CO/NO Emissions of Strongly-Pulsed Turbulent Nonpremixed Flames

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The CO and NO emissions of strongly-pulsed, turbulent diffusion flames are studied experimentally. In all cases the flames are fully-modulated (fuel completely shut off between pulses). The fuel is ethylene with an air co-flow. The emissions of NO and CO were measured at the combustor exit and on the combustor centerline. For short injection times the CO level substantially exceeds that of the steady (non-pulsed) flame, while the amount of NO is below the steady-flame level. The highest levels of CO and NO occur for short and long injection times, respectively. For sufficiently short jet off-times, as the flame structure interaction increases, levels of both CO and NO tend to reach the steady flame levels. These changes are consistent with an increased interaction between flame structures leading to a decreased air entrainment per amount of injected fuel and a longer flame length. The CO emissions, for a given fueling rate, are strongly dependent on both the injection time and jet off-time for a jet-on fraction less than approximately 50%. The NO levels are generally proportional to the fueling rate. The residence time based on the visual flame dimensions correlates reasonably well with the CO emission indices, reflecting incomplete combustions for low residence time of the flame structures; however, the NO levels do not directly scale with the residence time. Injection conditions exist that produce a more compact flame than the steady flame, and with significantly lower levels of NO but slightly higher CO emissions.