A burner comprising: a first cooling fluid jacket with an outer equivalent diameter, D; a first oxidant conduit disposed in fixed spaced relation to and generally concentrically within the first cooling fluid jacket, the first oxidant conduit having: an inlet; a first portion downstream of the inlet of the first oxidant conduit; a bend portion downstream of the first portion of the first oxidant conduit, the bend portion of the first oxidant conduit having a bend angle, α, of 45° to 120°; and a second portion downstream of the bend portion of the first oxidant conduit, the second portion of the first oxidant conduit terminating in an outlet end and having a flow axis and a length, L; and a fuel conduit having: an inlet; a first portion downstream of the inlet of the fuel conduit, wherein the first portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the first portion of the first oxidant conduit; a bend portion wherein the bend portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the bend portion of the first oxidant conduit; and a second portion terminating in an outlet end and having a flow axis wherein the second portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the second portion of the first oxidant conduit thereby defining an oxidant passage between the second portion of the fuel conduit and the second portion of the first oxidant conduit; wherein the oxidant passage has an inlet section, a transition section downstream of the inlet section, and an outlet section downstream of the transition section, wherein the inlet section has a cross-sectional area, A i, the outlet section has a cross-sectional area, A o, and wherein 0.8 ≤ L D ≤ 7 and 1.3 ≤ A i A o ≤ 5. whereby generally concentric means that the axis of one conduit is common with the axis of the other conduit or slightly displaced by up to 2 cm. a first cooling fluid jacket with an outer equivalent diameter, D; a first oxidant conduit disposed in fixed spaced relation to and generally concentrically within the first cooling fluid jacket, the first oxidant conduit having: an inlet; a first portion downstream of the inlet of the first oxidant conduit; a bend portion downstream of the first portion of the first oxidant conduit, the bend portion of the first oxidant conduit having a bend angle, α, of 45° to 120°; and a second portion downstream of the bend portion of the first oxidant conduit, the second portion of the first oxidant conduit terminating in an outlet end and having a flow axis and a length, L; and an inlet; a first portion downstream of the inlet of the first oxidant conduit; a bend portion downstream of the first portion of the first oxidant conduit, the bend portion of the first oxidant conduit having a bend angle, α, of 45° to 120°; and a second portion downstream of the bend portion of the first oxidant conduit, the second portion of the first oxidant conduit terminating in an outlet end and having a flow axis and a length, L; and a fuel conduit having: an inlet; a first portion downstream of the inlet of the fuel conduit, wherein the first portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the first portion of the first oxidant conduit; a bend portion wherein the bend portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the bend portion of the first oxidant conduit; and a second portion terminating in an outlet end and having a flow axis wherein the second portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the second portion of the first oxidant conduit thereby defining an oxidant passage between the second portion of the fuel conduit and the second portion of the first oxidant conduit; an inlet; a first portion downstream of the inlet of the fuel conduit, wherein the first portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the first portion of the first oxidant conduit; a bend portion wherein the bend portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the bend portion of the first oxidant conduit; and a second portion terminating in an outlet end and having a flow axis wherein the second portion of the fuel conduit is disposed in fixed spaced relation to and generally concentrically within the second portion of the first oxidant conduit thereby defining an oxidant passage between the second portion of the fuel conduit and the second portion of the first oxidant conduit; wherein the oxidant passage has an inlet section, a transition section downstream of the inlet section, and an outlet section downstream of the transition section, wherein the inlet section has a cross-sectional area, A i, the outlet section has a cross-sectional area, A o, and wherein 0.8 ≤ L D ≤ 7 and 1.3 ≤ A i A o ≤ 5. whereby generally concentric means that the axis of one conduit is common with the axis of the other conduit or slightly displaced by up to 2 cm.