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

Experimental measurements of visible flame heights, temperatures, and pollu tant emission indices in partially premixed swirling flames with a broad ra nge of central fuel tube equivalence ratios (Phi (F)) are reported. Two cas es of partially premixed swirling flames are studied; one with a constant f uel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the ov erall flame color changes from yellow to blue. Temperature measurements ind icate that the flame structures become thinner and temperatures increase co ntinuously with increasing partial premixing. Emission index, EINOx and EIC O, values decrease with increasing levels of partial premixing and reach a minimum value at Phi (F) approximate to 3, followed by an increase as Phi ( F) approaches the blowoff limits. The reduction in EINOx and EICO at optimu m partial premixing (Phi (F) approximate to 3), as compared with that for t he non-premixed swirling flame is at least 23% and 77%, respectively. The e mission index for NOx scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predic tions and measured results. (C) 2001 by The Combustion Institute.