Experimental study of nitrogen dioxide formation in combustion systems

Abstract

An experimental study has been performed to investigate the mechanism of nitrogen dioxide (NO2) formation in combustion systems. Two combustion flow fields were used. The first was a propane-fueled laboratory swirl burner, and the second was a simplified flow field consisting of double concentric jets, which simulated the mixing region of hot combustion gas with cold air in combustion systems. A comparison of NO2 measurements made with four different sampling probes in the swirl burner at an overall equivalence ratio of 0.37 demonstrated the possibility of the formation and existence of NO2 near the burner wall. The measurements in the double concentric jets were made by varying the cold air jet to combustion gas jet mass velocity ratio (from zero to about 6), the initial combustion gas temperature (from 1517 to 1839 K), and the initial NO concentration (from 10 to 250 ppm). NO2 seems to be formed in the turbulent mixing region surrounding the hot central potential core. The effects of the parameters on the amount of NO2 formed in the jets were examined. The NO2/NOx ratios obtained in the downstream region, where the probe-formed NO2 was negligible, were used to quantify the amount of NO2 formed in the jets. The amount of NO2 formed increased with increasing cooling rate of the combustion gas and initial combustion gas temperature, and the NO2/NOx ratio increased with decreasing initial NO concentration. The results of this parametric study could be explained, qualitatively, by a simple model calculation based on the NO-NO2 conversion mainly due to the HO2 mechanism. It was proved, therefore, that NO2 can be formed during the rapid cooling of hot combustion gas with cold air in the turbulent mixing region of combustion systems.