Effects of N₂ and CO₂ dilution on the combustion characteristics of C₃H₈/O₂ 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 NO_x 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 NO_x emission reduces remarkably comparing with N₂ dilution; under the same flame temperature, the NO_x 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.