Oxides of nitrogen emissions from turbulent jet flames: Part II—Fuel dilution and partial premixing effects

Abstract

Measurements of NO x emission indices, flame radiant fractions, and visible flame length were made for turbulent, nonpremixed, jet flames for which various amounts of inert diluent or air were mixed with the fuel. The objective of the study was to explore further the role of flame radiation in NO x production in jet flames. Vertical free jet flames were stabilized on a 4.12 mm diameter straight-tube burner. Four fuels, CH 4, C 2H 4, C 3H 8, and a 95% CO 5% H 2 mixture (by mass); three inert diluents, N 2, Ar, and CO 2; and air premixing were employed in parametric tests. Complementary dilution experiments were run with laminar jet flames using the three hydrocarbon fuels and N 2. For the turbulent flames, the results showed that the effects of dilution and premixing were strongly dependent on fuel type. Flame temperatures and NO x emissions increased when the more sooting fuels (C 3H 8 and C 2H 4) were diluted or partially premixed, resulting in increased NO x emissions. The opposite trend was observed for the nonluminous CO H 2 flames. Using the results reported here and from Part I [1] of this study, the effects of residence time, flame temperature, and departure from equilibrium on NO x emissions, regardless of what parameter affected the change, were well characterized by regressing characteristic NO x production rates as a function of nonadiabatic characteristic flame temperatures and global residence times. Separate regressions for the hydrocarbon and CO H 2 flames showed a weaker dependence of NO x on temperature for the hydrocarbons, suggesting that the prompt NO mechanism is quite active in these flames. The laminar flame experiments demonstrated the importance of the relative locations of NO x -producing regions and soot-containing (strongly radiating) regions of the flame.