Observation of Droplet Cluster Behaviors in a Premixed-Spray Flame by Laser Tomography

Laser tomography consisting of a high-speed digital CCD camera and an Ar-ion laser sheet were applied to a premixed-spray flame to visualize droplet cluster behaviors and to examine the formation and combustion processes of droplet clusters in the flame. By monitoring a time-series of cross-sectional images of the premixed­spray, it became clear that there were no distinct droplet clusters in the non-combusting case. When it was ignited, on the other hand, some portions of the spray stream disappeared rapidly probably due to the flame propagation through easy-to-bum regions, and unburned regions remained as droplet clusters flowed downstream. Furthermore, the droplet clusters seemed to act as sources of fuel-vapor and burned in the diffusion combustion mode, because their shape and size did not change rapidly in the downstream. Based on these observations, it was confirmed that, in the process of cluster formation, the phenomenon was very rapid and dominated by the flame propagation, and once the clusters were formed, the burning process of clusters was relatively slow and dominated by the diffusion combustion. In order to quantify the above mentioned phenomena, we defined cluster-based disappearance rate, ω, in stead of the conventional droplet evaporation rate or burning rate. As a result, ω was large in the upstream region of the spray flame where the flame propagation occurred intensively, while ω was relatively small in the downstream region where diffusion combustion was dominant.