Experimental analysis of a multi-magnetron microwave dryer prototype: electromagnetic field distribution and dielectric characterisation using apple pomace

Abstract

This study focuses on design, development, and evaluation of a prototype microwave dryer equipped with ten magnetrons, intended for the uniform and energy-efficient drying of high-moisture biomass. Apple pomace, a by-product of the apple juice industry, was selected as a representative model material due to its high water content and dielectric variability. The study focuses on the characterisation of electromagnetic field distribution inside the dryer chamber and the dielectric properties of apple pomace at varying moisture contents (10% and 70%). Experimental field mapping was conducted using electric and magnetic field probes, while dielectric parameters were extracted using coaxial transmission line measurements and NRW computational methods. Simulation results revealed significant field inhomogeneity due to multimode resonances and phase discrepancies between magnetrons. Drying performance was evaluated with respect to wave penetration depth, absorption uniformity, and power matching efficiency under different material loads. The results suggest that material moisture below 50% enables deeper microwave penetration and more stable operation. Although microwave drying of moist biomass has been extensively studied in theory and at laboratory or pilot scale, no industrial-scale systems currently exist that process apple pomace. The findings offer a reference for improving the construction and electromagnetic performance of industrial-scale microwave dryers, with relevance for various bio-based applications, including feedstock stabilisation and thermal processing.