Effects of partial premixing on pollutant emissions in swirling methane jet flames

Experimental measurements of visible flame heights, temperatures, and pollutant emission indices in partially premixed swirling flames with a broad range of central fuel tube equivalence ratios (Φ_F) are reported. Two cases of partially premixed swirling flames are studied; one with a constant fuel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the overall flame color changes from yellow to blue. Temperature measurements indicate that the flame structures become thinner and temperatures increase continuously with increasing partial premixing. Emission index, EINO_x and EICO, values decrease with increasing levels of partial premixing and reach a minimum value at Φ_F ≈ 3, followed by an increase as Φ_F approaches the blowoff limits. The reduction in EINO_x and EICO at optimum partial premixing (Φ_F ≈ 3), as compared with that for the non-premixed swirling flame is at least 23% and 77%, respectively. The emission index for NO_x scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predictions and measured results.