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A burner assembly includes a fuel nozzle and an air-fuel mixing cone coupled to the fuel nozzle. Fuel is discharged from the fuel nozzle into a mixing chamber formed in the air-fuel mixing cone. Air passes into the mixing chamber through openings formed in the air-fuel mixing chamber and mixes with

A burner assembly includes a fuel nozzle and an air-fuel mixing cone coupled to the fuel nozzle. Fuel is discharged from the fuel nozzle into a mixing chamber formed in the air-fuel mixing cone. Air passes into the mixing chamber through openings formed in the air-fuel mixing chamber and mixes with fuel to form a combustible air-fuel mixture in the air-fuel mixing chamber.

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The invention claimed is: 1. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cau

The invention claimed is: 1. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to only pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, and wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone. 2. The burner assembly of claim 1, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone. 3. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone, and wherein at least one of the air-admission slots is defined by first and second flame-anchor edges and a concave curved edge having a first end coupled to the first flame-anchor edge and a second end coupled to the second flame-anchor edge, the first and second flame-anchor edges are arranged to lie in spaced-apart relation to one another to define a downstream air-transferring channel therebetween, and the concave curved edge is located in a space between the first and second flame-anchor edges and the upstream nozzle-receiving opening of the inner end of the air-fuel mixing cone to define an upstream air-transferring aperture communicating with the downstream air-transferring channel. 4. The burner assembly of claim 3, wherein the first and second flame-anchor edges are separated by a uniform width dimension and the concave curved edge is defined by an arcuate section of a circle having a diameter that is greater than the uniform width dimension provided between the first and second flame-anchor edges. 5. The burner assembly of claim 4, wherein each of the first and second flame-anchor edges has a length that is about 3.5 times said diameter. 6. The burner assembly of claim 3, wherein the concave curved edge is arranged to intersect in two places a first reference line coincident with the first flame-anchor edge and to intersect in two places a second reference line coincident with the second flame-anchor edge. 7. The burner assembly of claim 3, wherein the concave curved edge circumscribes an arc of about 250 to 320 degrees. 8. The burner assembly of claim 3, wherein the first and second flame-anchor edges are arranged to diverge in an upstream direction toward the concave curved edge to cause the air-admission slot bounded by the first and second flame-anchor edges to have a lateral width that narrows as distance away from the concave curved edge increases. 9. The burner assembly of claim 7, wherein the concave curved edge is arranged to lie wholly in a space provided between a first reference line coincident with the first flame-anchor edge and a second reference line coincident with the second flame-anchor edge. 10. The burner assembly of claim 2, wherein at least one of the air-admission slots is bounded by first and second flame-anchor edges that are formed in the funnel-shaped side wall and arranged to converge in a downstream direction away from the upstream nozzle-receiving opening formed in the air-fuel mixing cone to cause the air-admission slot bounded by the first and second flame-anchor edges to have a lateral width that narrows as distance away from the upstream nozzle-receiving opening increases. 11. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone, wherein at least one of the air-admission slots is bounded by first and second flame-anchor edges that are formed in the funnel-shaped side wall and arranged to converge in a downstream direction away from the upstream nozzle-receiving opening formed in the air-fuel mixing cone to cause the air-admission slot bounded by the first and second flame-anchor edges to have a lateral width that narrows as distance away from the upstream nozzle-receiving opening increases, and wherein the at least one of the air-admission slots is also bounded by a concave curved edge located between the upstream nozzle-receiving opening and the first and second flame-anchor edges and arranged to interconnect upstream ends of the first and second flame-anchor edges. 12. