Modelling of microwave heating of water in a monomode applicator – Influence of operating conditions

Abstract

In this work, microwave induced natural convection in water was investigated experimentally and numerically. The modelling results showed that local overheatings (hot spots) are formed during microwave heating of water in a resonant microwave field. They are mainly localized in the axis of the heated liquid. As a result, natural convection develops which is chiefly induced in the strongest hot spot. The experimental and modelling approaches demonstrated that two characteristic periods can be distinguished in the time dependent temperature profiles of hot spots. While the microwave induced natural convection still develops in the first period of microwave heating, circulation of the liquid begins in the second. Moreover, it was shown that the deviation of only 5 MHz from the nominal frequency (2.45 GHz) changes the absorbed microwave power in water by about 20%. However, the simulated flow patterns at different microwave frequencies in the range between 2.4 and 2.5 GHz demonstrated that the primary circulation loop due to microwave induced convection has basically the same character – the upward direction in the axis of water and the downward direction near the PTFE cylinder wall. The obtained results also showed that the natural convection is not able to equalize the temperature over the entire volume of the liquid.