AN EXPERIMENTAL-STUDY ON DETAILED FLAME STRUCTURE OF LIQUID FUEL SPRAYS WITH AND WITHOUT GASEOUS FUEL

A spray has a complex group structure under the influence of eddies that vary greatly in size. It is highly probable, therefore, that its flame also has a complex group structure corresponding to the two-phase structure. In order to observe such a flame structure in detail, the light emissions in OH and CH bands, and the pedestal (Mie-scattering) and Doppler (velocity) signals of a laser Doppler velocimeter (LDV) were monitored simultaneously at and downstream of the flame front of sprays with and without gaseous fuels (dual-fuel/single-fuel sprays). These four signals were analyzed statistically and spectrally into the cross correlation, coherence, and phase. It was found that the transition in flame structure from a dual-fuel to a single-fuel spray was not radical but mild, and that the region of gas-phase reaction moved gradually from inside to outside of a droplet cluster. At the same time, small regions burning in diffusion mode appeared within the cluster burning generally in premixed mode. For a dual-fuel spray of a small liquid-fuel fraction, large-scale mother clusters burned in a premixed mode on the whole, whereas small-scale subclusters involved therein burned in an external group combustion mode. As the clusters moved downstream and evaporation of droplets proceeded, the flame of a single-fuel spray gradually approached that of a dual-fuel spray. The present study confirms the Fourier-like complex structure of the flames of dual-fuel/single-fuel sprays.