Optical characterization of droplet clusters and group combustion in spray diffusion flames

The clustering of fuel droplets and group combustion of droplet clusters are experimentally investigated in a spray burner. The burner was operated in the presence and absence of coflowing air around a pressure swirl injector which generates a kerosene spray. The Mie scattering images of the spray droplets were recorded by PIV technique, while planar measurement of droplet size was achieved by application of the ILIDS technique. The photographs of the spray flame and droplets gave evidence of distributed flame structure with the inner reaction zone containing several small diffusion flames indicating group burning of droplet clusters. The invariance of average droplet size downstream of the injector exit further supported the above observation and suggested significant reduction of gasification rate of droplets within the clusters. Application of Voronoi analysis to focused images of the spray droplets facilitated characterization of clusters of droplets. Accordingly, the Group combustion number was evaluated for each droplet cluster based on the measured cluster size and inter-droplet distance. The results reveal multi-scale droplet clustering and multi-mode combustion of the clusters during the spray burning process. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.