Experimental methods for evaluating heating uniformity in radio frequency systems

Heating uniformity is one of the most important considerations in developing radio frequency (RF) systems and processes in heating applications. Two different loads were used in this study to evaluate heating uniformity in a 12 kW, 27.12 MHz pilot-scale RF system with plate applicators. One method involved the use of a custom-designed water load, while the other used polyurethane foam. For the water load, 24 thermocouples simultaneously measured final temperatures of water and 1% carboxymethylcellulose (CMC) solution in both horizontal and vertical planes. Regulating the air gaps between the top electrode and the water or CMC samples resulted in different power couplings in the heating load ranging from 5.5 to 10 kW and corresponding heating rates of 5.3–9.4 °C min−1. The hottest spot was always located near the electrode centre. The temperature difference between the front and back sides of the RF cavity was less than 2 °C, but the difference between the right and left sides was as much as 10 °C. A relatively large zone was defined in the centre where the temperature difference was less than 4 °C. A symmetric design of the inductance positions and feeding strips improved the heating uniformity. The temperature distribution after symmetric design was further confirmed in polyurethane foams using a thermal imaging camera. Further research is needed to improve the heating uniformity with the help of computer modelling.