An experimental investigation on the vaporization and combustion of methanol and ethanol droplets

The vaporization and combustion of freely-falling methanol and ethanol droplets in dry and humid environments has been studied experimentally. From time-resolved data of the droplet size and composition, it is demonstrated that water vapor, either from the ambience or generated at the flame, can freely condense at the droplet surface and subsequently dissolve into the droplet interior. The associated condensation heat release and dilution of the droplet alcohol concentration can significantly modify the gasification behavior from that of the d²-law. Specifically, droplet gasification is characterized by an initial period during which the gasification rates of methanol and the entire droplet are enhanced due to the condensation heat release, an intermediate period during which water vapor still condenses although the overall droplet gasification rate is retarded due to water build-up, and a final, slow-gasification period during which the initially condensed water is gasified. The stucy further demonstrates that the alcohol droplets can sustain diffusional combustion even with substantial amounts of water dilution, and identifies a neutral state of water condensation/gasification such that the associated data can be advantageously used for numerical modeling of alcohol droplet combustion.