Numerical analysis of the autoignition of isolated wet ethanol droplets immersed in a hot and humid air atmosphere

Results of time-dependent, spherically symmetrical computations of the vaporization and combustion of ethanol and ethanol/water droplets are reported. Mixture-average transport was employed, along with a systematically reduced chemical-kinetic mechanism involving 15 overall steps among 17 chemical species, to speed the computations by a factor of about 100 over what would be required if full detailed chemistry had been used. Absorption of water from the gas surrounding the droplet and its diffusive transport within the liquid phase were taken into account, providing excellent agreement with previous experimental and computational results for the combustion of ethanol droplets in air. On the other hand, the assumption of rapid liquid-phase mixing produced very poor agreement when water condensation on the droplet surface or hydrous ethanol are considered. To characterize autoignition, we define the critical autoignition temperature Tc ∞ as the critical ambient temperature below which autoignition is not observed. Computations for autoignition of cold ethanol/water droplets in air showed that Tc ∞ decreases with increasing initial droplet diameters. In the range of parameters under consideration, ignition was found to take place always before complete vaporization of the droplet, and the ignition time was found to become longer with the increasing initial water content of the liquid ethanol droplet. On the contrary, addition of water vapor to the initial air atmosphere was found to shorten the ignition time, increasing ethanol vaporization rate as a consequence of the extra heat release associated with water absorption into the liquid.The authors want to express their gratitude to Professor Forman Williams in the conception and guidance of this work, in particular, and all the ongoing work on ethanol droplet vaporization and combustion. This work was supported by the projects ENE2015-65852-C2-1-R and PID2019-108592RB-C41 (MINECO/FEDER,UE)