Evaluation of Droplet-Evaporation Models for Gas/Liquid Flow

A report presents an evaluation of eight mathematical models of the evaporation of liquid droplets — models that are used in the numerical simulation of a variety of gas/liquid flows, including cooling sprays, burning liquid-fuel sprays, fire-suppression sprays, and air/fuel-premixing flows in combustors. Included in the study were two versions of a classical model that includes transient drop-heating effects, four versions of a heat-mass-transfer-analogy model, and two nonequilibrium models based on the Langmuir-Knudsen evaporation law. The models were used to predict evolutions of droplet diameters and temperatures, and the predictions were compared with experimental observations, for droplets of benzene, decane, heptane, hexane, and water vaporizing in convective airflows. All models performed nearly identically at low evaporation rates at gas temperatures significantly lower than the liquid-boiling temperatures. For gas temperatures at and above boiling temperatures, there were large deviations among the various model predictions. Nonequilibrium effects were found to become significant for initial droplet diameters <50 µm, and to increase with slip velocity. The models based on the Langmuir-Knudsen law agreed most closely with the experimental results, though not because they account for nonequilibrium effects; instead, the superiority of these models was attributed to the incorporation of a corrected heat-transfer equation.