Effects of Spacing and Arrangement of Droplet on Combustion Characteristics of Monodispersed Suspended-Droplet Cluster Model under Microgravity

摘要

For bridging between knowledge on droplet combustion and spray combustion, an experimental study was performed on autoignition and combustion of multiple droplet clusters. The monodispersed suspended-droplet cluster (MSDC) model with which arrangement, spacing and initial diameter of the droplet are well controlled has been developed. The effects of spacing and arrangement of droplet on combustion characteristics of the MSDC model in a high-temperature air were examined using microgravity environment in a drop shaft. The ignition delay and the burning time increased with decreasing the droplet spacing, regardless of the droplet number and the model dimensions. Larger droplet number with three-dimensional (3D) hexagonal closest packing (HCP) structure model resulted in longer ignition delay and longer burning time. 3D cubic closest packing (CCP) structure model showed rather longer ignition delay and much shorter burning time than 2D model. For 3D HCP model, an individual flame which enveloped each droplet was formed whole in the combustion duration with larger droplet spacing, while the group flame was formed whole in the combustion duration with smaller droplet spacing. When the droplet spacing was in the intermediate range, each droplet was ignited to form the individual flame, and each flame merged into the group flame. The diameter of the burning sphere decreased at the beginning of combustion, and turned to increase afterward. The transition from the individual flame to the group flame occurred around the time when the burning sphere diameter reached its minimum. The burning sphere diameter relative to the model diameter increased with decreasing the droplet spacing in the middle stage of combustion.