Lagrangian modeling of droplet evaporation

Abstract

Evaporation of liquid droplets in high temperature gas environment is of great importance in many engineering applications. Accurate droplet evaporation predictions are crucial in modeling spray combustion, since it is considered a rate limiting process. For this reason, the present dissertation aims are, first, to implement and validate Lagrangian droplet evaporation models that are usually used in spray calculations, including equilibrium and non-equilibrium formulations, and, second, to use these models to pursue a deeper insight on the physical phenomena that may be involved in droplet evaporation processes. In order to validate and assess these theoretical model predictions, an in-house code was developed and diameter evolution results from the numerical simulations are compared to experimental data. First, the model performance is evaluated for water in a case of low evaporation rate and, then, it is evaluated for n-heptane in moderate and high evaporation rates using recent experimental data acquired with a new technique. The Abramzon-Sirignano model is the only one which does not overestimate the evaporation rate for any ambient condition tested, when compared with experimental rate. From the results, it is also revealed 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. Furthermore, the incorporation of natural and forced convection effects on the droplet evaporation rate, by using an empirical correlation, is investigated, showing that including the Grashof number into the Ranz-Marshall correlation actually overestimates the evaporation rate for atmospheric pressure. Finally, the effects of ambient conditions on ethanol evaporation are investigated. Under ambient temperatures higher than the threshold temperature, the evaporation rate is enhanced with the increase of ambient pressure, contrary to what happens for cases when the ambient temperature is lower than the threshold temperature.