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 mi xing. Thus, inevitably, the spray flame also has complicated group str ucture, riot only corresponding to the complexity of two-phase flow, b ut 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 withou t the influence of the atomization process, the light emissions in the OH- and CH-bands, and Mie scattering from droplets, were monitored si multaneously in the flame of a premixed spray, i.e., a two-phase strea m with minimal slip between gas and droplets. These three kinds of opt ical signals were analyzed statistically and spectrally to yield autoc orrelation, cross-correlation, phase, and coherence in order to obtain the time-mean characteristics of the droplet clusters. The diameter a nd velocity of droplets in the flame were also monitored using a phase Doppler anemometer (PDA). It was confirmed experimentally that the bu rning mode of droplet clusters changed from external group combustion to internal group combustion as the evaporation and combustion of drop lets proceeded; that is, the combustion reaction occurred first outsid e the droplet clusters by preferential flame propagation through easy- to-burn regions of gaseous fuel and minute droplets, and then as the l ength scale of clusters decreased along the flow direction, the combus tion zone gradually invaded the clusters, and eventually small, dense clusters, or single droplets, burned in a diffusion combustion mode, a ccompanied by solid-body light emission from soot particles.