Effects of N^sub 2^ and CO^sub 2^ dilution on the combustion characteristics of C^sub 3^H^sub 8^/O^sub 2^ mixture in a swirl tubular flame burner

Premixed C3H8/O2 combustion has been investigated by adopting N2, CO2 and their mixture (50% N2-50% CO2 by volume) as the diluents, respectively. The oxygen content (mole fraction) in the oxidizer is varied from 0.13 to 0.35, which covers the operating conditions of EGR and OEC. The flame structures, temperatures, burning velocities and NOx emissions are examined under various oxygen contents. Detailed observations show that a steady, uniform tubular flame can be obtained over a wide range from lean to rich. By increasing oxygen content, the flame diameter increases slightly; the flame length decreases significantly, which is almost inversely proportional to laminar burning velocity. During decreasing the burning velocity to less than about 20 cm/s, a turning point appears and the flame length becomes extensively large. With the same oxygen content and equivalence ratio, by replacing half or all of the N2 diluent with CO2, the flame temperature and burning velocity decrease significantly. To obtain the same adiabatic flame temperature, the CO2 diluted mixture requires the highest oxygen content among the three diluents, however, its measured flame temperature is also highest. In the N2-CO2 diluted combustion, owing to thermal and chemical effects of CO2, the NOx emission reduces remarkably comparing with N2 dilution; under the same flame temperature, the NOx concentration reduces to almost 1/5 of that diluted by N2. The lean extinction limit raises by increasing CO2 content in the diluent, while the limit decreases with increasing oxygen content.