The relationship among the flame radiation, buoyancy, and NO_x emissions is examined for turbulent non-premixed jet flames with wide variations in coaxial air conditions. The major parameters used to modify the fuel/air mixing are fuel jet velocity, coaxial air velocity, and helium dilution. Measurements of NO_x emission, flame geometry, and flame radiation were made to explain the observed NO_x emission scaling. The overall 1/2-power scaling is observed in coaxial air flames, irrespective of coaxial air conditions, but for pure hydrogen flames, the degree of deviation from the 1/2-slope curve in each case is different. From the comparison between the results of pure hydrogen flames and those of helium-diluted hydrogen flames, it is observed that the deviation from 1/2-power scaling may be explained in two ways: the differences in the flame radiation and flame buoyancy in coaxial air flames. Helium dilution reduces deviation from the 1/2-power scaling in two ways: it reduces the effect of flame radiation and it reduces the effect of flame buoyancy through reduced flame length and increased momentum flux of fuel jet. From the radiation measurements, EINO_x is observed to be proportional to the radiant fraction to the two-thirds power for the momentum-dominated turbulent hydrogen jet flames. This observed tendency seems to be related to the 1/2-power NO_x emission scaling.
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