High-efficiency continuous-flow microwave heating system based on metal-ring resonant structure

Continuous-flow microwave heating has been widely applied in liquid food processing, and the heating efficiency relies heavily on the dielectric property of loads with fixed dimensions in a certain cavity. Specifically, rectangular waveguides are widely applied, and straight pipes are usually inserted through the center of rectangular waveguides. However, maintaining high efficiency is always challenging since the dielectric property will vary with temperature. Therefore, a simple and cheap resonant structure is proposed in this paper to maintain high heating efficiency for loads with a large range of relative permittivity in both modeling and experiment. First, the straight pipe surrounded by metal rings in a rectangular waveguide with a short circuit terminal was introduced to concentrate the electric field in the pipe. Then, a multiphysics model was established to calculate the energy efficiency of various load permittivity. The position of the short circuit terminal and the dimensions of the metal ring were further optimized. Finally, energy efficiency experiments of different aqueous ethanol solutions and liquid foods and point temperature rise experiments of continuous-flow water were carried out, and the results showed that the proposed structure can be used to maintain high energy efficiency over large dielectric dynamic ranges compared with the conventional microwave heating system based on rectangular waveguides. Moreover, the effect of different parameters, the pipe permittivity, the pipe wall thickness and the metal ring gap, on the heating efficiency was discussed to show the robustness of the proposed structure.