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone, wherein at least one of the air-admission slots is bounded by first and second flame-anchor edges that are formed in the funnel-shaped side wall and arranged to converge in a downstream direction away from the upstream nozzle-receiving opening formed in the air-fuel mixing cone to cause the air-admission slot bounded by the first and second flame-anchor edges to have a lateral width that narrows as distance away from the upstream nozzle-receiving opening increases, and wherein each of the first and second flame-anchor edges includes an upstream end located in close proximity to the upstream nozzle-receiving opening and an opposite downstream end located between a companion upstream end and the downstream combustion-discharge opening formed in the outer end of the air-fuel mixing cone, the first and second flame-anchor edges intersect at the downstream ends thereof, and the at least one of the air-admission slots is also bounded by an interior edge formed in the funnel-shaped side wall and arranged to interconnect the upstream ends of the first and second flame-anchor edges. 13. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone, wherein at least one of the air-admission slots is bounded by first and second flame-anchor edges that are formed in the funnel-shaped side wall and arranged to converge in a downstream direction away from the upstream nozzle-receiving opening formed in the air-fuel mixing cone to cause the air-admission slot bounded by the first and second flame-anchor edges to have a lateral width that narrows as distance away from the upstream nozzle-receiving opening increases, and wherein each of the first and second flame anchor edges intersects a narrow-diameter inner rim defining the upstream nozzle-receiving opening. 14. The burner assembly of claim 1, further comprising a burner housing including an interior region and wherein the air-fuel mixing cone is located in the interior region to expose the air-admission portal to primary combustion air extant in the air plenum and wherein the funnel-shaped side wall of the air-fuel mixing cone includes an exterior surface that terminates at a large-diameter outer rim and cooperates with a surrounding wall included in the burner housing to define means for diverting pressurized combustion air from the air plenum to generate a stream of secondary combustion air flowing past the unperforated outlet section of the funnel-shaped side wall to cool the funnel-shaped side wall of the air-fuel mixing cone and flowing through a secondary air channel defined between the large-diameter outer rim and the surrounding wall into a combustion zone provided in the burner housing and arranged also to receive the second-stage air-and-fuel mixture discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone. 15. A burner assembly for combining air and fuel to produce a flame, the burner assembly comprising a fuel nozzle including a shell formed to include several fuel-discharge ports and a fuel-transport passageway arranged to communicate fuel to the fuel-discharge ports to cause a stream of fuel to be discharged from the fuel-transport passageway through each of the fuel-discharge ports and mixing means for mixing the streams of fuel discharged through the fuel-discharge ports formed in the fuel nozzle with combustion air extant in an air plenum associated with the fuel nozzle to produce an air-and-fuel mixture that can be ignited in a mixing chamber to produce a flame, wherein the mixing means includes an air-fuel mixing cone formed to include an inner end defining an upstream nozzle-receiver opening, an outer end defining a downstream combustion-discharge opening, and a funnel-shaped side wall extending between the inner and outer ends to define a mixing chamber therebetween, the fuel nozzle is arranged to communicate with the mixing chamber via the upstream nozzle-receiver opening to discharge streams of fuel into the mixing chamber, and the funnel-shaped side wall includes an unperforated outlet section terminating at the downstream combustion-discharge opening and defining an outer region of the mixing chamber and a perforated inlet section extending from the upstream nozzle-receiver opening to the unperforated outlet section and having an upstream territory located adjacent to the fuel nozzle and a downstream territory interposed between the upstream territory and the unperforated outlet section and arranged to cooperate with the upstream territory to define an inner region of the mixing chamber, wherein the perforated inlet section of the funnel-shaped side wall is formed to include air-admission port means for defining an air-admission portal exposed to pressurized air extant in the air plenum and configured to extend away from the upstream nozzle-receiver opening and to decrease in effective size along a length of the funnel-shaped side wall as distance from the upstream nozzle-receiver opening increases to cause a greater volume of pressurized air to pass through an upstream portion of the air-admission portal into the upstream territory of the inner region of the mixing chamber in close proximity to the fuel nozzle to mix with the streams of fuel discharged by the fuel nozzle to produce a combustible fuel-rich air-and-fuel mixture in the upstream territory and to cause a relatively smaller lesser volume of pressurized air to pass through a downstream portion of the air-admission portal into the downstream territory of the inner region of the mixing chamber to generate in the downstream territory a first-stage