Nitrogen dioxide formation by the mixing of hot combustion gas with cold air

Abstract

NO, NO2, stable specirs concentrations and temperature have been measured in a double concentric jet facility to study the mechanism of NO2 formation by the mixing of hot combustion gases with cold air. The double concentric jets are designed to madel the mixing region in practical combustion systems and consist of a central hot combustion gas jet from a propane/air burner and an annular cold air jet confined in a cylindrical duct. The main objective of this study is to determine the effect of equivalence ratio, on NO2 formation. The burner was operated under a wide range of equivalence ratios from 0.62 to 1.60. NO was found to convert to NO2 in the mixing region of the jets through the HO2 mechanism (oxidation of NO mainly by HO2 radical). The degree of NO2 formation can be quantified from the NO2/NOx ratio measured at the exit of the duct. Under fuel-lean conditions, with decreasing the equivalence ratio, the NO2/NOx ratio increases up to 0.81 though the total NOx level quickly decreases. The effect of initial NO level on the, NO2 formation seems to be dominant in this case, which explains higher NO2/NOx ratio with lowering the initial NO levels. Under fuel-rich conditions, the NO2/NOx ratio is about 0.90 at equivalence ratios above 1.2. The additional supply of radicals from the diffusion flame, and the presence of unburned species in relatively cool regions probably yield the high NO2/NOx ratio under the fuel-rich condition.