Observation of combustion characteristics of droplet clusters in a premixed-spray flame by simultaneous monitoring of planar spray images and local chemiluminescence

In order to better understand the combustion behavior of spray flames, simultaneous measurements of droplet cluster visualization using laser tomography and local OH chemiluminescence and CH-band emission using a newly developed optical probe system named the Multi-color Integrated Cassegrain Receiving Optics (MICRO) are applied to a premixed-spray flame. Time-series planar images of droplet clusters and their transient structures during combustion are examined using an Ar-ion laser and a high-speed digital CCD camera. By observing the droplet clusters and local chemiluminescence simultaneously in the premixed-spray flame, it is confirmed that some portions of the spray stream disappear very rapidly due to preferential flame propagation, while other portions of the spray stream survive over a long period to form droplet clusters, disappearing gradually from their outermost portions, which seems similar to a diffusion flame. The disappearance speed of individual droplet clusters in the premixed-spray flame, instead of a conventional evaporation rate of a single droplet, is defined and calculated by processing the obtained droplet-cluster planar images. The disappearance speed for rapid preferential flame propagation through easy-to-burn regions in the upstream region of the flame is about 2.5 m/s. On the other hand, the disappearance speed when droplet clusters burn dominated by a diffusion combustion mode in the downstream region of the flame is approximately 0.45 m/s.