The Influence of Fuel Jet Precession on the Global Properties and Emissions of Unconfined Turbulent Flames

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Abstract
An investigation was conducted to characterize the shape and size, the global radiation, and the CO and NOx, emissions of precessing jet flames and to compare these results with conventional, free, turbulent jet diffusion flames. Jet precession was accomplished using a mechanically-rotated nozzle. Parameters varied include nozzle diameter, jet deflection angle, initial jet velocity (Reynolds number), dimensionless precession frequency (Strouhal number), and fuel type. Measured quantities included visible flame dimensions; NOx, NO2, and CO emissions; and radiant heat flux. Results of these experiments showed that two broad regimes of flame type exist for precessing jet flames. For precession Strouhal numbers (St p ) less than about 0.01, flames are non-luminous and, in comparison with conventional flames, are short and broad. For St p > 0.03, flames are highly luminous, and of a shape which is between that of a conventional jet flame and the low-St p flame. In general, radiant fractions increase with precession Strouhal number. High NO2-to-NOx ratios result for flames operating in the low-St p , regime. Concomitantly, CO emissions are relatively high in this regime. At higher Strouhal numbers, both NO2-to-NOx, ratios and CO emission indices fall and approach the values typical of conventional jet flames. NOx emission indices are influenced by jet precession and found to be functions of precession Strouhal number, nozzle size, fuel type, and jet deflection angle. NOx, emission indices are found to be somewhat lower than those of conventional jet flames, with the largest reduction being approximately 25%. Jet precession also causes a significant reduction in liftoff height relative to a conventional jet flame. This result is thought to be a consequence of the unique character of the flow field in the near-nozzle region of the precessing jet flames.