Evaluation of Droplet Evaporation Models and the Incorporation of Natural Convection Effects

Abstract

Evaporation of fuel droplets in high temperature gas environment is of great importance in many engineering applications. There are already several theoretical models proposed in the literature to represent this phenomenon by considering mass and energy transfer between the droplet and the surrounding gas. For that reason, this work aims to evaluate droplet evaporation models that are usually used in spray combustion calculations, including equilibrium and non-equilibrium formulations. In order to validate and assess these theoretical models predictions, an in-house code was developed and diameter evolution results from the numerical simulations are compared against experimental data. First, the models performance are evaluated for water in a case of low evaporation rate and; then, they are evaluated for n-heptane moderate and high evaporation rates using recent experimental data acquired with a new technique. Furthermore, the incorporation of natural convection effects on the droplet evaporation rate by using an empirical correlation is investigated. The Abramzon-Sirignano model is the only one which does not overestimate the evaporation rate for any ambient conditions tested when compared with experimental rate. The results also reveal that when a correction factor for energy transfer reduction due to evaporation is incorporated in the classical evaporation model, the predictions from this model and the non-equilibrium one cannot be differentiated, even if the initial droplet diameter is small. Additionally, taking natural convection effects into account by adding the Grashof number into the Ranz-Marshall correlation for Nusselt and Sherwood calculations actually overestimates the evaporation rate for droplet evaporation under atmospheric pressure.