Quasi-steady evaporation of deformable liquid fuel droplets

TL;DR

This study investigates how droplet deformation affects evaporation rates of liquid fuel droplets under convective flow, using DNS simulations to analyze the influence of Weber and Reynolds numbers on evaporation behavior.

Contribution

It introduces a quasi-steady evaporation model for deformable droplets and validates it against established correlations, revealing the impact of deformation on evaporation rates under various flow conditions.

Findings

Evaporation rate weakly depends on Weber number at low Reynolds numbers.

High Weber numbers significantly increase evaporation rates at high Reynolds numbers.

Deformation influences local evaporation flux distribution and enhances mixing in the droplet wake.

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 $1-12$ 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 $5 \%$. \textcolor{black}{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.