The flame properties surrounding a small size fuel droplet are difficult to investigate due to the infinitesimal size, weak chemiluminescence and relatively fast combustion process. The present work focuses mainly on the flame colour detection and analysis of the combustion of an isolated fuel droplet. The tested fuels are diesel and its water emulsions at different mixing concentrations. High speed colour imaging and digital image processing have been applied for tracking droplet ignition, secondary atomization, and flame chemiluminescence. Side ignition of the droplet reduced interferences in visualizing flame spread during ignition and propagation. Flame propagation starts from the heating side all around the droplet, before the development of the more dominant yellow sooty flame. Flame chemiluminescence during the overall droplet lifetime has been evaluated using the digital flame colour discrimination (DFSC) approach of analysis. This analysis has been used for demonstrating the effect of emulsifying diesel by water in enhancing its combustion characteristics. Droplet secondary atomization and sub-droplet generation has also been tracked and analysed. The chemiluminescence of the flame surrounding each individual sub-droplet has been analysed using the same DFSC scheme. From the analysis, it is shown that the sub-droplets burn in a premixed like mode, especially at a later stage, rather than the diffusive mode of the original droplet.
Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016.