Simulation of droplet evaporation under microgravity with a focus on irreversibility and entropy production

Droplet evaporation is a fundamental process in spray combustion systems, directly influencing ignition delay, combustion efficiency, and emissions. This study presents a comprehensive transient numerical model for single-component n-heptane droplet evaporation in a microgravity environment. The model captures spherically symmetric evaporation dynamics, solving conservation equations for mass, species, energy, and entropy in both gas and liquid phases under varying ambient conditions. Entropy generation, a key indicator of thermodynamic irreversibility, is incorporated to assess transport inefficiencies from heat and mass transfer. Model validation against experimental microgravity data shows good agreement in predicting the temporal evolution of droplet size and surface temperature, especially at higher ambient temperatures, with an average R2 of 0.83 and an average RMSE of 0.08 across the tested conditions. Results reveal that entropy generation is dominated by heat transfer in the gaseous phase, while mass transfer plays an increasingly significant role over time. The findings highlight the complex interplay between heat conduction, latent heat removal, and species diffusion, providing a thermodynamic basis for improving evaporation modelling in combustion and energy-conversion systems. This study lays the groundwork for future integration of chemical kinetics and combustion into entropy-informed droplet modelling frameworks.