An experimental and CFD modeling study of suspended droplets evaporation in buoyancy driven convection

A detailed analysis on the evaporation of acetic acid and ethylene glycol droplets is performed experimentally and numerically. The isolated droplet is positioned in a combustion chamber, suspended on a thermocouple and evaporated in buoyancy driven convection, following the thermal history throughout the droplet lifetime. The experiments provide quantitative and qualitative data on the evaporation physics of acetic acid, ethylene glycol and their mixture. The data are then modeled adopting the multiphase CFD code DropletSMOKE++, describing the flow field around the droplet, the heating rate and the evaporation process. The main novelty introduced in this work is a multiregion approach to describe the solid fiber, which allows to model the conjugate heat transfer with the liquid and the gas phase, as well as its impact on the droplet evaporation. DropletSMOKE++ results show a good agreement with the experimental data, regarding both the diameter decay and the liquid temperature, whose internal distribution in the liquid is shown to be highly affected by the heat flux from the fiber (which can contribute up to 30-40% in the total heat flux on the droplet). The effect of the thermocouple on the evaporation rate has been highlighted simulating the same experiments considering the solid as adiabatic, showing in this case a large underprediction of the vaporization rate and confirming the need of a detailed model for the fiber to correctly predict the vaporization phenomenon. The mixture evaporation has been investigated, emphasizing the importance of adopting a detailed thermodynamic model (which includes activity coefficients) and the impact of the mixture non-ideality on the evaporation process. The mixture also exhibits preferential vaporization, facilitated by the internal convection in the liquid phase.