State of the art of the fundamental aspects in the concept of microwave-assisted heating systems

As the processing of materials using clean energy becomes increasingly important, indirect conventional heating is being replaced by microwave-assisted heating. This method transforms energy directly into heat, providing rapid heating with instantaneous start-up, improved control and homogeneity of temperature distribution, and enabling a compact design. Therefore, the impact of microwaves in industry is growing in recent years. The complexity of incorporating microwaves into any industrial process resides in the need for a customised design. This is emphasised in the case of continuous flow microwave heating systems where the electromagnetic distribution and its transformation into heat, as well as the dynamics of the fluid to be heated under different operating conditions, are solved. For this purpose, and apart from the dielectric and magnetic properties of the material to be heated, which define how it behaves in relation to electric and magnetic fields when exposed to microwaves, there are several factors influencing the design of the process: the microwave generator and the transmission line, the geometry of the applicator where the electromagnetic field is distributed, the material container of the sample or the temperature measurement techniques to control the process. Additionally, numerical simulation is an essential tool for designing and predicting the suitability of any microwave system. This is the case of composite materials processing, where the application of numerical simulation tools arises from the need to achieve advanced materials more efficiently. In this paper, these fundamental aspects for the design of a microwave-assisted heating system are reviewed, presenting examples of various applications where this microwave technology is integrated.