Quasi-steady evaporation of deformable liquid fuel droplets

Abstract

This work covers the effect of droplet deformation on its evaporation rate under convective flow conditions. The evaporation behavior of a freely deforming droplet of single component jet fuel surrogate, n-decane, is investigated by varying Weber number ($We$) from $1\text{–}12$ and Reynolds number ($Re$) from 25 to 120 under high-pressure environment. These studies utilize interface capturing Direct Numerical Simulation (DNS). To validate the accuracy of the solver, the results are compared against correlations by Abramzon and Sirignano (Int. Journal of Heat and Mass Transfer, 1989) and are found to be in good agreement with a maximum difference of 5%. A quasi-steady evaporation approach is implemented to simulate this problem. The results suggest a weak dependency of normalized total evaporation rate (${\dot{m}}_{ND}$) on Weber number at low $Re$ flow. However, a strong correlation is seen between the total evaporation rate and $We$ at high $Re$. 20% enhancement in ${\dot{m}}_{ND}$ is observed at $We=12$ (highly deformed shape) when compared to $We=1$ at $Re=120$.

In these cases, the distribution of local evaporation flux on the droplet is found to be proportional to its curvature up to the point of flow separation which agrees with low $Re$ theories on droplet evaporation by Tonini and Cossalli (International Journal of Heat and Mass Transfer 2013), Palmore (Journal of Heat Transfer 2022). Beyond the flow separation point, evaporation flux distribution depends on the boundary layer development and flow evolution downstream of the droplet. For highly deformed droplets, a larger wake region creates favorable fuel vapor gradients and promotes mixing in droplet wake, hence higher evaporation flux.