Appearance, temperature, and NOx emission of two inverse diffusion flames with different port design

Experiments were carried out to investigate the appearance, temperature distribution, and NOx emission index of two inverse diffusion flames, one with circumferentially arranged ports (CAPs) and the other with co-axial (CoA) jets, both burning LPG with 70% butane and 30% propane. Flame appearances were investigated first with a fixed fueling rate at different airflow rates equivalent to air jet Reynolds numbers (Re) of 1000 to 4500; and then at a fixed airflow rate with different fueling rates equivalent to overall equivalence ratios (Φ  ) of 1.0 to 2.0. The CAP flame is found to consist of two zones: a lower entrainment zone and an upper mixing and combustion zone. The CoA flame in most cases is similar to a diffusion flame. The two-zone structure can be observed only at Re larger than 2500. The temperature distributions of the flames are similar at overall equivalence ratios of 1.0 and 1.2 for Re=2500Re=2500, except that the corresponding CoA flame is longer. The flame temperature is higher in the CAP flame than the CoA flame at higher overall equivalence ratios. A measurement of centerline oxygen concentrations shows that the oxygen concentration reaches a minimum value at a flame height of 50 mm in the CAP flame but decreases more gradually in the CoA flame. It can be concluded that there is more intense air–fuel mixing in a CAP flame than the CoA flame. Investigation of the emission index of NOx (EINOx) for both flames at Re=2500Re=2500 and overall equivalence ratios of 1.0 to 6.0 reveals that the EINOx curve of each flame is bell-shaped, with a maximum value of 3.2 g/kg at Φ=1.2Φ=1.2 for the CAP flame and 3 g/kg at Φ=2.2Φ=2.2 for the CoA flame.