Numerical modelling of liquid heating and boiling phenomena under microwave irradiation using OpenFOAM

摘要

In this work, a new successive multiphysics model that describes Microwave (MW) water heating until boiling condition was developed in OpenFOAM and validated against experimental data. The model was used to investigate the physics associated with MW water heating and boiling. The effect of anti-bumping granules was included in the study. The simulation results showed that during the MW heating stage (below the saturation temperature), the temperature distribution in the water sample is stratified, with high temperatures in the upper region and low temperature in the lower region. The temperature uniformity in the sample deteriorates significantly with the heating time, but with a decreasing rate due to the intensification of the natural convection. During the initial phase of the boiling process, the simulation results showed that the water sample is considerably superheated as no vapor bubble is formed in the bulk liquid and evaporation is only able to take place at the free surface. After a period , the water temperature at the anti-bumping granules reaches the saturation temperature and vapor bubble nucleation occurs. The vapor bubbles remove most of the superheat in the water and prevent the accumulation of superheat, mainly by enhancing the heat transfer to the liquid free surface and minorly by inducing evaporation within the bulk liquid. The results also showed that under MW irradiation, free surface evaporation dominates the overall evaporation process (about 90%). The models and findings could be useful to understand MW assisted processes that involve boiling, such as MW distillation process.