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

Abstract

We measured emission indices for NO x , CO, and HC for turbulent partially premixed flames formed by injecting rich methane/air mixtures through a central burner tube into a co-flow of air. The operating conditions included a broad range of burner tube equivalence ratios (Φ B) including the limiting cases of diffusion and stoichiometric premixed flames. At sufficiently high levels of partial premixing, a double flame structure consisting of a rich premixed inner flame and an outer diffusion flame is established similar to that previously observed in laminar flames. EINO x values remain approximately constant for 5 ≤ Φ B ≤ ∞ and then decrease slightly with decreasing Φ B at 3.5 ≤ Φ B ≤ 5. For 3.5 ≤ Φ B ≤ 1.5, EINO x decreases further with decreasing Φ B to a minimum at around Φ B = 1.5, followed by a sharp increase as Φ B approaches unity. The reduction in EINO x at Φ B = 1.5 as compared with that for the diffusion flame is approximately 25% and as compared with that for the stoichiometric premixed flame is approximately 35%. In addition, we measured temperature distributions and found that temperatures increase continuously with increasing partial premixing. We also estimated global residence times (τ 0) from flame length measurements and average velocities. The observed changes in temperatures and residence times are not sufficient to explain the observed changes in EINO x.