Reexamination of the scaling laws for NOx emissions from hydrocarbon turbulent jet diffusion flames

This paper presents an investigation on unconfined, vertical, turbulent jet diffusion flames of propane and ethylene issuing from a straight tube into quiescent air at atmospheric pressure and temperature. This work extends a recent investigation that we have conducted for turbulent methane jet diffusion flames. Measurements are reported for flame geometry (liftoff heights and flame heights), flame radiant fractions, and emission indices for oxides of nitrogen (EINOx). The test conditions analyzed covered a wide range of flow conditions varied by changing the nozzle exit internal diameter and the jet exit mean velocity. Three different-sized nozzles with inner diameters of 5, 6, and 8 mm were employed and the ranges of jet exit velocities and Reynolds and Froude numbers considered were 5-137 m/s, 8970-83876, and 315-384791, respectively. The main conclusions of the present study are: (i) the transition from buoyancy to momentum-controlled turbulent jet diffusion flames is related to the type of hydrocarbon, with the Froude number at which transition occurs ranking in the same order as sooting tendencies; (ii) the present data for propane and ethylene flames indicate that it is the hydrocarbon propensity for soot formation, rather than the flame regime (buoyancy or momentum), that is critical for establishing the main factors that control the NOx scaling; (iii) in common with previous studies, our data for propane flames show that the main factor that controls the NOx scaling is the flame volume, regardless of the flame-dominated regime; (iv) the present data for ethylene flames reveal that the evaluation of the effects of radiation is critical for obtaining reliable NOx correlations. A new correlation for EINOx is proposed for such sooty flames, which show good agreement with the experimental results. The correlation demonstrates the importance of the flame radiant fraction in the NOx scaling.