Observation of combustion mechanisms of acoustically levitated droplet clusters

In order to elucidate the microscopic combustion behaviors of droplet clusters, we took advantage of droplet clusters suspended in the air almost at the same position using an ultrasonic levitator. We observed images of OH Laser Induced Predissociative Fluorescence (LIPF) and Mie-scattering of droplets simultaneously to examine relations between the combustion reaction zone and unburned droplet regions for confirming droplet group combustion mode of the clusters. In addition, we observed time-series images of Mie-scattering of burning droplets illuminated by a laser-sheet, and light emission in CH band at the same time. Simultaneously at a specific point in the imaged region, OH radical chemiluminescence was detected by specially designed light collection optics, Multi-color Integrated Receiving Optics (MICRO), and velocity and diameter of droplets were measured by Phase Doppler Analyzer (PDA) to obtain Characteristics of spray. We calculated the diameter of each droplet in a cluster based on the Mie-scattering theory, and derived droplet group combustion number of the droplet cluster in time-series. As a result, number density of droplets in the cluster decreased monotonically in case of the low number-density clusters. In contrast, number density of droplets in the cluster decreased first and kept constant, and then decreased again in case of the high number-density clusters. This is explained due to the fact that the droplet group combustion mode changed from the internal group combustion mode to the single droplet combustion mode.