the flame temperature is minimized, thus reducing the emission of thermal NOx. The later part of the cone without the openings allows for burnout of the remaining CO created in the quenched first zone  83  of the cone. The shape of the openings allows for minimum flame retention without sacrificing flame stability. 
   A graph illustrated in  FIG. 8  shows that the effective size of the combustion air “openings” in an air-fuel mixing cone  20  made in accordance with the present disclosure and defined, e.g., by air-admission slots  90  and ports  115 ,  116  decreases as (1) the volume of cone  20  increases and (2) the distance from fuel nozzle  36  increases. This is in marked contrast to an increasing effective size of combustion air openings provided in a “typical” air-fuel burner. 
   The traditional approach is to use cones or mixing plates and to create a combustion zone within these plates. Cones or mixing plates typically use openings that are smaller at the base of the cone next to the fuel nozzle and become progressively larger as they move upward in the cone. The combustion air openings can be round with the smallest openings first and the largest last. If slots are utilized, then their orientation is also in the same fashion. They are small at the base next to the fuel nozzle and are progressively larger. 
   One reason for this difference is a fundamentally different approach to the emissions control and to the burner turndown. The prior burners were either designed for a constant airflow or for high turndown performance only, without the emphasis on burner emissions. The reason for the traditional layout of the openings is to allow minimum amount of air at the base of the flame next to the gas nozzle and maximum after the flame develops and is established. The opening size was progressively larger and sized according to the combustion zone volume. At minimum fire where the combustion zone volume is the smallest and where the flame intensity is the weakest, the air openings in the cone were sized to protect this flame and their open area was sized to only supply the air needed for that particular flame rate. The air openings would get progressively larger corresponding to the flame zone intensity. Such design allows for a good flame turndown control. However, it does not allow for NOx or CO emission control. 
   The slots/openings provided in air-fuel mixing conies in accordance with the present disclosure are sized and shaped to allow the largest volume of air to enter the cone next to the nozzle at the base of the cone and are smaller as the cone opens. The cone openings take up only half of the cone length. The remaining portion of the cone without openings serves as a protective zone. The reason for the opening size and shape is to provide flame with a cold-quenching zone, thus minimizing the flame temperature and reducing the emission of NOx. The later part of the cone without the openings allows for burnout of the unburned hydrocarbons and Co created in the quenched first zone of the cone. The opening shape allows for minimum flame retention without sacrificing flame stability. The cone openings are sized to allow 80 to 90 percent of air to enter the combustion zone at the base of the flame where the fuel is introduced. This approach allows emission control without sacrificing burner turndown or flame stability. Such opening and spacing are contrary to the traditional approach where a cone or mixing plates are used to create a combustion zone.