EVALUATION OF EQUILIBRIUM AND NONEQUILIBRIUM EVAPORATION MODELS FOR MANY-DROPLET GAS-LIQUID FLOW SIMULATIONS

A variety of liquid droplet evaporation models, including both classic al equilibrium and nonequilibrium Langmuir-Knudsen formulations, are e valuated through comparisons with experiments with particular emphasis on computationally efficient procedures for gas-liquid flow simulatio ns. The models considered are those used in droplet laden flow calcula tions such as direct numerical simulations for which large numbers of individual (isolated) droplet solutions are obtained. Diameter and tem perature evolution predictions are made for single-component droplets of benzene, decane, heptane, hexane and water with relatively large in itial sizes similar to 1 mm vaporizing in convective air flows. All of the models perform nearly identically for low evaporation rates at ga s temperatures significantly lower than the boiling temperature. For g as temperatures at and above the boiling point, large deviations are f ound between the various model predictions. The simulated results reve al that non-equilibrium effects become significant when the initial dr oplet diameter is <50 mu m and that these effects are enhanced with in creasing slip velocity. It is additionally observed that constant prop erties can be used throughout each simulation if both the gas and vapo r values are calculated at either the wet-bulb or boiling temperature. The models based on the Langmuir-Knudsen law and a corrected (for eva poration effects) analytical heat transfer expression derived from the quasi-steady gas phase assumption are shown to agree most favorably w ith a wide variety of experimental results. Since the experimental dro plet sizes are all much larger than the limit for non-equilibrium effe cts to be important, for these conditions the most crucial aspect of t he current Langmuir-Knudsen models is the corrected analytical form fo r the heat transfer expression as compared to empirical relations used in the remaining models. (C) 1998 Elsevier Science Ltd. All rights re served.