Droplet Buildup and Water Retention Prediction in Condensation Processes

A numerical model for the prediction of the buildup of a moist air condensate layer is presented. The model simulates the process of birth, growth, coalescence, and possible motion of each individual condensate droplet. Due to its computational intensity, such an approach is not feasible for the simulation of complex configurations of industrial interest, but allows for the identification of the major parameters influencing the water layer build up process. In particular, the model requires, as input data, the values of droplet advancing and receding contact angles, and computes the unsteady evolution of the droplet distribution. The droplet movement, as well as the moving droplet velocity, is detected by a force balance. Proper average values of practical interest, such as wetted area and water retention, are evaluated and compared with published experimental data. Such average data can be used as calibration values for global integral parameters usually required in order to incorporate the water layer effect within CFD computations in more complex geometries. The results show a good agreement with the experimental data and confirm that our model, although based on a small number of independent parameters (essentially the contact angles) and a relatively simple schematization of the complex physics behind the droplet history, predicts the water retention well within engineering accuracy. In particular, the hysteresis between advancing and receding angles seems the dominant effect.