A study of pollutant emission characteristics of partially premixed turbulent jet flames

We measured emission indices for NOx, CO, and HC for turbulent partially pr emixed flames formed by injecting rich methane/air mixtures through a centr al burner tube into a co-flow of air. The operating conditions included a b road range of burner tube equivalence ratios (Phi(B)) including the limitin g cases of diffusion and stoichiometric premixed flames. At sufficiently hi gh levels of partial premixing, a double flame structure consisting of a ri ch premixed inner flame and an outer diffusion flame is established similar to that previously observed in laminar flames. EINOx values remain approxi mately constant for 5 less than or equal to Phi(B) less than or equal to in finity and then decrease slightly with decreasing Phi(B) at 3.5 less than o r equal to Phi(B) less than or equal to 5. For 3.5 less than or equal to Ph i(B) less than or equal to 1.5, EINOx decreases further with decreasing Phi (B) to a minimum at around Phi(B) = 1.5, followed by a sharp increase as Ph i(B) approaches unity. The reduction in EINOx at Phi(B) = 1.5 as compared w ith that for the diffusion flame is approximately 25% and as compared with that for the stoichiometric premixed flame is approximately 35%. In additio n, we measured temperature distributions and found that temperatures increa se continuously with increasing partial premixing. We also estimated global residence times (tau(0)) from flame length measurements and average veloci ties. The observed changes in temperatures and residence times are not suff icient to explain the observed changes in EINOx (C) 1998 by The Combustion Institute.