Experimental investigation of the stability mechanism and emissions of a lifted swirl non-premixed flame

We report on the experimental investigation of a confined lifted swirl non-premixed flame by applying a novel Airblast nozzle [1]. 3D-Laser Doppler Anemometry, a non-intrusive, laser-based measurements 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 pre-combustion 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 large vortices along the jet, could possibly react with fresh unburnt mixture in a reignition 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 (PDF) 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 yield the existence of large scaled eddies in the shear layer between the downstream flow and the recirculated gases. Finally, the significant reduced NOx emissions in the lean area were also shown by means of emission measurements for elevated pressure conditions.