Scaling of nitric oxide emissions from buoyancy-dominated hydrocarbon turbulent-jet diffusion flames

Results of theoretical estimates and of measurements are reported on emissions of oxides of nitrogen from large methane, propane and natural-gas flames under buoyancy-controlled conditions. The theory employs reduced chemical-kinetic mechanisms for the reaction-sheet regime along with simplified mass, momentum and mixture-fraction balances for the turbulent flame height. Explicit expressions for local, instantaneous NO production rates are derived, involving rate constants of only fourteen elementary steps, and a two-reaction-zone description of the laminar flamelets is adopted, enabling average production rates to be obtained from asymptotics through approximation of a joint probability-density function for mixture fraction and scalar dissipation. The theory predicts significant contributions from the Zel'dovich mechanism in the oxygen-consumption zone and from the prompt mechanism in the fuel-consumption zone but negligible contributions from the nitrous-oxide mechanism and gives an increase in the ratio of prompt to Zel'dovich contributions with increasing strain rates, as extinction is approached. The experiments employ continuous gas analyzers with water-cooled quartz and stainless-steel suction probes and derive emission indices from measured ratios of NO x to CO 2 concentrations. The experimental results are in reasonable agreement with a Froude-number correlation derived from the theory over a range of Froude numbers from 10 to 5×10 5 , wider than available previously.

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