Tunable single mode resonant cavity suitable for continuous flow microwave processing of liquids

In this paper, the design and realisation of a single mode cylindrical resonant cavity suitable for continuous flow microwave processing of liquids is presented. The applicator utilizes a slightly perturbed TM₀₁₀ mode and the processed liquid flows through a Teflon tube of helix form placed in the applicator. The orientation of the tube within the applicator is mainly perpendicular to the electric field of the excited electromagnetic mode and, therefore, the resonance frequency of the cavity varies only slightly with the material properties of the processed liquid. This geometry furthermore allows fine-tuning of the cavity (shift of the resonance frequency about up to about 30 MHz) using a dielectric low loss cylinder (tuning element) in the axis of the cavity. Furthermore, an applicator-specific automatic tuner of simple construction and low cost was constructed to keep the applied microwave power constant during the processing of the liquid (independent of possible changes of the liquid material properties) as a function of temperature.