The effect of flame radiation on the scaling of nitrogen oxide emissions in turbulent hydrogen non-premixed flames

The relationship among the flame radiation, buoyancy, and NOx 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 NOx emission, flame geometry, and flame radiation were made to explain the observed NOx 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, EINOx 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 NOx emission scaling.