EMISSIONS CHARACTERISTICS OF LIQUID-FUELED PILOT STABILIZED LEAN PREMIXED FLAMES IN A TUBULAR PREMIXER-COMBUSTOR

Global emissions of NOx in a liquid-fueled lean-premixed tubular combu stor with a tubular premixer operating under atmospheric pressure are studied experimentally. The effects of equivalence ratio, premixer len gth, residence time, fuel type, and fuel atomization and dispersion ch aracteristics on NOx emissions are studied. Measurements of exhaust sp ecies concentrations are used as the primary indicator of the effectiv eness of premixing-prevaporization upstream of the combustor. Qualitat ive levels of prevaporization-premixing are determined from Mie-scatte ring signals measured at the exit of the premixer. Emission measuremen ts show that the equivalence ratio is the dominant operating parameter , with premixing length and residence time being less significant with in the present operating range. Ultralow NOx operation (< 10 ppmv at 1 5 percent O-2) is feasible for equivalence ratios less than 0.5. More significantly, small drops persist beyond the premixer even for very l ong premixers, and Mie-scattering measurements show considerable spati al inhomogeneity, while allowing ultralow NOx operation. One-dimension al evaporation calculations for single drop trajectories confirm that complete evaporation for typical drop, size distributions is lot possi ble with reasonable premixer lengths under atmospheric pressure. Fuel dispersion is found to be the most critical parameter for high combust ion efficiency, and adverse effects of poor fuel dispersion cannot be overcome by using longer premixers.