GROUP COMBUSTION BEHAVIOUR OF DROPLETS IN A PREMIXED SPRAY FLAME

A spray has a complex group structure under the influence of eddies which have a coherent structure and appear in the process of atomizing and mixing. Thus, its flame has inevitably a complicated group structure corresponding to the original two phase flow, and changes its structure depending on the characteristics of droplets, such as diameter, number density, mixing and vaporization rates, slip velocity, etc. In order to observe a detailed structure of spray flames with no influence of the atomization process, the light emissions in OH and CH bands, and Mie-scattering from droplets were monitored simultaneously in the flame of a premixed spray, i.e., a droplet suspension stream with minimal slip between gas and droplets. These three kinds of optical signals were analysed statistically and spectrally into auto-correlation, cross-correlation, phase and coherence. Diameter and velocity of droplets in the flame were also monitored using a phase Doppler anemometer. It was confirmed experimentally that the burning mode of droplet clusters changed from external group combustion to internal group combustion as the evaporation of droplets proceeded: the combustion reaction occurred first outside the droplet clusters by the preferential flame propagation through the fuel vapor originated from droplets, and then as the droplet number density of clusters decreased along the flow direction, the combustion zone gradually propagated into the clusters, and eventually dense small clusters or single droplets burned in diffusion combustion mode.