High-efficiency and uniformity continuous-flow microwave heating system based on impedance gradient structure

Continuous-flow microwave-assisted heating has been extensively applied in chemical engineering. A common method used for heating fluids is by using a tube in a cavity. However, it is challenging to maintain high heating efficiency owing to the temperature-dependent dielectric properties of different fluids, and the temperature of fluids is usually uneven. In this study, a multilayer ring structure is proposed based on an impedance gradient that covers a tube with a porous material inside it to achieve high heating efficiency and uniformity. A multiphysics model, including electromagnetic fields, fluid heat transfer, and free and porous media flow, was established to simulate the continuous-flow microwave heating process. The dimensions of the multilayer ring structure were optimized and manufactured. Energy utilization efficiency experiments and continuous heating experiments were conducted, which demonstrated that the proposed model achieved an efficiency > 90% with different aqueous ethanol solutions, while maintaining high heating uniformity compared with other heating models. Furthermore, the effects of the tube permittivity and porosity of the porous material on the heating efficiency were investigated to demonstrate the robustness of the proposed model.