air-and-fuel mixture 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 is established in the inner region of the mixing chamber and carbon monoxide, unburned hydrocarbon included in the first-stage air-and-fuel mixture flow from the inner region of the mixing chamber into the outer region of the mixing chamber formed in the unperforated outlet section, wherein the unperforated outlet section of the funnel-shaped side wall is separated from the air plenum to block admission of pressurized air from the air plenum into the outer region of the mixing chamber to establish a high-temperature emission-reduction burnout zone in the outer region of the mixing chamber causing carbon monoxide and hydrocarbon admitted into the outer region to be burned therein to generate in the outer region of the mixing chamber a second-stage air-and-fuel mixture characterized by a low nitrogen oxide content, a low hydrocarbon content, and a low carbon monoxide content that is discharged from the outer region of the mixing chamber through the combustion-discharge opening formed in the outer end of the air-fuel mixing cone, and wherein the air-admission portal is sized to provide primary air means for admitting from the air plenum about 80 to 90 percent of combustion air needed for combustion into the mixing chamber and the secondary air channel defined between the large-diameter outer rim and the surrounding wall is sized to provide secondary air means for admitting from the air plenum about 10 to 20 percent of combustion air needed for combustion in the combustion zone. 16. The burner assembly of claim 1, wherein the air-admission portal comprises first and second air-admission slots formed in the perforated inlet section of the funnel-shaped side wall and arranged to lie in spaced-apart relation to one another to define a field therebetween, a first small-size air-admission port formed in the field in the perforated inlet section of the funnel-shaped side wall and located in spaced-apart relation to the upstream nozzle-receiving opening and characterized by a first open-area size and a large-size air-admission port formed in the field of the perforated inlet section of the funnel-shaped side wall to lie between the upstream nozzle-receiving opening and the first small-size air-admission port and characterized by a second open-area size that is greater than the first open-area size. 17. The burner assembly of claim 16, wherein the air-admission portal further comprises a second small-size air-admission port formed in the field of the perforated inlet section of the funnel-shaped side wall and located between the first small-size air-admission port and the first air-admission slot and wherein the second small-size air-admission port is characterized by the first open-area size. 18. The burner assembly of claim 16, wherein the air-admission portal comprises a series of air-admission slots formed in the perforated inlet section of the funnel-shaped side wall of the air-fuel mixing cone, each of the air-admission slots is arranged to extend in a downstream direction along a portion of the length of the funnel-shaped side wall, and each of the air-admission slots is characterized by a lateral width that varies along a length of the slot and widens in places closer to the inner end of the air-fuel mixing cone. 19. The burner assembly of claim 1, wherein the air-admission portal comprises first and second air-admission slots formed in the perforated inlet section of the funnel-shaped side wall and arranged to lie in spaced-apart relation to one another to define a field therebetween and air-admission ports formed in the field and wherein the air-admission ports are progressively reduced in size as distance away from upstream nozzle-receiving opening increases. 20. The burner assembly of claim 1, wherein the air-admission portal comprises first and second air-admission slots formed in the perforated inlet section of the funnel-shaped side wall and arranged to lie in spaced-apart relation to one another to define a field therebetween, an upstream air-admission port formed in the field along a bifurcation reference line bifurcating the field to define a first field section between the first air-admission slot and the reference line and a second field section between the second air-admission slot and the reference line, a first downstream air-admission port formed in the first field section to locate the upstream air-admission port between the first air-admission port and the upstream nozzle-receiving opening, and a second downstream air-admission port formed in the second field section to locate the upstream air-admission port between the second air-admission port and the upstream nozzle-receiving opening and wherein one of the fuel-discharge ports is oriented to discharge a stream of fuel into the upstream territory of the mixing chamber along the bifurcation reference line. 21. The burner assembly of claim 20, wherein the upstream air-admission port provides an opening of a first size and each of the first and second downstream air-admission ports provides an opening of a relatively smaller second size.