Evaporation of Finite-Size Ammonia and n-Heptane Droplets in Weakly Compressible Turbulence: An Interface-Resolved DNS Study

Abstract

This study presents direct numerical simulation (DNS) of finite-size, interface-resolved ammonia and n-heptane droplets evaporating in decaying homogeneous isotropic turbulence. Simulations are conducted for each fuel to model the dynamics in a dense spray region, where the liquid volume fraction exceeds $\mathcal{O}(10^{-2})$. The focus is on investigating the complex interactions between droplets, turbulence, and phase change, with emphasis on droplet-droplet interactions and their influence on the evaporation process. The present study also explores how varying turbulence intensities affect the evaporation rates of each fuel, unveiling the differences in the coalescence and energy transfer from the liquid to the gaseous phase. The results reveal that, when comparing ammonia with n-heptane with equal liquid volume fractions, ammonia exhibits faster initial evaporation due to its higher volatility. However, this rate declines over time as frequent droplet coalescence reduces the total surface area available for evaporation. When numerical experiments are initialized with equal energy content, increasing turbulence intensity enhances the evaporation of n-heptane throughout the simulation, while ammonia evaporation soon becomes less sensitive to turbulence due to rapid vapor saturation. These findings are relevant to improving predictive CFD models and optimizing fuel injection in spray-combustion applications, especially under high-pressure conditions.