A STUDY OF EXTINCTION AND NOX IN STRETCHED TURBULENT PRE-MIXED FLAMES

An experimental study of pre-mixed turbulent counterflow combustion was conducted, in which the counterflow flame was established by means of two unconfined vertical and axially aligned opposed jets having an equal flow of pre-mixed natural-gas/air mixtures. The initial fuel jet velocities were varied to provide Reynolds numbers ranging from 10 000 to 20 000, and the fuel:air ratio was altered to accommodate equivalence ratios between 0.54 and 1.78. Measurements were made of mean temperature and concentrations of ions, NO(x) and major gas species. The onset of extinction on the stagnation plane was found to be independent of the nozzle-separation distance but strongly dependent on the equivalence ratio. A reduction in NO(x) formation was seen with a rise of either Reynolds number or burner-separation distance; however, the opposite trend was observed when nitrogen was deliberately added to the fuel to produce the fuel nitrogen component of total NO(x). Variation of the NO2:NOx ratio with jet velocity and radial position was consistent with the hypothesis that NO2 is formed in the cooler post-flame regions.