Experimental Investigation of the Stability Mechanism and Emissions of a Lifted Swirl Nonpremixed Flame

We report on the experimental investigation of a confined lifted swirl nonpremixed flame by applying a novel Airblast nozzle (Zarzalis, N., et al., 2005, Fuel Injection Apparatus, Patent No. DE 10 2005 022 772.4, EP 06 009 563.5). 3D-laser doppler anemometry, a nonintrusive, laser-based measurement technique, is adapted for the measurement of all three mean velocity components and of the six Reynolds stress components. The determination of the temperature and mixture field occurs by employing in-flame measurement techniques. Valuable information concerning the mixing procedure, the temperature distribution, the turbulence level, and the velocity field of the flame is provided. The results demonstrate that there is sufficient residence time in the precombustion area of the lifted flame in order to achieve spatial and temporal uniformity of the mixture, leading to a quasi-premixed state. It was also found that hot reaction products, carried upstream by an annular zone of reverse flow, react with fresh unburnt mixture in a re-ignition process. The determination of the flow pattern revealed the presence of an inner weak recirculation zone in the nozzle vicinity and a dominant external recirculation zone. The examination of the probability density function of the velocity measurements was also found to be a very useful tool in terms of the analysis of the turbulence structure of the flow. The bimodal distribution in the shear layer between the downstream flow and the recirculated gases yields the existence of large scale eddies. Finally, the significant reduced NO_x emissions in the lean area were also shown by means of emission measurements for elevated pressure conditions.