Group combustion behavior of droplets in a premixed-spray flame

A spray has complex group structure under the influence of eddy motion of the pow field which appears in the processes of atomization and mixing. Thus, inevitably, the spray flame also has complicated group structure, riot only corresponding to the complexity of two-phase flow, but also depending on the characteristics of droplets, such as diameter, number density, mixing and vaporization rates, slip velocity, and so on. bt order to observe the detailed structure of spray flames without the influence of the atomization process, the light emissions in the OH- and CH-bands, and Mie scattering from droplets, were monitored simultaneously in the flame of a premixed spray, i.e., a two-phase stream with minimal slip between gas and droplets. These three kinds of optical signals were analyzed statistically and spectrally to yield autocorrelation, cross-correlation, phase, and coherence in order to obtain the time-mean characteristics of the droplet clusters. The diameter and velocity of droplets in the flame were also monitored using a phase Doppler anemometer (PDA). It was confirmed experimentally that the burning mode of droplet clusters changed from external group combustion to internal group combustion as the evaporation and combustion of droplets proceeded; that is, the combustion reaction occurred first outside the droplet clusters by preferential flame propagation through easy-to-burn regions of gaseous fuel and minute droplets, and then as the length scale of clusters decreased along the flow direction, the combustion zone gradually invaded the clusters, and eventually small, dense clusters, or single droplets, burned in a diffusion combustion mode, accompanied by solid-body light emission from soot particles.